Figure 218 Configurations that Generate Flutter Echo Reverberation

Parallel Surfaces

Parallel Surfaces

Angled

Concave

Surfaces

\ / Double y Angled X Concave

Curved

Concave/Flat

Surfaces

Angled

Concave

Surfaces

Angled Semi

Concace Surfaces

\ / Double y Angled X Concave

Angled Semi

Concace Surfaces

Opposing Corners

Opposing Corners

Everest (1994) divides the reaction to a reflection into four regions according to the time delay. Region 1, where directional cues are determined, is less than 1 ms; Region 2 is up to about 50 ms, where the integrating effects of the ear occur; Region 3 is an intermediate zone, which depends mostly on reverberation time, and Region 4 is that of a long-delayed reflection where the curves in Fig. 21.6 flatten out and approach the horizontal.

Clark (1983) performed an interesting series of studies to measure the effects of the audibility of time delays, phase shifts, and frequency shifts in listening rooms having a stereo pair of loudspeakers at 10' (3 m) distance and 60° included angle. This standard stereo configuration, even when a monaural signal is fed to both loudspeakers, is preferred by most listeners who describe the sound as "fuller, more solid, or having depth" in spite of the fact that the arrangement produces significant comb filtering at the listener's ear. Clark then used a single loudspeaker with a 24" x 30" reflecting surface to recreate the comb filter effect of two loudspeakers. In this case, when the reflecting surface was in the "wall" orientation little audible effect resulted, much less than the stereo loudspeaker arrangement with a monaural signal. With the reflector in the "table-top" orientation the effect was more noticeable but still small. A third experiment was done wherein an electronic filter was used to generate a comb filter, fed to one loudspeaker. Here the result was very noticeable and degrading, giving a nasal quality to voice reproduction.

In an extension of this work, Clark (1983) used electronic manipulation to introduce phase shift, time delay, and frequency shift into the signal path. Phase shifts of up to 2700° were inaudible as were time delays up to 10 msec. He speculated that time delays of up to 30 msec would also be inaudible but did not test in this range. Where all three effects are present,

Revebration Time Room Volume

he judged that frequency shifts were the most audible. Stereo reproduction was preferred to single loudspeaker, even when a wall reflector was present. This likely is due to the fact that side-wall reflections help fill in the stereo comb filtering.

It is good practice, in control room design, to minimize the energy scattered to the mix position from surfaces immediately adjacent to the main loudspeakers. Likewise, it a good idea to provide a series of reflections that transition smoothly into a reverberant decay. This can be done using diffusion, surface orientation, and absorptive treatment to avoid high-energy reflections. In most rooms the side walls and ceiling are flared outward to direct the first reflection away from the mixer position. Rear walls are treated with absorption, diffusion, or a combination of the two.

Flutter Echo

In a music practice room or recording studio, where the source and receiver are close together, repeated multipath reflections known as flutter echo can be disturbing. The most commonly encountered example is between two parallel walls, but there are numerous geometric combinations capable of producing this phenomenon. Several examples are shown in Fig. 21.8 but these are not all-inclusive. Flutter can be treated with absorbing material, diffusing elements, staggered reflectors, or by canting the reflecting surfaces. The latter approach is not always effective since a third surface may complete the flutter loop. In the design of small rooms closed reflection loops of this type can be expected and treated with surface applied materials. A surface canted 1:12 suffices to eliminate the problem as long as the loop is not closed by other surfaces.

Reverberation

The reverberation time in small rooms is still an important design parameter. Figure 21.9 shows recommended values of the reverberation time for control rooms. The use of these data should be based on the program material to be played in the space. Rooms designed for the playback or broadcast of speech should be very dead; those for music may be somewhat more reverberant.

For average control room volumes preferred reverberation times fall into the 0.3 to 0.4 second range. In recording studios the reverberation times are variable, and range

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