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| November 15, 2003 Sound and Music: Room Interaction Loudspeakers don’t operate in isolation. They’re complex acoustic devices that interact with the equally complex acoustics of the environment in which they’re placed. Whereas October’s feature looked at how differences in the mid- and high-frequency dispersion of various loudspeakers influence their sonic character, this chapter will look at how low frequencies interact with the room. The basic rule for sound dispersion is that those sound frequencies with wavelengths longer than their source diaphragm spread out in all directions, whereas frequencies with shorter wavelengths than the diaphragm are beamed forward. Because low frequencies always have relatively long wavelengths, their sound will almost always spread out evenly in every direction. (The only exceptions are with dipole panel speakers and full-range horns, both of which are relatively rare special cases.) The basic rule for rooms is that the boundaries -- walls, ceiling, and floor -- will either absorb or reflect the soundwaves that strike them. The ratio of reflection to absorption will vary according to a number of factors, from the type of surface covering to the way the room was built. The reflection ratio will be much higher at low than at higher frequencies. The source of most of a speaker’s low-frequency output will be its bass driver, of course, but in the overwhelming majority of cases the speaker’s port -- the mechanism by which it vents into the room -- will also make a contribution. These days, many floorstanding models use multiple, smaller bass drivers mounted close together, to keep the front of the speaker nice and slim. In the best such designs, the outputs of the different drivers combine to act as a "point source." Bass soundwaves spread out from each speaker like ripples on a pond. Some waves travel directly to the listeners, but more are reflected off the room boundaries en route. If you’re interested in checking this, try taking a speaker outdoors. The farther away you get from reflective surfaces, the more bass-light your speaker will sound. Indeed, a loudspeaker’s "natural" sound is the sound it makes under "anechoic" conditions -- either in one of the special chambers built by larger speaker manufacturers for test purposes, or on top of a tall pole in the garden! When you bring the speaker back indoors, all those reflections from the nearby room boundaries are just one of three mechanisms that will boost the bass end of the sound spectrum significantly more than the mid and treble. (The other mechanisms are that two speakers will "couple" at the bass end, boosting frequencies whose wavelengths are longer than the physical separation between the speakers; and the room itself will have its own standing-wave modes or resonances at specific frequencies, which the speakers will excite.) Crucially, the reflected sounds will have traveled farther than those arriving directly at the listener from the speaker, and will therefore arrive slightly later, with an attendant shift of phase. A shift in phase means that the "shapes" of the direct and reflected sound waves get out of step with each other, causing unpredictable reinforcement and cancellation. This raises the major dilemma of loudspeaker placement. Provided the speaker is appropriately aligned, the cleanest (though not necessarily the most even) bass will be generated when the speakers are placed against a wall. The trouble is, while this might help the bass, it usually introduces unevenness and colorations in the midband. Some designers make their tradeoffs in one direction, some in the other. It all comes down to the way the direct sound interacts with the reflections from the wall behind the speakers -- by far the most important room boundary, because its reflections actually physically coincide with the direct sound. Sounds at a certain frequency -- whose wavelength is four times the distance between the drive unit and the wall -- will tend to be reflected exactly out of phase with the direct sound, and thus cancel each other out. This effect will be most pronounced when the speaker is quite close to a wall, both because the reflections will be strongest there, and because the cancellation frequency will be up in the midband, where it will be most audible. (The wavelength of middle C [256Hz] is around 56", for example, so this will suffer some cancellation when the bass/mid driver is 14" from the wall.) This is why most speakers are designed to be positioned at least 2’ out from a wall, maximizing the midrange smoothness at some cost in bass tautness and timing.
Some speakers are deliberately designed, in terms of relative bass output, to be sited close to a wall. Most speakers, however, are made to be placed in free space. The essential difference is that wall reinforcement will boost a speaker’s output substantially (by around 6dB) in the midbass region (50-100Hz) because the wall-reflected bass -- well below the frequency where phase cancellation will occur -- integrates with and augments the direct bass sound. Small speakers, such as two-way, stand-mounted minimonitors with 5.25" main drivers and enclosures with volumes of less than a third of a cubic foot, will usually benefit from being placed close to a wall. Larger speakers will tend to work better when clear of walls; although the occasional manufacturer will opt for wall reinforcement, the majority will attempt a compromise between the two. Some brands supply foam bungs to block the ports, which can be useful if a free-space design has to be put close to a wall. (A pair of socks is just as effective.) While it’s very significant in terms of sound, the increase in low frequencies that results from placing a speaker close to a wall is only one bass-boosting mechanism. Another key factor, and a thoroughly unpredictable one, is that of room modes, caused by the creation of and interaction between standing waves within a room. These modes are sounds whose wavelengths, both fundamentals and harmonics, correspond to the main room dimensions and interact with each other, the bass drive units, and the ports in very complex ways. Room modes are abrupt, substantial peaks and troughs through the bass region. It has always been difficult to do anything about these, though it’s worth experimenting with small adjustments of speaker position to try to achieve the smoothest results. That said, several speaker brands, including TacT, Bang & Olufsen, Meridian, Tannoy, and TAG McLaren, have begun introducing hi-tech solutions to smooth out room modes. These complex, flexible equalizers operate only in the digital domain and will not necessarily appeal to all types of audiophile, though they do seem to do the job effectively. It’s too soon to tell how much future impact DSP will have on a specialist hi-fi sector that still highly values analog techniques. What’s certain is that the room and the location of the speakers therein will continue to play major roles in the overall sounds of systems. ...Paul Messenger
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