Irregular Rooms Provide Desired Characteristics
Unfortunately, in many instances the room in which the hi-fi system is installed is not determined by how well the room is suited to such equipment. In most instances it is the living room. Although the living room of a particular house may be an ideal place for social gatherings, it might lack some important prerequisites for good acoustic performance. If the loudspeaker enclosure must be located in the living room, even though it is originally unsuitable, adjustments may be made to the room to bring it into a suitable condition to do justice to the hi-fi system.
It is seldom, however, that the living room is so poor that the loud speaker enclosure cannot be installed in it. Perhaps it is more than simple coincidence that the general optimum shape of a good listening room at home is very closely related to those dimensions which also make it easy to look at or to live in. One seldom finds perfectly square rooms in a house because they lack eye appeal; square rooms are also poor acoustically, for they easily set up undesired natural modes of vibration. Long narrow rooms are also seldom found in homes because they cause much difficulty in furniture arrangement and are equally “un-pretty" to the eye; acoustically, long narrow rooms introduce tubular resonances that are a function of the length of the room, which is also undesirable. Somewhere in between the perfectly square room and the long narrow room, there is a compromise that is of the right size esthetically and of suitable proportions acoustically. Statistical studies have shown that the room dimensions that provide the best results are those that fall in the ratio of approximately 1 : 1.3:1.6 (height to width to length). Naturally, these figures may be juggled with fairly good results. One would hesitate to completely discard a room for hi-fi purposes if its dimensions were 20 to 30 percent away from this optimum, for it should be remembered that this figure holds for a rectangular room only. Many of today's modern living rooms are combined living rooms and dining alcoves built in the shape of an ell, and the above figure of the overall ratio of the dimensions of a good room would not hold for the latter combination. In fact, an ell shape may be better than a perfectly rectangular room, for irregular shaped volumes tend to minimize room resonances. Room shape then is as important as room condition.
The first step in adjusting the room to the system is to select a room that is not square but roughly of the above proportions. If, perforce, one finds it necessary to use a room that deviates considerably from this ideal, compensations may be made through the treatment of the room that will considerably ameliorate the otherwise adverse acoustic conditions. However, more often than not, the listener's hi-fi room is close enough to the optimum. Most of his concern will be with the treatment of the room rather than with its shape.
Room Must Have Acceptable Reverberation Time
The prime concern in adjusting a room for satisfactory acoustic performance is to provide it with the right amount of liveness so that the musical reproduction will be real and vibrant, or "lifelike." The qualities that determine the liveness of a room are the physical volume of the room and the degree of reflection and absorption by the walls of the room in addition to the effects of furniture and people in the room. The combined effect of all these factors is summed up in what is called the reverberation time of the room. This was previously de fined as the time it takes for a given sound to drop 60 db from its original level after the sound producing element has stopped operating.
That the reverberation time of a room may be greatly changed simply by its decorative treatment is certainly well known to every body. When one walks into an empty apartment devoid of furniture, rugs, draperies, and the like, the least little whisper resounds through the place. However, as soon as the same apartment is made livable with the usual apertenances of home decorators, the "resounding" nature of the room, its reverberance, has been reduced. Speaking more technically, its reverberation time has been reduced.
Reverberation Time Dependent upon Room Size and Program Content
What then constitutes the proper reverberation time for the average listening room at home? Just as there are many factors that determine what the reverberation time may actually be, there are many factors that determine what it should be. The optimum reverberation time will vary with the frequency of the sound being radiated, with the type of program, and with the size of the room. Perhaps the most interesting of these factors is the matter of the program material. The articulation of a man's speech may be completely obliterated if he talks at a rapid-fire rate in an auditorium that is highly reverberant. His echoes come back in such profusion that his original delivery is completely lost. On the other hand, if slow organ music were to be played in the same auditorium the room would properly re-enforce the musical characteristic by allowing the pipes to "resound" throughout the edifice. Because of this question of the program material, we see at once that the proper reverberation time for a room must be selected by judicious compromise. In fact, researchers in the problem of room acoustics have actually classified the proper reverberation time of a room in terms of what composer's work is being performed.
Of course, even the acoustical engineer cannot be that precise in designing his buildings, even though they may be devoted entirely to the performance of a particular type of music. He may, however, use a sliding scale of optimum reverberation time for particular types of reproduction such as small classrooms intended for simple tutorial purpose, through medium sized auditoriums for solo work or small ensemble groups, to large concert halls and opera houses for the production of massive works. These sliding scales have been worked out with statistical methods after very many researches on existing structures of known acoustic quality of performance, in addition to laboratory analysis of the factors circumscribing the problem. From all these studies there has evolved a set of figures that are applicable to the home as well as to commercial buildings.
For the home, the reverberation time that will produce optimum listening conditions ranges from 0.75 second to 1.25 seconds, depending upon the room size and the type of music. These factors are tabulated in Fig. 17-1, from which is will be noted that for a given size room, organ music requires the longest reverberation time, while speech material requires the shortest reverberation time. How then do we go about achieving this desired room characteristic?
Liveness of Room may be Adjusted by Absorption Materials
Fig. 17-1. The optimum reverberation for a given room depends upon the
size of the room and the type of sound being produced. A large room
requires longer reverberation time. "Fast" music requires short
reverberation time.
As complex as the art may be that enables us to make the necessary adjustment to the room for optimum listening pleasure, the application of the principles is exceedingly straightforward and simple. In order to apply these principles intelligently, however, it is necessary to understand them in their simple technical aspects. We know that we can make a room live by making the walls reflective, as are the tile walls of the bathroom, or we can make a room dead by making the walls absorbent, as in a heavily draped foyer. The reverberation time depends then upon the amount of absorption of sound by the walls. In the acoustic laboratory there are many ways in which the reflection or absorption characteristics may be measured. As in all measurements, there must be a standard unit against which other quantities are measured. Dimensionally we use the inch; electrically we use the ohm. In acoustics, we use the term "sabin" as the unit in which absorption of materials is measured. This term stems from the name of the man who did much of the pioneer work in the field of room acoustics, Wallace Sabine.
One of the problems with which Sabine concerned himself was the matter of the absorption characteristics of materials. As in all measurements there had to be a standard "yardstick" of absorption.
Sabine decided to use literally "nothing" as his yardstick against which to measure absorption. His "nothing" was simply an open window.
Obviously, if the window is completely open it can reflect no sound at all; all the sound approaching it must go through it. The open window must then act as a perfect absorbing device, for it reflects no sound at all. Here then is the unit against which other materials may be compared. If a slab of material placed in the open window reflects half the sound that hits it, the other half must have entered the material to be completely absorbed by it and/or subsequently transmitted through it. Then we may say that this piece of material has an absorption co efficient of 0.5 as compared to the open window. The higher the value of the absorption coefficient, the more absorbent is the material (maxi mum value of 1.0 for the open window).
Absorption Varies with Type and Amount of Material
In this manner the absorption of many materials may be measured and tabulated for use where a knowledge of the sound absorbent properties of the materials is necessary for the computation of other acoustical data. Such a table is given in Fig. 17-2, listing the absorption coefficients of various types of material that may be found in the average living room, or for that matter in any room of the house. In addition to these we have listed the total absorption units of objects and individuals in the room; after all, not only do their ears absorb the sound, but their bodies absorb sound energy. Note that absorption units differ from absorption coefficients; the latter are simply numerical ratios with a maximum value of unity, showing what pro portion of the sound is absorbed by the exposed surface; the former units show the total absorption effect of an object on the sound energy.
One unit of sound absorption is produced by a material with a surface area of one square foot and having an absorption coefficient of 1.0. We will have more to say about this audience problem in later paragraphs. Our present concern is how to make use of the above tabulated data in adjusting the room acoustically.
Fig. 17-2. Absorption properties of material in the home, including individual
objects. Although the absorption varies with frequency, the value given
for 512 hz may be used satisfactorily because of variations prevalent
even in similar materials. ----- ------- COEFFICIENTS OF ABSORPTION
FOR USE IN APPROXIMATING THE TOTAL ABSORPTION OF A ROOM FOR PURPOSES
OF ADJUSTING IT TO THE PROPER REVERBERATION TIME (SEE TEXT)
A Simplified Approach to the Room Adjustment Problem
Stating the problem directly, we wish to determine how much and what kind of material we need to apply to a room of a particular size to achieve the optimum reverberation time as called for by the type of music we are going to reproduce in the room. Before going into the method of solving this problem, it is necessary to comment on the approach to the problem. It is possible to make a very accurate analysis of the treatment necessary for a room if all the pertinent statistics are available. It is not merely enough to know that a wall is plastered in order to know what its absorption coefficient is. There are several different types of plaster which may have different absorption characteristics. Even the way the plaster is supported will make a difference in its acoustical properties. Plaster on hollow tiles will have one characteristic absorption. When applied over metal lath and wooden studs, its absorption may be twice as high. Moreover, the actual factors of absorption of the various materials must be obtained from the manufacturers' published data concerning them, which often are not consistent. For reasons such as these, an exact paper analysis of a room and its treatment is at best a judicious approximation. It may be said, of course, that the professional man has available instruments for measuring the sound decay characteristics of rooms, enabling him to determine accurately the reverberation time of the room before and after treatment. With such instruments, the room may be very precisely adjusted.
However, where the layman must make use of published data concerning material and make educated guesses as to the type of walls he has in his room, or the quality of the wood on his floor, or the grade of the drapes that hang from the moldings, precision analysis is out of the question. However, this is not to infer that he cannot arrive at a close and workable adjustment to his room by making these educated guesses. Furthermore, because of the necessary approximations that must be made in the selection of the numbers that go into the solution of the problem, it is of advantage also to simplify the theoretical approach to the problem, so that the layman may proceed with a straightforward solution to the problem of adjusting the room for optimum listening conditions. The results obtained from the following simplified treatment of the subject will be within 10 to 15 percent of the more rigorous solution, but inasmuch as the numbers with which the layman will have to work are in many instances no more accurate than this, the simplified attack is fully justified. The problem to be worked out will demonstrate the workability of this rule-of-thumb method of adjusting the room.
Illustrative Problem on Room Adjustment
Stating the problem directly then, what we wish to determine is how much and what kind of material do we need to apply to a room of a particular size to achieve the optimum reverberation time, as called for by the type of music we are going to reproduce in the room.
Figure 17-3 contains a chart that enables us to determine the necessary total absorption units of a room of a given size for the particular type of music for which optimum results are desired. Let us choose the condition that will probably be the one most often selected - the average music condition. (It may be stated parenthetically, however, that by incorporating the proper variable adjustments in the room, we may convert it from an organ recital room to a speech room. This we shall discuss in due time.) Let us choose a room that is meant for average music, and which has dimensions of 10 feet high, 15 feet wide, and 20 feet long. What must the absorption of the room be? From the above dimensions, the volume of the room is 10 feet x 15 feet X 20 feet, which is 3000 cubic feet. For this volume, the chart shows ...
Fig. 17-3. Total absorption units required for a given room volume for
different types of program material.
... that the room will have to have a total absorption of approximately 150 units for average orchestral music.
We have, now, a quantitative measure of the room that will pro duce the desired results. Our objective is to adjust the room to this value. The next step is to use the data concerning the absorption characteristics of various types of material to realize this objective. The total absorption characteristic of a piece of material depends not only upon its absorption coefficient (how well it absorbs in reference to an open window) but on the expanse of the piece of material as well.
Thus, for a slab of gypsum plaster, which has an absorption coefficient of .04 and an area of 50 square feet, the total absorption of the section is 50 x .04, which is 2 absorption units. Now all we have to do is total up the absorption characteristics of all the surfaces of the room and change them, in part, so they equal the value we need (in this problem) of 150 units.
Completely Bare Room has too Little Absorption
Fig. 17-4. Sample calculations showing a given room in a completely untreated "live" state
has few absorption units. Same room when completely over-treated has a
high number of sound absorption units and is very dead.
For simplicity of illustration, let us assume that we start with a completely bare room, consisting of a wooden floor, four plaster walls and a plastered ceiling for the area in question of 3000 cubic feet.
Figure 17-4(A) illustrates this room, and at the same time tabulates the areas of the walls, ceiling, and floors along with the absorption coefficients for these sections. Notice that the individual area is multiplied by the absorption coefficient for the material of which that area is made to obtain the total absorption units for that area. After each area is calculated, they are all added together to give the final absorption of the whole enclosure. For the present instance, this figure turns out to be approximately 49 units, which is far less than the required 150 units. This means that there is not enough absorption, that there is too much reflection, that the room is too live. This is what we would expect from the postulated conditions of a bare room.
Completely Treated Room has too Much Absorption
Now let us go to the other extreme for this room and apply full treatment to the floor, walls, and ceilings. Our new conditions now will be those in which the floor is heavily carpeted from wall to wall, the walls heavily draped with velour trappings gathered in folds (draped to half their total area), and the ceiling treated with conventional sound absorbing acoustic tiles. The tabulation for this set of conditions is given in Fig. 17-4(B). Again all the various sections have been computed individually and then added up to give a final absorption of 502 units. This is far in excess of the required 150 units, and the room will be completely dead, as we probably expected from the statement of the problem. Somewhere between the totally bare room and the totally treated room is the final answer. It should be fairly apparent at this stage how the final treatment of the room is resolved.
We select the right amount of decorator's material not only to match the decor of the room but also to provide the right number of total absorption units so that these materials in conjunction with the un treated areas of the room approach the correct value.
Selecting the Absorbent Material
Fig. 17-5. Simplified method of adjusting the absorption of a room to produce the optimum reverberation for its size. (A) ROOM BEFORE ADJUSTMENT ROOM OF FIGURE CARPETED, OTHER 17.4 WITH WALLSUNTREATED FLOOR (B) DESIRED ABSORPTION UNITS = 150 FOR THIS ROOM (10 X15 X 20 = 3000 CUBIC FEET) FOR AVERAGE MUSIC AS OBTAINED FROM CHART IN FIG.I7 -3 (C) TOTAL SQUARE FEET OF ADDITIONAL TREATMENT ABSORPTION UNITS NEEDED ABSORPTION COEFFICIENT (FOR HEAVY VELOUR DRAPES FROM F G.17-2) 150 -106 44 - 126 SQUARE FEET .35 .35
To complete the problem, let us assume that the ceiling will re main in its original plastered condition, and that the floor will have 16 foot x 11 foot carpeting. How much drapery treatment will have to be added to the walls to arrive at the figure of 150 absorption units? These primary conditions of the problem are set forth in Fig. 17-5 (A), where it will be seen that the total absorption of the room in this state is 106 units. At some place in the room we have to add 44 units of absorption. Looking at the chart of Fig. 17-2, we see that a heavy weight velour drapery cloth has an absorption coefficient of 0.35. If we divide this factor into the total additional units needed, which in this case is 44, we will get the total square footage of this material that will provide the necessary additional absorption. Thus 44 divided by .35 equals 126 square feet, the total area of the drapery necessary to bring the room to 150 absorption units.
Since the room is 10 feet high, we need 12.6 running feet of this material. This amount of material may then be divided into several sections, say into four 3.15-foot sections, and these sections may then be hung on the several walls as befits the arrangement of the room with its doors and windows, as shown in Fig. 17-5 (D). Of course, in the hanging of the drapes, we have removed (covered up) some of the active surface of the bare plaster walls so there will be some reduction in the overall absorption of the room from the calculated value.
However, in relation to the absorption characteristics of the covering material, the amount lost from the bare walls themselves is rather insignificant, and may be neglected. One may, of course, if he desires to be very precise, calculate the amount of bare wall absorption that has been done away with because of the drapes (in this case 126 square feet) and add to the original figure for the drapes an additional amount to compensate for this loss.
Windows and Doors are Important
The reader will notice that we have left out some very important features of a room, namely the doors and windows. If the opening into the room is a simple archway, the room into which the archway leads becomes part of the acoustic problem, and its own characteristics must be added to the original room for the final evaluation of the necessary treatment of the combination. If, however, the opening into the room is a conventional door, the survey of the absorption units must be made on the basis of the absorption characteristic of the door material added to the other absorption factors, provided the door of the room is to remain closed when the hi-fi system is being used. On the other hand, if the door is open, the room beyond naturally be comes an acoustic element in the problem, as in the case of the archway.
Windows are treated in the same fashion. When the windows are closed, the room sees panels of glass, which have a particular absorption characteristic that must be added to the calculations. If it is summer and the windows are open, they constitute exceptionally good "absorbing" panels, for all the sound goes through them and none is reflected. It is remarkable how an open window will change the listening characteristic of a room. If, on a warm summer night, four living room windows are opened, and the open area is 9 square feet, the total open window area will be 36 square feet, with an absorption power of 36 units. If we originally designed the room for 150 units, it is thrown off by approximately 25 percent. For all practical purposes, the windows are devices that allow the room acoustics to be changed at will, although this was naturally not the original purpose of the window. There are more specific devices that may be used as manually adjustable elements within the room, but before we consider these, it is necessary to discuss the one or two more items that have been left out of our reckoning.
Furniture may be Highly
Absorbent In the main, the furniture of the room remains fairly well fixed in quantity. There are the usual easy chairs, sofas, side chairs, break fronts, and similar pieces in the average living room. These act as absorbing devices in exactly the same manner as do the walls of the room, and their characteristics may be approximated from the type of material from which they are made. Evidently, a large, well upholstered chair will be a good sound absorber, while a straight backed wooden chair will be a poor absorber. Absorption figures for typical chairs are given in Fig. 17-2.
Liveness is Affected by People in the Room
Now we get down to the matter of the people themselves. The absorption of the average human body will, of course, vary with its covering material as well as its size. An adult presents more of an absorption area to sound than does a child, and the clothes he wears similarly affect the amount of absorption. It will be realized from this simple statement how very approximate many of these calculations must be, for it is impossible to list all combinations of sizes and modes of dress in terms of sound absorbing constants. We must accept a figure that is representative of the average. Thus the adult represents a total of 4.2 absorption units when standing. When seated in a chair, he merges with the chair; if the chair is an upholstered one, the total absorption figure is only a little more than the chair itself represents.
Of importance, however, is the matter of the number of people in a room at any one time. If the room has been adjusted to the proper reverberation time for the immediate members of the family as the listening audience, an influx of ten party visitors will immediately add 40 to 50 absorption units to the room. This is a condition we often recognize when the guests pile in, for the sound becomes deadened and lifeless, and we invariably have to turn up the volume if we want to continue to have usable music coverage. The reverberation time has gone down because of the absorption added to the room.
Having learned how to permanently adjust the listening room, what can we do to provide a measure of adjustability to take care of contingencies that arise due to seasonal changes, for instance, with open doors and windows, or social affairs with their many guests, or even to different types of music. To have such adjustable devices is to add to the versatility of the high-fidelity system as a whole. A room liveness control is perhaps just as important as a treble boost control on an amplifier, or a presence control on a multi-speaker network. The liveness of the room is the final closing link in the acoustic chain be tween the loudspeaker and the ear.
Drawstring Drapes Provide Controllable Absorption
One of the easiest ways of providing variable acoustic adjustment in the home is to use drawstring drapes. There is a difference in absorption effect between a drape that is pulled out flat and one that is pulled together into folds. As shown in Fig. 17-2, a medium weight velour drape may have close to four times the absorption co efficient when it is bunched into half its area than when it is simply hung without folds over its entire area. Thus, although we cut down by half the total absorbing area when we bunch up the drape, we increase its absorption by a factor of 4; or we have a total gain of 2 in the absorption of the drape in folds. If the flat drape has an area of 160 square feet with an absorption coefficient of 0.13, its total absorption will be 160 x 0.13, which equals 21 units. Now, if we bunch up the drape to half its area and apply its new absorption figure, we get 80 x 0.49 = 39 absorption units. If we add to this figure the original absorption characteristics of the wall that has now been ex posed, 80 X .04 = 3 units, we have effected a total increase of absorption from 21 to 42 units simply by bunching the drapes. We have attained considerable control over the room by a simple means.
Venetian Blinds may be Used as Absorption Control
There is another simple expedient that the man at home may use to adjust his room if he has any reasonable area of venetian blinds over his windows. Some of today's homes utilize large picture windows in their living rooms, and frequently they are equipped with full width venetian blinds covering what amounts to a large expanse of wall. The blind may be used as an adjustable acoustic control by treating one side of the slats with an absorbent material and leaving the other side untreated. Suppose we had a window 8 feet long and 5 feet high. This amounts to an expanse of glass reflecting surface of 40 square feet. If the blind were pulled all the way up, or if it were left in the maximum open condition (slats horizontal), the room would feel the full absorption (or lack of it) of the glass. The figure for this would be 40 x .03 = 1.2 units, which provides very little absorption. Now suppose we left one side of the (wooden) blind untreated, and treated the other side with some common highly absorbent acoustic tile material cemented to the slats. We thus obtain a wide adjustment of acoustic absorption by varying the attitude of the setting of the slats of the blind. When the slats are all closed, with the smooth wood surfaces facing the inside of the room, there will be a probable absorption of approximately 40 x .2 = 8 units (taking .2 as the absorption coefficient of wood paneling). With the slats oriented so that the tiled sides are facing the room, the absorption would be 40 X .85 = 34 units (taking an average absorption coefficient of .85 for the tiles). Thus we can go from an open blind condition of 1.2 units gradually through a rising scale to 34 units.
It will be appreciated that this is simply an adaptation of the commercial means of controlling studio reverberation constants through the devices of rotating panels or sliding wall sections. When properly applied in the home, the method can be just as effective as in the studio. It is conceivable that many more such variable devices may occur to the reader that may fit directly into his home decorating motif. It will be realized, of course, that in a room that is made to be adjustable to different conditions, the room must be left live enough at one end of the adjustment so that it will never be overdamped under the most adverse conditions. Starting with this amount of liveness, the adjustable devices may then be designed to provide as much additional sound damping material as necessary to bring the reverberation to the other limit.
Enclosure Corner Should be Very Live
In treating the room for acoustic adjustment, one should be cautious about the manner in which the corners of the room are manipulated. When the corner of the room is to be used as a sound source for the placement of the enclosure, it should be completely devoid of any kind of acoustic treatment. The corner should be considered in the same light as the horn that is to go into the corner, namely that it be strong, rigid, and non-absorbing of acoustic energy. Thus, if the corner of the room is draped, much of the effectiveness of the acoustic radiation of the corner will be lost, especially if the horn is one that actually depends upon the walls of the room to complete the horn. These walls must be totally reflective, and so must be the floor for the horn action to be properly established. In the case of the self contained horn, in which the side walls are integral with the enclosure structure, the matter of the corner treatment is not quite as critical, although even in this case better results will be obtained if the corner retains its non absorbing qualities.
Absorption Material Should be Divided into Several Areas
In making the necessary acoustic adjustment to the room, it is advisable to break up the sound absorbent material into several smaller areas rather than one large area of treatment, where other commitments, such as interior decorating, will permit. Such a procedure will scatter the reflective and absorptive properties throughout the room, making the room more uniform from wall to wall. This will produce smoother acoustical performance of the room by the tendency to minimize excessive reflections from what might otherwise be a large bare surface. Reflections of this sort might still exist from such a wall, even though the room as a whole had been compensated by the treatment of some other wall. In general, broken up surfaces, differently treated, and with non-parallel walls (if possible) provide the optimum conditions for smooth room performance.