What does phase mean......

....and what does it do on a subwoofer?
Thanks

 

Im not sure you can use 'phase' on its own, certainly not in the sense you mean it. Rather I think you mean a phase difference.

Considering a simplistic case, you have 2 simple waves. If they are in phase, the peaks and troughs of the wave will co incide. If not, they are out of phase to some extent. If the peak on one wave is in the same spot as the trough of the other, then they are out of phase and you will get cancellation of the 2 waves.

For a sub, I think that the phase switch is there so that you can change the phase so that the bass from your mains does not interfere destructively with the sub's bass.

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I'm still not entirely sure.

 

I never quite understood why you would want to change the phase on the subwoofer... Wouldn't you always want it at 0 degrees (i.e. in phase with the original signal)? Or is it room effects that cause phase weirdness? What am I missing?

Dunc

 

IMO you shouldn't use the word "phase" when talking about speakers, you should use "polarity". But that's another discussion.

Loudspeakers work by moving their cones in and out which in turn creates pressure waves in the air. The cone is moved by running an electric current through a coil which causes the magnet inside the coil to move in and out, and the magnet is attached to the cone.

(Yes, I know this is simplistic).

The "problem" is that if you run the current through the speaker one way the cone moves in, but if you run the same current through the other way then the cone moves out.

This doesn't matter if you've only got a single speaker, but if you've got two or more speakers then they all have to be connected up the same way round. Imagine you have a pair of stereo speakers connected up, and one is connected the opposite way round from the other. Now imagine you feed the same signal into both speakers. The movements of one speaker will be the exact opposite of the other one - going in when the other goes out and vice versa. When the two resulting soundwaves have travelled to where you are listening, the high pressure part of the wave from one speaker will coincide with the low pressure part of the wave from the other speaker, they will cancel out, and you won't hear anything.

In pratice you rarely get exactly the same sound fed to two different speakers and the listening position is usually slightly off, so you'll never hear perfect silence, but even so when the speakers aren't all connected the same way round it sounds very peculiar. Assuming all the speakers but one are the same way round then you would describe the remaining one as "out of phase". Or if two speakers are connected differently you'd say that "they are out of phase".

 

Thanks for that NicholasB. I understand it better now.

So I will only know if my subwoofer is 'out of phase' if I can hear that it's out?
There's no definite way of knowing?

 

If your sub has two phase settings then get a fairly steady low frequency sound and play it through your front speakers and sub simultaneously. (Something with a steady bass dum beat would work). Then get someone to toggle the switch between the two positions while you listen. The position you want is the one where the bass sound is louder.

 

Phase as it relates to subwoofers

If you place your subwoofer closer to you than your stereo pair, then the time it takes a soundwave which has the same musical origin to reach you will be shorter, and the wavefront will arrive before that from the main speaker. Therefore the two waves which were in phase in the original sound will be out of phase and some superposition will result, either positively or negatively - but still affecting the sound you hear. The phase control allows you to compensate for the difference by introducing a delay to the sub output so that it arrives in phase with the sound from the main speaker. Hope this helps, I will post more later if you want a better explanation (typed in under a minute because I have to get back to work.)

 

Thanks EvilMudge - seems obvious now that you mention it! Guess I'm just being a muppet...

BTW, great nick - is there a story behind it?

Dunc

 

I suspect that using a processor's speaker delay function is going to produce a better result than playing with a subwoofer's phase control.

 

Hi


Another way to get your sub in phase (variation on what Nic said)-

You'll need a SPL meter and test tone(s).

(Assuming sub connected low level, all speakers set small).

Play a sine wave corresponding to your receivers crossover frequency (80Hz?) through your system, (using a normal stereo setting not DPL or other.)

Measure this at the listening position while adjusting the subs phase.
When the measurement is at it's highest, the sub is in phase with the mains.

An out of phase sub will sound 'slow' / 'delayed' / 'behind'.

Cheers
Boris

 

I've already done that with my Denon processor (inputting how many metres away the REL is from my viewing position), but I'm getting a new Velodyne on Monday (I hope) so will I still have to adjust the phase then?

 

Depends if it's the same distance away.

As a general rule (I think) there are only two useful phase settings on a sub: 0 and 180. If you have a sub that allows settings in between then the only reason you would want to use them is if there is no delay setting on your processor/amp (for example, if you're using a sub in an otherwise all-stereo system) and your sub is a different distance from the listening position than the main speakers. Setting the phase to 0 or 180 is simply the equivalent of deciding which way round to plug the cables in.

 

short of a signal generator, how would you do this though!?

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Actually the most useless of all subs are the ones with a 0 - 180 phase switch - your either perfectly positioned or you're screwed. The only reasons you wouldn't put an infinitely variable phase control on a subwoofer would be either cost or concern about colouration of the sound from the phase control (which costs even more to correct).
Nick, you are right to suggest that having the AV receiver introduce a delay to the sub channel would be the best bet, properly done digital processing can have a lot less in the way of nasty artifacts. However I haven't found a processor that has anything like the range of settings needed. Does anyone know of one?

 

I suspect that using a processor's speaker delay function is going to produce a better result than playing with a subwoofer's phase control.

I think EvilMudge is right, I haven't seen an amp/processor with a sub delay. And I agree that a phase switch is pretty useless, you need a dial for proper integration.

short of a signal generator, how would you do this though!?

You can buy some test CD which contains sine wave, sweeps etc.
I had one and loaned it to a friend - never saw them again.

 

http://www.snapbug.ws/sinewaves/

Should get you started.

Cheers,

Dunc