The delicate question of SPEAKER PLACEMENT.
by speaker expert ROY ALLISON.
THE process of choosing, buying, and installing high-fidelity components
is often accompanied by some anxiety and frustration. Perhaps the most exasperating
experience of all, however, is to bring home some speakers that you know are
good ones, only to find that they sound disappointing or even terrible in your
listening room.
If this should happen to you, the fault may lie with a component you haven't even thought about: the room itself. Any room has a major effect on what you hear from the speakers used in it. At low audio frequencies it actually modifies the acoustic-power output of a speaker as measured in the room, and it determines the spatial distribution of that power as well. Moreover, by means of its ability to absorb certain audio frequencies more readily than others, the room alters the frequency balance you hear when listening to the speakers. The acoustical characteristics of your listening room, and the placement of speakers in the room, are therefore of crucial importance in determining the quality of the sound field at your ears.
Room Dimensions
Sound energy reverberates in a room-that is, it persists for a time after leaving the speakers because of being reflected from one room boundary (wall, ceiling, or floor) to another. Ultimately, partial absorption at each encounter with a boundary or piece of furniture gradually diminishes the energy until it is no longer audible. At certain frequencies, however, it arrives back at its source (the speaker) at just the right moment to be reinforced strongly. This happens when the total to-and-fro length of the path followed by the reflected sound equals one wavelength of the sound or a multiple thereof. The reflected sound returns just in time to be in phase with the continuing output of the speaker.
Then the sound energy, comprised of what was left of the original impulse plus the augmentation by the speaker, makes another circuit of the room and is further reinforced as it reaches the speaker again. For as long as the speaker continues to emit sound of the same frequency (or frequencies), standing waves are set up in the room. The standing waves are room resonances, quite similar to the air-column resonances in organ pipes of various lengths.
The path length of the reflected sound is of course determined by the dimensions of the room, and a set of room resonances therefore exists for each dimension--length, width, and height. (A set of resonances also occurs for each combination of two and three dimensions. It is a simple matter to calculate the frequencies of the resonance modes involving single room dimensions. To find the lowest-frequency resonance mode of a dimension, simply divide 565 (half the speed of sound in feet per second) by the dimension in feet. If a room is 20 feet long, for example, its lowest resonance frequency is 28 1/4 Hz. Resonances for each dimension also occur at every multiple of the lowest frequency: twice the frequency, three times, four times, etc. Thus the set of 20-foot-length resonances is 28 1/4, 56 1/2, 84 3/4, 113 Hz, and so on. If the width dimension is 15 feet, the set of resonances for it is 372/3, 75 1/3, 113, 150 2/3 Hz, and so on. And if the height dimension is 10 feet, resonances occur at 56 1/2, 113, 169 1/2, 226 Hz, and on up. In rooms of typical size, resonances above these low frequencies are clumped together so closely that they tend to give uniform support to all frequencies. Therefore, the emphasis (or the suppression) of any single frequency above about 500 Hz is rarely prominent enough to cause problems.
However, at the lower frequencies we are considering, note that all three of the simple room dimensions have resonances at 113 Hz, and that two of the dimensions are resonant at 56 1/2 Hz.
Strong, probably annoying reinforcement could be expected for music tones near these frequencies in the room. But even so, the situation could be much worse. If two of the room dimensions were equal and the third were half of the others, there would be many groups of three identical resonance frequencies, and wide frequency expanses between them with little or no reinforcement from the room (see Figure 1). In other words, the low-frequency and transient responses of the room would both be very bad.
Simple logic informs us that the ideal room proportions are those for which the sets of resonance frequencies are separated by equal intervals. A good approximation is obtained when the dimensions are in the ratios of 1 to 1.26 to 1.59. This would be true for a room 8 feet high, 10 feet wide, and 12 3/4 feet long, or by one 9 feet high, 11 1/4 feet wide, and 14 1/4 feet long. These are not very large rooms, but ceilings higher than 8 or 9 feet are rare in home environments. One acceptable compromise would be to adopt a length-to-width ratio of 1.4 and use whatever ceiling height is available.
Suppose you are stuck with a room in which two dimensions are equal, or in which one is an exact multiple of another. You can break up a strong resonance mode in one direction by building a par tial room divider several feet out from one wall; cabinets or bookshelves on pole supports can serve this function very well. Alternatively, a wall can be covered by wood paneling, not more than 1/4 inch thick, applied over wall studs or furring strips. This is one of the few methods of resonance damping that is effective at upper bass frequencies (where coincident resonance frequencies are most frequent) but which does not have high absorption at high frequencies also.
------- Sound-pressure distribution (overhead view) when the combined second-order resonance of a room's length and width is stimulated nine columns of maximum pressure result.
------ Side view of the sound-pressure distribution in a room when the second resonance of the room's length is stimulated. Darkest areas represent zones of maximum pressure.
------ Side view of the sound-pressure distribution when the combination resonance of room height is activated. Pressure is least in centered vertical and horizontal planes.
Room Construction and Furnishings
If room proportions are reasonable to start with (or can be made so with minor modifications) the distribution of resonance frequencies will be fairly uniform.
But at low frequencies there are wide frequency intervals between resonances in rooms of average size, even when the distribution is very good. If the room boundary surfaces and furnishings absorb little sound energy, for example, reverberation will continue too long the room will provide too much reinforcement of sound at the resonance frequencies but very little reinforcement at the in-between frequencies. Music played in such a room sounds hollow, boomy, and--because of too little absorption at higher frequencies--screechy bright.
Fortunately, normal room-surface coverings and furniture do absorb some sound energy. In doing so they damp the room resonances. Damping reduces the amplitude of the peaks at the resonance frequencies and fills in the valleys be tween, producing a more uniform reinforcement throughout the room for all frequencies. Too much absorption, on the other hand, is nearly as bad as too little, for then the room has a lifeless quality that is decidedly unpleasant.
The right kind of acoustic treatment for a listening room involves a mixture of hard, rigid surfaces (such as real plaster walls and ceilings, solid doors, and glass window panes) that have low absorption. and soft padded surfaces (heavy drapes. upholstered furniture, and rugs) that have high absorption. Also, for all but the very lowest audio frequencies, open doorways are virtually perfect absorbers.
Ideally, there should be alternating surfaces of both kinds throughout the room. Usually it isn't practical to arrange this; the ceiling, for example, is most often uniformly hard. But in most cases it is at least feasible to avoid the condition in which opposite room surfaces are both entirely hard or completely soft. If the ceiling is plaster, much of the floor area should be covered by rugs with thick padding underneath, or even by wall-to-wall carpeting with a lighter padding. If one wall is nearly all papered or painted plaster, or wood paneling, the opposite wall should have large areas of draped windows, bookcases, or wall hangings. Furniture is obtainable in a wide range of mostly hard or mostly soft surfaces; it can be used to partially compensate for room-surface textures.
-------- Fig. 1. Distribution of resonant modes in two rooms. (A)
shows the coincidence of such modes in a room with cubical dimensions.
Resonant modes in another room (B) are well distributed and would minimize
formation of areas that would be "hot" or "dead" at
certain frequencies.
------- Fig. 2. Corner placement of a conventional bookshelf speaker
system as in (AI produces the greatest bass reinforcement, but it may
cause dips in the upper-bass response. Placements (B) and (C) may improve
frequency balance greatly owing to better room-boundary relationships.
One of the most difficult problems in room acoustics-particularly in modern home construction--is the lack of rigidity in walls, ceilings, and floors. A partition wall built of 2 x 4-inch studs on 16-inch centers, with thin wallboard and a skim coat of plaster on both sides, is really quite flexible. So is a large expanse of window glass. They yield readily to low-frequency sound-pressure waves, and they absorb or pass through the bass energy that should be reflected if it is to be reinforced equally with the rest of the frequency range. The result is a loss of bass in the sound field of the room that, in many cases, is quite significant.
Only a major job of carpentry or masonry can correct this condition in a room already built, for it requires the installation of a layer of additional thick-ness to the partition wall-a brick veneer or additional sheets of wallboard and plaster. A floor at the first-story level can be stiffened substantially by means of an adjustable lally column installed in the basement between the concrete basement floor and a joist or beam at the approximate center of the room's floor.
If that isn't practical, another layer of oak flooring thoroughly nailed across the existing floor will be of some value.
These are drastic reconstruction measures, to be sure. I recommend them only to those who are totally obsessed by a search for the perfect sound system.
The rationale would be that the room is part of the system-as indeed it is. The lucky few whose listening rooms already have rigid boundaries can congratulate themselves that they are getting all the bass their speakers are putting out.
Speaker Placement There is little need to elaborate on the general rules of placement for stereo speakers: they should be approximately equidistant from the center of the listening area and spaced apart by between one-third to two-thirds of that distance.
In four-channel or ambience-recovery setups, the two rear speakers should preferably be placed as far behind the listening area as is convenient; if that can not be done, nearly as good results can be obtained with the "rear" speakers placed to the left and right of the listening area. In either case, they should be as far away from the listening area as the front speakers are, or as nearly so as the room dimensions allow.
Within this general framework much variation is possible, and some experimentation--the only way to do it-is al most always advisable. When a speaker is placed at a location in a room where a standing-wave resonance has its maxi mum pressure, it will excite that resonance most efficiently. Conversely, if the speaker is placed at a point where that resonance has minimum pressure, it will excite the standing wave least effectively. Some degree of room-resonance control can therefore be obtained merely by moving a speaker a relatively small distance from its initial location.
-----This article is adapted from "Your Listening Room: The Final Component" which appeared in Boston's Phoenix.----
Every room-resonance mode has its maximum pressure at the room's corners. Half of the modes have maximum pressure at two-boundary intersections (floor and wall, ceiling and wall, or two adjacent walls), one quarter of the modes have maximum pressure at the center of a wall, and only one eighth of the modes have maximum pressure at the center of the room. In general, the bass output from any speaker is increased as the number of low-frequency resonance modes fully excited is increased. Moving a speaker closer to a wall intersection and then down toward the floor, there fore, gradually increases the average amount of bass power delivered to the room, but at the risk of strongly exciting some potentially undesirable combination of resonances. The only practical way to strike the best compromise in location is by trial and error.
There is another important aspect of speaker placement that has only recently received attention. When a conventional speaker is normally situated in a room, reflections from the three nearest room boundaries reduce the speaker's power output significantly in a rather narrow band of frequencies-usually in the 150-to 400-Hz range. The effect is difficult to avoid completely, but it can be reduced in severity by following the few simple rules illustrated in Figure 2.
1. Do not put the speaker at equal distances from the floor (or ceiling) and two adjacent walls. The most severe power-output dip occurs when a speaker is located on a line of symmetry from a room corner.
2. Turning the speaker cabinet so that one side, rather than the back, is against a wall is helpful if it can be done without compromising adequate radiation of the middle and high frequencies into the listening area.
3. The best placement (from the point of view of uniform bass-power output) has the side of the speaker cabinet against a wall, the woofer end of the cabinet resting on the floor, and the cabinet at least 2 1/2 or 3 feet from the nearest other wall. This is especially effective if the speaker system has a woofer-to-mid range crossover frequency of 500 Hz or lower. Unfortunately, it may result in so much bass output that the system becomes bass-heavy.
To repeat, there is no substitute for experiment in speaker placement. To ease your task, take advantage of the law of acoustical reciprocity. Put the left speaker in the chair in which you will normally be sitting while listening. Put on a record, set your amplifier in the mono mode, and disconnect the right speaker. Then move yourself around, exploring with your ears the general areas of all possible left-speaker locations. Note the location of your head where the sound is best, and put the left speaker there (or as close to it as possible). Repeat the process, putting the right speaker in your listening chair, etc.
If you try this method, you'll find that moving either yourself or the speaker affects the sound in similar ways-a fact worth remembering as you adjust for optimum final placement.
Next, return to your listening position and, using the same mono program source, assure yourself that the two speakers sound reasonably alike when you switch rapidly between them. If they do not, it's possible that your chosen speaker locations have too many differences between them to be compatible for stereo. For example, one may be surrounded by hard surfaces-bare walls and floor-while the other is amidst carpeting, upholstered furniture, and the like. If tone controls, or the controls on the speakers themselves, cannot compensate for these differences enough to produce a satisfactory, stable stereo image, you will have to repeat the whole speaker placement procedure, using an other wall or perhaps even another arrangement of room furnishings.
FINALLY, don't feel obliged to follow either hi-fi or decorating convention. There is no law, for example, that says both the left and the right speaker must rest against the same wall. If an odd, asymmetrical speaker placement turns out to be sonically the best and it is not aesthetically objectionable, then that is the place for your speaker.
Roy Allison, formerly Vice President of engineering at Acoustic Research. is now the President of Allison Acoustics. Inc.
Also see:
SPEAKER MYTHS---Get rid of that suspect informational baggage before you go out shopping, LARRY KLEIN
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