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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Bass in rooms below 150Hz? how many peaks might I expect to have? posted by Timbo in Oz on December 27, 2016 at 15:19:28:
Tim,
I used a Rane PEQ 55 for equalizing a pair of stereo subs (link below). It is a DSP-based PEQ but with analog controls and analog inputs & outputs. It has 10 filter bands and two channels that can be used as a 5 band stereo EQ or 10 band mono EQ. In their low frequency setting, the filter bands are adjustable from 12.5-200 Hz and 1/12 to 2 octaves and the gain is adjustable -24 dB to +12 dB, which makes them perfectly suited for bass equalization. Also, four of the 10 can be configured as high or low shelf filters which I found to be very useful. The analog controls make it very easy to set up. The PEQ 55 is discontinued but they get traded around on eBay for ~$500 which is in your price range.
I read in your profile that your room is 32x14x8 feet and L shaped. But the 32' might be a diagonal measurement, so the actual length of the room might be somewhat less, like 25-27 feet? If so you have a difficult room because the dimensions are close to being multiples of each other. In the rooms I've measured so far, the strongest modes have been the lowest order axial modes associated with the length and width of the room, 1-0-0 and 0-1-0. Given the length of your room, 1-0-0 is going to be down in the vicinity of 20 Hz and I don't think it will be much of a problem. 0-1-0 should be around 40 Hz. A concern I would have with your stated room dimensions is that 2-0-0 is also going to be in the vicinity of 40 Hz, and depending on the shape of the L you might have 1-1-0 in that same frequency range.
If your room measures like the theory predicts, there are two ways you can deal with those strong low order modes: with equalization and/or with careful placement. When setting up my first identical pair of subs, I restricted my placement options to the front 1/3 of the room and ended up with stereo subs placed on the front wall behind and just inside of the speakers. That required using A LOT of equalization: deep, narrow notch filters at 35 and 42 Hz, a shelf around 20 or 25 Hz to reduce excessive room/boundary gain, and a couple of milder correction bands up around 60-80 Hz used to get a decent blend with the main speakers.
And that was after building in as much bass trapping as I could reasonably accommodate given the room size & shape. I have a mix of broadband and tuned bass traps and they are very effective at cleaning up the waterfall/RT from the mid-bass on up but they don't have much effect on the 35 Hz and 42 Hz modes and they don't prevent dips and nulls due to 1/4 wavelength cancellations which are entirely determined by placement of the speakers, subs, and listening position.
After a few years my original pair of subs started to flake out and I had to get new ones, and during the transition I experimented with 4 subs (albeit not identical). Not long after I heard about Duke's Swarm. These things helped me let go of the audiophile mantra that subwoofers should be placed near the main speakers for best integration. I ended up repositioning my subs based on two simple principles. First, if you don't want to excite a room mode, place the sub in one of its anti-nodes. For example, if you place a sub at the midpoint of one room dimension, it won't excite the odd-order modes associated with that dimension. Or if you place it at 1/3 length of one room dimension, it won't excite the even-order modes associated with that dimension. Second, if you place two subs symmetrically on opposite walls, they will cancel all odd-order modes associated with the room dimension they span. Given that the strongest two modes in my room are 1-0-0 at 35 Hz and 0-1-0 at 42 Hz, I placed the two subs at the midpoints of the two long walls.
I had great results with this subwoofer placement when combined with small stand-mounted speakers. It gave me flat response from both subs from the measurement limit (10-15 Hz) up to 60 Hz where the crossover kicks in, aside from a narrow null at 35 Hz, which is perfect because my main speakers have enough output 35 Hz to excite that mode moderately but not strongly enough to cause noticeable boom. I no longer needed any equalization, so I sold the Rane. And it only took two subs, not four.
So, to make a long story short, my advice is to start with two subs and work on placement first, taking advantage of mode structure to avoid exciting your room's worst modes. Second, use bass trapping to reduce the effect of higher order modes and to generally reduce reverberation and tighten up the bass. If that's not satisfactory, then add additional subs and equalization.
Dave
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Follow Ups
- Long reply - Dave_K 14:02:47 12/29/16 (1)
- Maaate, thanks for the ideas. - Timbo in Oz 23:38:09 12/29/16 (0)
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