Foaming at the ears

Unless you are a student of sound absorption – and if you are, please send over the course prospectus – its importance in ensuring our listening environments are optimised may be unappreciated, in fact overlooked. Imagine, for example, what the expensive in-car entertainment system in an equally expensive car might sound like if too little sound absorbing material or the wrong type of sound absorbing material were used?

In this post, Focal Press guest blogger Mike Sorenson, a structural engineer and master cabinet maker and the author of www.AcousticFields.com/blog audio blog, tells us about the two main types of sound absorbing materials on the market and their properties.

There are two types of acoustic foam: open cell (porous) and closed cell (non-porous). Each has different absorption properties. With closed cell foams, gas, typically carbon dioxide, is injected into a substance that is in a state somewhere between a solid and a liquid creating pockets or cells, which are then sealed to keep moisture out. Once solidified, each cell is a tiny sound-absorbing chamber, uniform in shape and size. Open Cell Foams are similar, but the chambers are irregular in their size (both width and depth) and shape. A good example of an open cell foam is a common kitchen sponge. Open cells absorb sound well, but not consistently and predictably.

Predictability and consistency in sound absorption costs more since closed foams are more complicated to manufacture, but they do enable us to detect subtle changes in sound absorption.. Open cell foams, by contrast, are less expensive, but rates and levels of absorption are much harder to control.

The differences in rate and level are important. For comfort and safety, we must hear and process all frequencies in an “absorption balance”. Some open cell foam can absorb up to 100% at some frequencies, and over-absorption is not good.

So, in general terms, the advice is, dig deep and cough up for the expensive foam. It’ll pay dividends in the long run.