Sound System Design Reference Manual
Sound System Design Reference Manual
Sound System Design Reference Manual Table of Contents Preface ............................................................................................................................................. i Chapter 1: Wave Propagation ........................................................................................................ Wavelength, Frequency, and Speed of Sound ................................................................................. Combining Sine Waves ........................
Sound System Design Reference Manual Table of Contents (cont.) Chapter 6: Behavior of Sound Systems Indoors ......................................................................... Introduction ....................................................................................................................................... Acoustical Feedback and Potential System Gain ............................................................................. Sound Field Calculations for a Small Room ..............
Sound System Design Reference Manual Preface to the 1999 Edition: This third edition of JBL Professional’s Sound System Design Reference Manual is presented in a new graphic format that makes for easier reading and study. Like its predecessors, it presents in virtually their original 1977 form George Augspurger’s intuitive and illuminating explanations of sound and sound system behavior in enclosed spaces.
Sound System Design Reference Manual
Sound System Design Reference Manual Chapter 1: Wave Propagation Wavelength, Frequency, and Speed of Sound Sound waves travel approximately 344 m/sec (1130 ft/sec) in air. There is a relatively small velocity dependence on temperature, and under normal indoor conditions we can ignore it. Audible sound covers the frequency range from about 20 Hz to 20 kHz. The wavelength of sound of a given frequency is the distance between successive repetitions of the waveform as the sound travels through air.
Sound System Design Reference Manual Combining Sine Waves Referring to Figure 1-2, if two or more sine wave signals having the same frequency and amplitude are added, we find that the resulting signal also has the same frequency and that its amplitude depends upon the phase relationship of the original signals. If there is a phase difference of 120°, the resultant has exactly the same amplitude as either of the original signals.
Sound System Design Reference Manual Combining Delayed Sine Waves If two coherent wide-range signals are combined with a specified time difference between them rather than a fixed phase relationship, some frequencies will add and others will cancel. Once the delayed signal arrives and combines with the original signal, the result is a form of “comb filter,” which alters the frequency response of the signal, as shown in Figure 1-4.
Sound System Design Reference Manual A typical example of combining delayed coherent signals is shown in Figure 1-5. Consider the familiar outdoor PA system in which a single microphone is amplified by a pair of identical separated loudspeakers. Suppose the loudspeakers in question are located at each front corner of the stage, separated by a distance of 6 m (20 ft). At any distance from the stage along the center line, signals from the two loudspeakers arrive simultaneously.
Sound System Design Reference Manual Subjectively, the effect of such a comb filter is not particularly noticeable on normal program material as long as several peaks and dips occur within each one-third octave band. See Figure 1-6. Actually, the controlling factor is the “critical bandwidth.” In general, amplitude variations that occur within a critical band will not be noticed as such. Rather, the ear will respond to the signal power contained within that band.
Sound System Design Reference Manual Effects of Temperature Gradients on Sound Propagation Effects of Wind Velocity and Gradients on Sound Propagation If sound is propagated over large distances out of doors, its behavior may seem erratic. Differences (gradients) in temperature above ground level will affect propagation as shown in Figure 1-8. Refraction of sound refers to its changing direction as its velocity increases slightly with elevated temperatures.
Sound System Design Reference Manual Effects of Humidity on Sound Propagation Contrary to what most people believe, there is more sound attenuation in dry air than in damp air. The effect is a complex one, and it is shown in Figure 1-11. Note that the effect is significant only at frequencies above 2 kHz. This means that high frequencies will be attenuated more with distance than low frequencies will be, and that the attenuation will be greatest when the relative humidity is 20 percent or less.
Sound System Design Reference Manual
Sound System Design Reference Manual Chapter 2: The Decibel Introduction In all phases of audio technology the decibel is used to express signal levels and level differences in sound pressure, power, voltage, and current. The reason the decibel is such a useful measure is that it enables us to use a comparatively small range of numbers to express large and often unwieldy quantities.
Sound System Design Reference Manual 3. What power level is represented by 4 milliwatts? As we have seen, the power level of 1 milliwatt is –30 dB. Two milliwatts represents a level increase of 3 dB, and from 2 to 4 milliwatts there is an additional 3 dB level increase. Thus: –30 + 3 + 3 = –24 dB 4. What is the level difference between 40 and 100 watts? Note from the table that the level corresponding to 4 watts is 6 dB, and the level corresponding to 10 watts is 10 dB, a difference of 4 dB.
Sound System Design Reference Manual The normal reference level for voltage, E0, is one volt. For sound pressure, the reference is the extremely low value of 20 x 10-6 newtons/m2. This reference pressure corresponds roughly to the minimum audible sound pressure for persons with normal hearing. More commonly, we state pressure in pascals (Pa), where 1 Pa = 1 newton/m2. As a convenient point of reference, note that an rms pressure of 1 pascal corresponds to a sound pressure level of 94 dB.
Sound System Design Reference Manual Sound Pressure and Loudness Contours We will see the term dB-SPL time and again in professional sound work. It refers to sound pressure levels in dB above the reference of 20 x 10-6 N/m2. We commonly use a sound level meter (SLM) to measure SPL. Loudness and sound pressure obviously bear a relation to each other, but they are not the same thing. Loudness is a subjective sensation which differs from the measured level in certain important aspects.
Sound System Design Reference Manual Figure 2-3. Frequency responses for SLM weighting characteristics Figure 2-4.
Sound System Design Reference Manual Inverse Square Relationships When we move away from a point source of sound out of doors, or in a free field, we observe that SPL falls off almost exactly 6 dB for each doubling of distance away from the source. The reason for this is shown in Figure 2-5. At A there is a sphere of radius one meter surrounding a point source of sound P1 representing the SPL at the surface of the sphere. At B, we observe a sphere of twice the radius, 2 meters.
Sound System Design Reference Manual that the measuring microphone is far enough away from the device to be in its far field, and he can also calculate the imaginary point from which sound waves diverge, according to inverse square law. This point is called the acoustic center of the device. After accurate field measurements have been made, the results are converted to an equivalent one meter rating.
Sound System Design Reference Manual In other voltage measurements, dBV refers to levels relative to 1 volt. Rarely encountered by the sound contractor will be acoustical power levels. These are designated dB-PWL, and the reference power is 10-12 watts. This is a very small power indeed. It is used in acoustical measurements because such small amounts of power are normally encountered in acoustics.
Sound System Design Reference Manual Chapter 3: Directivity and Angular Coverage of Loudspeakers Introduction Some Fundamentals Proper coverage of the audience area is one of the prime requirements of a sound reinforcement system. What is required of the sound contractor is not only a knowledge of the directional characteristics of various components but also how those components may interact in a multi-component array.
Sound System Design Reference Manual increases another 3 dB. Continuing on at D, we place the sound source in a trihedral (3-sided) corner, and we note an additional 3 dB increase as sound power is radiated into one-eighth-space. We could continue this exercise further, but our point has already been made. In going from A to D in successive steps, we have increased the directivity index 3 dB at each step, and we have doubled the directivity factor at each step.
Sound System Design Reference Manual The data of Figure 3-1 was generalized by Molloy (7) and is shown in Figure 3-3. Here, note that Dl and Q are related to the solid angular coverage of a hypothetical sound radiator whose horizontal and vertical coverage angles are specified. Such ideal sound radiators do not exist, but it is surprising how closely these equations agree with measured Dl and Q of HF horns that exhibit fairly steep cut-off outside their normal coverage angles.
Sound System Design Reference Manual Isobars have become popular in recent years. They give the angular contours in spherical coordinates about the principal axis along which the response is -3, -6, and -9 dB, relative to the on-axis maximum. It is relatively easy to interpolate visually between adjacent isobars to arrive at a reasonable estimate of relative response over the useful frontal solid radiation angle of the horn.
Sound System Design Reference Manual The values of DI and Q given in Figure 3-6 are the on-axis values, that is, along the axis of maximum loudspeaker sensitivity. This is almost always the case for published values of Dl and Q. However, values of Dl and Q exist along any axis of the radiator, and they can be determined by inspection of the polar plot. For example, in Figure 3-6, examine the polar plot corresponding to Diameter = λ. Here, the on-axis Dl is 10 dB.
Sound System Design Reference Manual The Importance of Flat Power Response If a radiator exhibits flat power response, then the power it radiates, integrated over all directions, will be constant with frequency. Typical compression drivers inherently have a rolled-off response when measured on a plane wave tube (PWT), as shown in Figure 3-7A.
Sound System Design Reference Manual The rising DI of most typical radial horns is accomplished through a narrowing of the vertical pattern with rising frequency, while the horizontal pattern remains fairly constant, as shown in Figure 3-8A. Such a horn can give excellent horizontal coverage, and since it is “self equalizing” through its rising DI, there may be no need at all for external equalization.
Sound System Design Reference Manual Using Directivity Information A knowledge of the coverage angles of an HF horn is essential if the device is to be oriented properly with respect to an audience area. If polar plots or isobars are available, then the sound contractor can make calculations such as those indicated in Figure 3-9. The horn used in this example is the JBL 2360 Bi-Radial. We note from the isobars for this horn that the -3 dB angle off the vertical is 14°.
Sound System Design Reference Manual Horns may be stacked in a vertical array to improve pattern control at low frequencies. The JBL Flat-Front Bi-Radials, because of their relatively small vertical mouth dimension, exhibit a broadening in their vertical pattern control below about 2 kHz. When used in vertical stacks of three or four units, the effective vertical mouth dimension is much larger than that of a single horn. The result, as shown in Figure 3-11, is tighter pattern control down to about 500 Hz.
Sound System Design Reference Manual
Sound System Design Reference Manual Chapter 4: An Outdoor Sound Reinforcement System Introduction Our study of sound reinforcement systems begins with an analysis of a simple outdoor system. The outdoor environment is relatively free of reflecting surfaces, and we will make the simplifying assumption that free field conditions exist. A basic reinforcement system is shown in Figure 4-1A. The essential acoustical elements are the talker, microphone, loudspeaker, and listener.
Sound System Design Reference Manual Figure 4-2. Electrical response of a sound system 3 dB below sustained acoustical feedback The Concept of Acoustical Gain Boner (4) quantified the concept of acoustical gain, and we will now present its simple but elegant derivation. Acoustical gain is defined as the increase in level that a given listener in the audience perceives with the system turned on, as compared to the level the listener hears directly from the talker when the system is off.
Sound System Design Reference Manual Adding a 6 dB safety factor gives us the usual form of the equation: Maximum gain = 20 log D0 - 20 log Ds + 20 log D1 - 20 log D2 - 6 In this form, the gain equation tells us several things, some of them intuitively obvious: 1. That gain is independent of the level of the talker 2. That decreasing Ds will increase gain 3. That increasing D1 will increase gain.
Sound System Design Reference Manual How Much Gain is Needed? The parameters of a given sound reinforcement system may be such that we have more gain than we need. When this is the case, we simply turn things down to a comfortable point, and everyone is happy. But things often do not work out so well. What is needed is some way of determining beforehand how much gain we will need so that we can avoid specifying a system which will not work.
Sound System Design Reference Manual As we saw in an earlier example, our system only has 7.5 dB of maximum gain available with a 6 dB safety factor. By going to both a directional microphone and a directional loudspeaker, we can increase this by about 6 dB, yielding a maximum gain of 13.5 dB — still some 16 dB short of what we actually need. The solution is obvious; a hand-held microphone will be necessary in order to achieve the required gain.
Sound System Design Reference Manual
Sound System Design Reference Manual Chapter 5: Fundamentals of Room Acoustics Introduction Absorption and Reflection of Sound Most sound reinforcement systems are located indoors, and the acoustical properties of the enclosed space have a profound effect on the system’s requirements and its performance.
Sound System Design Reference Manual When dealing with the behavior of sound in an enclosed space, we must be able to estimate how much sound energy will be lost each time a sound wave strikes one of the boundary surfaces or one of the objects inside the room. Tables of absorption coefficients for common building materials as well as special “acoustical” materials can be found in any architectural acoustics textbook or in data sheets supplied by manufacturers of construction materiaIs.
Sound System Design Reference Manual More recent publications usually express the absorption in an enclosed space in terms of the average absorption coefficient. For example, if a room has a total surface area of 1000 square meters consisting of 200 square meters of material with an absorption coefficient of .8 and 800 square meters of material with an absorption coefficient of .1, the average absorption coefficient for the entire internal surface area of the room is said to be .
Sound System Design Reference Manual Figure 5-4. Interference pattern of sound reflected from a solid boundary Figure 5-5.
Sound System Design Reference Manual Consider a sound wave striking such a boundary at normal incidence, shown in Figure 5-4. The reflected energy leaves the boundary in the opposite direction from which it entered and combines with subsequent sound waves to form a classic standing wave pattern. Particle velocity is very small (theoretically zero) at the boundary of the two materials and also at a distance 1/2 wavelength away from the boundary.
Sound System Design Reference Manual theory allows us to make simple calculations regarding the behavior of sound in rooms and arrive at results sufficiently accurate for most noise control and sound system calculations. Going back to our model, consider what happens when the sound source is turned off. Energy is no longer pumped into the room.
Sound System Design Reference Manual It is easier for us to comprehend this theoretical state of affairs if energy growth and decay are plotted on a decibel scale. This is what has been done in the graph. In decibel relationships, the growth of sound is very rapid and decay becomes a straight line. The slope of the line represents the rate of decay in decibels per second.
Sound System Design Reference Manual Figure 5-8. Calculating reverberation time Figure 5-9.
Sound System Design Reference Manual by a single number, α. Only one step remains to complete our model. Since sound travels at a known rate of speed, the mean free path is equivalent to a certain mean free time between bounces. Now imagine what must happen if we apply our model to the situation that exists in a room immediately after a uniformly emitting sound source has been turned off. The sound waves continue to travel for a distance equal to the mean free path.
Sound System Design Reference Manual Figure 5-11. Reverberation time chart, English units Figure 5-12.
Sound System Design Reference Manual Rather than go through the calculations, it is much faster to use a simple chart. Charts calculated from the Eyring formula are given in Figures 5-10 and 5-11. Using the chart as a reference and again checking our hypothetical example, we find that a room having a mean free path just a little less than 3 meters and an average absorption coefficient of .2 must have a reverberation time of just a little less than .5 seconds.
Sound System Design Reference Manual Another identifiable characteristic, particularly of small rooms, is the presence of identifiable resonance frequencies. Although this factor is ignored in our statistical model, a room is actually a complicated resonant system very much like a musical instrument. As mentioned previously, if individual resonances are clustered close together in frequency the ear tends to average out peaks and dips, and the statistical model seems valid.
Sound System Design Reference Manual indicated by the density of the dots on the page; near the source they are very close together and become more and more spread out at greater distances from the source. The reverberant field is indicated by the circle dots. Their spacing is uniform throughout the enclosed space to represent the uniform energy density of the reverberant field. Near the source the direct field predominates.
Sound System Design Reference Manual Critical Distance (DC) The distance from the acoustic center to the circle-black boundary is called the critical distance. Critical distance is the distance from the acoustic center of a sound source, along a specified axis, to a point at which the densities of direct and reverberant sound fields are equal. Critical distance is affected by the directional characteristics of the sound source.
Sound System Design Reference Manual Critical distance also is affected by the absorption coefficients of room boundary surfaces. Figures 5-17 and 5-18 illustrate the same sound source in the same size room. The difference is that in the first illustration the room surfaces are assumed to be highly reflective, while in the second they are more absorptive. The density of the black dots representing the direct field is the same in both illustrations.
Sound System Design Reference Manual Suppose we place a small non-directional sound source in a room having R = 200 m2. If we measure the sound level at a distance 0.25 meter from the acoustic center and then proceed to walk in a straight line away from the source, the level will at first decrease as the square of the distance. However, about 1 meter from the source, the inverse square relationship no longer applies.
Sound System Design Reference Manual relationship is often indicated by the simple expression: 4W/cSα. W represents the output of the sound source, and the familiar expression Sα indicates the total absorption of the boundary surfaces. Remembering our statistical room model, we know that sound travels outward from a point source, following the inverse square law for a distance equal to the mean free path, whereupon it encounters a boundary surface having an absorption coefficient α.
Sound System Design Reference Manual We can agree that if the source of sound in a given room is non-directional, the equation for R is probably accurate for all practical purposes. It would also seem that the equation could be used for a room in which absorption was uniformly distributed on all boundary surfaces, regardless of the directivity of the source. Where we run into trouble is the situation of a directional source and absorption concentrated in restricted areas.
Sound System Design Reference Manual room constant with some confidence, then we can estimate the sound pressure level that will be produced in the reverberant field of the room for a given acoustical power output. Figure 5-22 enables us to determine by inspection the room constant if we know both α and the total surface area. This chart can be used with either SI or English units.
Sound System Design Reference Manual Once the critical distance is known, the ratio of direct to reverberant sound at any distance along that axis can be calculated. For example, if the critical distance for a talker is 4 meters, the ratio of direct to reverberant sound at that distance is unity. At a distance of 8 meters from the talker, the direct sound field will decrease by 6 dB by virtue of inverse square law, whereas the reverberant field will be unchanged.
Sound System Design Reference Manual Chapter 6: Behavior of Sound Systems Indoors Introduction The preceding five chapters have provided the groundwork on which this chapter is built. The “fine art and science” of sound reinforcement now begins to take shape, and many readers who have patiently worked their way through the earlier chapters will soon begin to appreciate the disciplines which have been stressed.
Sound System Design Reference Manual Acoustical Feedback and Potential System Gain Just as in the outdoor case studied earlier, if we have a microphone/amplifier/loudspeaker combination in the same room and gradually turn up the gain of the amplifier to a point approaching sustained feedback, the electrical frequency response of the system changes with the gain setting. The effect results from an acoustic feedback path between the loudspeaker and the microphone.
Sound System Design Reference Manual Step 1: Calculate relative sound levels produced by the talker at microphone and listener. We begin with the sound system off. Although the calculations can be performed using only relative levels, we will insert typical numbers to get a better feel for the process involved. The microphone is .6 meter from the talker, and at this distance, the direct sound produces a level of about 70 dB. Since DC for the unaided talker is only 1 meter, the microphone distance of .
Sound System Design Reference Manual The loudspeaker is mounted at the intersection of wall and ceiling. Its directivity index, therefore, is assumed to be 6 dB. In this room, the critical distance for the loudspeaker is 1.4 meters. This is almost the same as the distance from the loudspeaker to the microphone.
Sound System Design Reference Manual Calculations for a Medium-Size Room Consider a more typical (and more complicated) situation in which the sound system is used in a larger room and in which a directional microphone is employed. Figures 6-4 and 6-5 show a room having a volume of 918 m3, a total surface area of 630 m2 and α = 0.15. The first step is to calculate the room constant, and we would do well to examine the actual distribution of absorptive material in the room.
Sound System Design Reference Manual In the frequency range of interest, the loudspeaker is assumed to have a directivity index along its primary axis of 9 dB. From Figure 6-6 we find the corresponding critical distance of 4.2 meters. The loudspeaker’s directivity index at a vertical angle of 60° is assumed to be -3 dB, with a corresponding critical distance of 1 meter. The unaided talker has a directivity index of 3 dB and his critical distance must therefore be 2 meters.
Sound System Design Reference Manual However, all of the preceding calculations have assumed that the microphone is an omnidirectional unit. What happens if we substitute a directional microphone? Figure 6-7 shows the additional geometrical relationships needed to calculate the increase in gain produced by a directional microphone. Note that the distance from talker to microphone is still .6 meters and that the talker is assumed to be located along the major axis of the microphone.
Sound System Design Reference Manual Calculations for a Distributed Loudspeaker System Figure 6-8 shows a moderate-size meeting room or lecture room. Its volume is 485 m3, surface area is about 440 m2, and α is 0.2 when the room is empty. For an unaided talker in the empty room, R is 110 m2. However, when the room is fully occupied, α increases to 0.4 and the corresponding room constant is 293 m2.
Sound System Design Reference Manual From our calculations of critical distances, we see that the unaided talker will produce a sound level at the listener of 59 dB in an empty room and about 55 dB with a full audience. For a usable working delta of -6 dB, the calculated acoustic gain at the listener’s position is about 5 dB in an empty room and about 9 dB when full. Can we get more gain by turning off the loudspeaker directly over the microphone? Not in a densely packed array such as this.
Sound System Design Reference Manual Level at listener = L - 20 log (Dct/Ds), where Dct is the critical distance of the talker. The assumption made here is that the level at the listener is entirely made up of the talker’s reverberant field and that that level will be equal to the inverse square component at Dct. Now, the system is turned on, and the gain is advanced until the loudspeaker produces a level L at the microphone.
Sound System Design Reference Manual General Requirements for Speech Intelligibility The requirements for speech intelligibility are basically the same for unamplified as for amplified speech. The most important factors are: 1. Speech level versus ambient noise level. Every effort should be made to minimize noise due to air handling systems and outside interferences. In general, the noise level should be 25 dB or greater below the lowest speech levels which are expected.
Sound System Design Reference Manual The last point is illustrated graphically in Figure 6-12, adapted from the Peutz paper. Each of the diagonal lines corresponds to a particular reverberation time. Each shelves at a point corresponding to a direct-to-reverberant sound ratio of -10 dB. Note that the shelf may lie above or below the 15% figure depending upon the reverberation time of the room. This agrees with other published information on intelligibility.
Sound System Design Reference Manual Also, local acoustical conditions may exist which are not taken into account by statistical theory and, therefore, not covered by the Peutz findings or any of the other equations we have studied. Such localized dead spots or zones of interference may not be discovered until the sound system is installed. In large reverberant spaces, sufficient flexibility should always be built into the sound system design to allow for such surprises.
Sound System Design Reference Manual The analysis shown in Figure 6-14 indicates that when each of the two horns is powered by one watt, the reverberant field in the room (read directly from Figure 5-21) is 94 dB-SPL. The direct field level provided by each horn over its coverage angle is about 85 dB-SPL. This produces a direct-toreverberant ratio of -9 dB, and an inspection of Figure 6-13 tells us that the system will have marginal intelligibility.
Sound System Design Reference Manual Thus, the direct-to-reverberant ratio will be 83-92, or -9 dB. This is still not good enough, but we must remember that more than half the listeners will be closer to a loudspeaker than 4 meters. Another very important point we have not yet considered is the fact that the distributed loudspeakers are aimed almost totally into the audience, with its absorption coefficient considerably greater than α of .12.
Sound System Design Reference Manual The Role of Time Delay in Sound Reinforcement The preceding example mentioned time delay as a means of preserving naturalness in a distributed system. This comes about by way of the Haas (or precedence) effect (5), which is illustrated in Figure 6-16. If two loudspeakers are fed the same signal, a listener mid-way between them will localize the source of sound directly ahead (A).
Sound System Design Reference Manual System Equalization and Power Response of Loudspeakers It is customary to equalize all professional sound reinforcement systems for two reasons: overall response shaping and control of feedback. The overall response may be made smoother for a more natural effect through the use of broadband equalization and through the proper choice of drive components themselves.
Sound System Design Reference Manual At A, we see the on-axis (solid curve) and power (dotted curve) response of a 2-way system making use of a ported LF horn unit and an older type HF radial horn. When such a system is equalized for smooth power response, as in the case of the standard mid-house equalization procedure, then the on-axis, or direct field response of the system will have a couple of “bumps” in its response. This will have the effect of making both speech and music sound unnatural.
Sound System Design Reference Manual System Design Overview There is a rational approach to indoor sound reinforcement system design, and it can be broken down into the following steps: 1. Lay out the coverage requirements, generally starting with a central array. Determine the drive requirements for each element in the array. 2.
Sound System Design Reference Manual
Sound System Design Reference Manual Chapter 7: System Architecture and Layout Introduction Typical Signal Flow Diagram Just as the building architect interprets a set of requirements into flexible and efficient living or working spaces, the designer of a sound reinforcement system similarly interprets a set of requirements, laying out all aspects of the system in an orderly fashion.
Sound System Design Reference Manual will simplify things by considering only a single microphone path through the system to a single loudspeaker. For the moment, let us consider only the abbreviated console flow diagram shown in the upper part of Figure 7-1A. Microphone ratings in use today state the unloaded output voltage when the unit is placed in a sound field of 94 dB SPL. Normal speech level at an operating distance of .
Sound System Design Reference Manual Figure 7-1B.
Sound System Design Reference Manual Step Two: We now have to determine what the nominal operating level of the system should be for the farthest listeners, which we will assume are some 20 meters away from the loudspeaker. Let us further assume that the reverberation time in the room is no greater than 1.5 seconds in the range from 250 Hz to 2 kHz and that the average noise level room is in the range of 25 dB(A).
Sound System Design Reference Manual Step Three: For a simulated microphone input of 72 dB SPL, adjust the HF and LF outputs of the DSC260 for nominal levels of 0.4 Vrms. Then, advance the LF gain control on the MPX600 amplifier until a reference level of 60 dB SPL has been reached at a distance of 20 meters. Following this, increase the level of the HF section to reach the same value. Details here are shown in Figure 7-1.
Sound System Design Reference Manual Figure 7-3. Calculation of resistance in wire runs Constant Voltage Distribution Systems (70-volt lines) Many distribution systems in the United States make use of the 70-volt line for powering multiloudspeaker paging systems. In Europe the 100-volt line is common. In either system, the full output power of the driving amplifier is available at a line voltage of 70 Vrms or 100 Vrms, respectively.
Sound System Design Reference Manual LREV = 126 + 10 log WA - 10 log R, where WA is the continuous acoustical power output from the transducer and R is the room constant in m2. In using this equation, we assume that the space is fairly reverberant at very low frequencies and that the value of absorption coefficient at 125 Hz (the lowest value normally stated for materials) will be adequate for our purposes.
Sound System Design Reference Manual transmission coefficient for a direct radiator as a function of cone diameter. The solid curve is for a single unit, and the dotted curve is for two units positioned very close to each other. In addition to the double power handling capability afforded by the two units, the dotted curve shows a 3 dB increase in transmission coefficient at low frequencies.
Sound System Design Reference Manual Case Study A: A Speech and Music System for a Large Evangelical Church: 1. Basic Description and Specifications: The fan shaped architectural design shown in Figure 7-7 is common for modern evangelical churches in that it accommodates many people, all seating positions with good sightlines. The major acoustical problem is likely to be the curved front fascia of the balcony and the curved back wall itself.
Sound System Design Reference Manual The system consists of a central array of left, center, and right stereophonic music channels; speech will be reinforced over the center channel only. Delayed coverage for the balcony area will be provided by a ring of seven flown loudspeakers, and under-balcony coverage will be augmented by a ring of fifteen soffit mounted loudspeakers. The main array over the platform should be designed for nominal horizontal coverage in excess of 120 degrees.
Sound System Design Reference Manual The other approach is to use pre-existing building blocks, such as the SP225-9 system. Specifically, four of these systems can be arrayed with sufficient space between them for good stereo presentation and splayed to produce an included coverage angle of 135°. Rigging is integral in the SP Series, so that problem is solved. A center pair of SP225-9 units should be located side by side and splayed along their common back angle to give 120° coverage.
Sound System Design Reference Manual The three HF sections in each main array should be powered by one section of an MPX600 amplifier (200 watts into 16 ohms). Each one of the four main arrays should also have an SP128S subwoofer module. These would be powered the same way as the LF sections of the SP128S units. Level calculations are given: Level 102 dB 133 dB 113 dB Power 1W 1200 W 1200 W Distance 1 meter 1m 10 m The electrical diagram for the main array is shown in Figure 7-8. 3.
Sound System Design Reference Manual Figure 7-10. Plan and front elevation views of large liturgical church Figure 7-11.
Sound System Design Reference Manual Entering values and rearranging: For the next step in the analysis we need to determine the resulting reverberant level in the room. 10 log Efficiency = 92 -109 - 5 = -22 The efficiency is then 10-22/10 = 10-2.2 = .63% The total contribution to the reverberant field will be from all 18 loudspeakers working at once. We can then calculate the radiated acoustical power as: 18 x 10 x 0.0063 = 1.134 acoustical watts.
Sound System Design Reference Manual 1. That when a single loudspeaker produces a level of 85 dB SPL at the farthest listener, the resulting reverberant level is 96 dB SPL. 2. That the mid-band reverberation time in the room is 2.5 seconds. As our final step in the analysis, we can check the probable system speech intelligibility performance, according to Peutz’ Articulation loss of consonants (Alcons) by means of the chart shown in Figure 6-13.
Sound System Design Reference Manual Case Study C: Specifications for a Distributed Sound System Comprising a Ballroom, Small Meeting Space, and Social/Bar Area. 1. General Information and Basic Performance Specifications: 1.1 Ballroom Description: The size of the space is 33 meters long, 22 meters wide, and 8 meters high. A stage is located at the center of one short side, and the room may be used for banquets, displays, and social events such as dancing.
Sound System Design Reference Manual 2. Determine the power allocation for each loudspeaker. 2.3 Social Area System: 1. Suggest a stereo layout of loudspeakers that will provide all patrons with satisfactory sound. 2. Determine power requirements and distribution method. 3. Specify disco components that will produce a level of 115 SPL dB in the middle of the dance floor. 3. Answers to Exercises: 3.1 Ballroom System: 1. Use the square array, with center-to-center overlap.
Sound System Design Reference Manual Figure 7-15. Meeting space layout. Plan view (A); side section view (B) Figure 7-16. Meeting space system, signal flow diagram.
Sound System Design Reference Manual single loudspeaker will, at a distance of 7 meters, produce a level of 105 dB. The added contribution of the eight neighboring loudspeakers will increase this by 3 dB, making a maximum level capability of 108 dB. Level variations will be 1.4 dB. Because of the wide-band capability of the loudspeakers and relatively high power required, a low impedance distribution system should be used.
Sound System Design Reference Manual The total level at the test position is thus 82 dB when all 12 loudspeakers are powered with 1 watt, or 85 dB with 2 watts per loudspeaker. With 20 watts per loudspeaker, we would have a comfortable 10 dB margin over our target value of 85 dB SPL. The system will be “coasting” most of the time, and a single stereo amplifier, with loads wired in seriesparallel, will suffice. 2. Disco system. There are a number of possibilities here.
Sound System Design Reference Manual For a stereo system, we would need one DSC260 digital controller for frequency division and other signal processing. Subwoofer requirements can be met with four (one in each corner) JBL 4645B systems. Drive requirements would be two MPX1200 amplifiers and one section of a DSC260 controller. The amplifiers can deliver 800 watts continuous power per channel into 8 ohms. With their half-space reference efficiency of 2.
Sound System Design Reference Manual Figure 7-19. Disco system, signal flow diagram.
Sound System Design Reference Manual Recommended Reading: Books and Anthologies: 1. D. and C. Davis, Sound System Engineering, second edition, Howard F. Sams, Indianapolis, 1987. 2. J. Eargle, Electroacoustical Reference Data, Van Nostrand Reinhold, New York, 1994. 3. Various, Sound Reinforcement, an anthology of articles on sound reinforcement from the Journal of the Audio Engineering Society, volumes 1 through 26. (Available from the AES) Papers: 4. C. P. and R. E. Boner, “The Gain of a Sound System,” J.
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