Why does acoustic foam sound “boxy” or “boomy”?
We hear this statement from more and more people who have tried foam-based portable vocal booths and then switched over to our portable Carry-on Vocal Booth Pro. The Carry-on utilizes Producer’s Choice Blankets, and because of this, the end product is a much better sound.
According to seasoned voice over actors who have had an opportunity to experience different options, the foam-line portable vocal booths as well as the full-size (walk-in) mobile vocal booths, tend to result in a “boxy’ or “boomy” sound. After testing the Carry-on Vocal Booth Pro, they realized that sound is “dead-on” resulting in a clearer and better recording quality.
It is easy to see why the Carry-on Vocal Booth beats foam based portable vocal booths in the area of portability, ease of set-up, practicality, longevity and durability. However, the most important part of a vocal booth – the sound quality — needed some investigation.
INVESTIGATING FOAM BASED PORTABLE VOCAL BOOTHS AND the CARRY-ON VOCAL BOOTH PRO
We decided to make a comparative analysis of foam-based portable booths and the Carry-on Vocal Booth Pro, based on Producer’s Choice brand acoustic absorption material.
The first thing that comes to mind is that foam needs to be attached to a hard surface. And all hard surfaces are reflective.
Therefore, the sound that goes into the foam is immediately reflected right back, but at a somewhat reduced energy.
The Carry-on Vocal Booth is designed in a way that it does not need to have solid backing, allowing the sound to go out. Producer’s Choice sound absorption material reduces the energy of the sound as it comes through it and the ambient room sound that comes back gets absorbed and shield the microphone from surrounding noise.
- Internal volume
The other obvious difference is the internal volume of the booth. Due to the thickness of the foam itself, foam based portable booths tend to have a very small internal volume. This in combination with the hard reflective surfaces makes the booth sound “boxy” or like you are speaking in a cave. Reflected sound waves come back, although somewhat subdued.
On the other hand, tight foam based vocal booths can only harbor the microphone and the actor himself has to speak from outside the booth. This allows for the outside noise to leak in.
With the Carry-on Vocal Booth, the actor is surrounded with sound absorption material from all sides except the back. The sound is being projected forward. In fact, the booth canopy creates a shield from the above and the sides. The body of the actor serves as a shield from the back. All of this effectively creates a more isolated environment than foam based portable booth structures can provide.
- Physics of sound absorption
Less obvious, but not the least important difference is the difference in sound absorption between porous materials like acoustic foam and sound absorbing fibers used in Producer’s Choice absorption panel. In both cases, the sound energy transforms into heat but the mechanism is different
First of all, the way Acoustic foam absorbs the sound energy is by trapping sound waves in a pores and channels of the foam. Diameter of the channels, its tortuosity 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.
Fibrous sound absorption panels reduce sound energy by dissipating its energy. When the air gets into fibrous thickets – the waves are trying to move and shake each of these fibers and spend its energy on doing so.
To use a visual analogy – think of a patch of seaweed in the ocean waters. As the waves get into that, the energy gets spent trying to move the seaweeds and then it calms down.
Why does it matter?
We compared the noise reduction graphs of acoustic foam and acoustic blankets to see if there is anything in these charts that can explain the “boominess” of the sound.
In the graph below you can see which frequency being attenuated by two different thickness of foam and by acoustic blankets Producer’s Choice with comparable overall NRC rating.
COMMENT TO DRAWINGS:
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).
– Fig. 1 –
As you can see in Figure 1, Producer’s Choice acoustic 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. What this means is that 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.)
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 “boomy”.
Also this uneven distribution of sound absorption tilts the NRC of the acoustic foam up. When in fact this sound absorption pattern is not natural and results is poorer sound quality, compare to acoustic blankets Producer’s Choice.