Why Does Acoustic Foam Sound ‘Boxy’ or ‘Boomy’?

When it comes to acoustic room treatment, many people immediately turn to acoustic foam. But, does it really work that well?

As a company that develops products for acoustic room treatment, we tend to hear a lot of grumbles about acoustic foam-treated rooms mainly sounding “boxy” or “boomy.”  Is this true? So, we decided to investigate this problem to see if this general complaint can be best explained based on actual test results and general physics of sound absorption. In order to do this, we decided to make a comparative analysis of foam-based portable booths and the Carry-on Vocal Booth Pro, which is based on the Producer’s Choice acoustic absorption material.

Introduction to Sound Absorbing Materials

Sound-absorbing materials absorb most sound energy, striking them and reflecting very little, making them very useful for the control of noise.  The major uses of absorbing materials are almost invariably found to include the reduction of reverberant sound pressure levels and, consequently, the reduction of the reverberation time in enclosures or rooms.  Although all materials absorb some incident sound, the term “acoustical material” has been primarily applied to those materials that have NRC of at least 0.5 — in other words absorbing at least 50% of sound

Porous sound absorbing materials can be classified as cellular, fibrous and granular

• Porous Cellular Sound Absorbing Material is a solid that contains cavities, channels or interstices so that sound waves are able to enter through them.  To be effective sound absorber, the cells have to have an opening in order for sound waves to enter the materials through a multitude of small holes or openings.  Those pores “open” into continuous twisted channels which have a great importance for the absorption of sound. Open pores can be “blind” (open only at one end) or “through” (open at two ends).

Porous Cellular absorbers, typically open cell rubber foams or melamine sponges, absorb noise by friction within the cell structure.

• Porous Fibrous Sound Absorbing Materials are composed of a set of continuous filaments that trap air between them.  Fibers can be classified as natural or synthetic (artificial). Natural fibers can be vegetable (cotton, kenaf, hemp, flax, wood, etc.), animal (wool, fur felt) or mineral (asbestos) and are essentially completely biodegradable. Also, natural fiber processing is more economical and environmentally friendly. Porous Fibrous sound absorbers absorb noise by vibrating the fibers within the material and also by air friction trapped between the fiber filaments of the material.

Absorbing sound spontaneously converts part of the sound energy to a very small amount of heat in the intervening object (the absorbing material), rather than sound being transmitted or reflected. There are several ways in which a material can absorb sound. The choice of sound absorbing material will be determined by the frequency distribution of noise to be absorbed and the acoustic absorption profile required.

Comparison of Sound Absorption Performance of Acoustic Foam and  Producer’s Choice Acoustic Panels

As mentioned above, there is an important difference in the sound absorption mechanism between porous cellular materials like acoustic foam and sound absorbing fibers like cotton, which is used in Producer’s Choice Sound Absorption Panels.

Acoustic foam absorbs the sound energy by trapping sound waves in a pores and channels of the foam.  Diameter of the channels, its tortuosity (shape) and length of the channels are all contributing factors in the sound absorption.  Sound waves get into the open cells of the foam and lose their energy through friction between the air particles and the void walls of the material it is passing through.  Foam attenuates airborne sound waves by increasing air resistance, thus reducing the amplitude of the waves.

In Fibrous Sound Absorption, panels sound reduces its energy by trying to vibrate the fibers within the panels.  When the air gets into fibrous thickets – the waves are trying to move and shake each of these fibers and dissipate its energy.

To see if there is indeed a difference in the sound absorption pattern, we compared the noise reduction graphs of acoustic foam and acoustic blankets by frequency to see if there is anything that can explain the “boominess” of the sound.

In the graph below you can see frequencies being attenuated by acoustic foam in two different thicknesses and by Producer’s Choice Acoustic Blankets with comparable overall NRC rating.

NOTE: In the graph below we compiled Noise Reduction Coefficient (NRC) data done by the same Acoustic testing lab.

For Acoustic foam performance are taken from published Auralex Foam acoustic test results for products known as “Aur3in Wedge” ( 3 inch thick Acoustic foam Yellow line) and “Aur 4in wedge”  ( 4 inch Acoustic foam Blue Line).
Results for Acoustic blankest are the actual test result for acoustic blanket hang flat (Brown line) and hang in pleated manner (Red line).

Figure 1

As you can see in Figure 1, Producer’s Choice Sound Absorption Panel’s absorption curve is pretty much linear, meaning that the frequencies from mid range to high range are being absorbed at about the same level of 90% – 100%,  where acoustic foam’s absorption curve is nonlinear. In other words, the mid frequencies are being absorbed at a higher rate than high frequency, which is leaving the bigger gap between low frequency and high frequency.  In other words, you would hear more of the low frequency, then less of a midrange frequency and then more of a higher frequency again!  We believe that this gap or jump from low frequencies to higher frequencies sound creates the “boominess” of the sound.

To demonstrate this point even more  we show in the Figure 2,  how the increase of the foam thickness makes this “midrange frequency gap” even more dramatic:

Now we added the sound absorption curves of 1 inch acoustic foam (Blue line), 2 inch acoustic foam (Green Line) and Producer’s choice blanket that is tested flat, not pleated. (Brown line) (Figure 2.)

Figure 2

As you can see in the low range all of the tested materials performed poorly, (10-30% absorption). In the mid frequency range, acoustic foam absorbs as much as 120% of the sound, more than the acoustic blankets and the thicker the foam the more midrange frequency is getting absorbed.

At the high frequency range all tested materials in all thicknesses absorbed close to 100% of the sound.

We believe that this absorption pattern is especially dramatic in a smaller enclosed space; this is why the foam based acoustic booths sound “boom”.   This “boxy” sound with a low frequency resonance (or boominess), might feel more or less expressed depending on the user’s voice.

Also this uneven distribution of sound absorption tilts the overall NRC of the acoustic foam up. When in fact this sound absorption pattern is not natural and results is poorer sound quality, compare to Producer’s Choice Acoustic Blankets.