Speaker installation is an essential consideration in any hi-fi system; a poor acoustic arrangement can defeat the characteristics of the finest electronic components. Similarly, a poor acoustic arrangement can defeat the potential of the finest speakers. Acoustics is the science of perceived sound, and its basic concern is the clarity with which sounds can be heard in a room or other listening space. That is, certain physical properties of a room affect the quality of sounds radiated by a speaker. Since the clarity of sound perception involves a subjective judgment, acoustics has both psychological and physical aspects.
Many important characteristics of an acoustical situation can be measured; in other words, there are various quantitative approaches to evaluation of room "response." The experienced technician makes a clear distinction between poor acoustic arrangements and unsatisfactory speaker characteristics.
Common trouble symptoms caused by poor acoustics or speaker defects are:
1. Echoes
2. Resonances
3. Room distortion
4. Speaker distortion
5. Rattle
6. Weak output
GENERAL DISCUSSION
There are four basic acoustic factors that need to be considered by the hi-fi installation technician.
These factors are (1) room noise level, (2) reverberation characteristics, (3) optimum loudness, and ( 4) room sound distribution. The same sound-level meter used to check the balance of a stereo amplifier is suitable for measurement of the room noise level. Noise tends to mask the sound radiated by a speaker. Since noise often has a dominant frequency, the masking effect is greatest at this frequency. For example, if room noise is dominated by the rumble of street traffic, the acoustic balance can be improved by judicious boost of the lower audio frequencies.
On the other hand, room noise in rural areas is often dominated by medium- or high-pitched background noises. In this situation, the acoustic balance can be improved by appropriate boost of medium or high audio frequencies.
Reverberation is another important room characteristic. Sound must "hang on" long enough to allow musical notes or spoken words to sound natural, but not to the extent that one word tends to blur the next word in normal speech. It has been determined that it is often desirable to have low frequencies reverberate longer than high frequencies in a given room. Whether or not specific measurements are made, or whether evaluation of reverberation is made by ear, we are basically concerned with how long it takes sound to decay in the room, and how the decay time varies at different frequencies. Since commercial sound-analysis equipment is comparatively expensive, the hi-fi installation technician is usually dependent upon practical experience in this area.
Loudness and clarity are separate considerations.
Optimum loudness occurs at a level that makes the listener unaware of this factor. Contrary to popular opinion, an abnormally loud reproduction of sound does not necessarily mean clearer sound. Echoes, an aspect of reverberation, will increase the total amount of sound, although often at the expense of intelligibility. Room size may be an essential factor in planning the installation. That is, good acoustics at one location in a room should not be provided at the expense of marginal acoustics at other locations in the room. This aspect of acoustic planning involves primarily the types of speakers to be used, and their placement in the room.
It is obvious that various rooms have different acoustic characteristics, apart from size. In other words, methods of providing good acoustics in a living room that is furnished with wall-to-wall car pet and plush furniture, plus heavy drapes, differ considerably from the methods that are exploited in a recreation room with tiled floor and paneled or bare walls, plus wrought-iron furniture. We will find that any good hi-fi speaker system, with the L and R speakers properly spaced, will provide good acoustic characteristics in an acoustically damped room.
On the other hand, the same hi-fi speaker system might be completely unsatisfactory in a highly reflective recreation room. One basic approach to acceptable stereo reproduction in a room with strong reflections is to install an oppositely directed speaker package, as shown in Fig. 6-1. In most cases, this is the optimum plan.
Fig. 6-1. Speakers mounted at opposite ends of cabinet.
Fig. 6-2. Open-back speakers in corners of a room.
Fig. 6-3. Preferred arrangement of speakers in a "dead" room.
Open-back speakers can also be used to advantage in a highly reflective room, if located as shown in Fig. 6-2. This is a good basic layout that can often be used in other situations; the speakers are placed in the corners at floor level. The corner walls and floor serve as sound directors. It is less desirable to mount the speakers half-way between the floor and ceiling. If the floor location is impractical, the best alternative is to mount the speakers at the ceiling in opposite corners. Next, if the installation is to be made in a "dead" room with little or no reverberation, better stereo reproduction will often be obtained with the speaker placement shown in Fig. 6-3.
Either open- or closed-back speakers may be used, and some advantage is usually realized by placing the speakers at floor level. Fig. 6-4 shows the conventional arrangement using closed-back speakers; the disadvantage in this example is that good stereo reproduction is obtained only along the center line of the room. If the listening positions are in this line, there is no objection to the plan.
Fig. 6-4. The conventional arrangement is less desirable.
Fig. 6-5. Poor placement of speakers in an L-shaped room.
Fig. 6-6. Preferred placement of speakers in an L-shaped room.
Fig. 6-8. A mono speaker in the center improves the acoustical characteristics.
Fig. 6-7. This placement gives poor sound distribution and a serious "hole
in the middle."
Fig. 6-9. An anechoic room.
The basic considerations become modified in accordance with substantial variations in room shape.
Aside from the rectangular room, the L-shaped room is most often encountered by the hi-fi installation technician. Although a do-it-yourselfer tends to in stall stereo speakers as pictured in Fig. 6-5, this is an example of poor practice, inasmuch as the radiation from the L speaker is not only impeded, but undergoes many more reflections than the radiation from the R speaker. The preferred speaker arrangement in this situation is shown in Fig. 6-6. Although a portion of the room is excluded as a stereo listening area, the advantage of this arrangement is that the acoustical characteristics in the main area are much better than in the example of Fig. 6-5.
Next, let us consider the characteristics of the arrangement shown in Fig. 6-7. In this example, the speakers are too far apart; the result is poor distribution of sound and a serious "hole in the middle." Customers who have had little or no experience with stereo reproduction sometimes believe that they must hear the L and R separation prominently. A listener with this misconception will tend to place the L and R speakers so far apart that an obtrusive ping-pong effect results. The goal of realism is accordingly lost in this overemphasis on separation.
An occasional situation is encountered in which the R arrangement of Fig. 6-6 is not feasible because of room layout, decor, etc. In such a case, the arrangement of Fig. 6-7 will have much better acoustic characteristics if a third speaker is added between the L and R speakers, as shown in Fig. 6-8. This speaker is essentially a mono speaker, inasmuch as it is driven by both the L and the R signals. A separate volume control must be included with the mono speaker, and adjusted for best acoustic balance.
Fig. 6-10. Development of initial reflected wavefronts.
Fig. 6-11. Principle of acoustic focusing.
Fig. 6-12. Large objects produce sound shadows.
Fig. 6-13. Shape of hall designed for optimum acoustic characteristics.
Fig. 6-14. Sine wave corresponding to compressions and rarefactions in
a sound wave.
Fig. 6-15. Phase representation in an acoustic wavefront.
Only an anechoic chamber such as that illustrated in Fig. 6-9 is completely free from reverberation or acoustic reflection. No domestic installation is completely "dead," and reflected wavefronts are promptly set up and trail close behind the main acoustic wavefront, as exemplified in Fig. 6-10. An acoustic wavefront travels at a speed of 1100 feet per second, so that it takes only 20 milliseconds for the wave to reach the opposite wall of a 22-foot room. Thereupon, the main wavefront becomes a reflected wavefront, and the listening area is soon populated by a multitude of reflected wavefronts. However, each reflection entails a substantial energy loss, and re-reflections may fall below the threshold of audibility, except in special cases. An extreme example of reflection and acoustic focusing is a "whispering gallery," as exemplified by St. Paul's Cathedral in London. A whisper at one side of the cathedral is distinctly audible at the other side, a hundred feet distant.
Acoustic focusing is provided by the walls and floor of a corner in a room, when a speaker is mounted in the corner. However, sharp focusing is obtained by means of curved surfaces (ideally parabolic contours) in architectural design. The principle of the whispering gallery is illustrated in Fig. 6-11.
"Sound shadows" are produced by interfering surfaces in the path of an acoustic wavefront, as shown in Fig. 6-12. As in the case of an acoustic reflector, an object must have a comparatively large surface area to produce a significant sound shadow. Note also that a limited area which shadows high audio frequencies will have little effect on low frequencies.
It follows from the basic principles of acoustics noted previously that there are many factors which must be taken into account if a room or hall is to be designed for optimum sound reproduction. Carnegie Hall is an example of outstanding acoustic design; a plan view is seen in Fig. 6-13. A basic acoustic rule of thumb states that the best shape for a listening room is a height/width/length ratio of 1: 1.27: 1.62. We know that sound waves are propagated as compressions and rarefactions in the distribution of air molecules; however, it is helpful to represent a sound wave in sine-wave form, as shown in Fig. 6-14. This representation clearly points out the phase characteristic. Phase relations in an acoustic wavefront may be visualized as shown in Fig. 6-15. It is evident that reflected wavefronts will aid or oppose, depending on their phase relation. This phase relation changes as the listener changes his position in the room. In most installations, this variation is not troublesome, because the reflected waves decrease in amplitude rapidly. However, in highly reflective rooms, we may find certain locations that are virtually "silent zones" due to wavefront cancellation.
The same principle applies to installation of stereo speakers in any location-that is, the speakers must be connected to operate in phase, as shown in Fig. 6-16.
Fig. 6-16. Incorrect phasing of stereo speakers will produce cancellations
in the sound wavefronts.
Analysis of Faulty Reproduction
(B) Evaluation of reproduction with speaker connections interchanged.
Fig. 6-17. Preliminary analysis of distorted reproduction.
To avoid confusion between amplifier defects and speaker defects, the preliminary check shown in Fig. 6-17 is helpful. If the connections to the two speakers are interchanged, the distortion will either remain in the same channel, or will appear in the other channel. If the distortion remains in the same channel, the defect is in the speaker. On the other hand, if the distortion appears in the other channel, the defect is in the amplifier. Fig. 6-18 shows the interconnections for a typical component-receiver system. To verify that the speakers are phased correctly, the tuner may be driven by an AM generator, and the radiated sound may be analyzed as you walk through the line midway between the speakers. If the audio tone does not rise and fall in intensity, the speakers are phased correctly. On the other hand, if the audio tone rises and falls in intensity as you move your listening position, the speakers are out of phase. To correct the trouble, simply reverse the connections of one of the speakers.
Deteriorated stereo reproduction sometimes occurs in do-it-yourself installations because of excessive cable capacitance from the tuner to the multiplex adapter. This trouble will not be confused with speaker defects if AM and mono-fm reproduction is compared with stereo-fm reproduction. It is good practice to keep the shielded cable for the adapter input as short as possible. The capacitance of conventional cable ranges from 20 to 100 pF per foot, and the loading effect of a long cable can seriously reduce the amplitude and . shift the phase of the higher-order frequencies in the composite stereo signal. For this reason, some manufacturers supply pre cut lengths of high-quality, low-capacitance cable for this interconnection. If you do not use precut cable, it is good practice to choose a low-capacitance type, and to mount the adapter as near the tuner as may be convenient.
Fig. 6-18. Interconnections for a typical component-receiver system.
Fig. 6-20. A crossover network.
Faulty reproduction due to speaker defects can be corrected by the hi-fi technician, provided that some major malfunction has not developed. For example, children sometimes push knitting needles, pencils, or other potentially damaging objects into speaker grilles. Cracks, holes, or tears in a cone will impair the fidelity of sound reproduction. Therefore, the entire cone should be inspected carefully (Fig. 6-19). Damaged cones should be replaced; however, at a customer's request, minor holes or tears can be repaired with all-purpose cement. Patching of comparatively large holes is poor practice, and is regarded as a last-resort measure. Any lack of complete bonding is a potential source of rattle. Remember that when an enclosure is reassembled, all parts must be securely tightened; a loose screw or nut is very likely to cause trouble.
If we find that the moving coil is rubbing against a pole piece, replacement is generally advisable. Sometimes, if the design permits and if damage has not occurred, it may be practical to re-center the moving coil. A substantially scuffed or open voice coil should not be repaired unless a replacement is unobtainable. Experienced technicians know that a complaint of speaker rattle may not indicate a speaker defect, but may be tracked down to some vibrating surface or object on or near the speaker.
For example, knick-nack display cabinets are frequent offenders. Such cabinets are acoustically self resonant, and can "store" considerable acoustic energy. In turn, the contents and shelves tend to vibrate and produce sound interference. Cabinet doors should be securely latched. To avoid possible mis understanding, a hi-fi installation technician may feel it advisable to discuss some basic acoustical principles with his new customers. This precaution is helpful in forestalling future accusations such as "Why wasn't I told about that?"
Fig. 6-19. Inspect speaker cone carefully for possible defects.
Crossover networks are part of a speaker system.
A crossover network comprises filter circuits used in multiple-speaker system to separate the high and low audio frequencies into separate groups that are fed to individual speakers designed to reproduce the high and low frequencies, respectively. A typical crossover network is shown in Fig. 6-20. Capacitor defects are the most common sources of trouble.
To check out a crossover network, connect the out put of an audio oscillator to the input terminals of the amplifier. Connect suitable values of power resistors in place of the woofer and tweeter speakers.
In the first test, connect a VOM or TVM across one terminating resistor. In the second test, connect the meter across the other terminating resistor. Note that a VTVM is not entirely suitable in this test, because of its ground capacitance. Rolloff rates for a cross over filter are chosen at widely different values by various designers. However, the important point is that the high-frequency and low-frequency rolloffs should be consistent, so that the overall frequency response will be flat. We will encounter rolloff rates from 6 dB to 18 or 24 dB per octave. In general, transient response is better with slow rolloff, al though a greater demand is placed on the tweeter for power output.
In some amplifiers, the crossover network is connected to intermediate stages instead of directly to the speakers. Moreover, a few amplifiers have electronic crossovers instead of LC filter configurations as shown in Fig. 6-20. However, the same basic test procedure is appropriate. As noted previously, some systems employ a woofer, midrange (squawker), and tweeter assembly. In this situation, a test involves measurement of three frequency responses, instead of two. The essential consideration is that the combined responses should show an overall flat frequency characteristic.
ANALYSIS OF COMMON SYMPTOMS
An analysis of the common trouble symptoms listed earlier in this Section is presented in this section.
1. Echoes
When echoes are a problem in a speaker installation project, there are several possible remedial courses. In any given situation, the best method will depend both upon the physical properties of the acoustic area, and also upon the viewpoint and desires of the customer. A hi-fi buff will go to much greater lengths in the pursuit of good acoustical conditions than will an interior decorator whose chief concern is eye-appeal.
Factors contributing to objectionable echoes in a speaker installation are:
a. Speakers may distribute sound energy to better advantage if located at the opposite end of the room.
b. Draperies or tapestries may be needed on a wall to provide sonic absorption (see Table 6-1).
c. Speakers may operate more satisfactorily if located in ceiling corners, instead of floor corners.
d. Echoes may be less troublesome if the speakers are mounted in opposite ends of a cabinet (see Fig. 6-1).
Table 6-1. Typical Acoustic Absorption Coefficients
2. Resonances
Acoustic resonance cannot develop in the absence of reverberation (sonic standing waves). Any enclosure that is acoustically coupled to the surrounding air will act as a sonic resonator at a particular frequency or frequencies. Fig. 6-21 shows a classic ex ample of sonic standing waves. The reduction of acoustic resonances is accomplished by minimizing the pertinent reflections, by reducing or eliminating the coupling to the sonic resonator, or by a combination of both approaches.
Possible causes of perceptible resonances in a conventional hi-fi installation are:
a. Display cabinets or hollow decorative structures may need to be closed off from the surrounding air, or lined with a special acoustic material.
Installing Hi-Fi Speakers
b. Speakers may have been mounted in cabinets of poor acoustic design by a do-it-yourselfer.
c. Speakers may have been located under a table or in a nook that develops acoustic resonances.
d. A thin wall behind a speaker may act as a sonic resonator; try relocating the speakers.
3. Room Distortion
Fig. 6-21. Examples of sonic standing waves.
Room distortion takes on various forms and often changes with the conditions of use. Some forms of distortion, such as echoes or resonances are comparatively simple to analyze; on the other hand, complaints such as "poor stereo effect," or "poor sound quality in the daytime, and good sound quality at night," are more difficult to analyze. The customer sometimes fails to recognize basic acoustical situations; for example, a customer may be unaware of the fact that a highly reflective recreation room will have improved acoustical characteristics when a large number of people are present. A person has an acoustic absorption coefficient of 0.44, whereas smooth wood has a coefficient of only 0.04.
Possible causes of room distortion are as follows:
a. Poor stereo effect can be caused by improper speaker placement (see Figs. 6-5 and 6-7).
b. Changed sound quality at night compared with daytime is generally caused by drawing drapes over large picture windows.
c. Changed sound quality in cold weather compared with hot weather is usually due to closing windows and doors in the listening area.
d. Branched portions of an L- or T-shaped room have poor acoustic characteristics (Fig. 6-6), and are not regarded as listening areas.
e. A large screen or similar object may have been added to the room decor, producing sound shadows (Fig. 6-12).
4. Speaker Distortion
In some installations, the room acoustics are better suited to a particular type of speaker. That is, the trouble symptom may not be due to a speaker defect, but to the installation of an unsuitable type of speaker. An experienced hi-fi installation technician will quickly recognize this source of difficulty. Another type of difficulty occurs when a do-it-your selfer has installed speakers that do not match the amplifier characteristics satisfactorily. Knowledge of components is required in this situation, and the experienced technician will suggest a comparative trial of conventional combinations when this trouble symptom occurs.
Possible causes of speaker distortion are as follows:
a. Open-back speakers operating in wrong corners in a highly reflective room. Replace with oppositely directed closed-back speakers (see Fig. 6-1). Alternatively, try locating the open-back speakers as indicated in Fig. 6-2.
b. Basic trouble due to "hole-in-the-middle" (see Fig. 6-7) . Relocate speakers as shown in Fig. 6-6.
c. Basic trouble due to a sound shadow zone, as shown in Fig. 6-12.
d. Speaker phasing incorrect (see Fig. 6-16).
e. Serious impedance mismatch between amplifier and speaker; use speaker with appropriate input impedance.
f. Speaker defect such as shorted turns or high resistance connection.
5. Rattle
As noted previously, a nontechnical person may confuse an environmental rattling with speaker rattle. That is, a rattle developed by a nearby cabinet, or by a decorative lamp placed on the speaker en closure might be confused with a speaker defect.
Therefore, the hi-fi technician should keep the possibility of customer confusion in mind when this trouble symptom is reported. Experienced technicians can recall case histories in which a deceptive rattle was actually caused by an amplifier defect, in stead of an environmental or a speaker defect.
Possible causes of rattle in a hi-fi installation are as follows:
a. Object placed on speaker enclosure.
b. Loose door or shelf in nearby cabinet.
c. Loose connection to a speaker terminal.
d. Assembly screws not tight in speaker enclosure or speaker assembly.
e. Speaker damage, such as a pencil pushed through the grille.
f. Amplifier defect that simulates speaker rattle (see Fig. 6-17).
6. Weak Output
Weak output from a speaker can be caused by various defects, most of which fall into the category of electrical troubles. Weak output may or may not be accompanied by distortion. For example, if a do-it yourselfer has been tampering with a speaker, and the voice coil is obstructed by iron particles or other foreign matter, weak output will be accompanied by rasping noises and/ or loud popping and crackling. On the other hand, an insulation defect that causes a low-resistance shunt path across the voice coil produces weak output without substantial distortion. If a speaker is located at a considerable distance from the console, it is good practice to use a correspondingly larger size of conductor, in order to keep the PR loss within reasonable limits.
Possible causes of weak output in a speaker installation are:
a. Corroded or high-resistance connection in the speaker circuit.
b. Deteriorated insulation causing a low-resistance shunt path.
c. Plug not fully inserted into connector.
d. Defective stranded lead in speaker circuit that introduces substantial resistance.
e. Internal defect in speaker (less likely than external troubles) .