One of the first things to consider when creating a space designed for optimum sound, be it a dedicated home theater, recording studio or vocal booth, is how that sound is managed. As a useful component of acoustical treatment, sound deadening foam is one of the first materials people turn to for correcting a room’s flutter echoes, standing waves, and mid and high-frequency problems. But because foam is present in so many places and used in so many ways, the question frequently comes up, “What’s the difference between acoustical foam and the everyday stuff?”
This is a valid question, and one that shouldn’t be dismissed. After all, acoustical and conventional foams do look the same, and they generally feel the same as well. However, acoustical foam treatment is a specially engineered material with many unique traits that differentiate it from the foam in your couch cushions or the mattress on your bed. These traits combine to create a special product that is designed for a singular purpose. What follows is a listing of the differences between conventional and acoustical foam, and why close enough isn’t good enough when it comes to sound treatment.
Fire Retardancy – While not necessarily a performance characteristic, the fire retardancy of acoustical foam may be its single most important trait due to safety. In studios and home theaters, acoustic treatment is often implemented in open areas where possible ignition sources like cigarettes, candles and extensive wiring and electronics may exist. Proper sound foam should have an acceptable fire retardancy rating that meets all pertinent local safety and building codes. The testing method for foam fire retardancy is assessed by ASTM E84. That fire retardancy makes true acoustical foam much safer than conventional foam when used in the same way.
Durability – Because acoustical foam is to be used in settings where it will be in direct contact with people, it needs to be made to handle accidental contact. Acoustical foam is made to be “non-dusting” so it resists crumbling over time. In a location where the foam would never be touched, it would be a non-issue, but in places like studios where multiple people can fill a tiny room, or home theaters with kids or friends, foam can be bumped, brushed, poked and scratched. Acoustical foam is designed to hold up to this kind of abuse longer than traditional foam would if used in the same way.
Firmness – Just like comfort foam, the firmness of acoustical foam is also an important consideration. But while a user’s personal comfort preference dictates the firmness needed for traditional foams, firmness in acoustical foam plays a role in its ability to treat sound. Low firmness foam does a better job at treating high-frequency sound waves, while firmer foam is better for treating low-frequency waves. Putting non-acoustical foam of unknown firmness in a room can leave voids in a sound scape, while allowing other sound frequencies to run rampant. Acoustical foam features a firmness that strikes a balance between absorption and diffusion at both high and low frequencies for the best overall treatment.
Cellular Structure – A physical trait of acoustical treatment that separates it from sofa foam is the way the foam is made. One way of evaluating cellular structure is an analysis of cell size, which is measured as a Pores Per Inch rating, or PPI. This is exactly what it sounds like: the count of foam cells within a linear inch of the material. Conventional furnishing and comfort foam has a PPI of 60-70 cells while some porous, specialty foams like dryfast foam have a PPI as low as 25 or 30. Acoustical foam typically has a PPI of 80. Greater PPI makes for a more sonically absorbent product. For an example of the difference a few PPI can make, a 12″ x 12″ x 3″ tile of 80 PPI foam would have more than 127 million additional cells compared to a 60 PPI foam with the same dimensions.
Appearance – It may seem trivial, but the appearance of the foam constitutes another major difference between acoustical and conventional foam. Acoustical foam is manufactured to be much more consistent and uniform than conventional foam. A few air bubbles in a mattress won’t affect performance one bit and will never be seen, but in a studio, those same air bubbles would look unsightly in a wall diffusor. Because of the care taken in manufacture, you can trust that foam purchased months down the road will match previously purchased materials. And of course, the patterns cut into the foam, be they wedge, spirals, pyramids or eggcrate, all have an impact on the way the material handles sound.
On the surface it may seem all foam is created equal. In reality, conventional foam and acoustical foam are like apples and oranges. They’re both fruit, but they don’t taste anything alike. So if you need to treat the sound in a space, make the right choice with true acoustical foam and don’t pick a bad apple.