About Loudspeakers In Rooms

In General

To begin with, it is impossible to seriously think about or work with loudspeakers without considering the room in which you are listening. At the same time, there is a common and unfortunate tendency to view the room as “the enemy,” as a confusing agent that masks the “true” output of the loudspeakers. This view leads to the idea that the optimum listening environment is the anechoic chamber, where all you hear is the loudspeaker.

Well, I’ve listened to music over loudspeakers, in stereo, at length, in anechoic chambers. I’ve got to tell you that that way of listening, while certainly fascinating and not really “bad-sounding,” really isn’t suitable for mixing or production work. Trust me.

In fact, and this isn’t generally known in the recording industry, the early reflections provided by the room are “good” data for our hearing, and they contribute mightily toward the perceived timbre and spatial details of the music coming from the speakers. The most useful and beneficial early reflections are those that come from the side walls of the playback room. Therefore, these walls should be completely reflective – they should be hard, flat surfaces: acoustic mirrors, if you know what I mean. The reason for this is that our hearing mechanism “accumulates” all of the early reflections of any given sound in the room and integrates them with the direct sound, to come up with both a perceived timbral character of that sound and also a location in space for that sound. If we filter an early reflection by hanging up Sonex® or some similar high-frequency absorber, we are in fact changing the perceived timbre of the loudspeaker, rolling off the high end! Holy Moly!

So, those early reflections make a positive contribution to the quality of what we hear coming out of the loudspeakers. What’s not so good, in playback rooms, is reverberance, that huge array of reflections that show up at our ears after the early reflections have been integrated with the direct sound and which hang around during the time the sound dies away. This is the stuff that masks and confuses the details in the recording. The dividing line between early reflections and reverb is around 50 milliseconds after the direct sound. The ideal playback room, to my way of thinking, has perfect broadband reflections for the first 50 milliseconds (so that we can get the richest possible collection of timbral and spatial cues coming out of the speakers for our ears to make use of) and then an quick shut-off after that (so that we can hear the following sounds from the loudspeakers). Pretty interesting, eh?

	Energy Time Curve (ETC) of Direct Sound, Early Reflections and Reverberance in a small room.

Take a look at Figure 1. It shows typical early reflections and reverb for a small room. That bunch of 10 or so reflections that reach the listener in the first 50 ms. are gathered with the direct sound, neurologically, in our hearing system, to form a neural template of timbre and location-in-space. The early reflections are not separately perceived at all (due to the so-called Precedence or Haas effect). Reflections after 50 ms. are heard as reverberance, slap, and echoes, depending on how widely they are spaced.

In Stereo

Meanwhile, assume stereo for all of this. Three-channel stereo and surround are more complicated versions that I’ll mention below. The (stereo) speakers and the listener, by established tradition, should be located at the points of an approximately equilateral triangle, which is to say that the speakers should be as far from each other as they are from the listener. See Figure 2.

	Idealized arrangement of a listener and stereo pair of loudspeakers.

The most important thing here is that the listener must be on the median plane! For critical listening and production work, being six inches off the median plane is enough to completely screw up the localization of phantom images.

Multiple listeners can share the median plane, in front and back of each other. Logically, if oddly, the presence of a listener right in front of you on the median plane doesn’t disrupt your perceived center phantom image at all – you “hear” right through the listener in front.

There are two fundamental options for stereo speaker placement: in the front wall (soffited) or out in the room. Out-in-the-room speakers can either be free-standing monitors (if you have the room) or “In-Yer-Face”® meter-bridge monitors. Mounting speakers in the front wall solves a bunch of low and mid-frequency diffraction and reflection problems, particularly if you build a “monitor shell” in which to put them (see my generic room, Figure 3). Free-standing speakers, particularly really good ones, give you much more depth in imaging. “In-Yer-Face”® monitors are the simplest, cheapest and most popular. They present serious problems in terms of bass response, plus some problems with imaging depth, while yielding fairly good definition and levels without running into power problems, because they are so close to you. Also because they are so close, it is difficult for multiple listeners to reliably hear them with the similar perspective.

In Surround

Surround sound is coming. If you want to tackle it, you are probably best off starting with an array of free-standing speakers around you, because if you soffit the front speakers, you are also probably going to need to soffit the rears, and now you are into severe construction problems. Meanwhile, surround meter-bridge monitors are a truly bizarre concept. It’s my belief you need identical speakers all the way around. Happily, you can use small speakers and a subwoofer – that’s the prevailing format – which really eases the bass problem. I think you’ll be surprised at how cool it can sound, once you get the hang of it.

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