Why All Acoustic Foams Are Not Created Equal
By Mike Sorensen
Our ears are really like roller coasters for sound energy. Sound energy strikes the outer ear and gets a boost, then slides into the middle ear canal, and finally runs into the eardrum. We must be conscious of this complex process and have a thorough working knowledge of human hearing, so that we feed our ears the energy best for our ears.
Our eardrums have three connected parts that many of us will remember from our science classes in school: the hammer, the anvil, and the stirrup. Sound energy pushes the first set of bones or the hammer, which in turn tugs at the anvil, which then throws itself into the fluid filled inner ear where all of our neurons, or nerve cells, reside. These three ear parts move in a very elaborate pattern so that no matter what note is played, every frequency contained in that note is represented.
This three-part movement is preceded by the original pressure wave entering the ear canal. This wave stimulates small hairs that transmit nerve impulses to the brain. These hairs, like antennas, detect loudness so quickly that we have the time necessary to cover our ears if needed. They detect pitch, tempo, and a host of others. In order for our ears to work best, they must be considered in any room acoustic treatment, especially when choosing foams commonly used for vocal and music reflection control.
The ear works well within a relatively narrow band of frequencies compared to that of other animals. Its center is a 1″ opening surrounded by another device (outer ear) that amplifies and channels energy into the hole or middle ear canal. This canal cannot handle the pressure overloads that come with excess sound energy.
Slow And Steady
Our ears work best if they are fed consistent information that rises and lowers in amplitude over a narrow bandwidth. We must be able to hear the subtle difference between a whisper at a movie theater by a disturbed patron and the soft whisper from a lover. If one does choose to endure sound energy produced beyond the consumption levels preferred by ear design (rock concerts, anyone?), we will not hear everything, or hear the things we should, especially when it comes to music.
Music demands everything from our auditory system. We are listening to music in whatever form we choose for an emotional connection. It’s an individual thing. Remember, our ears’ primary function is to hear speech, which operates in a very narrow frequency band compared to music. Music changes the whole dynamic with increased frequency range and dynamics. We want the sound absorption performance in our rooms to account for the ear’s need for slow and steady to ensure we do not have brain overload causing us to miss meaningful subtleties.
Our ears are finely tuned instruments that rival any man-made one. They are composed of a series of parts that work together to send signals, complete with all the musical information, to our brains. Human hearing works well in lower pressure. Lower reverberation pressure from room boundary surface reflections and a clearer, more defined direct signal is required in small-room acoustics for both speech and music. Our ears work within their determined “bandwidth.” Therefore, we must build absorption products that work within this bandwidth. The rate and level of sensitivity to absorption of frequencies within this bandwidth needs to be gradual, smooth, and consistent.
Watch And Listen
When we work with the acoustical process of sound absorption, we must consider this ear and brain interaction when designing, manufacturing, and choosing absorption products. Since our ears can detect changes in sound pressure energy at the molecular level, we need to create products that match our ears’ performance and complement the hearing processes.
Sound absorption is one of three things that happens to sound energy. Our ears are sensitive to over-absorption, but also very sensitive to under-absorption. When reflections from our room boundary surfaces send too much energy to our ears, these different reflected signals can confuse our brains. We need to send our brains energy that is scientifically managed so that we hear music in all its complexity and sonic splendor.
Sound Absorbing Products
There are many sound absorbing products currently in the marketplace. One popular technology is acoustic foam. This lightweight foam can easily be manufactured into many different sizes and designs. It comes in two flavors: open cell and closed cell. These terms refer to the manufacturing method used to produce the foam. Open cell foam has pores or open areas in the surface of the foam. Closed cell foam is exactly what the name specifies−the cells used to produce the foam are actually closed and sealed. Each type of foam has different absorption properties.
Closed Cell Foams
To create closed cell foams, gas is injected into a specific solid-type material actually halfway between a solid and a liquid. After the gas, such as carbon dioxide, is injected, numerous pockets or cells are created, each sealed with a head over it. If we are using our closed celled foam in a liquid environment, the closed cell with its “cap” will keep moisture out. If we are using it in an acoustical environment to absorb sound energy, each cell offers us a miniature sound-absorbing chamber, uniform in shape and size.
Open Cell Foams
Open cell foams also have individual cells or chambers, but their shape is irregular and their size (both width and depth) is very irregular. A good example of an open cell foam is a common kitchen sponge. If you examine it closely, you see many pores of various shapes and sizes. These open cells absorb liquids well, but do not have a consistency and predictability in the amount that they absorb. They work the same way with sound energy. They can over-absorb or under-absorb certain frequencies.
Predictable And Consistent
If your design goal is predictability and consistency in sound absorption, then you must turn to closed cell foams which are more complicated to manufacture and much more expensive. This predictability and consistency lends itself well to the way we hear and process sound energy with our ears. Our ears can detect the subtlest changes in sound absorption within small rooms. It can detect over-absorption at any frequency and under-absorption at others. Open cell foams are notorious for their many different rates and levels of absorption.
Rate And Level
Why is rate and levels of absorption important? The human ear needs to hear all the frequencies in the room represented in an “absorption balance” in our small to large rooms. We do not want to over-absorb. One of the distressing features of open cell foam is that it can absorb up to 100% at some frequencies and that its response curves have large peaks and troughs. We must let the ear and brain hear all frequencies. We must reduce room wall reflections gently, not completely. We do not want to change too much of our sound energy to heat. Once it is absorbed and converted to heat, it is lost forever.
A smoother rate and level of absorption is what the ear was designed to hear. It is designed to detect minute changes in sound, and to process these subtle changes to our brains. Remember, these changes occur at the molecular level in our ears, which are primarily designed to hear speech. We need to have a balanced rate and level of absorption so that the ear and brain can spend its time processing these small but emotion-producing nuances in our music.
About the author
Mike Sorensen is a structural engineer and master cabinet maker and the author of www.AcousticFields.com/blog audio blog.