## D d

A demonstration of comb filtering in which direct sound from a loudspeaker is acoustically combined with a reflection from a surface at the diaphragm of a microphone. (A) No surface, no reflection. (B) Placing the microphone 0.7 in from the surface creates a delay of 0.1 ms and the combination of the direct and the reflected rays shows cancellations at 5 and 15 kHz and every 10 kHz. (C) A delay of 0.5 ms creates cancellations much closer together. (D) A delay of 1 ms results in cancellations...

## Sine Wave Language

Amplitude relationships for sinusoids, which apply to sine waves of electrical voltage or current, as well as to acoustical parameters such as sound pressure. Another term which is widely used in the audio field is crest factor, or peak divided by rms. Amplitude relationships for sinusoids, which apply to sine waves of electrical voltage or current, as well as to acoustical parameters such as sound pressure. Another term which is widely used in the audio field is crest factor, or peak divided...

## Absorbent in Patches

Applying all the absorbent in a room on one or two surfaces does not result in a diffuse condition, nor is the absorbent used most effectively. Let us consider the results of an experiment that shows the effect of Comparison of the modal pattern for a 5 x 7 meter two-dimensional room and a non-rectangular room of the same area. This sound field of the 1,0 mode is distorted in the nonrectangular room and the frequency of the standing wave is shifted slightly.8 The 1,3 mode for the 5 x 7 meter...

## The Simple Sinusoid

The sine wave is a basic waveform closely related to simple harmonic motion. The weight (mass) on the spring shown in Fig. 1-1 is a vibrating system. If the weight is pulled down to the -5 mark and released, the spring pulls the weight back toward 0. The weight will not, however, stop at zero its inertia will carry it beyond 0 almost to +5. The weight will continue to vibrate, or oscillate, at an amplitude that will slowly decrease due to frictional losses in the spring, the air, etc. The...

## Reverberation Chamber Method

The reverberation chamber method of determining the absorption coefficients of absorbing materials automatically measures the average value. This chamber is a large room with highly reflective walls, ceiling, and floor. The reverberation time of such a room is very long, and the longer it is, the more accurate the measurement. A standard sample of the material to be tested, 8 X 9 ft in size, is laid on the floor and the reverberation time measured. Comparing this time with the known...

## Future Dynamic Range Requirements

If the peak instantaneous sound levels and noise thresholds are regarded as determining dynamic range requirements, much greater ranges are required. Fiedler's study6 has shown that a dynamic range of up to 118 dB is necessary for subjectively noise-free reproduction of music (see Fig. 5-12). He considered the peak instantaneous sound level of various sources, as shown at the top of the figure, and the just-audible threshold for white noise added to the program source when the listener is in a...

## The Precedence Effect

Source, even though reflections from other directions are involved. The sound energy integrated over this period also gives an impression of added loudness. It should not be too surprising that the human ear fuses all sounds arriving during a certain time window. After all, our eyes fuse a series of still pictures at the cinema, giving us the impression of continuous movement. The rate of presentation of the still pictures is important there must be at least 16 pictures per second...

## Noise Transmitted by Diaphragm Action

Although very little airborne sound energy is transmitted directly to a rigid structure, airborne sound can set a wall to vibrating as a diaphragm and the wall, in turn, can transmit the sound through the interconnected solid structure. Such structure-borne sound might then cause another wall at some distance to vibrate, radiating noise into the space we are interested in protecting. Thus two walls interconnected by solid structure can serve as a coupling agent between exterior airborne noise...

## Timbre vs Spectrum

Timbre has to do with our perception of complex sounds. The word is applied chiefly to the sound of various musical instruments. A flute and oboe sound different even though they are both playing A. The tone of each instrument has its own timbre. Timbre is determined by the number and relative strengths of the instrument's partials. Tonal quality comes close to being a synonym for timbre. Timbre is another subjective term. The analogous physical term is spectrum. A musical instrument produces a...

## Frequency Range of Speech and Music

It is instructive to compare the frequency range of the various musical instruments with that of speech. This is best done graphically. Figure 5-11 includes the ranges only of the fundamental tones, and not of the harmonic tones of the instruments. The very low piano and organ notes, which are below the range of audibility of the ear, are perceived by their harmonics. Certain high-frequency noise accompanying musical instruments is not included, such as reed noise in woodwinds, bowing noise of...

## Reverberation Calculation Example

The goal now is to correct the reverberation of curve A of Fig. 7-23. It is evident that much absorption is needed at midband frequencies, a modest amount at higher frequencies, and very little at lower frequencies. The need is for a material having an absorption characteristic shaped more or less like the reverberation curve A. Skipping the laborious thumbing through of handbooks, 3 4-in acoustical tile seems to have the right shape. Giving no thought at this point to how it is to be...

## Frequency Response Measurements

As previously mentioned, the impulse response measured by the ETF program is saved in buffers and can be used to compute a variety of results, otherwise called postprocessing. Frequency response is one of these postprocessing options. Since the gate times used for the frequency-response measurements can be changed as desired in postprocessing, measurements do not have to be repeated nearly as often as with other types of measurement systems. Figure 26-6 shows the steps ETF uses to convert the...

## Achieving Track Separation

Achieving 15- to 20-dB intertrack separation requires intelligent effort and attention to detail. Without such separation, the freedom of establishing relative dominance in the mix down is sacrificed. The following methods are employed to yield the required separation Adjusting the acoustics of the studio. Using microphone placement and directivity. Use of contact transducers or electrical instruments.

## Effect of Airspace Behind Absorbent

Low-frequency absorption can also be improved by spacing the absorbent out from the wall. This is an inexpensive way to get improved performance within limits. Figure 9-9 shows the effect on the absorption coefficient of furring 1-inch glass-fiber wallboard out from a solid wall. Spacing 1-inch material out 3 inches makes its absorption approach that of the 2-inch material of Fig. 9-8 mounted directly on the wall. 125 250 500 1 kHz 2 kHz 4 kHz Frequency - Hz The thickness of glass-fiber...

## Ratios vs Differences

Imagine a sound source set up in a room completely protected from interfering noise. (The term sound-proof is avoided because there will be much sound in it.) The sound source is adjusted for a weak sound with a sound pressure of 1 unit, and its loudness is carefully noted. When the sound pressure is increased until it sounds twice as loud, the level dial reads 10 units. This completes observation A. For observation B, the source pressure is increased to 10,000 units. To double the loudness,...

## 50 100 150 200 250 300 Frequency Hz

The standard deviations are 4.67 and 8.13 dB for the best solution and the worst case, respectively. Long-term spectrum. The standard deviations are 3.81 and 6.28 dB for the best solution and the worst case, respectively. Long-term spectrum. The standard deviations are 3.81 and 6.28 dB for the best solution and the worst case, respectively. space (representing different listener and loudspeaker positions). These nodes move around the space until either (a) the difference in...

## Influence of Reverberation on Speech

An illustration of the effects of reverberation on the intelligibility of speech. Understanding the word back depends on apprehending the later, lower level consonant ck, which is masked by reverberation if the reverberation time is too long. arbitrary level of 0 dB at time 0, after which it decays according to the reverberation time of the room, which is assumed to be 0.5 seconds (60 dB decay in 0.5 seconds). The ck consonant sound, peaking 0.32 seconds later, is 25 dB below the ba sound peak....

## Sound Waves in the Free Field

Ractical acoustic problems are invariably associated with people, buildings, rooms, airplanes, automobiles, etc. These can generally be classified either as problems in physics (sound as a stimulus) or problems in psychophysics (sound as a perception), and often as both. Acoustical problems can be very complex in a physical sense, for example, thousands of reflected components might be involved or obscure temperature gradients might bend the sound in such a way as to affect the results. When...

## Area of Audibility

Curves A and B of Fig. 3-8 were obtained from groups of trained listeners. In this case, the listeners face the sound source and judge whether a tone of a given frequency is barely audible (curve A) or 20 50 100 300 1 kHz 3 kHz 10 kHz 20 kHz The auditory area of the human ear is bounded by two threshold curves, (A) the threshold of hearing delineating the lowest level sounds the ear can detect, and (B) the threshold of feeling at the upper extreme. All of our auditory experiences occur within...

## Placement of Materials

The application of sound-absorbing materials in random patches has already been mentioned as an important contribution to diffusion. Other factors than diffusion might influence placement. If several types of absorbers are used, it is desirable to place some of each type on ends, sides, and ceiling so that all three axial modes (longitudinal, transverse, and vertical) will come under their influence. In rectangular rooms it has been demonstrated that absorbing material placed near corners and...

## Conclusion

A computer program has been developed that allows automated selection of positions for listeners and loudspeakers within listening rooms. The criterion for optimum listener and loudspeaker Comparison of the modal response for the two listener and loudspeaker arrangements of five equidistant matching loudspeakers. Comparison of the modal response for the two listener and loudspeaker arrangements of five equidistant matching loudspeakers. positions within the room is the minimum standard...

## Some European Designs

A glance at several Swiss control rooms and studios will broaden your view of the application of Schroeder diffusors. I am indebted to Helmuth Kolbe, a consultant in acoustics, for data on the rooms to be described, and which he designed.7 The first example is Studio Sixty of Lausanne, Switzerland, owned by Wolfgang Ehrlich, which is busy around the clock on jobs from Paris and even from the United States. A good example of the use of diffusors in a studio is shown in Fig. 21-15. The lower...

## Helmholtz Resonators

The Helmholtz type of resonator is widely used to achieve adequate absorption at lower audio frequencies. There is nothing particularly mysterious about such resonators in fact they pop up in various forms in everyday life. Blowing across the mouth of any bottle or jug produces a tone at its natural frequency of resonance. The air in the cavity is springy, and the mass of the air in the neck of the jug reacts with this springiness to form a resonating system, much as a weight on a spring...

## Drapes as Sound Absorbers

Drapes are a porous type of sound absorber because air can flow through the fabric under pressure. Variables affecting absorbency include weight of material, degree of drape, and distance from the wall. Data are scarce, but Fig. 9-13 compares the absorption of 10, 14, and 18 oz sq yd velour hung straight and presumably at some distance from the wall. One intuitively expects greater absorption with heavier material. However, the greater absorption in going from 14 to 18 oz sq yd than in going...

## Writing Speed

The B& K 2305 graphic-level recorder has a widely adjustable writing speed. A sluggish pen response is useful when fast fluctuations need to be ironed out. When detail is desired, faster writing speeds are required. A too slow writing speed can affect the rate of decay as it smooths out the trace, as will be examined. In Fig. 7-9, the same 63-Hz decay is recorded with five different pen response speeds ranging from 200 to 1,000 mm sec. The instrument-limited decay for each is indicated by...

## Impedance Tube Method

The Kundt tube has been applied to the measurement of the absorption coefficient of materials. Used in this way it is commonly a standing- wave tube or an impedance tube. No matter what name it bears, it is a very handy device for quickly and accurately determining coefficients. It also has the advantage of small size, modest demands in terms of supporting equipment, and it requires only a small sample. This method is primarily used for porous absorbers because it is not suited to those...

## Increasing Reverberation Time

Low-Q Helmholtz resonators are capable of shortening reverberation time by increasing absorption. High-Q resonators can increase reverberation time through storage of energy as described by Gilford.2 To achieve the high Qs necessary, plywood, particleboard, masonite, and other such materials must be abandoned and ceramics, plaster, concrete, etc., used in resonator construction. By proper tuning of the resonators, the increase in reverberation time can be placed where needed in regard to...

## Analog and Digital Signal Processing

N the early days of sound recording, signal storage was a major problem. The final recording was laid down directly, without benefit of stop-and-go recording of portions that could be patched together later. This had the advantage of minimizing the number of recording generations, but the format was a stringent one with little latitude for artistic enhancement. With the introduction of high-quality signal storage (magnetic tape or digital memory) many creative decisions, normally reserved for...

## Diffraction by the Zone Plate

The zone plate can be considered an acoustic lens. It consists of a circular plate with a set of concentric, annular slits of cunningly devised radii. If the focal point is at a distance of r from the plate, 50 100 300 500 1kHz 3kHz 5kHz 10kHz An estimation of the effectiveness of a sound barrier in terms of sound (or noise) attentuation as a function of frequency and barrier height. (After Rettinger.4) the next longer path must be r + X 2 where X is the wavelength of the sound falling on the...

## Selection of Space Internal Factors

If the space being considered is within a larger building, other possible noise and vibration sources within the same building must be identified and evaluated. Is there a printing press on the floor above A machine shop Is there a noisy elevator A reinforced concrete building efficiently conducts noises throughout via structural paths. Audio video work space might require support activities such as duplication, accounting, sales, telephone, each with its own noise generation potential. Serious...

## The Corner Reflector

In an art museum with large Dutch paintings on display, the eyes of certain subjects seem to follow as one walks by. Corner reflectors are like that. There seems to be no way of escaping their pernicious effect. The corner reflector of Fig. 10-9, receiving sound from the source S, sends a reflection directly back toward the source. If the angles of incidence and reflection are carefully noted, a source at B will also send a direct, double-surface reflection returning to the source. A source at...

## Growth of Sound in a Room

Referring to Fig. 7-2A, let us consider a source S and a human receiver Hin a room. As source S is suddenly energized, sound travels outward from S in all directions. Sound travels a direct path to H and we shall consider zero time (see Fig. 7-2B) as that time at which the direct sound reaches the ears of listener H. The sound pressure at H instantly jumps to a value less than that which left S due to spherical divergence and small losses in the air. The sound pressure at H stays at this value...

## Introduction to the fourth edition

The science of acoustics made great strides in the 20th century, during which the first three editions of this book appeared. This fourth edition, however, points the reader to new horizons of the 21st century. A newly appreciated concept of distortion of sound in the medium itself (Chap. 25), a program for acoustic measurements (Chap. 26), and the optimization of placement of loudspeakers and listener (Chap. 27), all based on the home computer, point forward to amazing developments in...

## Double Windows

Between the control room and the studio a window is quite necessary, and its sound transmission loss should be comparable to that of the wall itself. A well-built staggered stud or double wall might have an STC of 50 dB. To approach this performance with a window requires very careful design and installation.3 A double window is most certainly indicated a triple window adds little more. The mounting must minimize coupling from one wall to the other. One source of coupling is the window frame,...

## Formation of Voiced Sounds

If the symbolic boxes of Fig. 5-3 are elaborated into source spectra and modulating functions, we arrive at something everyone in audio is interested in the spectral distribution of energy in the voice. We also get a better understanding of the aspects of voice sounds that contribute to the intelligibility of speech in reverberation, noise, etc. Figure 5-4 shows the steps in producing voiced sounds. First, there is the sound produced by the vibration of the vocal cords, pulses of sound having a...

## Reverberation Time of Helmholtz Resonators

Some concern has been expressed about the possibility of acoustically resonant devices, such as Helmholtz absorbers, ringing with a reverberation time of their very own adding coloration to the voice and music signals. It is true that any resonant system, electronic or acoustical, has a certain time constant associated with it. The Q-factor (quality factor) describes the sharpness of tuning of the Helmholtz resonator as shown in Fig. 9-37. Once the tuning curve has been obtained experimentally,...

## Evaluation of Sound Absorption

The absorption coefficient is a measure of the efficiency of a surface or material in absorbing sound. If 55 percent of the incident sound energy is absorbed, the absorption coefficient is said to be 0.55. One square foot of this material gives 0.55 absorption units (sabins). An open window is considered a perfect absorber because sound passing through it never returns to the room. It would have an absorption coefficient of 1.0. Ten square feet of open window would give 10 sabins of absorbance....

## Acoustical Characteristics of a Studio

Sound picked up by a microphone in a studio consists of both direct and indirect sound. The direct sound is the same as would exist in the great outdoors or in an anechoic chamber. The indirect sound, which immediately follows the direct, is the sound that results from all the various nonfree-field effects characteristic of an enclosed space. The latter is unique to a particular room and may be called studio response. Everything that is not direct sound is indirect, reflected sound. Before...

## Binaural Localization

Stereophonic records and sound systems are a relatively new development. Stereo hearing has been around at least as long as man. Both are concerned with the localization of the source of sound. In early times some people thought that having two ears was like having two lungs or two kidneys, if something went wrong with one the other could still function. Lord Rayleigh laid that idea to rest by a simple experiment on the lawn of Cambridge University. A circle of assistants spoke or struck tuning...

## Appraisal of Room Resonances

Even though the room proportions are favorable and highly recommended, it is well to verify axial mode spacing. Only axial modes are considered because the tangential modes are 3 dB down and the oblique modes are 6 dB down with respect to the more powerful axial modes (Ref. 3 in Chap. 15). All of the length, width, and height axial mode resonance frequencies to 300 Hz are listed in Table 23-1. These constitute the low-frequency acoustics of this room. The spacings of these modes determine the...

## Reflections from Parabolic Surfaces

A parabola has the characteristic of focusing sound precisely to a point (Fig. 10-5). It is generated by the simple equation y x2. A very deep parabolic surface, such as that of Fig. 10-5, exhibits far better directional properties than a shallow one. Again, the directional properties depend on the size of the opening in terms of wavelengths. Figure 10-5 shows the parabola used as a directional sound source with a small, ultrasonic Galton Whistle pointed inward at the focal point. Plane waves...

## Diffraction of Sound by Large and Small Apertures

Figure 11-2A illustrates the diffraction of sound by an aperture that is many wavelengths wide. The wavefronts of sound strike the heavy obstacle some of it is reflected, some goes right on through the wide aperture. The arrows indicate that some of the energy in the main beam is diverted into the shadow zone. By what mechanism is this diversion accomplished Figure 11-2A illustrates the diffraction of sound by an aperture that is many wavelengths wide. The wavefronts of sound strike the heavy...

## The Initial Time Delay

Every recording bears indelibly the marks of the room in which the sounds were recorded. Beranek1 made an intensive study of concert halls around the world. He noted that those rated the highest by qualified musicians had certain technical similarities. Among them was what he called the initial time-delay gap. This is the time between the arrival of the direct sound at a given seat and the arrival of the critically important early reflections. He was impressed by the fact that halls rating high...

## Refraction of Sound

Refraction changes the direction of travel of the sound by differences in the velocity of propagation. Diffraction is changing the direction of travel of sound by encountering sharp edges and physical obstructions (chapter 11). Most people find it easy to distinguish between absorption and reflection of sound, but there is often confusion between diffraction and refraction (and possibly diffusion, the subject of the next chapter). The similarity of the sound of the words might be one cause for...

## Perception of Reflected Sound

In the preceding section, reflected sound was considered in a rather limited way. A more general approach is taken in this section. It is interesting that the loudspeaker arrangement Haas used was also used by dozens of other researchers and that this is basically the familiar stereo setup two separated loudspeakers with the observer (listener) located symmetrically between the two loudspeakers. The sound from one loudspeaker is designated as the direct sound, that from the other, the delayed...

## Variable Resonant Devices

Resonant structures for use as sound absorbing elements have been used extensively in the Danish Broadcasting House in Copenhagen1. One studio used for light music and choirs employs pneumatically operated hinged perforated panels as shown in Fig. 24-7A. The effect is basically to shift the resonant peak of absorption as shown in Fig. 24-7B. The approximate dimensions applicable in Fig. 24-7A are width of panel 2 ft, thickness 3 8 in., holes 3 8 in. diameter spaced 1-3 8 inch on centers. A most...

## Mode Pressure Plots

It is easy to say that the modal pattern of a given room creates a very complex sound field, but to really drive this point home several sketches of sound pressure distributions are included. The one dimension organ pipe of Fig. 15-2 is a starting point that can be compared to the 1,0,0 mode of Fig. 15-14 for a three-dimensional room. The pressure is higher near the ends (1.0) and zero along the center of the room. Figure 15-15 shows sound pressure distribution when only the 3,0,0 axial mode is...

## Localization of Sound Sources

The perception of a direction to a source of a sound is, at least partially, the result of the amazing encoding function of the external ear, the pinna. Sound reflected from the various ridges, convolutions, and surfaces of the pinna combines with the unreflected (direct) sound at the entrance to the auditory canal. This combination, now encoded with directional information, passes down the auditory canal to the eardrum and thence to the middle and inner ear and on to the brain for...

## Rfz Vs Ambisonic

DSP Convolution With Anechoic Audio Material Auralization of the room is achieved by convolving the binaural impulse responses with music. 3D model of a listening room with five matching speakers in the ITU configuration. The model illustrates how the chosen variables describe the corner detail, soffit, and allocation of acoustical treatment to various sections on the room's boundaries. 3D model of a listening room with five matching speakers in the ITU configuration. The model illustrates how...

## Energy Time Curve Measurements

ETF also generates energy-time curves which are useful for determining the time and level of reflections versus that of the direct sound. Energy-time curves show the level as a logarithmic quantity, making lower-level behavior more visible. In ETF, the energy-time curves can be viewed as full-range levels, or they can be broken into various octave bands for checking the levels of specific ranges of frequencies. Figure 26-14 shows the process by which ETF 5 generates energy-time curves from the...

## Selected Absorption Coefficients

Carpet heavy with latex backing on foam or 40 oz hair felt Acoustical tile, ave, 1 2 thick Acoustical tile, ave, 3 4 thick Floor linoleum, Asphalt-tile, or cork tile on concrete Owens-Corning Frescor, painted, 5 8 thick, Mounting 7 Plaster, gypsum or lime, smooth finish on tile or brick Plaster gypsum or lime, smooth finish on lath Gypsum board 1 2 on 2 x 4s, 16 on centers chord 45 height 16 empty chord 35 height 12 empty chord 20 0.25 height 8 empty chord 20 0.3 height 8 filled 5 32 thick, 4...

## Audio Listening Room Dimensions

The acoustics of the space is a vital part of both the recording and reproducing process. In every acoustical event, there is a sound source and some sort of receiving device with an acoustical link between the two. Disc or tape recordings have the imprint of the acoustics of the recording environment recorded on them. If the sound source is a symphony orchestra and the recording is made in the performing hall, the reverberation of the hall is very much a part of the orchestral sound. If the...

## Sound Reflections in Inclined Spaces

I he perceptual effect of sound reflections depends on the size of the room. The situation in a recording studio, control room, or listening room is quite different from that in a music hall or large auditorium. The case of sound reflections in small rooms is considered first in this discussion. Imagine two people in a small room as illustrated in Fig. 16-1. The first sound of the person speaking to reach the listener is that traveling a direct path because it travels the shortest distance....

## Uu o

The Flutterfree is a non-absorptive flutter echo control molding. It is a quadratic-residue diffusor based on the prime-7. It can also serve as slats on a slat-type Helmholtz low-frequency absorber. Peter D'Antonio, RPG Diffusor Systems, Inc. and the Audio Engineering Society. by well depth, and the high-frequency limit is determined principally by well width. Manufacturing constraints place a limit of 1 in on well width and 16 in on well depth, beyond which the units become diaphragmatic. To...

## Room Shape

How can a room be built to achieve maximum diffusion of sound This opens up a field in which there are strong opinions some of them supported by quite convincing experiments and some just strong without such support. There are many possible shapes of rooms. Aside from the general desirability of a flat floor in this gravity-stricken world, walls can be splayed, ceilings inclined, cylindrical or polygonal shapes employed. Some shapes can be eliminated because they focus sound, and focusing is...

## Quiet Air for the Studio

I he background noise levels in recording studios, control rooms, and listening rooms must be kept under control if these rooms are to be of maximum use in their intended way. Hums, buzzes, rumbles, aircraft noises, tooting auto horns, dogs barking, or typewriter sounds are most incongruous if audible during a lull in a program. Such sounds might not be noticed outside the studio when they are a natural part of the situation, but during a pause or a quiet musical or speech passage they stand...

## Performance of Diffraction Grating Diffusors

In designing an audio space of any kind, the acoustician has three building blocks at his disposal absorption, reflection, and diffusion. In the past, it was a common experience to find too much reflection but too little diffusion in the room to be treated. Only absorption and a limited number of geometrical tricks were available for the designer to carry out his assignment. Absorption, reflection, and diffusion are now in better balance, at least potentially. The effects on the incident sound...

## Time Delay Spectrometry TDS Measurement Techniques

In response to this need for better measurement techniques, the late Richard C. Heyser refined a measurement method known as time- delay spectrometry (TDS) in the late 1960s. By the early 1980s, a TDS measurement system was implemented commercially by Techron in their TEF Analyzer. This small but very powerful digital instrument was implemented on the platform of an industrial portable PC. TEF used time-delay spectrometry to derive both time and frequency response information from measurements....

## Acousti Softs ETF Program

In 1996, AcoustiSoft introduced their ETF Loudspeaker and Room Acoustics Analysis Program. The program was one of the first software-based measurement programs intended to turn an ordinary multimedia PC into a proquality measurement tool. At the time of this writing, the program is in its fifth version and is an MLS-based measurement program. The program contains calibration features for lab-quality precision and accuracy. The way the program works is that the operator connects a test...

## Perforated Panel Absorbers

Perforated hardboard or plywood panels spaced from the wall constitute a resonant type of sound absorber.12-15 Each hole acts as the neck of a Helmholtz resonator, and the share of the cavity behind belonging to that hole is comparable to the cavity of the Helmholtz resonator. In fact, we can view this structure as a host of coupled resonators. If sound arrives perpendicular to the face of the perforated panel, all the tiny resonators are in phase. For sound waves striking the perforated board...

## Simplified Axial Mode Analysis

Apply what you learned about axial modes to a specific rectangular listening room or studio. The dimensions of our specimen room are 28 x 16 x 10 ft. The 28-ft length resonates at 565 28 20.2 Hz, the two side walls 16 ft apart resonate at 565 16 35.3 Hz, and the floor-ceiling combination resonates at 565 10 56.5 Hz. These three axial resonances and the train of multiples for each are plotted in Fig. 15-21. There are 27 axial resonance frequencies below 300 Hz, and for this exercise, the horde...

## The Quick Sound Field

The concept of a Quick Sound Field i.e., live, but dry , which comes from Acoustic Sciences Corporation Ref. 3 in Chap. 19 can be applied to the small e.g. 4 x 6 ft or 6 x 8 ft voice recording room. The realization of a practical quick sound field is based on a multiplicity of half-round Tube Traps as shown in Fig. 23-4. A quarter-round Trap is mounted in each wall-wall and ceiling-wall intersection to provide absorption to control the normal modes. Half-round traps alternate on the walls and...

## The Middle

Transmitting sound energy from a tenuous medium such as air into a dense medium like water is a serious problem. Without some very special equipment, sound originating in air bounces off water like light off a mirror. It boils down to a matter of matching impedances, and in this case the impedance ratio is something like 4,000 1. Consider how satisfactory it would be to drive the 1-ohm voice coil of a loudspeaker with an amplifier having an output impedance of 4,000 ohms Clearly not much power...

## Complex Waves

Speech and music waveshapes depart radically from the simple sine form. A very interesting fact, however, is that no matter how complex the wave, as long as it is periodic, it can be reduced to sine components. The obverse of this is that, theoretically, any complex periodic wave can be synthesized from sine waves of different frequencies, different amplitudes, and different time relationships phase . A friend of Napoleon, named Joseph Fourier, was the first to develop this surprising idea....

## Example Combining Decibels

Let's say it is warm in our studio and a fan is brought in to augment the air conditioning A C system. If both fan and the A C are turned off, a very low noise level prevails, low enough to be neglected in the calculation. If the A C alone is running, the sound-pressure level at a given position is 55 dB. If the fan alone is running, the sound-pressure level is 60 dB. What will be the sound-pressure level if both are running at the same time Combined dB 10 log 10 10 10 10 61.19 dB If the...