The Listening Room Deserves Acoustic Consideration
Unfortunately, too little attention has been accorded the home listening room in the general question of hi-fi reproduction in the home. Probably the main reason for this is that the hobbyist must first be introduced to, and learn to understand, the newest tools of the art -- the loudspeaker and the enclosure. Secondly, of course, there doesn't seem to be much a man can do with his living room unless he is willing to undertake a major rebuilding and refurbishing project. Fortunately, however, if one has a basic understanding of what it takes to make a good listening room, it will be found that there is indeed much that can be done to make sure that the many dollars spent on the reproducing equipment itself are not blown out of the nearest window or lost in the nearest drapes. The vast store of knowledge that has been accumulated on the acoustic problems of concert halls may readily be applied to the listening quarters in one's home. If we are engaged in making "miniature music halls" in our own homes, as all manufacturers of hi-fi equipment claim, we must extract what we can from this background of knowledge and apply it in home-size doses to get the best possible performance out of our hi-fi system.
Rooms are Basically Enclosures
It will be recognized that a room is an enclosure. There is no basic difference between the room in which we listen and the enclosure in which the loudspeaker is mounted. There are, of course, differences in degree. We will, therefore, find that the study of the room in which we listen is virtually an extension of our previous analysis of baffles and enclosures in general.
It will be recalled that in the wall infinite baffle, the baffle and the room were actually one and the same. Although the other baffles and enclosures described in Part 2 may not seem to be as closely connected with the room problem, our further analysis in the next section will show that a great interdependence does exist between these two elements. It will therefore help in understanding the problem of room acoustics to consider a room in the same category as an actual loud speaker enclosure.
It will be recalled that enclosures fell into two general classifications. The "enclosure" type (such as the bass reflex) is a resonant device. It has its own resonating characteristic determined by its size and the opening into it. Then there is the anti-resonant enclosure (such as the horn baffle), which theoretically attempts to attain a truly un resonant condition by approaching as large a horn mouth as possible, consistent with the design flare of the horn. In a similar vein, we may have resonant listening rooms and listening rooms which are anti-re sonant. The degree to which they are one or the other is dependent upon their shape and the quality and quantity of sound absorbent material (or lack of it) that is deployed about the room. In general, "live" rooms, rooms that are resounding and reverberant, are more resonant than rooms that are "dead" and muffled.
We must not mistakenly use the word resonant as if it were a desirable quality. True, we speak of the fine resonance of a stringed instrument, or the full bodied resonance of a baritone's voice. These are resonances that are deemed desirable because they aid in the projection of the instrument or the voice. However, if a violin had a single peaked resonant spot in the middle of the D-string tones, or if the baritone could elicit good projection at only one or two notes in his allotted vocal scale, their music would be erratic in volume, unsmooth in general projection, and difficult to listen to. Under these conditions, resonance is undesirable. Just as we controlled the resonance peak of the loudspeaker in order to get smooth operation, so we must temper the resonance of the room so that the room will perform properly.
Rooms May Have Many Resonances
Unlike the loudspeaker with its one major resonance, rooms and enclosures may in general have several resonances or "modes" of vibration. Acoustically, there is no great difference between a rectangular room and a long tube. There is a difference in dimension, of course. In the long tube, the major dimension is its length, and the frequency to which it responds most easily is closely related in wavelength to the actual length of the tube itself. A tube whose length is a half wave length of the sound being transmitted into it will reproduce that frequency very efficiently from its open end. However, this is not the only frequency that the tube will radiate. It will transmit all the harmonics (overtones) of this one note, if these overtones existed in the original signal, because the tube is still an integral number of half wavelengths of these overtones. In like manner, rooms that are roughly rectangular in shape may have very many natural resonances or modes of operation, depending not only upon the length of the room, but upon the other dimensions as well. These other dimensions can develop their own resonance modes. It is therefore to be expected that the normal living room will exhibit very many natural resonances that are completely a function of the size of the room and its relative dimensions, and partly a function of the liveness of the room.
Live Rooms Have Small Degree of Sound Absorption
Very resonant rooms are characteristically "live" rooms. The best example of a live room in the home is a tile lined bathroom. These hard smooth surfaces reflect sound almost as efficiently as mirrors reflect light. We are all good singers when it comes to vocalizing in the bathroom, because the walls reflect our feeble voices back to our ears and magnify our vocal powers many-fold. Our ears receive the same magnified impression of the sound from the walls of the bathroom that our eyes receive from the large concave mirror used for shaving or cosmetic purposes. The constant and repeated reflection of the sound from one hard wall to the other, with little absorption by these hard smooth surfaces builds up the sound around the ear, giving it life, or "liveness." The fact that this quality of liveness in any room is a function of the normal mode of vibration of the room and the reflection properties of the room leads to a situation that may be detrimental to smooth audio reproduction.
Fig. 15-1. Standing waves for low frequencies may be set up in a room
leading to "hot spots" or "dead spots" for different
locations along the direction of the standing wave.
ALSO: 1/2 WAVELENGTH STANDING WAVE SET UP AT 25 HZ, 1 1/2 WAVELENGTH STANDING WAVE SET UP AT 75 HZ, 2 WAVELENGTH STANDING WAVE SET UP AT 100 HZ, ETC.
Live Rooms Introduce Sound Pressure Irregularities
The reflection of sound from one wall to the other and the normal mode of vibration of the room set up "hot" spots and null points of sound energy for different frequencies and for different areas of the room. Because rooms are large "enclosures," they vibrate and respond most readily to long wavelengths (low frequencies). Thus, as shown in Fig. 15-1, a standing wave may be produced for a particular frequency.
Under these conditions there will be in certain spots a peak condition of sound pressure and in other spots there may be a complete cancellation or reduction of sound pressure. This condition often makes for disagreement as to what one person hears from the system and what someone else hears. For instance, if the listener were at a spot where there is re-enforcement due to the standing wave, he would hear "plenty of bass." If his companion were standing at a point of cancellation, he would hear much less bass.
This is not such a completely hypothetical condition. It is a serious situation that must be recognized in listening to systems. The reader may prove this to himself if he has an oscillator, or even a test frequency record. Select some note down in the 100-hz range and re produce it at fairly good volume. As the note is being sounded, move slowly across the room, and you will clearly hear the great differences in sound pressure for that note as you change position throughout the room.
As mentioned above, this state of affairs may cause differences of opinion between two listeners located in different parts of the room as to whether or not a certain portion of the low frequency response is missing from the system. In making a truly objective listening test, it is therefore mandatory that a system be heard from many different positions of the room. Obviously, if the listener's easy chair happens…
Fig. 15-2. Standing waves may change the manner in which listeners in
different parts of the room hear the speaker output. Irregularities and
abnormal high and low level pressures are introduced at those frequencies
for which standing waves are set up.
…to be located at a hot spot, or a null point, for a particular small band of frequencies, he will consistently be subjected to frequency irregularities in the response of the system. The matter of the location of the listener is thus of considerable importance not only for low frequencies but in connection with dispersion of the high frequency beam from the loudspeaker system as well. Secondly, the actual placement of the loudspeaker in the room, irrespective of the listening ear, will greatly affect the final performance. These two latter matters will be discussed in the succeeding sections. It is of importance at this point, however, to realize that because of the resonance condition of the room due to its normal mode of operation and due to reflection from the walls, a live room may produce many irregularities of response and materially affect the smoothness of the system, as illustrated in Fig. 15-2.
Sound Decays Slowly in Live Room: Long Reverberation Time
A "live room," in which there are excessive reflections from the walls, will have a high "reverberation time." The reverberation time of a room is a measure of the way in which a sound decays after the source of sound has stopped. This factor is defined as the time (in seconds) required for the sound energy to decay 60 db, which is equivalent to one millionth of its original power value, as shown in Fig. 15-3.
The actual value of the reverberation time of a room is a function of the volume of the room as well as the reflective properties of the walls for a particular frequency in question. Obviously, if the walls are highly reflective, the sound will continue to bounce back and forth with little -r o .; SECONDS absorption at the reflective surfaces, and it will take a long time for the sound to reach the low level of -60 db in relation to its original intensity. Similarly, if the room is very big and the walls are correspondingly further away, the sound will not be attenuated as quickly by the absorbing characteristics of the walls, and the sound energy will be sustained longer within the room. Thus, in general, a large room has a high reverberation time constant, and reflecting walls produce a high reverberation time constant.
High Reverberation Time Produces In-articulation
Now what does this reverberation time mean in terms of listening effects? Let us take the case of the live room, the one that is highly reverberant. As soon as the first note leaves the loudspeaker, it will begin to travel into the room, hitting the walls, ceiling, floor, doors, and windows, and will be reflected back and forth for a long period of time before it is reduced by the necessary 60 db. However, while this first note is bouncing around, other notes are coming out of the loudspeaker. As a result, the first note may still be audible when the second note is produced, and there is overlapping of the notes, as indicated in Fig. 15-4. This leads to indistinct reproduction and poor articulation of the program. Therefore, too live a room is detrimental to good musical reproduction, on the basis of cleanness of reproduction of the individual notes.
Fig. 15-3. After the sound source has been stopped, the sound in the
room begins to decay. The rate at which it decays is a function of the
reverberation of the room.
Reverberation time is de fined as the time in seconds for the sound to decay 60 db.
It would also appear that the degree of overlapping of these notes would be a function of the type of music being reproduced. If, as an extreme example, a composition were being played in which there were large gaps between notes as well as a slow tempo, a larger reverberation time might be acceptable, for then one note would not run into the other. However, in a fast staccato movement there would be a severe loss of definition. There is no "optimum reverberation time" for all applications. Therefore, in addition to the factors of room size and reflective conditions (which determine the reverberation time of a…
Fig. 15-4. When a room is very "live" its reverberation time
is high and the decaying sound overlaps subsequently heard notes. This
leads to in-articulation for fast speech and to in distinct musical reproduction.
… particular room) we must remember that the actual musical composition will determine whether the reverberation time given by the previous factors is suitable. Although the constructor cannot do anything about the type of music that comes out of his loudspeaker (other than select it, of course) he can do something about his listening room. These matters will be discussed in Section 17.
Live Rooms Sound Louder
Another characteristic of the highly reverberant room is that the reproduction seems louder than normal for moderate audio powers.
This condition exists both objectively and subjectively. Since a high reverberation time means that the sound is not being absorbed but is moving around in "space," the reflected sound is there waiting for the ear to hear it as well as the direct sound coming from the loud speaker. In other words, the ear receives additional acoustic power because of the reflections. This increases the overall acoustic pressure at the ear, which makes the sound louder to the ear.
Fig. 15-5. A very live room gives the listener a feeling of largeness
of the room due partly to the many acoustic images that appear to come
from areas beyond the wall boundaries. This provides an artificial enlargement
to the original source of sound increasing the "acoustic perspective" of
the music. --------- APPARENT ADDITIONAL SOUND SOURCE DUE TO REFLECTIONS
\ NEW ACOUSTICALLY "REMOVED" WALL
Live Rooms Sound Bigger
Subjectively, a very live room also gives the illusion of largeness.
A small room with highly reflective walls may produce a feeling of psychoacoustic spaciousness because of the moving around of the sound within the enclosure. We may consider the reflecting walls as creating new "acoustic sources" for the original sound, as shown in Fig. 15-5.
The reader will recognize the optical counterpart of this demonstration.
The ear in the location shown will hear the direct ray from the loud speaker and also the reflected ray, which will appear to be emanating from a spot well in back of the wall itself. Thus the highly reverberant room will be psychoacoustically enlarged.
Live Rooms Reduce Sound Directivity
It is also a characteristic of live rooms that the directivity characteristic of the reproducing unit is considerably altered by the reflective properties of the room. Where the walls are highly reverberant, the several radiating beams from the loudspeaker become greatly dif fused about the room by successive reflections, effectively increasing the uniformity of the sound reproduction throughout the room in respect to the higher frequencies. (See Fig. 15-6.) In fact, some constructors have made use of this principle in building their tweeter units into small enclosures in the upper ceiling corner of the room so that multiple reflections from the smooth walls in the ceiling area and from the ceiling itself will help diffuse the high frequency radiation.
In the room that it too live, we may say then that deteriorated articulation of the program will occur, that irregularities of low frequency response will be produced within the room, that the system will sound louder, that the sound will appear to come from a larger area, and that the sound in general will be more diffused throughout the room.
Fig. 15-6. Due to the many reflected rays in a live room the directivity
of , the sound as it reaches the listener is a function of the room liveness
plus the speaker directivity characteristic itself.
Dead Room Has High Absorption of Sound
Let us now examine the other extreme, the dead room. This is a room characterized by a high degree of absorption of the sound at the walls of the room. It is necessary to mention an important difference between the performance characteristics of the dead room and the live room other than that of reflective properties. We discussed above the matter of the normal modes of operation of the rectangular enclosure (the natural resonance of the room itself). However, resonances can be set up only where there are reflective surfaces. (For the sake of technical accuracy, it should be stated that an open end of a tube may also be characterized as a reflective surface, inasmuch as its abrupt termination causes impedance discontinuities and wave reflections back into the tube.) In our present instance, where we are dealing with total enclosures, we shall proceed on the assumption that to obtain normal resonances of the room we must have reflective walls.
It follows that although the frequencies of the normal vibrations within the room are governed by the dimensions of the room, the intensities of the resonances are governed by the reflectivity of the walls themselves.
Suppose the walls were totally absorbing? The entire room would act as a perfect "sink" for the sound energy leaving the loudspeaker. The sound would all be lost in the completely absorbing surface, no waves would be reflected, and there would be no normal mode of vibration of the room. This, it will be realized, is the reason for treating the insides of the bass-reflex enclosure with damping material; it is thus prevented from setting up its own mode of resonance apart from that dictated by its volume and the port opening size. Accordingly, then, a total absorbing room will not only reduce the wall-to-wall reflections but will also reduce the normal mode of resonance of the enclosure.
Dead Rooms Sound Softer
Because of the great lack of reverberance in the room with a very short reverberation time constant, the room sounds dead. In relation to a live room, the sound seems lower in intensity, because the ear does not benefit from any reflected sound. In fact, the sound hitting the highly absorbent walls will be completely lost in the absorbing material, and will be lost to the ear. In effect, the overdamped room is wasteful of acoustic power delivered to it, just as the overdamped loudspeaker enclosure is wasteful of acoustic power delivered to its interior. The analogy between the loudspeaker enclosure and the listening room carries over completely.
Dead Room Sounds Smaller
In the dead room, there is also the psychoacoustic effect upon the ear of the lack of sound coming to it from sources other than the loudspeaker directly. In the case of the very live room, the reflected acoustic images create an apparent source of sound that seems to be behind the walls. This gives a synthetic depth to the source of the sound; it seems to come from a large area. In the dead room, no such reflection exists. As a consequence, the sound all appears to emanate from the one point source of the loudspeaker itself, without benefit of sound images from the wall. The end effect is to produce a feeling of a small sound source playing directly to the listener in cramped quarters. There is no acoustic "spread" to aid the acoustic imagination.
The loss of ambient sound around the listener's head thus shrinks the sound source in perspective as well as in amplitude (Fig. 15-7).
Dead Room Makes Sound More Directive
In the matter of the dispersion characteristic within a very dead room, the total characteristic is controlled almost completely by the speaker characteristic itself, and by the placement of the speaker in the room. Where there are no reflections from the wall, there is no diffusive assistance to the various rays of sound coming out of the loudspeaker. In such a room then, the high frequency distribution of the system depends solely upon the loudspeaker and baffle combination. If the speaker high frequency beam is sharp, the position of the ...
Fig. 15-7. Dead room eliminates sound reflections to the observer, and
the sound appears to come more directly from the loud speaker. The acoustic
spread of the sound source is thus narrowed down and the room shrinks
psychologically.
... listener in the room will determine how much and how many highs he will hear. As far as the low frequencies are concerned, however, the dead room exhibits fairly uniform distribution characteristics free of standing wave effects. There are few low frequency hot spots or null points throughout the room, due to the absence of standing waves.
This means that the dead room has uniform low frequency response characteristics without hills or valleys, but at the expense of reduced apparent power.
Reverberation Time Depends Upon Room Size
It was pointed out above that the reverberation time of a room is a function of the volume of the room and the absorption characteristics of the wall surfaces. The room size is rather closely related to the degree of reverberation desired in the system. In general, small rooms should have less reverberation time than large rooms. When the listener is in a small room, he naturally finds himself closer to the loudspeaker enclosure and closer to the sound reflecting surfaces. Reflection will therefore reach the listener sooner from these close walls than from the walls of a larger room. Consequently, if it is desired to maintain the same clarity of separation between notes in the small room as compared to the larger room, the time taken for the reflected sound to reach the ear in the small room will have to be reduced.
Accordingly, the smaller room should have a reverberation time smaller than that of a large room, for the same type of program material. In the large room, where the ear is far from the source of sound and far from the reflected sound, time must be allowed for the reflected sound to reach the ear from the greater distances of the comparatively far away walls, and so the reverberation time may be longer.