Can REW be used to phase speakers

[ghstudio]

I just started to use REW and it's both interesting and frightening. Interesting to see how much information one can get...frightening to find out just how badly my $10K system measures. I almost instantly discovered "standing waves" and some nasty valleys. I am measuring 5:1 speakers (NHT's al around with a James sub-woofer).

That led me to try feeding multiple speakers to see if together they were more balanced. I found a big difference when I changed the phase of one of the speakers. (note: they were all wired in phase, assuming that the speakers and amp were marked correctly). I was able to significantly reduce some of the standing waves by doing this.

However, I made a leap of faith that wiring two speakers so that they were the flattest (50-4000hz) was better than following the wire color phasing. But I'm not at all sure that this is valid.

What do you think? is there any merit in finding the flattest curve using two speakers?

Among other problems, I also have a nasty dip at 70hz.....I have to move the sub around a bit to see if location is the problem.

[hifisponge]

I recall that if you send a mono signal, like the test tone from REW, to two speakers you will get phase cancellations in your reading. This is why you are only supposed to measure one speaker at a time. It may have just been luck that reversing the phase of one speaker resulted in smoother response, it doesn't necessarily mean that your wiring was wrong to begin with. I think you can use the impulse response to see if your speaker is connected properly. It would seem to me that the impulse spike would show a negative value, but I'm not certain.

[grn1969c10]

I am very interested in the responses to your question. I am working with the phasing of my subs and they aren't playing well together. You might want to post a graph comparison showing before and after the phase shift of the speaker.

How are you sending the signal to your speakers? You say you are measuring a 5.1 system. Apparently there are issues (signal formats, patents, licensing, etc.) which force REW to use a mono signal which can be applied to the receiver in stereo (two channel), but not in any of the dolby digital surround sound modes. If you have discreet amplifiers for all speakers I guess you could bypass the reciever and split the signal to each of the amps. I'm reaching a bit here because I have no idea what kind of hardware exist in a $10k system!

[hifisponge]

grn1969 -

Your situation is a bit different since the sub channel is mono to begin with. To get the proper phasing of your two subs you would simply measure them playing together. If there is a dip at the crossover point, try flipping the polarity on one of the subs. Measure again. If the dip increases, return that sub to the original polarity and flip the other one. Measure again, and chose the polarity settings on both subs that gives you the smoothest blend. You can also use the same approach with the phase dial on the subs, moving it 15 degrees at a time and measuring in between.

- Tim

[grn1969c10]

Thanks Tim. I started a new thread so as not to hijack ghstudios thread with my long winded rambling! This phase thing is a lot more complex than it looks. I've got the hang of the measurements tab in REW but I'm still trying to learn about other features such as the impulse response, RTA, etc.

[ghstudio]

All you need to test two speakers together is a Y cable so you can take the output from REW and feed it into your left/right stereo inputs. Then you can move your speaker wires around, connecting whichever two you want to test to left and right. (almost all processor/receivers have an "unprocessed" setting or some sort).

It's a little easier to do when you have a separate amp because then you don't have to fiddle with the speaker wires as much. All you have to do is change where you plug the left/right front speakers in so they feed thru the speakers you want to test together. But to test for phase, I switched one of the speaker wires anyway (again...some amps have speaker phasing switches so you don't even have to touch the speaker wires.

In both cases, when you get the two speakers you want to test connected...then you have to balance the volumes on both speakers (to 75db) individually before you do the REW test.

While this sounds like a lot of work, if you have a processor or receiver with built in volume setting test tones (e.g. lexicon) and a sound meter, it actually goes very quickly.

But I still don't know if it's worth doing.....

[grn1969c10]

However, I made a leap of faith that wiring two speakers so that they were the flattest (50-4000hz) was better than following the wire color phasing. But I'm not at all sure that this is valid.

Yeah, I'm still hoping for some comment on that one. When you say it's flatter, is that because a null was reduced bringing the level up, a peak was reduced bringing the level down, or both? Would it matter?

In my very limited experience, the nulls have usually had a steep slope and been confined to a narrow frequency region, whereas the peaks have tended to cover a broader range of frequencies. I would assume that any reduction of a harsh null making the response flatter would be preferable (Even if a speaker were wired backwards).

What if flatter meant reducing a large peak? If the peak was reduced, was it destructive interference? Usually, people say destructive interference is a bad thing. But what if you were going to use EQ to cut it anyway?

Is your response flatter throughout the room? I have read that EQ should be used sparingly as its effects may vary considerably at different listening positions. These phase effects - good grief, I have no idea how they will change with location.

On a side note, has changing speaker distance settings in the receiver made any difference?

Ewww. I just did a Preview Post and that was a lot of question marks! Sorry. Totally cool topic though!

[Bruce]

ghstudio,

Think about what each speaker is playing for a minute . . . .

Typically, the front L&R&C will not be playing the same freequencies at the exact same time as the side or rear surrounds, ergo you won't commonly get the scenario of peaks and nulls you see in your described test environment of front and surround speakers.

Also remember, the side & rear surrounds typically have a set amount of delay applied to them in a 5.1 system playback, which may further invalidate your testing results with respect to front and surround tests.

So, I think your testing methodology is not realistic with respect to the real-time playback of a 5.1 (7.1 or whatever multichannel) system.

It does have some validity with L&R mains and a sub(s). Likely the center channel is like the surrounds though.

[ghstudio]

Bruce.....I am suggesting using a mono signal across two speakers. If you place your microphone in the middle of the two, there will be no delay effect (wire delay is immeasurable over the distances we are talking about). So you have two speakers playing identically. In phase, they should produce higher levels than out of phase where the vibrations are cancelling each other out.

Once you have done this with one pair of speakers, you then can continue to go around your surround system one speaker at a time to make sure they are all in phase.

I think this would work....it should in theory. In fact, the non measurement approach just calls for playing something and seeing which phasing produces louder bass....or the lease localization of the speakers.

[Bruce]

gh,

But the mic location equidistant between the 2 speakers is NOT the listening position, so what do you gain ? Nothing of any value !

Again, my point is that surround speakers and main speakers (as an example) are never playing the same thing to have a phase problem, plus the delay to the surrounds completely negates any phase testing you are trying to test for.

[ghstudio]

Before I start to EQ, I want to make sure the speakers are all in relative phase, including the sub which has variable phase.

Once that's done, then I can go on to do EQ from the listening position.

There is no point in spending time EQing the room if the speakers aren't connected correctly. Of course they should be if one follows the connection labeling and has wire that shows polarity, but I just want to check. I'm not a trusting soul.....

[fredk]

Could the cancellation you are seeing be due to speaker positioning? It could well be that you are getting cancellation from reflected sound such that the reflection is exactly out of phase with the direct sound from the other speaker.