Does your current PA technology need to reach a larger, more distributed audience? The most common solution is to add PA speakers. However, if you need to provide sound to a really large area, it is unavoidable that you will place the additional speakers far away from the main PA system. For the sound to remain coherent, you need to time-delay the sound signal. This is the delay line. Why is this required? Without a delay, the quality of the PA application suffers. This is because sound is air pressure moving through space. The challenge: People stand at differing distances from the different speakers – and indoors, they can be around corners, behind obstacles or on different levels. Read on to find out what you should consider with a delay line.
Distance affects sound in two ways:
Speaker delay as an aid for distance
High frequencies are more prone to interference (diffusion) as they propagate through space. This is because they have a short wavelength. As the distance between the speaker and the audience increases, higher frequencies decline superproportionally faster than lower frequencies. This results in the sound becoming muddy and unintelligible in the back of a large space. This is especially true for open-air events.
The volume of the sound decreases by half with each doubling of the distance from the sound source. Let's assume that we have a large hall or a shopping centre, where the sound pressure level (SPL) that must be generated by speakers on a stage or a central PA system is much too loud for the audience in the front area – at least when the audience in the back section of the venue should also hear something.
Speaker delay as an aid for obstacles, corners or angles
Diffusion and absorption prevent high frequencies from spreading as far as low frequencies. They also prevent high frequencies from penetrating obstacles and going around corners. The result: Areas under balconies, especially when combined with a certain distance, behind doorways and around corners, experience a noticeable drop in high frequencies. The solution: Additional speakers cover areas that do not have a direct line of sight of the main PA system.
How you can tell that you need additional speakers
The short answer: by testing! Play a representative piece over the PA system and listen to it in all areas where an audience will be standing later. Note whether frequencies drop significantly anywhere and whether speech intelligibility is intact. Keep in mind that the reverberation will be lower when there are people in the space later – this makes speech intelligibility better, but also absorbs high frequencies superproportionally to low ones. Listen to determine if there are any "dead spots" or if the sound is otherwise significantly different in different areas. If this is the case, you have the solution: You need more speakers.
Use digital aids
An old saying of PA technology states that the best meter is your own ear. Completely true. Yet a few tangible parameters can help. SPL meters and spectrum analysers deliver concrete data and (hopefully) confirm your perception.
General rule: no more than 6 dB deviation in the entire audience area.
If you run white noise over the PA system and apply a real-time analyser (RTA), you can also visually observe frequency troughs or peaks.
What to look out for when using additional speakers
The goal must be to find a delay line speaker (or several speakers) that covers the desired PA zone in at least an acceptable manner. It needs the right frequency, coverage and dynamic characteristics that you would choose if that area were your only concern. If you do that, you're off to a good start, but there are a few additional considerations:
Make sure that your additional PA speakers] are of similar quality to your main speakers. Of course, with the help of equalisers you compensate for some inconsistencies, but the PA application is evidently more elegant if the speakers match.
Compared to the main speakers, your additional speakers may require less power.
Your additional delay line speakers probably won't need as much bass as the main system. The reason is that below 200 Hz (see also: frequency response for speakers) the sound often cuts through obstacles much better and avoids corners in a much more audible manner.
The best positioning of additional speakers
The positioning of wall-mounted speakers is a topic in its own right. On a balcony or in a box in a theatre, for example, you have to mount the delay line speakers higher up so that they are not in the field of vision of the audience. Railings and pillars can also be used. The best solution is to position the speakers as close as possible to the horizontal line of the main speakers. This reduces the horizontal offset.
In reality, the best position for your delay line speakers depends on aesthetics and mounting possibilities.
Calculating delay and why you need delay at all
Once the additional speakers are installed, you will need to synchronise them with the PA system. This is the only way for the sound from the delay line speakers to reach the audience at the same time as the sound from the main speakers. If you don't do this, the sound will arrive with a delay, causing a poor sound pattern.
What else is important for the delay line and speakers
Even if you perfectly calculate the delay for each of your delay speakers, there are still a few more challenges waiting.
Do I maintain the stereo image despite the additional delay speakers?
In a wide-open field, the answer is easy: You can get a stereo image with delay speakers as well (but this is only appropriate for a stationary audience). If you want to provide sound in a hall under a balustrade or around a corner, things becomes more difficult. Here, stereo separation of the signal can in turn lead to unfavourable phase shifts. In addition, visitors standing in different positions would experience audibly different mixes. So pay close attention to where the delay speakers are placed. In a theatre box, two delay speakers would have to cover a stationary audience from two sides as well as directly in front of the stage.
Use a mixer to rectify any imbalances in the frequencies
Depending on the distance to the delay speakers, you may only need to boost the highs or high-mids because the bass cuts through quite well. In addition, in delay lines weak frequencies can be supported, but other frequencies can accumulate. Whether and to what extent this is necessary depends on the music that your PA system is supposed to play back. That's why it makes sense to create your own delay mix in addition to the main mix. On most mixers, you have the option of using an output for individual side mixes.
The Haas Effect
You can learn a lot more about the Haas Effect here. In short, our brain interprets a tiny difference in the timing of the sound as a change in the direction of the sound. Or, to put it another way, the directional perception referring to the sound source changes when the timing of a speaker changes slightly. You can also use this effect in a targeted manner in a delay line. Deliberately delay additional speakers and sidefills by a few milliseconds. This gives the impression that the entire sound heard by the audience is coming from the stage.