Live sound - Delay towers

This is the name given to the towers that are set up at large live events, for the purpose of diffusing the sound over extended distances away from the stage. P.A systems are set up on the towers (in addition to the P.A's present onstage) which consent a virtually uniform sound over very wide surface areas. Due to the distance between the P.A systems, the sound waves they generate will not be perfectly in phase with the waves coming from the stage as a result of their "limited" speed. We have seen how sound intensity diminishes as distance increases, with a 6dB loss every time distance doubles. This is a problem which needs to be resolved, seeing that when setting up an amplification system for very large halls, it is of absolute importance that the sound is as uniform as possible at every point in the room. Obviously a spectator standing right in front of the stage will receive a sound intensity that is far greater than that of a spectator standing 40 m. away from the stage! Since we can't, for obvious reasons, massively increase the volume on the P.A ^{[17 ]} we'd need to set up an extra loudspeaker 30 m. away from the stage in order to reinforce the sound front, as shown in the following figure:

Example of sound reinforcement

The new loudspeaker provides for the intensity loss due to distance, but another problem arises as a consequence: the sound signal coming from the stage takes a certain amount of time to reach listener 2, whereas the electric signal which feeds the reinforcement arrives immediately. This means that the wave coming from the reinforcement arrives before the one coming from the stage, thus generating an unnatural overlapping effect. A first step towards resolving this problem consists in applying a delay to the signal going to the reinforcement, in order to try and make sure that the two signals reach listener 2 together. Let's make a few calculations to pin-point the quantities we're dealing with:

s=v x t da cui t=s/v da cui t=30/344=87ms

So, by delaying the reinforcement signal by 87 ms (approximately), the two signals arrive together, and this in itself is already quite good. However, we still haven't resolved the problem, seeing that the signal coming from the stage has very much weakened (that's why we added the reinforcement!) and therefore the reinforcement signal prevails to such an extent that an unnatural effect is created in seeing the musicians play but hearing the sound they are producing come from the reinforcement. The Haas effect [Haas effect] resolves this problem quite neatly. Indeed if we delay the reinforcement again by a rate within the Haas zone, as a result of the precedence effect what will happen is that the direction of the sound perceived by the listener will be that of the wave coming from the stage, because now it arrives before the reinforcement signal, even though the latter has an intensity that is considerably greater (this is true up to 10 dB differences between the two signals- beyond this, the Haas effect is no longer present and the signal, even though delayed, is so strong that it covers the other one). In the previous example, by applying a 110 ms delay we'd fulfil our aim. This system is applied very frequently at large open-air concerts, due to the large surface-areas that need to be covered.

Arena reinforcements

Spreading out of a mono sound's image