[James Romeyn]

This was posted here because of Eric's knowledge concerning boundary effects; he is welcome to move it to a better circle if he sees fit.
 
The sub described below is being considered because the drivers sound great.  The only choice is to build a sub enclosure or give the drivers away.

A single mono sub will have four 10s in a sealed enclosure.  Each 10" needs only .75' for an anechoic -3 dB of 40 Hz.  The current plan is to fit the drivers into an approximately 7'-tall triangle-shaped enclosure fitting snugly in a corner w/ the drivers facing the room center.  (Optionally if it will help, one 10" could fire downward facing & elevated above the floor, another 10" could fire upward facing & spaced below the ceiling.  In this case only two of the four drivers would fire toward the room center.  This may increase gain but at the cost of transparency & transient speed.)

If memory serves I read that boundary gain in the bass range for an average room is +9 dB @ 20 Hz.  Consistent w/ that, the above 10s measured, in a sealed enclosure, in two different rooms, sited on a floor but away from vertical walls, -3 dB @ 30 Hz.

What is the predicted effect on the -3 dB cutoff point if the above sub is moved to a front wall corner?  Room is about 16.8' x 26.8' x 8' ceiling; speaker wall still undetermined.   

Sealed systems roll off at 12 dB/octave.  A sealed system w/ a -3 dB cutoff @ 30 Hz in-room will roll off at the rate of .8 dB per Hz.  Meaning I need +4 dB of gain @ 25 Hz.  Is +4 dB of gain a reasonable expectation just by moving the sub far from a vertical wall to being snug in a corner? 

Thanks for everyone's consideration.

     

[ekovalsky]

Jim

I'm not an expert on boundary effects but what I do know is corner placement maximizes the output of a subwoofer, though at the expense of even response.  Basically dB per watt will be maximized, as will the dB difference between response peaks and nulls.  Even in your fairly spacious room you will likely need some type of EQ (preferably DSP) to get a decent frequency response from corner subs.  And proper integration with main channels is only possible if you use appropriate high and low pass crossovers and have control over time delay to compensate for location and also phase shift or delay in the passive or active crossover filters.

TacT has been advocating the use of corner woofers for a long time.  Crossovers from the corner woofer to main channels can be quite high (300-400hz) in these systems.  From the standpoint of a VMPS afficionado like yourself, it would be interesting to combine the V60 ribbon system (i.e. sans the small bottom woofers) with a pair of good corner woofers/subwoofers crossed over at 350hz or so with 24dB/octave digital filters, optimized of course in the frequency and time domain.

Here is a pic of the TacT W210 subwoofer (2 x 10" per side).  If you want I can send you construction drawings of this design and also the larger W410 (4 x 10" per side).  Both use octagonal enclosures with front vents and woofers angled towards the adjacent walls.

[ctviggen]

The problem with corner loading is that all room modes (length, height, width) end in the corner.  That's why corner filtering (using broad or low band absorption) works well.  One aspect of this is that enforcement of certain modes (typically, the worst modes) occurs.  TACT probably recommends this placement because it does increase (3dB per wall is the commonly quoted figure, though it's hard to determine exactly what's being increased by 3dB -- overall output?) output, and because they can reduce the peaks that are caused.   If you're not using TACT or some other peak-removing tool, I'd recommend against corner placement.  In fact, placement in a null that occurs away from the wall can be useful.  If you are using TACT, then you can take out the peaks, assuming they aren't too huge.

Since I don't have a TACT, I have two of my subs placed on absorbers to reduce the height modes (and provide decoupling) and placed away from the corners, along the front wall.  I can't put them into the room, or I would.

[ekovalsky]

I agree with everything Bob says.  Avoid corner placement if you have no way of controlling output at the resonant frequencies.  You will be better off putting the subs at or near a null point which will give flattest output, but also reduced SPL.   If you can manage the resonances with EQ then you will probably see about 6dB more SPL from the sub compared with placement away from the walls. 

Getting that 6dB bump in SPL by only swapping amplifiers will require 4x more watts so it is not insignificant particularly if the sub has low sensitivity.  Most sealed systems have significantly lower sensitivity than ported designs.

Another reason why corner placement (with EQ) is good is that the driver will be equal distances from the adjacent walls which reduces early reflections which can cause time smear at the listening position.  Also the number of resonances will be minimized, although those present will be higher in amplitude.