# Sub sounds delayed?



## atledreier (Mar 2, 2007)

Hi!

After much grief and sweat and tears I've finally got my fronts set up properly.
Now i've concentrated on my subwoofer.

I have my fronts crossed over at 80Hz (12dB/oct).

My SVS PB12+/2 is right behind my couch. This is the spot where it gives the best frequency response in my room. I also like the tactile feel when it thumps away. I've set the receiver delay by looking at the frequency response. The flattest response I got with the sub set at 6.7m (The longest my pre will go). My fronts are set to 3.0m (Their 'real' distance).

Now, somehow it sounds pretty good with music, I don't notice any significant delay there, maybe just a little. But with movies it's getting annoying. A good example are the lightning strikes in WotW. The initial crack is awesome, then a split second later the awesome low frequency shaking of my couch and body. Other sudden impacts as well. Every single shot in Shoot'em up is delayed like this. The constant rumbles are fine, timing is not critical with those, and the level of the thumps are fine. But it sound delayed by quite a bit. 
So I decided to try to measure this delay.
I used REW and impulse graph to try to see what was happening.

Now, I'm not exactly sure what I am seeing, but it seems to relate to what I'm hearing, so...

I see the initial impulse from one of my mains ( I chose the right one). Then the usual reflections from that where I'm used to see them. Then, after about 35ms there is a large impulse, less defined than the initial, but still a definite impulse. I'm assuming this is the subwoofer. It moves in time when I use the distance control in my pre, and also change when I use the phase control on my subwoofer.
So I thought I'd bring it in as close as I could. I set the distance for my mains to 0.0m, and the subwoofer at 6.7. This is the longest delay difference my pre will allow. I also set phase on my sub to 0 degrees, as this brought the delay down further. Now, Doing this I got my impulse delay down to around 23-25ms, but still a significant delay. I run my subwoofer through a BFD. I seem to recall reading somewhere that this introduced a slight delay, but I think that was in the order of 1-2ms, not 20-30.
Am I seeing what I'm hearing in the impulse plot? Is this two totally unrelated things? 
Lookign at Iilka's subwoofer measurements, group delay can be in the region of 30-ish ms. Is this group delay? How does that sound?

Any input? It's really annoying to have thumps delayed like that.


----------



## brucek (Apr 11, 2006)

*Re: Sub sounds deayed?*

Why aren't you setting the sub distance in the receiver to its actual distance (plus a foot for the BFD)?


----------



## brent_s (Feb 26, 2007)

*Re: Sub sounds deayed?*

I think I'm thinking the same thing Bruce is. Your sub is much closer to you than your mains/surrounds, not further away as you have it set. If the timing is correct, the frequency difference between distances should be a result of comb filtering. Delaying the sub that much gives the overlap signal produced by the mains time to decay before the sub plays it, giving you a measurably flatter frequency response. I say start with correct distances and work from there.

I have an antique processor, no sub delay. My sub is about 7' from the main listening seat, right beneath the right rear surround. Mains are about 13-14' away to the front. I don't notice any delayed bass with music or movies.

-Brent


----------



## atledreier (Mar 2, 2007)

Well, that's sort of my point. Of course I started with the real distance. The timing was off so I started experimenting.

A sub's phase control, is that just a delay or what exactly does it do?

And where do you measure a PB12+/2 from. It's not exactly a point source when you're 2' from the closest side of it.

Besides, setting the sub to farther away should make the preamp/DSP delay the fronts, not the sub.


----------



## OvalNut (Jul 18, 2006)

I agree with the posts above. Set it to the actual distance, plus 1 foot for the BFD. Then, use the Phase dial to get the best frequency response in the crossover region.

Tim
:drive:


----------



## brucek (Apr 11, 2006)

> A sub's phase control, is that just a delay or what exactly does it do?...


The phase control is just a 2nd order all pass filter with a very low Q, so that it offers a linear phase response over the frequency range of interest, for a sub in this case. It approximates what the time (distance) delay does in your receiver except it accomplishes it in the analog domain....



> Of course I started with the real distance. The timing was off so I started experimenting.


If you see the flash on the screen and then you hear it a second later, then you are waaaaaaayyyyy off. Something is amiss. Start from scratch. Set the sub and all the speakers to their actual distance and the sub phase at zero from your listening position. Then do a sub response and a mains + sub response and adjust the phase for the smoothest crossover area. Don't try and use the impulse time delay in REW to set distance. It uses relative time and not absolute time.



> And where do you measure a PB12+/2 from


Not important enough to matter. A foot doesn't change the sound to the point where you actually hear and see a delay in the subwoofer as you described.



> setting the sub to farther away should make the preamp/DSP delay the fronts, not the sub


Well, I believe the receiver sets the actual delay of the furthest speaker to zero, and then delays the rest relative to it. But, it isn't important. What is important is to set the actual distances and then use the phase control to get a smooth frequency response...

brucek


----------

