What is phasing in the context of speaker setup?

[MrSpeedy]

I hear lots of people mentioning phase when setting up relatively large speaker setups (like 4 subs - 2 per side, and 2 tops - 1 per side.

Although setups are often larger than this.

 

What is the effect of phase, what is it?

 

I am aware of phase reverse buttons on my crossover, and it does make the sound being produced different.

 

When would you use the Phase reverse?

 

Thanks in advance

 

Toby

[matt c]

Well, it's setting which way the speaker cone moves.

 

If you have a pair of speakers in your living room, and one speaker set different to the other, then the sound will be off. Basically, one speaker is pulling air in when the other is pushing air out, so they cancel each other out a little.

 

One situation where you may reverse them, is when you've got an extra set of speakers halfway down a long room, such as a church.

 

Switching the phasing of the "middle" speakers will make these speakers produce noise that is in phase with the noise from the main speakers at the end of the room. Since the sound travels a lot slower than the electricity, if you are half a wavelength out with the sound, then switching the speaker phase makes it right.

[mackerr]

Well, it's setting which way the speaker cone moves.

 

If you have a pair of speakers in your living room, and one speaker set different to the other, then the sound will be off. Can't quite explain it, but basically, one speaker is pulling when the other is pushing, so they cancel each other out a little.

 

What is commonly called "phase" on console input strips is really "polarity", which changes all positive values to negative and negative to positive. In relation to speaker setups phase is the relationship in time between the arrival of the same signal from 2 different sources. The amount of offset in time between the 2 arrivals will cause the signal to be anywhere from fully in phase, a given frequency matching exactly, to many hundreds of degrees out of phase. Where the signal is in phase it will add positively, where it is a multiple of 180º out of phase it will cancel perfectly, any other offset it will add or cancel in smaller values.

 

Because at any given time offset the phase offset will be different at different frequencies this causes what is known as "comb filtering" because the alternating summation and cancellations cause frequency response anomalies that look like the teeth of a comb.

 

Phase and polarity are not the same thing, although the share certain behaviors.

 

When speaking about "phase" in regard to speaker setups the issue is usually how 2 different drivers in the same speaker stack combine at and around their crossover point. Generally they should be in phase (not necessarily in polarity) at the crossover point. This will give the expected perfect summation to make up for the 3dB down point of each speaker at the crossover frequency. Maintaining good phase response at crossover is done with filter design, and in the case of modern DSP speaker controllers, with digital delay to get the signals aligned in time.

 

Mac

[mackerr]

In the previous post I said the signal would cancel at multiples of 180º out of phase. What I meant to say was "odd multiples of 180º out of phase". Even multiples put the signal back in polarity, but at multiples of 360º out of phase.

 

Mac

[mackerr]

One situation where you may reverse them, is when you've got an extra set of speakers halfway down a long room, such as a church.

 

Switching the phasing of the "middle" speakers will make these speakers produce noise that is in phase with the noise from the main speakers at the end of the room. Since the sound travels a lot slower than the electricity, if you are half a wavelength out with the sound, then switching the speaker phase makes it right.

 

This is not even a maybe. No. The time offset caused by the electronic signal coming out of the mains and remote speakers at the same time, but arriving at an audience member at different times cannot be changed with the polarity switch. It needs to be corrected with a delay device which will delay the audio in the remote speakers till it matches the acoustic sound from the mains in time.

 

If switching the polarity created enough of an offset to correct the delay it would only be at 1 single frequency. Since the wavelength of a 100Hz sine wave is about 10', the polarity switch would correct a 100Hz tone at about 5'. Of course everything above 100Hz would be progressively farther and farther out of phase.

 

Mac

[mervaka]

can I clear a couple of things up here:

• polarity affects the ampliftude of a signal. reversing polarity inverts the waveform with respect to its amplitude.

• phase is the relative angular time lag (or lead) of two instances of a single frequency. if a waveform is lagging by 90 degrees, it is lagging by a quarter of its wavelength.

[matt c]

I stand corrected.

 

You learn something new every day...

[paulears]

In all fairness, Matt you weren't so much wrong, but more just using one answer to prove the question was right. The bit about pushing air in and out at the same time is quite valid, and does happen when somebody uses a cable with the conductors swapped at one end - so one speaker (using the wrong terminology, but the common one) is out of phase. It's just working backwards, so technically is a polarity issue. I never get too cross about this wrong use, because plenty of high end mixer manufacturers used the word 'phase' on the polarity switch, and used the phase symbol too. If they get it wrong then who are we to argue with it? Nobody talked about polarity until quite recently in the history of the development of sound. I guess they all knew the process wasn't phase at all, but a sine wave 180 degrees out does look the same in a diagram. Look at the stuff you see about m/s mic techniques, they all tend to talk about phase cancellation and polarity doesn't get a look in - when it's polarity that's doing the cancellation.

 

If you have two speakers and one is reversed polarity, then the sound goes thin, and the bass vanishes to a large degree. Some people do it on purpose for monitors either side of a mic stand, so the monitors get picked up less by the mic, as they tend to cancel. I have to admit I've never had a lot of luck with this. You certainly get the cancellation, but the singers always moan the monitors sound odd - which they do.

 

With music, trying to do anything that depends on matching wavelengths usually fails because which particular wavelength are we talking about.

 

The hifi brigade who have golden hearing can hear all sorts of things we can't - and they seem to spend a great deal of time matching wavefronts - by moving the vertical plane of the LF and HF drivers so the output from bass and treble drivers lines up. The ones I have heard that do this do sound better, but I suspect they're probably just better loudspeakers. I've never heard a single product that has one version aligned and the other not - so can't say for certain. PA wise, we've been having different parts of the sound coming from different locations for a long time - and it never really seems a problem. Tannoy marketed their dual concentric on the point source theory, but it never really took off in a big way.

 

I suggest that we should always talk about polarity but make sure the incorrect use of phase is linked to it - because that's what would be googled. Out of phase is more common than any search phrase with polarity in it.

[mackerr]

I never get too cross about this wrong use, because plenty of high end mixer manufacturers used the word 'phase' on the polarity switch, and used the phase symbol too. If they get it wrong then who are we to argue with it? Nobody talked about polarity until quite recently in the history of the development of sound. I guess they all knew the process wasn't phase at all, but a sine wave 180 degrees out does look the same in a diagram. Look at the stuff you see about m/s mic techniques, they all tend to talk about phase cancellation and polarity doesn't get a look in - when it's polarity that's doing the cancellation.

 

Paul, I have to disagree. Just as you can't abide my use of mic'd, I can't abide continuing the confusion between phase and polarity. I agree that the word phase has come into common usage for polarity because console manufacturers used the greek letter Ø which is the common scientific symbol for phase, to indicate the polarity switch. However unless you are combining 2 identical signals with one of them polarity reversed, it is not polarity that is doing the cancellation. Cancellation happens over a range of phase offsets to a varying degree, as does summation. Summation happens in the plus or minus 1/4 wavelength area around 0º of phase shift. Cancellation happens in the plus or minus 1/4 wavelength area around 180º of phase shift.

 

Phase and polarity are 2 very different things. Changing the polarity of a complex signal reverses to voltage (amplitude) of every part of the signal, and when combined electrically with a non-inverted version of the same signal will result in complete cancellation. To put some part of the signal 180º out of phase requires the specification of what frequency will be 180º out of phase, and all other frequencies will be some other amount out of phase, resulting in different amounts of cancellation and summation at every frequency when combined electrically. This will result in what we commonly call a comb filter.

 

Polarity is absolute, phase is related to time and frequency.

 

A related confusion that we have here in the US has to do with our power distribution system. Our common residential distribution is 240V single phase, yet it has 2 hot wires that are out of polarity with each other. Despite the fact that they are out of polarity, and the 240V is the difference between them, they are just a transformer balanced feed off a single phase of the 3Ø power distribution system. In a commercial building with 3Ø power, all 3 phases are used, and they are 120º out of phase with each other.

 

Now I think I'll start mic'ing up the band. ;)

 

Mac