I just wanted to mention that I did make an error by saying that many feet of absorptive material are required to see any
low end absorption results. You do not need multiple feet of thickness to absorb low frequencies, and you don't necessarily need a super thick panel to have some degree of low frequency absorption. However, it is still true that there is a physical limitation of how well a certain thickness of absorptive material can absorb low frequencies. The Auralex FAQ offers an explanation with the answer to "What is NRC?"
, found at the bottom of the linked page; low frequency cut-off (f) is a function of the thickness (t) versus the ¼-wavelength of the lowest frequency affected by the material. With (c) being the speed of sound in air, the function is f=c/4t. Thus a thicker panel with have a greater low frequency performance. This is a physical property, but to understand real life performance you should recognize a few things about how room treatments and sound waves work to achieve absorption of low frequencies.
In real-life application, 4" panels will be effective to some degree absorbing long wavelengths. The effectiveness varies on how the panels are used, however. First, a mounted panel will absorb a soundwave multiple times—initially when the wave first passes through, then again when it reflects off the wall and passes back out. Second, air gaps have a significant effect on low end absorption. Spacing panels out even an inch or two from the wall will increase low frequency NRC coefficients. Third, that function for low frequency cut-off works only if a soundwave is passive through directly perpendicular (at a normal incidence of 0 degress) to the panel. In reality, sound waves enter at all angles of incidence, and greater angles increase the effective depth. Auralex offers this illustration
Fourth, keep in mind that measuring the performance of panels is done in an environment with multiple panels set up in a room. For example, GIK's test report for the measurement of the Tri-Traps (here
) has a set up of eight traps placed strategically in a room of certain dimensions. The performance measured is partly dependent on placement. In all acoustic treatment environments, sound waves will bounce back and forth off walls, passing through panels multiple times and being absorbed to some degree each time, and so they really will be affected by multiple feet of absorptive material. For another reference, Auralex's test report for it's LENRD bass traps (here
) has a setup of a whopping 48 units stacked along the four corners of the test room, and shows exactly how the room treatment affects the frequency response of the room.
In the end, an acoustic treatment aimed at smoothing or flattening low end response will be very expensive, probably intrusive, and likely ineffective. While a bass trap or panel will absorb below 100Hz, you'll have a hard time flattening peaks or raising valleys in the response significantly using a room treatment. By examining the Auralex report for its LENRDS, you see that even 48 of them in a room can do little for the frequency response below 125Hz. However, bass traps are a good option when you're looking to reduce room boom and gain some low end clarity. The first step in getting even bass response is finding the best location for your subwoofer. After that, proper equalization is cheap and effective. You can obtain a BFD, mic, preamp, and cabling to EQ your sub for less than the cost of a few GIK panels.
For anyone who's looking to get the best performance from their Epik sub, I recommend combing room treatments, a sub isolation platform or spikes (depending on your floor), and an EQ for a complete package.