7.1.4 Reflection-controlled rooms
For the home listener, or sound engineer in the control room of a studio, the ideal would be an acoustic that allows them to "listen through" the system to the original acoustical environment that the sound was recorded in. Unfortunately the room in which the recorded sound is being listened to is usually much smaller than the original space and this has the effect shown in Figure 7.4.
FIGURE 7.4 The effect of a shorter initial time delay gap in the listening room.
Here the first reflection the listener hears is due to the wall in the listening room and not the acoustic space of the sound that has been recorded. Because of the precedence effect this reflection dominates, and the replayed sound is perceived as coming from a space the size of the listening room, which is clearly undesirable.
What is required is a means of making the sound from the loudspeakers appear as if it is coming from a larger space by suppressing the early reflections from the nearby walls, as shown in Figure 7.5.
FIGURE 7.5 Maximizing the initial time delay by suppressing early reflections.
Examples of this approach are: "live end dead end" (LEDE) (Davies and Davies, 1980), "Reflection free zone" (RFZ) (D'Antonio and Konnert, 1984), and controlled reflection rooms (Walker, 1993, 1998). One way of achieving this is to use absorption, as shown in Figure 7.6.
FIGURE 7.6 Achieving a reflection-free zone using absorption.
The effect can also be achieved by using angled or shaped walls, as shown in Figures 7.7 and 7.8.
FIGURE 7.7 Controlled reflection room (in the style of Bob Walker) for free-standing loudspeakers (from Newell 2008).
FIGURE 7.8 An example controlled reflection room, Sony Music M1, New York, NY. (Photo by Paul Ellis of The M Network Ltd; Acoustician: Harris, Grant Associates).
This is known as the "controlled reflection technique" because it relies on the suppression of early reflections in a particular area of the room to achieve a larger initial time delay gap. This effect can only be achieved over a limited volume of the room unless the room is made anechoic, which is undesirable.
The idea is simple: by absorbing, or reflecting away, the first reflections from all walls except the furthest one away from the speakers the initial time delay gap is maximized. If this gap is larger than the initial time delay gap in the original recording space then the listener will hear the original space, and not the listening room.
However, this must be achieved while satisfying the need for even diffuse reverberation, and so the rear wall in such situations must have some explicit form of diffusion structure on it to assure this. The initial time delay gap in the listening should be as large as possible, but is clearly limited by the time it takes sound to get to the rear wall and back to the listener. Ideally this gap should be 20 ms but it should not be much greater or it will be perceived as an echo. In most practical rooms this requirement is automatically satisfied and initial time delay gaps in the range of 8 ms to 20 ms are achieved.
Note that if the reflections are redirected rather than being absorbed, then there will be "hot areas" in the room where the level of early reflections is higher than normal. In general it is often architecturally easier to use absorption rather than redirection, although this can sometimes result in a room with a shorter reverberation time.