Phase Tests

**dlark** · **07-23-08, 10:24 PM**

Hi All

A bit of background. I am a part time muso and electronics guy who spends most of his spare time working on stuff for the band, home recording, building amps etc and chanced on this site whilst looking for some software like REW!

I have a 2.1 system that gets used occasionally when time allows. I have a sub on loan for the system but it sounds boomy despite the phase setting and I am suspecting as one option that there could be a phase reversal in the wiring of one of the bookshelf speakers.

I am about to load REW up to a laptop so I can do a check in the living room on the system but I am not sure what I should be looking for in the graphs REW is going to give me.

Any pointers would be great.

thanks
Derek

**thxgoon** · **07-24-08, 02:08 AM**

Hey dlark, Welcome to the Shack!

I'm not as well versed at REW as some. Reversing the phase on just one of your speakers would hurt your imaging and is not a great idea. You could reverse the phase on both speakers or on the sub and see what that does. Take a few readings with REW in each config and see what they look like. Chances are that moving the sub little bit could solve your problem.

**07-24-08, 05:16 AM**

Are you sure you are not referring to polarity?

Phase and polarity are not interchangeable.

'In phase' and 'out of phase' refers to the time relationship between the two sources or relative to an additional reference.

Polarity, on the other hand, refers to the response of the response to a stimulus. In this case, the cones of both sources should move towards the audience in response to the application of a stimulus - not both moving backwards, nor one moving forward while the other moves backward.

For a dynamic speaker, a small battery can be used to determine or verify the terminal markings and thus the polarity.

**dlark** · **07-24-08, 05:26 AM**

Thanks for the quick feedback.

Well I guess it is all terminology but a speaker that is 180 deg out of phase would have polarity reversal, which is what I was getting at.

So if my bookshelves have been wired up internally incorrectly so say one of the drivers is connected reverse polarity I would get some weird results by my guessing.

I like the idea of taking measurements then reversing polarity and retesting, that certainly would give some data to base a factual repsonse on.

I was hoping to avoid pulling the speakers down and looking inside.

I have tried three different brand subs in differing locations and get pretty similar results to the ear so I was starting to look for a more complex answer to the question.

thanks
Derek

**07-24-08, 08:26 AM**

Quote:

dlark wrote:

Well I guess it is all terminology but a speaker that is 180 deg out of phase would have polarity reversal, which is what I was getting at.

My intent isn't to argue, but this is not correct.

The case you state would only be 'equivalent' for a steady state invariant sine wave.
As the difference is that one signal lags in time by one half cycle!

This is like saying that if we both run a race, and our strides are equal length and at the same rate (in other words, we run at the same frequency!), that if you start one stride before I do, that as long as we remain stride for stride that we will arrive at the same time - which obviously we will not, as one is 1/2 stride (cycle) ahead.

A better way to think of this is in response to an impulse. The initial behavior of the speaker to the initial impulse determines the polarity. The difference in response with respect to time after the impulse is the phase. And relative variance to the initial time of impulse is the absolute phase, while the comparison of the response of the two speakers with regards to each other is the relative phase.

I hope that helps a bit.

**tonyvdb** · **07-24-08, 09:29 AM**

Derek, May I ask what you have your sub's crossover set at and what sub are you using? The phaze being out will not cause it to be "boomy" its either your room shape or sub placement that is most likely the issue. You may need to add some bass traps to absorb the so called boomy frequencies.

**Otto** · **07-24-08, 09:39 AM**

Hi dlark.

Touch a battery across the speaker terminals. If the cones of each speaker move in the same direction, then they are wired correctly. Of course, you want to orient the battery in the same way each time.

You could also allow music to play with the sub only. If you still find it boomy, you'll know it has nothing to do with your main.

Good luck.

**thxgoon** · **07-24-08, 12:48 PM**

How is the sub hooked up?

Try adjusting the phase on the sub and if possible try moving it to a different location. If all 3 subs sounded boomy it's likely the room or settings somewhere.

**tonyvdb** · **07-24-08, 01:37 PM**

he is only using one sub so it shouldn't be phase cancellation and he is using bookshelf speakers so I doubt that they go low enough to affect the phase on them either.

**thxgoon** · **07-24-08, 02:15 PM**

I think he's thinking it's the phase between the bookshelves and the sub at the crossover point that is over reinforcing those frequencies so by changing the phase on the sub he could intentionally cancel some of that out.

dlark - have you tried anyting with REW?

**dlark** · **07-24-08, 07:15 PM**

Hi, and thanks to all for the replies and great ideas

The bookshelves are quite old Goodman 1010's and the orginal sub was a wharfedale 15 now replaced with a front firing 12 in a smaller box, not sure of the branding, also had a smaller front firing 8 in there from my studio set up to try.

The Goodmans had the low freq drivers changed at some point in their long lives. Hence my suspicion on their wiring. I will start with the battery test, thanks for that suggestion, then move on to REW if that doesn't show an issue.

The setting on the sub is what I would call "subtle" ie not lots of volume and roll of at a mid point setting. Phase switch set so that it sounds the best to my ear, ie the less boomy setting. This set up is in a reasonable size lounge room, carpet floors, lots of soft furnishings, drapes on the windows etc.

I have done some measurements with REW in my studio to get acquainted with the software some time ago but I don't have it on the laptop just yet. Too many other interruptions like family, work etc getting in the way of the project. I guess this project also will allow me to get better trained in how it works and what I am seeing.

My original technical education is as a professional engineer in the radio frequency communications space but I haven't played with these toys for a long long time.

Oh and the sub is fed from the tape out section of the receiver!

regards
Derek

**dlark** · **07-26-08, 04:38 AM**

Ok, here is the latest ...

set up rew and looked at left and right channels. Found one of the bookshelves giving poor results on spectrum test.

Replaced both bookshelves with some home made ones that I know are ok and voila, very listenable.

Big thanks to all

regards
Derek

**thxgoon** · **07-26-08, 01:24 PM**

Nice, good to hear you got it sorted out. Wonder what was wrong with your speakers...

**dlark** · **07-26-08, 07:38 PM**

Hi

Got one pulled down at the moment, it did have polarity reversed. Also found the tweeter is poling so needs a major fix, will pull the other one down also when I get a minute.

Importantly though my mate and I had a lot of fun using REW in the living room, he is a real home theather junkie and having only scratched the surface of its features it might become a bit contagious.!!

thanks
Derek

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