The cocktail party effect: Speech intelligibility and room acoustics

We’ve all been to restaurants or events where the acoustics are so poor that you need to raise your voice in order to be heard. To make matters worse, when everybody else in the room is doing the same, the noise levels quickly get out of hand. We know that this type of environment can make conversation very difficult and uncomfortable for anybody, let alone sufferers of tinnitus or hearing loss.

Of course this problem is not limited to dining spaces. For obvious reasons, speech intelligibility is critical in classrooms and learning spaces, which led to the development of the BB93 guidelines for the acoustic design of schools. And increasingly, modern homes are being designed with a proliferation of hard surfaces and high ceilings, but with less of a regard for acoustics. This brings the problem of poor acoustics and long reverberation times into in many households.

Preventing sound from being reflected against hard surfaces is the key to creating rooms that are acoustically functional. Carpeted floors or rugs, soft furnishings and curtains all help, but sometimes are not sufficient, or are just not used in the interior décor.

In this case, the traditional approach, seen in many recording studios, restaurants and schools, is to use fabric wrapped panels which contain an absorbent core material. These can be hung on walls and ceilings and are available in a wide range of sizes and finishes. They can even become artworks, abstract images, or larger than life photos with digital printing.

The total area of panels will determine how much the reverberation time (the time it takes for sound to die out) is reduced. Usually it is difficult to over-treat a room, but a general rule of thumb is to cover 25 per cent of the total room area (including the walls, floor and ceiling).

Generally speaking, for speech applications, a 50mm thick panel is sufficient (absorbing effectively down to about 150Hz), whereas for music that has more bass content 100mm panels will control the lower frequencies. Where the budget is smaller or LF performance is not so critical a 25mm thick absorber will still cut down on reverberation, improving speech intelligibility.

Aesthetically, many people are able to incorporate such panels into a domestic or commercial environment by matching fabric colours closely to the décor, by deliberately making them stand out as feature pieces or a combination of both. However, there are also many rooms where a panel type solution is not desired and an invisible solution is preferred.

There are a small number of stretch walls and ceiling solutions on the market that can meet these demands. Ideally a durable, highly engineered fabric should be sought, with acoustic transparency provided by micro perforations. This allows sound to pass through the fabric into an acoustically absorbent backing material. This gives the same performance as a fabric wrapped panel, but with the advantages of a wall to wall appearance, and corresponding increased surface area of absorption.

Stretch fabric walls and ceilings like this are generally tensioned within a discreet frame that is installed around the perimeter of the existing wall or ceiling. Installation is very fast, creating minimal disruption and mess. Services such as lights or smoke alarms can easily be accommodated, as can corners, curves and so on.

Philip Nash suffers tinnitus and hyperacusis, and being particularly sensitive to noise, recently decided to have an acoustically absorbent stretch ceiling installed in the dining room at his home. I asked Philip to explain why he did this and what effect it has had.

“Supper parties – for most people the biggest worry is what to cook and which wine to serve. However, for me it would be how long would I be able to last before the noise became too much and knowing the ringing in my ears would rise and be worse for days.”

“Since taking the decision to install the absorbing surface in the ceiling, supper parties have become enjoyable again. The noise level does not increase as people can hear each other and don’t need to raise their voices. Our friends have been amazed by the effectiveness of the installation. The downside is I can now clearly hear my wife talking about me at the other end of the table… her downside too!”

By Martin Rawlins, sales & marketing manager at RPG Europe