ASC SubTrap
Everybody hates modes. We all want to get rid of those pesky room modes that damage our sound. To most people modes are pests like mosquitoes, squirrels and raccoons that rampage your skin, damage the house and tip over garbage cans. Acoustic Sciences Corporation likens modes (also known as standing waves) to acoustic feedback … they exaggerate room interaction and must be eliminated. Indeed ASC has been designing product (TubeTraps and SubTraps) to “trap” those darn modes for years.
Let’s discuss room modes in some more detail. Most of us are unaware that a standing wave or mode will appear at every frequency between any 2 parallel surfaces. Yes sir, you have standing waves at 20 Hz, 50 Hz, 100 Hz, 500 Hz, 1000 Hz, 5000 Hz and so on. Those modes are a function of wavelength and help carry the acoustical energy.
Ever notice that it’s harder to get adequate loudness outside? Why does the Marching Band sound at football games seem to disappear when the band turns-about facing away from you? Without a far surface to reflect the energy back, loudness is simply much harder to maintain. In certain ways walls, reflections and standing waves can be our friends.
You can verify the standing wave effect for yourself. Just play a 1000 Hz tone (wavelength about a foot) in your listening room and you’ll hear the sound change in level as you move your head a few inches. That’s because a standing wave has formed. However, this isn’t a big deal in your listening room because at frequencies with shorter wavelengths you’ll get the same effect everywhere in the room. A flutter echo is another example of this phenomenon but which is easily cured with acoustical absorption.
Standing waves only become a significant problem at lower frequencies (below 100 Hz although the modal region is evident up to roughly 300 Hz in most rooms) where the wavelengths are very long (50 feet at 20 Hz, 10 feet at 100 Hz) and there can be large magnitude differences from seat to seat and frequency to frequency in a given seat. Remember that these variations can have both positive and negative level differences. So “trapping” a standing wave can only be useful where level is too great at a given frequency. In other words, you can’t trap a ghost and even if you could, what would you have ….most likely not enough bass at the offending frequency.
We must also remember trapping or canceling modes won’t necessarily fix the problem. Let’s say that you had a modal problem at 70 Hz (with 8-foot ceiling height, this is a common issue) where the frequency of 70 Hz was 6-db too prominent. If you had a perfect trap, one that would swallow 70 Hz completely, then you’d just be trading too much 70 Hz for too little. The amount of absorption may be as important as the frequency.
Which brings us to the ASC-SubTrap. This product is specifically intended to provide acoustical absorption at 70 Hz, the most typical ceiling height in modern construction. This potentially helps ameliorate a common standing wave build up problem typical in small rooms where the modes tend to stack up between 60 and 80 Hz and in rooms that have 16-foot length or width dimensions (a 16-foot width has a basic axial mode of 35 Hz with a 2nd harmonic at 70 Hz.) Therefore the 18-inch SubTrap 70 Hz absorber directly attacks the ceiling height mode in typical North American listening rooms which, according to ASC, causes “vertical mode coupling” a form of “feedback.” The SubTrap is said to reduce the tendency for “inarticulate slurring of bass” at 70 Hz. The product comes as a 15-inch model to accommodate 6-8 inch subwoofers, 18-inch ($438) for 10-12 inch subs and 22-inches ($548) which is intended for 15-inch subwoofers. All are said to “provide ample absorption at the crucial 7-9 foot ceiling resonances.”
How well does this work? I tested the SubTrap by finding a room with an 8-foot ceiling (my main listening room has an 18-foot vaulted ceiling.) My lower walkout level in the house has an 8-foot ceiling but doesn’t contain a system, except for the Boston Acoustics all-weather system in my sauna (yes, it’s a real sauna …. can do 220 degrees F, although I mostly use it at 190 and yes, I’m of Finnish descent and yes, you can use water on the rocks) but it has a large open area (no walls shorter than 23 feet) which means that I could evaluate the SubTrap’ ability to deal with the 70 Hz/8-foot ceiling mode in isolation.
I experimentally searched for subwoofer and listener positions that excited the ceiling mode by using a sub/microphone positions that induced interaction with the ceiling mode (74 Hz in this case.) Because the room was very large there were no related compounding modal interactions at 74 Hz and I could see the effect on the axial floor/ceiling mode in isolation. In this example if you inspect the graph you’ll see modal excitation at 37 Hz, 56 Hz, 74 Hz, 92 Hz, 115 Hz and 152 Hz along with a deep notch at 52 Hz.
However, even when I found a position where the floor/ceiling mode was clearly in excitation there were no exaggerated modal tendencies in this room at any practical listening position. In other words, it took a lot of experimentation to find a weaker subwoofer location/listening position combination. And, even then the combination shown in the graph yields a reasonably smooth subwoofer response. The subwoofer was 5.5-feet from a corner at the floor/wall junction and the microphone was positioned 9-feet from the end wall where a listener directly in front of a left channel speaker would sit if a system were to be installed in a likely fashion in this space.
Then I installed the 18-inch SubTrap. This involved placing the SubTrap under the Velodyne SPL-1000 10-inch powered subwoofer. I then repeated measurements. What happened? Well examining Graph 1 we see that when the SubTrap was installed there were 1.5 dB increases in output at 28Hz, 40 Hz and 56 Hz and 3 dB reductions at 52 and 108 Hz. Also there was a deep 12 dB cut at 152 Hz. In other words, the SubTrap had no apparent affect on the 74 Hz ceiling mode and a small worsening of modal activity at unrelated frequencies; and there were larger frequency to frequency variations with the SubTrap installed. It’s more than likely that the latter effect was simply a function of raising the subwoofer 18-inchs by placing it atop the SubTrap.
The major affect, a relatively large absorption at 152 Hz, was apparently the 2nd harmonic of the ceiling mode. Actually this isn’t too surprising; the SubTrap occupies a 3.4-cubic foot gross volume and seems unlikely to have the capability of significant absorption of a frequency that has a full wavelength of 16-feet even with its “two-stage acoustic circuit” that couples acoustic resistance with an “interior air cavity” “enhanced by the sound canceling (sic) effects of the tuned acoustic core.”
I’m not arguing with the design of the SubTrap, the idea of a tuned device intended to address a common room situation is beautiful. But, in this case it just seems that the intended effects happen an octave higher than the claimed frequency. It is true that most rooms lack sufficient low frequency absorption but generally speaking tuned devices may not always be the best solution. In this case, there doesn’t seem to be an immediately obvious useful application for this device.
Graph 1: The solid curve is the subwoofer response with the subwoofer placed on the floor. Note the modal/room effects especially the lack of modal excitation at 52 Hz. The dotted trace is with the 18-inch SubTrap installed under the subwoofer. There is increased modal effect at the lower frequencies, somewhat greater frequency to frequency variation, along with a 12 dB notch at 152 Hz. The former appears to be associated with the raised subwoofer position and the latter is apparently a function of the SubTrap.