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Acoustics 101 Course by John Dahl of THX - page 2

By Patrick Hart

Room Reflections

Bass frequencies are generally those tones from 20Hz up to ~200Hz. Beyond 200Hz are the mid frequencies with the high frequency range beginning at 2000Hz. The distinction between the bass frequencies and the mid and high frequencies is profound because of the difference in the way each of these sets of frequencies act within the room. Those above ~200Hz are usually not long enough (5.65 feet = 200 Hz) to interact aggressively with any of the room's dimensions, a typical 8' high ceiling for instance. But they can cause havoc with blurring of images or spectral imbalances caused by comb filtering.

The comb filtering phenomena has seldom been explained with such clarity as did John Dahl.

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From the slide above we see the proverbial problem of the direct sound versus the indirect sound. (And remember, without room treatment and at most typical listening positions we're getting roughly the same amount, 50% direct and 50% indirect sound.)

The reflected sound is t2. Because it travels a longer distance to reach our ears its arrival is delayed by a few milliseconds. As you can see in the two charts (to the right in the slide), that particular frequency can become either additive in nature or it can exactly cancel the direct frequency. These are the two extremes. In reality the distribution of any particular frequency in time is almost always somewhere in between these purely additive or completely canceling extremes. What we get instead in those t2 frequency waves are the following series of additive and canceling effects which, as frequency increases, start to resemble the teeth of a men's comb.

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These t2 waves look quite different than the t1 sine waves which reached your ears a few milliseconds earlier. So it becomes easier to understand why, left untreated, these mid and high frequency reflections become so destructive to the primary sound.

The strongest and potentially most destructive room reflections are axial reflections. Like a pool ball caroming off a pool table's sidewall cushion, axial waves bounce or are reflected off of only one surface before they reach the listener's ear. Typically these reflections are from either of the two horizontal walls to our right and left in our theater or from the floor or ceiling.

The ceiling and floor reflections can be distracting in that we localize from up-to-down fairly well. So we may be more aware of their presence. The sidewall reflections meanwhile tend to be a bit more insidious because, as John previously pointed out, we don't localize quite as well. All four of these reflections, however, constitute one of the main challenges to achieving accurately placed images across the front soundstage.

There are a couple of ways to cope with these first axial reflections. We can either try to absorb them or diffuse them. In most cases the best approach is to try to absorb the mid and high frequencies in equal proportion to each other. What we are trying to achieve is a very broadband reduction of all frequencies above the ~150Hz level so that the spectral balance of the (hopefully) flat response speaker system you've chosen remains flat all the way to your ears at the listening position. Absorption is usually recommended over diffusion in most instances because more living room/theaters and even dedicated home theaters have too high a proportion of revererant sound in the first place. The goal is a revereration time of 0.3 seconds across the full frequency bandwidth and this is usually difficult to achieve without adding more absorption.

Slap Echoes

A variation of the axial reflection is the slap echo.

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The difference here is that slap echoes occur between two parallel surfaces. This form of acoustic interference is very common in living room layouts wherein the sofa at the listening position is very close to the room's rear wall. Slap echoes need to be absorbed to avoid a bright zingy sound which interferes with the acoustic character of the soundtrack.

Common solutions covered by John Dahl were both absorptive and diffusive in nature. Plus, we learned later on that where you place the absorptive material and where (and when) you place the diffusive material is also tied in with proper placement and type of surround speakers. Thus, it became more obvious as we went along how all the speaker systems' ultimate performance where intimately tied in with the room and proper placement of absorptive and diffusive "systems" within the room.

 

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