Sound beam focusing and beam forming is possible using the multi-channel delay facility in Signal Wizard 3. Each channel can be delayed from 1 to 12285 sample points. Hence when operating Signal Wizard at a codec sample rate of 96 kHz, a signal can be delayed by up to 0.128 seconds with a resolution of 10 s. When sampling at 3 kHz, a signal can be delayed by up to 4.095 seconds with a resolution of 0.33 ms. Delays are introduced to each channel very easily using the delays sliders, shown opposite. The sliders introduce delays in real time, and once set can be stored in the unit’s configuration memory for standalone operation.
The diagram illustrates how sound produced by eight loudspeakers can be focused to a maximum intensity at a point P. This is located 2 m from the plane of the loudspeakers, which are spaced 0.5 m apart. The focal point is achieved by ensuring that the sound travelling from all speakers arrives simultaneously at P. Since they are at different distances from this point, progressive delays must be introduced to speaker 2 to 4. The table shows the distance for each loudspeaker, L, and the delays that must be introduced. The delay is very easy to calculate. For example, L1 is located 2.828 m from P, whereas L4 is at a distance of 2.062 m. The difference is 0.766 m. To ensure the sound from L1 arrives at the same instant as that from L4, we must delay the sound from L4 by the time it takes sound to travel 0.766 m. The speed of sound in air (at 20C) is 343.2 m/s. Hence the delay is 0.766/343.2 = 2.232 ms. Calculations for L2 and L3 are performed in the same way.
By adjusting the delays for each speaker, it is possible to establish a point of maximum intensity over a wide azimuth range, or indeed produce parallel, converging or diverging beams.
Auditory masking using time delays:In this extraordinary example, delays of 1 ms have been applied alternately to the left and right channels of a sound track. Using headphones, the delay is perceived as a reduction in volume, i.e. the sound appears to switch between the right and left headphones. In fact, both are operating at the same volume. This can be proved by listening to the track using only a single headphone.