Originally Posted by SherazNJ
I made a post in DIY section here but no one bothered to reply. I'm getting the fabric this tuesday. I measured the corners for bass trap and I have 10 inches on each side. Would it be enough to provide a good bass trap?
I understand that the main reason of bass trap is to absort low frequencies (below 40). How does it help to produce a better sound? I mean it is not repelling the waves. I tested with SPL yesterday and found high level of SPL in corners. How Installing corner Bass traps is going to help when all these are high frequencies?
It was an 80 hz sines wave.
In spite of what some of the vendors of bass traps would like you to believe, traps will do next to nothing at 40 Hz. Traps work to absorb the energy in the sound waves. They do that through friction between the moving air particles and the material of the trap. They are most effective when the air particles are moving the most, and least effective when the particle are moving the least. Now look at a wave:
The particle velocity will be moving fastest on the points of the curve that are sloped, (the upslope and downslope), and slowest at the peaks and troughs, where the particles slow down and reverse their movement. They will be perfectly still at the exact peak and trough points. If a particle starts at rest on the left side of the curve and then accelerates up to the peak, that is is the point of maximal velocity, in that 1st quarter wavelength. This is the best place to "catch the wave" and absorb it. This is the concept behind the "1/4 wavelength theory" of acoustic absorption.
Bass traps are very effective at frequencies where the 1/4 wavelengths are shorter than the thickness of the trap. A 100 Hz soundwave has a wavelength of 11.25 feet. 1/4 wavelength would be 2.8 feet. In other words, you would need a 2.8 feet thick bass trap to effectively absorb 100 Hz soundwaves. That is not very practical in most rooms. Another way you can get that much trapping is to use an 8" thick trap and "straddle" a corner with it. The wave will go through the trap, some absorption will take place, then it will reflect in the corner behind the trap and more absorption will take place as it tries to go back out through the trap. In this way, you can get fairly effective trapping to 100 Hz.
Getting to 40 Hz is far tougher. A 40 Hz wave has a wavelength of over 28 feet. 1/4 wavelength is about 7 feet. Building a bass trap that thick is *very* impractical in most rooms. The only way to effectively absorb a 40 Hz wave is with a "tuned resonator" trap. Those are expensive and complex, and are best left to specialists for construction AND installation.
So, how does one get flat response at 40 Hz and below? You really only have 2 choices: location and EQ. You have already been playing around with the location of your speakers and subs. You can also play around with your seating location. As you do this, you should be targeting the placements that yield the least nulls. Don't worry too much about the peaks at this point because the EQ can fix the peaks. However, EQ can do very little to fix the nulls. The nulls are cancellations caused by a positive wavefront arriving at the same time as a negative wavefront from a reflected wave. These two wavefronts would be "out of phase" with each other and they would cancel each other out. The problem for EQ is that to fix this null problem, you need to add boost. However, the paradox is that, when you add boost, you just get more cancellation. You add more energy to the system but you get no more acoustic output.
Since you can''t fix nulls with EQ, try to fix them with placements of the subs and listening position. Once you've found the locations for everything where you have no nulls, THEN add your EQ, (Audyssey), and it will knock down the peaks and you'll end up with flat response.
I know Braveheart has you messing around with the subwoofer Variable Phase Control knobs trying to get the subs in phase with each other. I've never had much luck doing that. The problem with these Variable Phase Controls is that they use a center frequency, (usually 80 Hz.) That is fine if the phase issue is right at 80 Hz. However, the problems are often at other frequencies, they're usually "broadband" and cover the entire subwoofer range. An 80 Hz control won't address those properly. In addition, the Variable Phase Controls have an effect above and below 80 Hz also, and they change the phase angle of frequencies above and below 80 Hz. This can negatively impact the FR above and below frequency of the Variable Phase Control, and cause group delay. If you want to get one subwoofer in phase with another subwoofer, it is better to use an external "delay" circuit that delays the entire signal than to adjust the phase of part of the signal.
In terms of getting the speakers and subs in phase with each other, I personally have had better luck using gain-matching of all the subs, setting all their phases to "0" and then running Audyssey. After running Audyssey, the only issue left to deal with is the crossover frequency null, which I showed a graph of, and correction for, the other day. I use the receiver's subwoofer Distance setting for this purpose. I like the Distance setting because it is a delay added to the ENTIRE signal, not just a small part of it around 80 Hz. It doesn't change the phase angle of any frequency and it doesn't add any group delay.
I said the other day that I would explain how the Distance setting can affect the null at the crossover point. Here is that explanation:
In a multi-speaker system, often the individual speakers and subwoofers end up being different physical distances from the listening position. In addition, all the processing involved, (codec decoding, Bass Management, Audyssey EQ, etc.) can add some latency to the signal. Combined, Theses will cause the signals from different speakers to arrive at the listening position at different times. This problematic for the mains because it will affect the "imaging" of sounds. It is problematic for the subs because soundwaves that arrive at different times than the speakers' soundwaves will be "out of phase" and will cause cancellations in the frequencies shared by the speakers and subs, (i.e., the frequencies around crossover frequency.) Those cancellations are depicted in the graph I showed a few days ago. Here is another example of the same thing:
In the above example, the subwoofer was initially set to the same "distance" as the speakers. However, it had some internal latency in it's internal DSP. The sub was set to fire at the same time as the speakers, but the internal delay caused it to be late. The 80 Hz and above wavefronts from the speakers arrived before the 80 hz and below wavefronts from the sub. When the subwoofer's wavefronts arrived they were later and out-of phase. They were still on the positive side of the wavefront while the sub was already on the negative side. The positive and negative cancelled each other to 0 and there was no energy being produced at 80 Hz.
By lengthening the subwoofer Distance setting, I caused the subwoofer to fire FIRST, and then the speakers were delayed until the subwoofer "caught up" to them. Both wavefronts arrived at the same time, reinforced each other instead of cancelling each other, and the net effect was flat response through the crossover range.
The difference in sound is night and day. The system with the null sounded thin and weak and lifeless. The same system with the "flat" response sounds powerful and full, with punch and kick. The bass is articulate and distinct instead of boomy and muddy. This is the way I go about setting up subwoofer systems now. It works for me. Here is the commentary from the owner of the most recent system I set up: http://www.avsforum.com/t/1309798/my-ht-2ch-setup/780#post_23735788
(I also posted just below that with the measurements.)