# How to "bass crawl" to place dual subs?



## Philnick (Jul 9, 2008)

I'll be rebuilding my basement theater in a few months - it had to be demolished to fix a structural problem in my basement, and I'm going to dual subs in the process. (My new AVR is supposed to be able to correct each sub independently, in addition to decoding Atmos and DTS:X.)

I've read about doing a "bass crawl" by putting the sub in the main listening position and seeing where it sounds loudest near my intended location for it. 


1) Should I simply do the same thing with each of the two subs, one after the other, to place them?

2) Should I be playing the first sub I've placed while crawling for the second, to detect and avoid destructive interference?

3) At what point in the process should I run the room correction program? Once after the first sub is placed and then again after placing the second one?

4) I'm planning on placing my downward-firing sub behind the MLP to the right (in a corner) and my forward-firing sub on its side under the screen, near the opposite corner. (The room is 14' wide by 15'9" deep. I'll be sitting 2/3 of the way back, 10'6" from the screen.) Does this seem reasonable?​


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## theJman (Mar 3, 2012)

1) Yes, the same process needs to be done with both subs.

2) No, each is done separately. In theory, if both sound good they should be more constructive than destructive.

3) After you have identified two good locations run the room correction with both subs powered on. It's important to level match them using some type of measurement system to ensure you're starting at the same relative output at the main listening position. Gain matching, even on identical subwoofers, is less accurate because each can have a slightly different gain structure in their controls. It's also likely the locations of the subwoofers will not be the same distance from the listeners so matching gain on their respective amp controls will not yield the same output.

4) That could work, but predicting where subwoofers should be placed is hit-or-miss. The room (and your ears) will ultimately decide what works best. Each situation is unique.


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## Philnick (Jul 9, 2008)

Should I slide the couch over a few feet in order to put the sub being tested on the floor? Putting the sub - particularly a downward-firing one - on the seat of a couch doesn't sound like it would work very well.


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## Philnick (Jul 9, 2008)

theJman said:


> . . .
> 
> 3) After you have identified two good locations run the room correction with both subs powered on. It's important to level match them using some type of measurement system to ensure you're starting at the same relative output at the main listening position. Gain matching, even on identical subwoofers, is less accurate because each can have a slightly different gain structure in their controls. It's also likely the locations of the subwoofers will not be the same distance from the listeners so matching gain on their respective amp controls will not yield the same output.
> 
> . . .


I have a hand-held Radio Shack sound level meter (the one with analog VU meters) that I can use for level-matching.

What can I use as a sound source for level-matching and for doing the bass crawl in the first place? Should I just use the place in my receiver's auto room correction, after running the test, that lets you travel a test tone manually from speaker to speaker, or is there something I could download and play?


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## theJman (Mar 3, 2012)

Philnick said:


> Should I slide the couch over a few feet in order to put the sub being tested on the floor? Putting the sub - particularly a downward-firing one - on the seat of a couch doesn't sound like it would work very well.


You could do that, or just lay it on the couch with the driver pointing out. Since bass is omnidirectional the room modes should be the same regardless of driver orientation.




Philnick said:


> I have a hand-held Radio Shack sound level meter (the one with analog VU meters) that I can use for level-matching.
> 
> What can I use as a sound source for level-matching and for doing the bass crawl in the first place? Should I just use the place in my receiver's auto room correction, after running the test, that lets you travel a test tone manually from speaker to speaker, or is there something I could download and play?


That test tone generator sounds like it would be fine. You basically want a standard signal that you can use on both subs to set the level equally.


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## Philnick (Jul 9, 2008)

Thanks! It'll be a few months until I get the room rebuilt, but you've given me a lot to chew on.

BTW, I'm wiring the in-wall line-level runs from the AVR to the wall jacks for the subs using coax, with coax-to-RCA adapters at either end, and RCA cables terminated with whatever connector is needed at the AVR and each sub.

I'm assuming that coax will make decent in-wall line-level cables for the sub signals.


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## perceval (Oct 17, 2012)

Best way is to use the REW software.

You can have REW play a pink noise, install the mic at your listening position and use the RTA settings mentioned in the post.

Then you can view in real time the changes happening when you move either of the subs.

PS. if you get cancellations that seem almost impossible to fix, try flipping the phase on one of the subs. 

Follow directions from here:

https://www.hometheatershack.com/fo...rying-out-speaker-placements.html#post1612934


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## basscleaner (Aug 22, 2011)

Could you send your height, because your room dimensions may not be acoustical? 
If "yes", that means you have troubles with bass sound quality, caused room, and you need to find the better dimensions.


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## Philnick (Jul 9, 2008)

It just occurred to me that playing the downward-firing - and thus omnidirectional - sub in the main listening position would allow me to crawl for the loudest bass in _both_ locations (in the front left and rear right parts of the room), rather than dragging each sub there plugged into a different jack.

Once I decide on the two locations, _then_ I'd wire up a sub in each location.

Does this make sense?


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## basscleaner (Aug 22, 2011)

Possibly, we didn't understand to each other. When I told you about dimensions I meant, that for subs optimal position problem the main is to have correct room dimensions. These called the Acoustical dimensions, because they define bass behavior for small room and, as a result, the thimbre room quality. If you want to know, is there any corresponding to this term, you could send me room dimensions and I'll test by A.Lim code use for it.


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## theJman (Mar 3, 2012)

Philnick said:


> It just occurred to me that playing the downward-firing - and thus omnidirectional - sub in the main listening position would allow me to crawl for the loudest bass in _both_ locations (in the front left and rear right parts of the room), rather than dragging each sub there plugged into a different jack.
> 
> Once I decide on the two locations, _then_ I'd wire up a sub in each location.
> 
> Does this make sense?


I'm not quite sure I understand what you're saying but there is one point to clarify; bass is omnidirectional, irrespective of driver orientation. Front firing, down firing, side firing, it all pretty much does the same thing. There are some advantages/disadvantages to each alignment, but bass waves will propagate in all directions from every one of them.


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## theJman (Mar 3, 2012)

Philnick said:


> Thanks! It'll be a few months until I get the room rebuilt, but you've given me a lot to chew on.
> 
> BTW, I'm wiring the in-wall line-level runs from the AVR to the wall jacks for the subs using coax, with coax-to-RCA adapters at either end, and RCA cables terminated with whatever connector is needed at the AVR and each sub.
> 
> I'm assuming that coax will make decent in-wall line-level cables for the sub signals.


Coax typically has good shielding so that should work fine. Even so, I would try to keep the coax runs away from the electric cabling.


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## needspeed52 (Aug 2, 2008)

perceval said:


> Best way is to use the REW software.
> 
> You can have REW play a pink noise, install the mic at your listening position and use the RTA settings mentioned in the post.
> 
> ...


Correct me if I'm wrong, would it not be easier and just as accurate to place sub(s) at MLP and move mic around the room with the RTA settings and observe the response for best location(s)?


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## blekenbleu (Jan 4, 2007)

My guess is that your original second proposal would be most expedient:
* Place one subwoofer in the primary listening position, playing pink noise
* crawl for "best" bass (either by ear or ideally real-time spectral display of calibrated mic)
* place and play subwoofer pink noise at "best" bass position, along with subwoofer at listening position.
* crawl for optimal second bass position with both subs playing,
accounting for constructive and destructive interferences.


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