Beating bad sound

Does your audio sound like you have your head down the toilet? Chronic feedback or simply not enough volume? We’ve all been there before, read the book and got the T-Shirt. So why does it still happen? Andrew Smith, explores.

Audio – it’s by far the most difficult part of ‘Audio-visual’ and is almost always the poor relation to the visual part of any installation. A few 100v line ceiling speakers for the voice re-enforcement and a pair of bog standard cabinet type loudspeakers linked to a £100 8 ohm amplifier for the programme sound. After all, it worked on a previous boardroom so there’s no reason why it shouldn’t work on this project.

Well that’s precisely why it won’t work. Or at least won’t work as well as it should. The audio system was designed (if indeed it was designed rather than just some random equipment selection based on guesswork and things that ‘have worked before) for another project, not this undertaking.

We can’t “see” audio like we can see a video image – to measure audio properly you need specialist test measurement equipment and in the general audiovisual environment it’s much more difficult to engineer audio than it is video/data.

Let’s look at the average boardroom installation. Typically I find this contains a number of ceiling loudspeakers (which are usually 100v line) to provide the audio for the video conferencing and a pair of 6” or 8” cabinet style loudspeakers for programme sound. These are usually recessed into the rear projection wall or simply surface mounted near the projection screen.

So does this type of basic installation work? Well yes – it does. But only to a point. Could something much better be designed and installed – almost certainly!

As a general rule, the larger the space the more difficult it will be to design a suitable audio system. The exception to this rule is normally the broadcast environment where extremely high specification systems are often required or facilities such as audio analysis laboratories or radio studios.

So how we do design a suitable audio system?

Firstly we start with the space and its’ function. Is it a boardroom, lecture theatre or rock concert venue? How many people does it seat? Is it multipurpose or dedicated to just one task? How large is it? Does the space divide?

What will actually be happening in the space? Video conferencing? Presentations? Critical analysis? A screening room for new release feature films?

And what about the acoustic environment? Hard surfaces everywhere may cause problems with reflections and feedback on voice re-enforcement. The last thing you want is your audio system creating feedback or having a 5 second delay on the reverberation.

Close attention needs to be paid to the acoustics of the space. If the area reverberates then a specialist acoustic consultant may need to be consulted to provide advice on passive or active damping system to reduce reflections, reverberations and aid focussing. Typically a model of the space is built using a specialist software package to model the reverberation times and to define what surface treatments are required to control the space acoustically. A computer generated auralisation can then be produced to hear what the space will sound like.

So how do we design a system for a difficult space? Firstly, the installer needs to fully understand the function of the space. How will the space be used, by whom and for what. Seating layouts, potential equipment mounting positions and acoustic criteria are all as important as the function of the space. This means that before everything else, the dealer or consultant has to listen to their customer and understand what the customer wants to achieve from their audio system. Having established the needs the dealer or consultant then needs to design a system that meets those needs, with sufficient headroom for any future developments.

The key to this is to use a top end modelling package such as EASE. This package allows you to place loudspeakers within the space, input the characteristics of the speaker and calculate the sound coverage, including a RASTI value for speech intelligibility.

In a small environment it isn’t necessary to create an EASE model, but it is important to check the dispersal pattern on your loudspeakers will fill the area with sound. This can be simply drawn up using a package such as AutoCAD. Draw out a plan and section of the room, then add in the speaker coverage from the mounting position.

But how many dealers actually do this? Not many I bet! It only takes a few minutes but would ensure that the audio coverage is suitable for the space.

The more complex the system, the more you need to design it properly.

Take a large auditorium seating 400 or more people. Creating an EASE model for this type of space is pretty much essential, particularly if the space is multi-purpose and the AV system has to be ‘all things to all men’. What are the layout configurations for the room? Where can the loudspeakers be mounted? What is the source material? Will the system be used for life safety? All this and more needs to be thought of.

Typically, in a stadium, environment the PA system is used as part of the life safety system. The safety/fire licence will demand a specific RASTI value that must be achieved from the system. If the system doesn’t achieve this level of speech intelligibility, then it won’t meet the terms of the safety licence. If that happens then the safety licence may well be withdrawn and the stadium unable to trade. Try telling the MD of Old Trafford that his new £500K PA system doesn’t meet the requirements of the stadium fire licence!

As an industry we need to take audio a lot more seriously. The big pictures on the screens are very sexy, but this is the AUDIOvisual industry. If we pay as much attention to the sound as we do the picture, then the quality of our systems will improve endlessly.

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