Sub Phase Effect

[CdnTiger]

After some quick and dirty REWing, I've tested a few different speaker positions and have, for now, decided on what seems to be the best performance setup. My sub has a 0/180 phase switch, and I've tested the sub's integration with the mains on both settings. The graph below shows the 0 degree setting in red while the 180 setting is purple. I've left the sub on the 180 setting. I don't have any room treatments yet (planned) or a BFD - any suggestions on ways of improving the response? I also experimented with changing the sub distance in my receiver, but it had absolutely no effect on the 180 setting response. I didn't try it at the 0 setting. Is that normal?

I cringe at the thought of doing all these measurements with pen and paper like I first started doing! Thank you REW!

[Blaser]

Hi CDN!

I think this is a typical problem for subs placed apart (i.e not colocated). There are always tradeoffs. I have 2 subs and when I separate them, I have to chose between increase in bass below 40 Hz and cancellation above, or vice versa.

For your case, I think if you have the sub in a corner, or far from speakers, you may try/consider placing it between speakers, or just below/behind screen.

To decrease this effect, set your speakers to small, you may also chose a higher speakers crossover (I think you are running them as large).

Anyway, with movies, this effect should not be so important, as the LFE channel has much more bass than what's sent to the fronts.

Only one thing for me is sure.... you need a BFD!! By the way, I also prefer the 180o setting.

B Rgds
Blaser

[CdnTiger]

So having two subs doesn't necessarily help to even out bass response, eh? I only have one sub, but was thinking that maybe I should have bought two smaller subs to get better in-room response. From what you're saying, this isn't always the case.

My sub is currently in front of and between the front speakers, about half the distance between the centre speaker and right speaker. My options for sub placement are very limited right now, with the current location providing a better standalone sub response than near the front corner of the room. The gold trace is the current location between centre and right speakers and the blue trace is near the right front corner.

My speakers are set to small in the receiver, unfortunately the only crossover settings the receiver supports are 100, 150, and 200 Hz . I've got it set at 100 Hz. When the HDMI 1.3 pre/pros come out this fall I'll be upgrading this component (speakers are being upgraded this spring).

[Blaser]

Having 2 subs may of course help smooth out the FR but they are far more tricky and time consuming to adjust. Moreover, I don't know why but I have obtained smooth response with separated subs, but I had other issues such as: at some frequencies, one of the may become localizable.... Anyway it seems my ears are too sensitive to bass and I can even not place it in a corner, but always behind TV (but maybe this is just my room).

2 colocated subwoofers should give you same FR but an increase of 6 db (i.e more headroom). I think if you're considering 2 subs just to smoth out the FR, this is probably not a good idea, but with a BFD or FBQ you will do it better and certainly much cheaper.

Of course the brounish FR is much better, indeed it is even not that bad at all, you even have a natural house curve, but a BFD will improve the shape!!

Can you post your subwoofer FR alone!! This will be more revealing if you need a BFD or not.!!

Hope this helps!

B Rgds
Blaser!

[CdnTiger]

The graph I posted above with the blue and gold traces is the standalone subwoofer response. As mentioned above my receiver's lowest crossover setting is 100 Hz, so that's what I've used.
However, after you brought it up I took another look and noticed that my sub-only response at 180 phase has a peak between 80 and 100 Hz. I think my next measurement will be to set my sub's crossover to 80 Hz with my receiver's staying at 100 Hz. Maybe the sub's earlier crossover will smooth out the peak between 80 and 100 Hz.

I am planning on eventually getting a parametric equalizer but am holding out for now because: there's the possibility of a new product being developed purpose built for home theater (do a search on these forums), I want to solve as many problems as I can passively with room treatment first, and there haven't been any BFDs for sale on the used market recently (the BFD is over $150 Canadian new after taxes).

Thanks for your help so far Blaser. I'll let you know how the new crossover setting works out for me.

[brucek]

Your reponse is actually quite good for not having any equalization..... but a BFD would still help.

brucek

[CdnTiger]

Well, here's the response of my speakers (set to small on the receiver) and sub together. Both traces have the receiver's crossover set to 100 Hz, the red trace has the sub's crossover at 80 Hz while the gold trace has the sub's crossover at 100 Hz. It looks like I'll be keeping the sub's crossover at 80 Hz. However I've noticed that my sub causes interference on my HD CRT TV, so it looks like now I'll have to do some adjustments for that issue . Note that I offset the red trace for ease of comparison, not that it should make a difference (I don't think).

Unfortuately, my REW experimentation will have to be postponed for awhile now as I've gotta go pack for my vacation. Not sure if that calls for or ! I'm sure I'll have internet access where I'm going and will be keeping up with the happenings here.

Later,
Cdntiger

[Blaser]

Happy Holliday!!

Keep in touch!

[CdnTiger]

I'm back and looking like some weird cross between a leopard and a lobster - apparently I need to be more careful to get even coverage with the sunscreen!

I've now settled on a sub position (until I build my component rack then we'll see if any more options open up). However I still can't see any difference in setting different subwoofer distances on my receiver. I just ran 3 measurements at 7.5', 17.5', and 25' and all three had exactly the same frequency and impulse responses. I used the full range REW sweep to test the integration between sub and main speakers.

I was hoping to try to reduce a dip at about 90Hz by adjusting the sub distance on the receiver. Am I missing something or shouldn't that subwoofer distance alter anything?

[Wayne A. Pflughaupt]

However I still can't see any difference in setting different subwoofer distances on my receiver. I just ran 3 measurements at 7.5', 17.5', and 25' and all three had exactly the same frequency and impulse responses.

In that case I’d just set the correct figure in the receiver and not worry about it. It looks like you are getting the best results with the receiver set to 100 Hz, and the sub set to 80 Hz. The crossovers can introduce their own phase anomalies in the area around the crossover that can affect response as well, as you can see.

I want to solve as many problems as I can passively with room treatment first,

Room treatments (assuming you mean bass traps) reduce low frequency signal decay – i.e., “tighten up” and dampen the sound. This often can somewhat reduce some peaks and valleys in response, but ultimately the equalizer is the best tool for smoothing response.

Regards,
Wayne A. Pflughaupt

[CdnTiger]

I may try bass traps in the future, although my room isn't all that conducive to their installation. I can only put a trap in one of my front corners, and the rear of my room is a weird shape that's open to the laundry area and central furnace.

I do have some triangles and rectangles on the way from eighthnerve, so I'll post some before and afters once I receive them and have a chance to play around a bit. I'm not really expecting too much to happen in the bass frequencies though. Now if only I could hurry up and scrape together the coin I need for new speakers...