In general terms, controlling sound is all about improving intelligibility or our ability to comprehend what is being communicated. Or more simply stated: taking the clutter out of the sound so that you can clearly hear the message. In a church, this may be the spoken word. In an airport, it may be flight announcements. In a factory, it may have to do with safety paging. In a hotel lobby or restaurant, it may simply be an attempt at controlling the reverberant time to make communication between patrons more comfortable. And in a recording studio, controlling the acoustics allows us to create a predictable outcome so that the recording will translate to other audio systems with relative confidence.

Without treatment, sound will echo off the walls, floor and ceiling and reach a point where the room’s ability to handle and dissipate energy has been exceeded. For instance, a teacher quietly speaking in a classroom is very different from one yelling above a room full of excited kids. Once the room’s natural threshold is exceeded, conversation and communication requires much more attention. This causes an effect known as ‘ear fatigue’ – whereby we have to work hard at listening and speak louder to be heard in an attempt to overpower other competing sounds.

These competing sounds are called reflections. They can be powerful primary or first order reflections that echo off nearby surfaces, or be secondary reflections that create a reverberant field. Controlling the ambiance or reverberant field is generally done by mounting acoustic panels on the walls or hanging them from the ceiling. Adding sound absorption to a room can easily turn a dreadful sounding space into one that is comfortable, quite effective for communication. 

Sound Absorption: a Thermodynamic Transfer

When loud music is playing, place your hand on the loudspeaker, floor, nearby furniture or even a window and you will feel the vibrations. Sound energy travels through air, solids or liquids in the form of vibrations and when the medium is set into motion, it inevitably generates heat. Sound absorption is in fact an energy transfer function. The scientific term for this occurrence is called a thermodynamic transfer.

When sound penetrates a Broadway acoustic panel, it causes the minute glass wool fibres in the panel to vibrate. The same thermodynamic transfer occurs only this time, it is very efficient due to the minute size of the glass strands and their ability to vibrate freely with very little energy. By distributing acoustic panels around the room, the echo is quickly attenuated.

In the world of acoustics, bass or low frequencies are difficult to control due to their long wavelengths. With sufficient power, bass will pass through just about anything. This means that it not only requires more energy to generate bass (think elephant versus mouse), it is a lot tougher to stop once it gets going (think trying to stop a freight train versus stopping a bicycle). High frequencies are much less of a problem as the shorter wavelength is much less powerful

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The easiest way to absorb low frequencies is to increase the thickness of the panel. One can more or less predict the required thickness of an acoustic panel by employing ‘quarter wavelength’ calculations. But actual acoustic testing generally delivers surprising results. Unless the panel is sufficiently dense, bass will pass right through. This is the problem with low density foam; it is ineffective at absorbing bass. On the other hand, if the panel is too dense; the high frequencies will simply bounce off and reflect back into the room. Broadway panels are designed to do both; they provide balanced absorption.

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