This is the name given to the towers that are set up at large live events for the diffusion of sound over extended distances away from the stage. P.A. systems (in addition to the P.A.'s present onstage) which grant a virtually uniform sound over very broad 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. This is because of their "limited" speed. We have seen how sound intensity diminishes as distance increases, with a 6 dB 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 for the sound to be as uniform as possible everywhere. Obviously a spectator standing right in front of the stage will receive a sound intensity that is far greater than someone 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:
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 rear listeners, 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 rear listeners at the same time. Let's make a few calculations to pin-point the quantities we're dealing with:
s = v x t ==> t = s/v ==> t = 30/344 = 87 ms
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, since 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 signal again by a value 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 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 with 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.
We can use the Haas effect to widen the stereo image of a mono instrument on our mix. Let's take a mono guitar for example, very well played, but mono... With reference to the following figure, we'd send a copy of the signal on the left channel and a delayed copy of a time-period within the Haas zone on the right channel. This will widen the guitar's image but it will also move it towards the left, seeing that the dominant direction is the one that reaches the listener first. Let's lift the channel by 3 dB to balance things out, although our volumes are now unequal and we'll have to modify the other instruments to balance them out. This isn't necessarily a bad thing, because by distributing the volumes left and right we will reach the kind of variety which is necessary in order to avoid a flat mix consisting in sounds that have simply been put one on top of the other, whereas we should be aiming at applying a creative approach and produce a rich and dynamic mix.
[17 ] Public Address: the loudspeaker systems that are placed on the sides of the stage and directed towards the hall.