Why All Acoustic Foams Are Not Created Equal
By Mike Sorensen

   By Guest Blogger   Categories: Audio Equipment

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.

Three Parts

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.   

Human Hearing

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

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.

Ears-Instrument

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

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.

RELATED POSTS:

3 Comments

Tell us what you think!

*

  • Sloane said on Aug 09, 2012 at 4:29 PM

    POSTED ON BEHALF OF MIKE SORENSON

    Hi Floyd,

    Thank you for your considered feedback. I appreciate your points but would like to point you to the following research and independently verified lab results we have achieved.

    Open celled foams are efficient as a general rule from 100 cycles and up. They get more efficient with frequency increase. They are efficient based on using the energy conversion process you have described.

    Closed cell foams can be manufactured to control density, rigidity, and cell configuration through the expansion and cooling process. One can vary the density from 1 lb. / cu. ft. through 11 lbs. / cu. ft.along with other variables to reach certain acoustic benchmarks below 100 cycles. You are correct. Closed cell foams have been used for other purposes and perform more of a support or structural paradigm in those areas.

    Certain types of closed cell foams can be made to absorb energy below 100 cycles. Based on actual builds, I have found that the rate and level of absorption is amazingly smooth below 100 Hz., especially in the polyethylene based foams if designed properly. One can manipulate many variables during the manufacture process. If one manipulates the expansion and cooling process correctly, certain lower frequency, acoustical benchmarks can be achieved.

    I discovered the lower frequency, absorption capabilities of closed cell foams, while I was searching for an internal, cabinet fill material for a diaphragmatic absorber I was designing. I finally settled on activated carbon for the diaphragmatic absorber and had it tested at Riverbank Labs. The test results are available at the following link.

    http://www.acousticfields.com/wp-content/uploads/2012/01/ACDA-12.pdf

    I have also designed a broadband absorber using the same diaphragmatic, activated carbon fill approach. Here is Riverbank Labs data on that unit:

    http://www.acousticfields.com//wp-content/uploads/2012/01/ACDA-10.pdf

    Proper positioning of low frequency absorption within a small room is critical to the sound presentation at the listening position. Broadband and frequency specific absorption techniques, each have their respective positions within small room acoustical environments and finding that balance in relation to room volume and listening pressure levels is all part of the acoustical, room tuning process.

    Regards,
    Mike

  • Foaming at the ears said on Jul 09, 2012 at 9:28 AM

    […] 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 […]

  • Floyd Toole said on Jul 07, 2012 at 7:21 PM

    I’m sure that this article was well intended, but it has several errors of fact and interpretation. The author needs to read some serious texts on the topics of how we hear and how acoustical absorbing materials perform their functions before attempting to teach these topics in lay terms. As it stands it is not instructive.

    As a simple example, in the discussion of closed- and open-cell foam no mention is made of the fact that sound absorption occurs as a result of the effort required to move air molecules within the “fibrous tangle” of the skeleton of open cell foam. Sound energy is converted into heat in the process, and absorption occurs. Closed cell foam is for mattresses and packing material not sound absorption. There is an optimum range for the flow resistance of the open cell foam material if it is to be most effective. The thickness of the foam layer relative to the wavelength of the sound determines the frequency range over which the material is an effective absorber. The effectiveness of foam as an absorber is very similar to that of fiberglass or mineral wool of the same thickness.

    Although my own book “Sound Reproduction – the acoustics and psychoacoustics of loudspeakers and room”, Focal Press 2008 – does not dwell on these particular topics, they are discussed. I suspect the author would find much of interest in it.

The Latest From Routledge