A

ABSOLUTE PHASE

In the recording and reproduction chain, there are many opportunities to reverse polarity, especially when hooking up speaker wire betwen the amplifier and the speaker. For most people there is no difference in the sound when the polarity is reversed. However, some audiophiles argue that there would be a difference in the sound if the polarity were to be changed for something like an explosion and if the reproduction could reproduce all the low frequencies in the sound. The sound of an explosion would have positive polarity, you see.

ACOUSTIC FEEDBACK

If a portion of the sound output from loudspeakers can somehow excite the turntable or stylus of the system, a new, unwanted signal is created, which is in turn amplified by the electronics and reproduced by the speakers. If this signal is in phase with the original signal at a particular frequency (usually bass) a tremendous resonance (oscillation, a form of positive feedback) can occur in the room. Even small amounts of acoustic feedback can cause a considerable amount of distortion.

The careful positioning of speakers and turntable is the usual remedy to the problem. Placing the turntable anywhere near the speakers is usually a mistake.

An effect similar to acoustic feedback occurs when vibrations from walking in the room are transmitted up to the turntable from the floor. Generally if a turntable is positioned so that it is free from this kind of vibration, it will also be safe from acoustic feedback.

CD players and amplifiers are a lot less susceptible to acoustic feedback, but it does exist, and precautions sometimes help.

Another example of acoustic feedback (this time at high frequencies) is the squeal that occurs in microphone hookups where output from the speaker re-enters the system through the microphone.

No matter what the frequency, even if the gain of the amplifier is not high enough to cause oscillation, distortion in the form of a boost in response at certain frequencies will result from acoustic feedback. This can cause even the best speakers to sound "boomy". Ironically, the general recent improvement in low frequency response of quality systems has simply intensified the problem, so that acoustic feedback has become a major problem in quality sound installations.

This is the company founded by Ed Villchur (see Dedication) and Henry Kloss (later of KLH and Advent). Here began the invention of acoustic suspension and many other principles and techniques of modern speaker design.

Mounting a speaker in a hermetic or air-tight enclosure traps a fixed quantity of air behind it. The springiness of this air can effectively replace the suspension spring used in speakers to return the speaker cone to its "normal" position, hence the term "acoustic suspension" (either "air suspension" or "pneumatic suspension" would be more accurate). The ADVENT, the AR 3, and the EPI 100 are (old) examples of acoustic suspension speakers.

Acoustic suspension speakers are traditionally much less efficient than bass reflex speakers and therefore need more powerful amplifiers.

The major advantage of the design is simplicity and therefore simpler manufacturing and lower initial cost. However, any savings may be eaten up by the need for a more powerful amplifier. This is the hidden but unavoidable cost of inefficient speakers. 

A method of reproducing sound waves that is analogous to how they are in air. Analog sound waves can be converted into digital information bits in an A to D converter.

back to top

B

BAFFLE

A baffle is a mounting for a speaker which assists sound radiation. A baffle is important because many sound wavelengths are longer than the speaker. The ideal baffle is flat and extends forever, creating a hemispherical sound wave.

The baffle is usually part of the cabinet. Sometimes special baffles are built for testing purposes.

When a small speaker is placed near the wall, the wall becomes part of its baffle.

The term infinite baffle is another way of describing the ideal baffle mentioned above. Actually, infinite baffle is often used to describe any situation where the back wave from the speaker is completely isolated from the front, as when a speaker is mounted in a hole in a wall.

A variation of the bass reflex design. A speaker enclosure is divided into two chambers with the woofer mounted between them. One of the chambers is then vented with a tuned port. The tuned vent acts like a filter which limits the frequency responce to a desired bandwidth, thus the term band-pass. This type of design is used primarily for sub-woofers and bass units of satellite systems.

BANDWIDTH

The range of frequencies in which amplitude remains constant. Bandwidth is generally the subject of much controversy and confusion, since there is no standard method of measurement. Should speaker bandwidth be measured in a living room or in an anechoic chamber? How far should the microphone be from the speaker: Should the mike record reflected sound? Etc.

bandwidth is another way of saying frequency response. 

(See SPEAKER SPECIFICATIONS)

A speaker in an infinite baffle or a sealed enclosure uses only its front wave to produce sound. In a bass reflex or tuned port design, part of the back wave of the speaker is used to reinforce the front wave usually in the bass.

The back wave excites a resonant system which then radiates sound. This resonant system has two components: mass (actually air in this case), and the stiffness of the air suspension in the box.

The resonant system using the backwave is far more efficient than the primary system. Very little power excites great sound from bass reflex systems in the frequency range to which the port is tuned (generally below 100 Hz).

The increased output obtained in a bass reflex design can be used by the designer either to improve efficiency or to extend low frequency response.

An advantage of bass reflex design over acoustic suspension design is that the relatively higher efficiency of the former permits the same amount of bass to be produced with far less woofer travel or excursion. Since this generally increases linearity of the response, it produces less distortion, particularly in the low bass.

(Also see BANDWIDTH, THIELE, TUNED PORT)

A speaker designed for low frequencies only, usually for use in conjunction with two or more satellite speakers. 

A bass unit is different from a sub-woofer in that it does not claim full bass coverage and it is designed for use with specific satellites.

(See DIRECTIONALITY.)

back to top

C

CAPACITOR

An electronic element in which two metallic plates are close to each other but not in contact, with voltage on one plate affecting the voltage on the other. A capacitor will not conduct a direct current, but will allow alternating current to pass.

Capacitors have an impedance with respect to frequency which declines at a regular rate of 6 dB per octave. At low frequencies a capacitor therefore has high L, making it an excellent filter for keeping low frequencies away from a tweeter. Therefore, capacitors are common elements in crossovers.

The magnitude or size of a capacitor in microfarads.

(See IMPEDANCE.)

DAMPING

Any form of resistance or friction, electrical, mechanical, or acoustic.

Most of the damping of woofers is caused by the amplifier (electrical damping). Fiberglass in speaker enclosures is another form of damping (acoustical damping). In tweeters damping may be accomplished by means of a magnetic fluid bathing the voice coil (See FERROFLUID).

In general, damping reduces the amplitude of resonance

The careful control of damping is important in speaker design because resistance or damping is a factor in the calculation of Q, which greatly modifies speaker response across a wide audible frequency range.

The ratio of a speaker's impedance to the output impedance of the amplifier which drives it. Since speaker impedance is usually around 8 ohms, and amplifier output impedance is generally less than .1 ohms, damping factors may reach or surpass 100.

Most amplifiers have comparable damping factors. An amplifier can be designed to create more damping effect, but it is not certain that a higher damping factor is better. Amplifier damping is often related to negative feedback, which is not necessarily a good thing. 

(See NEGATIVE FEEDBACK.) 

Damping fluid is not necessarily ferrofluid, but ferrofluid is so useful and versatile that it has completely replaced other types in loudspeakers.

(See FERROFLUID)

dB refers to the ratio between two power levels. 10 dB is equivalent to a power ratio of 10; 3 dB is equivalent to doubling; -3 dB is equivqalent to halving. 20 dB is multiplying by 100. 100 dB means multiplying by 10,000,000,000. 

(see SPL, DYNAMIC RANGE.)

A way of storing and processing bits of information in a format that is suitable for computers to manipulate. Before digital information can be heard from a loudspeaker it has to be converted to analog in a D to A converter.

(See ANALOG.)

At its highest frequencies, a speaker directs almost all of its sound straight ahead . At low frequencies, sound radiates equally in all directions. The former effect, referred to as "beaming", can be a problem in speaker design.

Beamed sound has a psychological effect which can be pleasant: it causes voices to sound more "up front". It can also be unpleasant.

STEREO EFFECT is exaggerated with beamed sound. If you get good dispersion, there's a lot of sound arriving at your ear in complex patterns: the first reflection, the second, etc. making it confusing to judge when the sound actually happened. The phase information used by the mind to derive the stereo image is emphasized when beamed. This is one of the appeals of earphones, where virtually all sound is beamed with no dispersion.

Some designers choose to use drivers in their directional range (or beaming range) in order to obtain the desirable effects of beaming. If the designer is trying to achieve a more linear speaker (with respect to dispersion) he will wish to eliminate beaming. This can be done by crossover design, using crossovers to deprive drivers of frequencies where they beam, or through driver design, since drivers can be designed so that they don't produce any sound at frequencies where they would beam. 

Though directionality is generally thought of as a high frequency problem, it also occurs at the midrange frequencies at the "high-end" of the woofer.

Dolby Labs is a company prominent in the design and manufacture of hardware and systems to modify the reproduction of sound, most notably for the "Dolby Prologic" and "Dolby Digital" systems.

(See HOME THEATER.)

The original appeal of the domed tweeter is that it physically resembles the spherical waveform which an ideal loudspeaker should produce. Actually, the real advantage is that the dome shape gives the tweeter a very rigid structure so that it does not deform at high frequencies as much as a cone tweeter, e.g. the dome shape resists deformation and therefore has a more uniform response.

The conventional domed tweeter is convex, with a voice coil of the same diameter.

Also see SOFT DOME TWEETER, INVERTED DOME TWEETER)

A speaker generally has some low frequency at which there is a dramatic increase in distortion; reviewers generally refer to this phenomenon as "doubling." The origin of the term may be the fact that most speakers have a tendency to produce the harmonics of any tone they are required to reproduce, and one such harmonic is double the original frequency. Also the loudness seems to double. In reality, it is the third harmonic which sounds less pleasant than the second, so that what is labeled doubling may in effect be trebling.

Another type of distortion often called doubling occurs when a speaker causes a low-frequency vibration in the grill-cloth assembly. Another occurs when a speaker goes beyond its normal limits of travel and hits up against its suspension or frame (bottoms out).

The difference between the loudest and the softest sounds. This is normally expressed in dB. The dynamic range of human hearing is about 130 dB. The dynamic range of symphonic music is about 100 dB. For a typical CD it is about 80 dB; for a typical LP, about 40 dB. The lower limit of dynamic range is determined by background noise; the more noise, the less dynamic range. The upper limit of dynamic range is determined by physical restraint: amplifier power, for example; for human hearing it is physical pain; for absolute sound, it is the elastic limits of air.

back to top

E

ELECTROSTATIC RADIATOR

The simplest form of electrostatic radiator is a capacitor in which one of the plates is allowed to move, causing sound waves. The force is created by the alternating electric field which comes from the amplifier. Adding a polarizing voltage to one plate of the radiator will usually improve efficiency and lower distortion, so many electrostatic designs use power supplies of their own. Very often high voltages are involved (up to 3000 volts) because efficiency increases with voltage. This voltage can be obtained from house current, in which case the speaker will plug into the wall, or from the audio signal. This latter technique causes some leakage and loss of signal, leading to clipping and a bit of distortion.

The area of the plate reached by electric current gets smaller as frequency increases. (It is, after all, a capacitor.) Designers can control what part of the plate current goes to by use of electrodes in the plate, by use of multiple plates, and by controlling the conductivity of the material itself.

Polyvinylidene (of Saran-wrap fame is often used for plates because it has the right combination of conductivity and low weight. Mylar can also be used.

Years ago, recording studios used speakers which were chosen primarily for efficiency rather that bandwidth. People in recording studios had no idea what kind of high and low frequency response was going onto their records. The introduction of the equalizer allowed them to change the power to the speaker to make up for its deficiencies, and a lot of studios can now hear stuff they couldn't hear before. Therefore the quality of sound on records, particularly in the last five years, has improved a great deal. This has in turn caused speaker manufacturers to become more and more interested in the high and low frequency response of their products.

The improvement in records has also had beneficial effects on cartridge design. In turn, the improvement in cartridges allowed the recording industry to make yet another round of improvement through better hearing. This is, perhaps, the best example of symbiotic effect in audio. Also true, of course, with speakers and other components.

The introduction of CD's and digital technology has accelerated this process even more.

A component with lots of knobs, each one representing a certain bandwidth, or frequency division (an octave or fraction of an octave) which allows you to boost or decrease energy going to a speaker in that range. The result is that you can compensate for speaker limitations or acoustic distortion in the room itself. Now common in recording studios and becoming more common in living rooms.

back to top

FERROFLUID

Ferrofluid is a trademark of Ferrofluidics, Inc. It is truly a space age material, a magnetic fluid, originally created for lubricating ball bearings in space. The liquid has tiny magnetic particles attached to the fluid base the way soap attaches particles of dirt. This fluid flows toward stronger magnetic fields.

In speakers, ferrofluid can be used in the gap of a tweeter magnet. Because of the fluid's magnetic properties, it is held in the gap and cannot run out. The presence of this fluid helps cool and therefore protect the tweeter by improving conductivity between the coil and the frame. ferrofluid also helps lower distortion because it prevents air movement in the gap. (This eliminates the possibility of spurious whine or whistle.)

Once sold at more than $100 an ounce, now a bit cheaper, this fluid is more in the class of optimization than luxury. The net result is that the loudspeaker can take greater power input and produce greater amounts of sound without burning out tweeters.

Something which limits the bandwidth of transmission. A filter can be electrical, mechanical or acoustic. Typically, filters are hi-pass, low-pass or band-pass, referring of course to frequencies. For example, the suspension of an automobile is a low-pass filter, hopefully blocking all but the very slowest bumps from the tender sensibilities of the passengers.

(See CROSSOVER, BAND-PASS.)

Fletcher and Munson proved conclusively that at low volume levels, the responsiveness of the ear to both high and low frequencies declines. In other words, as you reduce volume, both bass and high treble disappear before the midrange sounds do. Many amplifiers have a loudness control to boost low and high-end response at low volume settings and thus compensate for this particularity of the human ear. 

(See LOUDNESS, HUMAN HEARING.)

An example of frequency is a single musical note. Frequency was originally described as cycles per second, now simplified to Hz. (See HERTZ). The frequency of a second hand on a clock is one cycle per minute 1/60 Hz.A single frequency is a series of identical waves. The numerical value of the frequency is the number of complete waves that pass a single point in one second.

(See OCTAVE, HARMONICS, WAVELENGTH.)

back to top

HARMONICS

Frequencies which are multiples of a basic frequency. For example, a 110 Hz note will have harmonics of 220, 330, etc. (known as the second, third harmonics, etc.) also called overtones. An ideal transducer should reproduce only the signal it receives, without introducing any of its harmonics. In reality, any physical or electrical device inevitably introduces some harmonics of the tone it receives, and this is know as harmonic distortion.

Since harmonic distortion is particularly easy to measure, it often occurs in specifications of audio equipment.

Even-numbered harmonics are pleasant to the ear; odd-numbered HARMONICS, not so pleasant. A flute, for example, produces mainly even-numbered harmonics; a saxophone produces many odd-numbered harmonics.

An abbreviation (pronounced HURTS) meaning cycles per second. Named after a German physicist who did outstanding work in acoustics.

(See FREQUENCY.)

Systems for the reproduction of movies and other audio/visual productions include as a minimum a television set. The most common configuration nowadays also includes a Dolby Prologic receiver and four, five or six speakers, which include the conventional Left and Right stereo speakers as well as a center speaker , a pair of rear channel speakers and perhaps a sub-woofer. dolby has introduced a newer system called "Dolby Digital" or "AC-3", which includes an amplifier with five equal channels. These can provide greater flexibility and realism than the Dolby Prologic system. The most elaborate Home Theater systems include large screen TV monitors and digital sources, such as laser disks, digital tapes or digital broadcasts from satellites.

Research into human hearing has shown that stereo or spatial information is usually considered more important by listeners than wide-bandwidth response. Listeners will usually choose narrow-bandwidth stereo sound over wide-bandwidth mono. Audiophiles might prefer the latter, and some even prefer wide-bandwidth mono to wide-bandwidth stereo.

It has generally been considered that stereo information is derived principally from the difference in timing in the sounds emanating from two different speakers. However, intensity of sound also plays a role in imparting spatial information, particularly at lower frequencies, and beaming also has an effect. 

(See DIRECTIONALITY, DISPERSION , and PHASE.)

Biologically, sound is transmitted from the eardrum to bones to the cochlea (a snail-like organ.) Because of the way it's built, various frequencies go to various parts of the cochlea. There are lots of nerves throughout the organ: various frequencies excite various nerves. The timing of the sound is transmitted into digital pulses, and a part of the brain acts as a decoder to tell you what frequency came to which ear when. A survival tool, "Down that path you get stomped by a rhino," and therefore remarkably sensitive.

Human hearing is not only sensitive; it's also educable. Example, few people hear the 15,750 Hz sound of the flyback transformer on a TV before it's pointed out to them; most do, after it has been. This constant re-education of hearing and continual widening of perceptual abilities can be a problem for the audiophile whose hearing improves faster than he can afford better equipment, or who enjoys high fidelity less as it teaches him to hear better.

The importance of hearing is seldom fully appreciated. It is actually a more direct sensation of reality than seeing. To say "hearing s believing makes a lot more sense than "seeing is believing."

It is now beginning to be accepted that better hitters actually hear the ball better as well as seeing it better. It is certain that in most sports, good hearing may be as important as good vision.

When using firearms in sports, as in trapshooting, sound levels become destructive. Firing 25 rounds without earplugs can cost you a portion of your hearing ability for life, and no amount of money spent on audio equipment will do much good.

back to top

IMPEDANCE

The ratio of voltage (or electrical pressure) to current (or electrical flow or quantity). Neatly expressed in OHM'S LAW.

In a speaker, impedance is affected by resonance and by crossover components, so it is not necessarily uniform as a function of frequency. (In fact, hardly ever.)

The lower a speaker's impedance, the more current will flow into it from the amplifier. Most speakers offer enough impedance to protect the amp, but hooking several speakers in parallel to the same output terminals can effectively simulate a short circuit and blow the amp or at least a fuse. On the other hand, connecting speakers in series increases impedance and hence reduces amplifier efficiency.

(See INDUCTANCE, CAPACITANCE, RESISTANCE, OHM'S LAW.)

A type of electrical behavior very similar to capacitance. Any inductor in an electrical circuit will increase its impedance to an alternating current as frequency rises. This is called inductance. Inductors are therefore low-pass filters and often used as elements of crossovers to prevent high frequencies from getting to a woofer.

The coil of the woofer itself has its own inductance and therefore to some degree acts as part of its own crossover.

(See IMPEDANCE, CAPACITANCE, RESISTANCE.)

Also called a choke or a coil, usually made by winding a number of turns of wire around a form, which sometimes encloses a magnetic core. An inductor acts as a low-pass filter.

(See CAPACITOR)

(See BAFFLE)

INVERTED DOME TWEETER

An important variation of the domed tweeter, invented in 1968 by Winslow. Burhoe. Shares the advantages of the domed tweeter, plus others.

Essentially, the inverted dome results in even greater rigidity than the domed tweeter because the dome, once inverted, can be reinforced (by the voice coil) at its weakest point, half way between the perimeter and the center. Moreover, the inverted design allows the use of a smaller-diameter voice coiL. Since dispersion is related to the diameter of the radiating surface, and since at very high frequencies the diameter of a tweeter's radiating surface is equal to the diameter of the coil, the inverted dome tweeter also provides generally superior dispersion.

Using the same amount of wire in a smaller-diameter coil also results in a longer voice coil which stays in the gap of the magnet more effectively, creating a more linear response. (Again, less DISTORTION.)

The practice of putting one woofer in front of another connected by a sealed enclosure, so that only one is exposed to the outside air. This system has the effect of increasing moving mass, thereby providing greater bass extension at the cost of efficiency.

back to top

KLIPSCHORN

Book shelf speakers seldom have efficiency above one percent. For the Klipschorn (an invention of Paul Klipsch), it's something about 30%. The horn-loading makes it far more efficient, but the speakers really have to be big. The Klipschorn principle uses the walls of the room as effective extensions of the speaker itself. This effect applies only to sub-woofer frequencies. 

LINEAR, LINEARITY

As opposed to non-linear. When dealing with cause and effect, an effect is linear when it is exactly proportional to the input. When you pull on a spring with a specific force, twice the force should produce exactly twice the deflection. In a speaker, twice as much power from the amplifier should produce exactly twice as much sound.

All devices have some degree on non-linearity, hence distortion.

In audio, a term which usually refers to the principle (documented by Fletcher and Munson) that the response of the human ear to sound is not regular, but varies enormously with the intensity or loudness of the music.

At very low levels both bass and treble frequencies seem to decline more than the midrange frequencies. Many audio amplifiers therefore have some form of loudness control which causes the amplifier to boost certain frequencies at low-volume playback.

Ideally a loudness control should affect both high and low frequencies. Actually, many so-called loudness controls affect only bass response, therefore doing only half the job. This is worth investigating when buying an amplifier.

Ideally, at concert-hall levels of sound, the frequency response of the amplifier should be flat whether the loudness control is engaged or not. Accurate compensation requires two controls, a gain control in addition to the loudness control. The gain control must be used to compensate for gain variations in all elements of the system (speaker efficiency, room acoustics, room size, source level, amplifier gain, etc.) to set the amp to the one gain setting at which loudness contour will provide a realistic balance.

Loudness systems which consist only of a button to convert the volume control into a loudness control and back again can't usually provide exact compensation. As a result, many audiophiles have never heard music with proper loudness compensation.

Understanding loudness compensation is important because properly used it can provide the same sonic balance from a system at low listening level as at full volume. Loudness controls, though, are generally misunderstood and often are used simply to increase bass response (sometimes because people feel nervous about any deviation from "flat" response and fell vaguely that the loudness button is "legal" while the bass control is not.)

back to top

MAGNET

The source of the magnetic field which interacts with electrical current flowing through the voice coil wire to produce the force which moves the speaker cone.

Alnico, which used to be the material of choice, has been mostly replaced by ceramic ferrites. In cases where the magnet has to be very small or very strong, a neodymium alloy is now used.

The combination of magnet, its steel housing and the voice coil. These parts provide the power and the force to move the loudspeaker.

NEGATIVE FEEDBACK

In general, feedback is the application of a portion of the output signal to the input signal of any system which has gain. The most common usage of feedback is in audio amplifiers, since distortion in the output signal of an amplifier can generally be reduced by feeding a percentage of the output signal back into the input of the circuit with polarity reversed (hence the term "negative feedback").

In fact since all design is a matter of trade-offs, any decrease in distortion obtained by this technique must be paid for by a decrease in gain in the amplifier. Since negative feedback is essentially a corrective technique with an inevitable trade-off cost, a better solution is to create an amplifier with so little DISTORTION that negative feedback is not required to correct it.

Since negative feedback changes the damping factor of an amplifier (and therefore changes Q, therefore changing the frequency response of the speaker) some attention should be paid to this question when choosing an amplifier since speakers may sound quite different when driven by amplifiers with different amounts of negative feedback.

Random sound at all frequencies. Produced everywhere, even by the collision of molecules in the air around us. (This noise is just below the level of sound audible to humans, but exists nonetheless.) In audio, noise is also caused by the flow of electric current through transistors, vacuum tubes, and resistors.

Since there is always noise in the listening environment, sound-producing systems must be turned up loud enough to mask the noise. This fact may become a problem with components, like amps, which produce their own noise, since internally-generated noise may increase faster than the musical signal as volume is increased. Though modern high fidelity equipment usually generates so little noise that this not much of a problem, some internally-generated noise always shows up nevertheless in almost all component-generated sound. (Also see DISTORTION in PART TWO.)

Systems which are to be used in a noisy environment should include relatively efficient loudspeakers. In such environments, low efficiency loudspeakers may require too much power in order to come up to levels which effectively mask ambient noise. 

(Also see WHITE NOISE AND PINK NOISE.)

back to top

OCTAVE

A frequency term: doubling or halving a frequency. Called an octave because it is eight notes of a musical scale.

Also see HARMONICS.

Same as harmonics

PASSIVE RADIATOR

A loudspeaker component that looks like a woofer but isn't, for it has neither voice coil nor magnet. Like a tuned port, a passive radiator is part of a secondary sound producing system used either to improve efficiency or to widen bandwidth by utilizing the real woofer's back wave.

A passive radiator may be loaded with more mass than can be obtained in a tuned port of standard size, so it may be the solution in a particularly small cabinet. A sales advantage of the passive radiator is that it often makes the buyer think he is getting a large woofer when he isn't.

Sometimes called polarity, when it refers to the plus/minus of interconnections. In a more general sense, it refers to the timing of a waveform. Particularly important in stereo. Two speakers connected to the amplifier out of phase so that one is pushing (waveform is increasing) while the other is pulling (waveform is decreasing) will usually sound flat. Mis-phasing can cancel out most of the bass, particularly if the speakers are close together. Speaker hook-up wire is generally color-coded or otherwise coded so that both speakers can be grounded to the negative output connection of the amp, putting them "in phase".

PHASE is also important in each individual speaker system. Suppose you have a woofer and tweeter producing the same sound simultaneously. The timing or phasing of the sounds will be influenced by cross-over design, mass, , impedance etc.

In the more general meaning of phase, one complete cycle of a sound wave can be considered to be 360 degrees of phase. 180 degrees is one half way through the sound cycle, etc. As frequency rises, inductance alone can cause phase shifts of as much as 90 degrees. At the cross-over point phase shifts as great as 120 degrees are not uncommon. For this reason, crossovers are sometimes wired 180 degrees out of phase (by reversing wires) for a net reduction in mis-phasing or mis-timing.

Another interesting area of phase change is around woofer resonance. Right at resonance there is generally minimum phase shift, but phase does shift dramatically just above and just below resonance.

In a speaker with high Q, phase shift around resonance will be greater. There is some dispute and discussion as to whether one can actually hear this phase shift. Since variations in frequency response are often associated with variations in phase response, it's difficult to know which one is being heard. It's generally accepted that drastic shifts in phase are audible as distortion. To put it differently, it's desirable to have a system in which phase shifts only gradually with change in frequency response

A special case of phasing (either 0 or 180 degrees, plus or minus).If you touch a battery's terminals to those of a woofer, the woofer will move in one direction, either in or out. The convention is that if the woofer cone moves out, the positive terminal of the battery is touching the positive terminal of the woofer. In this case the polarity is positive. If the battery's terminals were reversed, the woofer cone would move in, representing negative polarity , or a reversal of polarity.

Produced by DOLBY Labs for home theater systems, using four amplifier channels for Left, Right, Center and Rear (Ambient) speakers. 

back to top

RESISTANCE 

The simplest form of impedance. Resistance opposes the flow of current equallly at all frequencies in a wire, loudspeaker or other electrical device. The more resistance, the less current.

(See also IMPEDANCE, INDUCTANCE, CAPACITANCE)

The mathematical expression of the relationship between voltage, current and impedance: impedance equals voltage divided by current. In its simplest form impedance is merely resistance.

Proportional to the square root of the product of mass and stiffness. (These factors are also found in the equation for Q.)

Speaker manufacturers do not always indicate the resonant frequencies of their speakers in their specifications. (Usually dealers will know.) Generally, the lowest the resonance the deeper the bass (assuming that everything else is all right with the speaker.)

Usually a speaker with low resonance is well made.

The smaller the box or the larger the woofer, the higher the resonant frequency. Therefore a fifteen inch woofer requires a huge box to keep its resonant frequency low enough.

An important quality in speaker components like woofer cones and tweeter domes, since the more rigid the radiator, the more it acts like a theoretical or ideal device.

An uncontrolled resonance, generally in a small part of a tweeter or woofer that's loose and shouldn't be, or is allowed to vibrate uncontrolled by the voice coil. Generally, a sign that a speaker is not well-made.

(See RESONANCE)
 

back to top

S

Very small speaker enclosures with only a tweeter or a mid-range and a tweeter. These are designed for high frequencies only and are usually combined with a bass unit, sub-woofer or other main speakers.

Satellites can be used as main speakers, when combined with a sub-woofer or bass unit or as rear channel speakers.

(See HOME THEATER)

A dome tweeter made out of relatively soft material, which allows radiation from only a portion of the dome (mostly adjacent to the bond to the voice coil)

(See DOME TWEETER.)

Some loudspeakers have controls which allow the owner to modify their sound. Usually there's a knob to change tweeter response, and in three-way or four-way systems, a knob or knobs to change the intensity of midrange response.

The purpose of these controls is to permit the speakers to be "tuned" to the room in which they will perform and/or to the ear of the listener. Some individuals spend a great deal on speakers then fail to get maximum performance from them by neglecting to find the optimum settings for controls (a process which may take quite a bit of time and experimentation.)

There's no system so good that it can't be improved. Therefore minor adjustments to speaker and electronic component controls may easily create an improvement in overall sound. The listener should feel free to experiment until he arrives at the sound quality which suits both the environment and his own hearing. The ear, not some arbitrary ideal setting (e.g. flat controls) should be the ultimate test.

Something to be used with extreme skepticism. 

Most specifications which are used to promote and sell loudspeaker are simply those which are easily made. The equipment used to produce them is easily obtained and easy to use. So, most specifications you may read about a piece of equipment are not generally very important to the performance of the unit. Every time someone invents a new piece of test equipment, one finds new specs getting into speaker literature.

FREQUENCY RESPONSE is a specification where there has been confusion for years. That's because there is no simple or agreed-upon way of measuring the frequency response of a speaker. Microphone placement, room size and shape, reflecting surfaces, etc. may vary and give immensely different specifications for the same speaker. For example, it's quite easy for frequency response specifications to go from +/- 2 dB to +/- 10 dB simply by repositioning the microphone, creating perfectly "accurate" yet completely different specifications for the same speaker.

Historically the meaning for frequency response has been "any audio output in that frequency". In these terms, a response curve reading 15 Hz to 25 KHz is completely meaningless because both these frequencies are beyond the range of human hearing.

Furthermore, they are both outside the range of the usual test equipment used by speaker manufacturers to test their products. This means that such specifications are either being made up by copywriters and engineers, or that they are judging by any kind of audio output. (For example, if you put 15 Hz into many speakers, you'll hear something. It won't be 15 Hz, but one of its harmonics: 30, 45, Hz etc.)

On-axis measurement of frequency response is also tricky. Lots of speakers have response up to about 16 KHz +/- 3 to 5 dB if the microphone is placed dead center in front of the speaker. That kind of measurement gives good specifications. However, anywhere but straight ahead the actual power output at the highest frequencies may be negligible compared to other frequencies. (See DIRECTIONALITY.)

CROSSOVER frequency specifications? Who cares? The usual reasons for including them: they're easy to specify and fill out some of the white space on the page.

A basic tenet of science is that measuring techniques inevitably affect the phenomenon being measured. A good example of this occurs in the field of amplifiers. For years amplifiers have been judged by an inexpensive piece of equipment to measure it. There are amplifiers which sound better with their high harmonic distortion than others with better specifications, but it's the ones with the best specs which sell best. In speakers, a new piece of equipment has just become available to measure phase distortion, and now all kinds of specifications relating to phase are appearing in the industry's literature; yet it's a relatively unimportant consideration. In this sense, the importance given to specs by reviewers may not be helping the public.

Audiophiles with extremely sensitive hearing claim that they can hear differences between various forms of speaker wire. Scientific evidence for this is very slight. Even straight wire has some inductance, and there is always at least some capacitance between two wires in proximity to each other. It turns out that there are some wires which have detectable reactance at audible frequencies and therefore to infer that there may be some audible effects.

There are many conjectures about how wire can cause a difference in sound, such as oxidation in the wire, magnetic eddy currents within thick wires, rectification between wire strands, and interference between high and low frequencies.

What is certainly true is that the thinner and longer the wire, the more resistance. Speakers usually have impedance of several ohms. In order that the resistance of the wire be small compared to that of the speaker, one half ohm would be too much. 

Sound Pressure Level is expressed in dB and is defined in terms of the practical hearing range of human hearing. For the human ear, silence is defined as 0 dB SPL. The threshold of pain for the human ear is at about 130 dB SPL. 

(See dB.)

An acoustic resonance of high Q, generally an annoying form of distortion, usually in the bass, caused by the shape and dimensions of the room in which music is being reproduced. When the distance between two parallel room surfaces is one fourth of the wave length of a given frequency, or an odd multiple of the wave length (three times, five times, etc.) the pressure of the reflected wave is in phase with the wave itself, creating a room resonance at that frequency.

On the other hand, if the distance between the two room surfaces is one half the wave length, or an integral multiple thereof, the pressure of the reflected wave is out of phase with the wave itself, creating a canceling at the frequency which is another form of distortion but a bit less noticeable.

Of the two types of distortion, the former is the greatest problem, because it tends to augment or complicate acoustic feedback.

Careful repositioning of speakers, careful repositioning of objects in the room (like furniture) and the use of sound-absorbing materials on room surfaces are the classic solutions to standing waves. The fewer parallel planes there are in the room's surfaces, the less likely that there will be standing waves, so that room acoustics are generally improved by angling a wall or two, and/or the ceiling, when the room is being planned.

Organ pipes are based on standing waves.

(See Resonance, Q.) 

(Also see HUMAN HEARING)

If a click is heard by both ears at once, it will appear to be straight ahead or behind the listener. If the sound reaches one ear slightly sooner, the sound will seem to emanate from that side. 

This is the principal cause of stereo effect, a shift in timing or phase.

However, a change in volume in one ear will have a similar stereo effect. Because of studio mixing techniques, many stereo recordings use only this volume differential to convey stereo information, and overlook or ignore the primary information, timing. This stereo is not as complete, but is particularly common in pop recordings, where one track is recorded at a time and then added to the final tape.

At very low frequencies, however, phase information in two channels is almost nil because of the wavelength of the sounds. Conversely, at frequencies where wavelength comes close to the size of the space between the ears, (250-10,000Hz), the perception of directionality increases.

A separate speaker enclosure (often with its own power amplifier) dedicated only to bass frequecies, e.g., from 15 - 150 Hz. The theory is that most stereo speaker systems lack one or two octaves of the deepest bass, and that this can be added with a separate speaker. Unfortunately many so-called sub-woofers are still missing one or two octaves of the deepest bass.

Sub-woofers are often configured as one monaural channel, on the assumption that low frequencies have very little stereo information.

(See also BAND-PASS)

back to top

THIELE

An Australian electrical engineer whose name has become quite famous in audio circles. Thiele's work was based on the fact that any loudspeaker system can be expressed in classic electronic symbols. He then applied filter theory (a special branch of electronic analysis) to speaker design.

The advantage of Thiele's work is that it makes it possible for the designer to avail himself of the immense body of statistics and research that has gone into electrical filter theory in order to obtain the desired frequency and phase response in a new design with minimum trial and error.

Thiele's theories have been taken up and popularized by Richard Small, who did a Ph.D. on Thiele's work.

Today's speaker designers often use filter theory, analyzing loudspeakers as analogous to electronic circuits. This approach is particularly useful for designs using either a port or a passive radiator, for Thiele's work is particularly applicable to the selective tuning of the port or radiator to achieve desired frequency response. Still, the Thiele approach is only one of several methods which may be used to arrive at the desired result.

These are the values of the basic parts of the electronic circuit analogue proposed by THIELE and popularized by Small. They are related to the physical properties of a woofer, e.g. mass compliance, resonance, Q, Bl, etc. They are commonly circulated between manufacturers and design engineers because there are a number of cookbook recipes which use the Thiele -Small parameters for determining box size and port tuning.Computer programs are now common both for measuring the parameters and for designing the enclosures based on them.

Any device which converts mechanical energy to electrical energy, or vice versa. Therefore, speakers, microphones, generators, alternators, motors, cartridges, etc. In speakers there are generally three types of transducers: dynamic, or conventional magnet and coil and paper cone; electrostatic, and piezo-electric effect.

In the dynamic type, sound comes from the movement of a coil in a magnetic field when current passes through it. In electrostatic, variations in voltage on two plates causes one plate to move in respect to the other (one plate effectively replaces the magnet of the dynamic speaker) transmitting a sound wave to the air in the room. In the piezo-electric effect, current causes a change in the shape of the material, a solid, causing it to vibrate and transmit sound waves to the air. (All these effects also work in the opposite direction.)

Transducers are particularly important in high fidelity because they are used in speakers, microphones, and cartridges, and they all have to be good for the sound to be good.

A term often used ambiguously. Specifically, transient response is the ability of a system to reproduce sudden changes, such as clicks or pops, especially at the beginnings of continuous sounds,rather than a steady waveform (sine wave).

In reality transient response is a reflection of frequency response. Pass a pulse through any system with a certain frequency response, and the accuracy of reproduction of a transient waveform will correspond to the limitations of the system's frequency response. A square wave can be used to indicate transient response. If there's a deficiency in high end response there will be a rounding of the square wave and a change in the vertical edge.

Though transient response is usually thought of as a high frequency function, there is also such thing as low frequency transient response. Failure to handle this results in a tilt of the horizontal section of the wave.

Some people confuse high Q with poor low frequency transient response, because drums seem to be muffled in a high Q system, and this can be erroneously labeled "poor low frequency transient response". Actually, poor low frequency transient response is of limited importance since the human ear does not usually recognize the absence of low frequency harmonics (or sub-harmonics).

One type of transmission line is a type of bass reflex system with the added characteristic of providing a long path for the rear wave to follow so that by the time it gets out of the enclosure it is in phase (in a certain frequency range) with the front wave. Not particularly useful, since this design offers neither the frequency range nor the efficiency of a tuned port. It is however appropriate for those who prefer a softer, well-damped bass.

A second type of transmission line, less common, uses a cabinet design which absorbs all the rear energy from the woofer, making it behave as if it were mounted in an infinite baffle.

A tuned port is simply a tube or opening allowing air from inside the speaker box to escape or enter. Note that the air in a tuned port has the property of acoustic mass. It moves in the port without being compressed, has inertia, and therefore has its own resonant frequency. The port is generally tuned (dimensioned) in the vicinity of the woofer's resonant frequency in order to augment bass output. A tuned port can be used either to increase efficiency or to increase frequency range, depending on the goals of the designer.

(See PASSIVE RADIATOR, BASS REFLEX.)

back to top

WAVELENGTH

Inverse to frequency, waves are short at high frequencies and long at low frequncies. The corresponding range for 20 to 20,000 Hz is 50 feet to 1/2 inch. Wavelength is the distance from crest to crest, similar to ocean waves.

(See FREQUENCY.)

NOISE is often used in testing speakers because it's easy to obtain. Also, a lot of companies like to use noise as a signal source when measuring specifications because it does not show up peaks in the frequency response as a pure tone would.

WHITE NOISE: noise in which there is equal energy per frequency division. (For example, the same amount of energy between 100 and 200 Hz as between 1100 and 1200). 

PINK NOISE has equal energy distributed logarithmically, for example, the same energy between 100 and 200 Hz as between 1000 and 2000. Pink noise corresponds fairly closely, on an average basis, to music. (More energy at low frequencies.) In listening tests with speakers, Pink noise is a better listening tool than White noise. In white noise there is a great exaggeration of treble. Surf is an excellent example of white noise.

High frequency sound is attenuated in air (particularly humid air) so that the closer you are to surf (white noise) the shriller it sounds. Even the humidity in your living room will to some degree influence the treble response of your speakers. On cold winter mornings, sound travels well, not just because the air is cold, thus dense, but also because of its dryness. (Very cold air rejects humidity.) Probably no-one will seriously suggest getting a dehumidifier for your living room in order to improve the high end response of your sound system. Still, acoustically speaking, it would help.