Phase

Firstly let me say that I am not exactly technically minded and would appreciate An Idiots Guide to Phase

What is meant by Phase? - in phase - out of phase - Phase differences - Absolute phase - phase at crossover - phase differences - 90 degrees and other phase angles re subwoofer etc - Phase anomalies etc.

Thanks People!

 

It took me ages to find this website but I knew it was out there

Have a look at the first few links under 'Basic Wave Phenomena'

http://www.kettering.edu/~drussell/demos.html

 

Thanks for that I spent some time on that page and as I couldnt find an empty beer bottle to suspend between rubber bands I am forced to drink one!!
It seems a wee bit more complicated than I had at first suspected but I am determined to get an understanding of phase - It's taking me back all those years ago when I studies wave addition etc in Physics.
Guess Ill have to get "Focus" out and play "Moving Waves" havent heard that in ages!

 

Phase can get pretty complex but all you need to know in terms of subwoofers is relative phase at the same frequency. This is pretty simple.

Waves have peaks and dips in their amplitude (or pressure). How often the wave peaks and dips in one second of the frequency. If you had two of these waves in a room (one from a speaker and another from the sub) they should combine to produce a wave with 6dB greater amplitude. This is when two waves are perfectly 'in phase' with each other.

But what if the subwoofer is set further back in the room than the main speakers? The sound from the sub will 'start' from a different point. This means that the peaks and dips of each wave will not match up anymore and the sound will not combine perfectly. If the distance is ‘just so’, the peak of one wave will be exactly where the dip of the other is. These waves will combine and exactly cancel each other out. Hence, 'out of phase' with each other. This is part of how active cancellation headphones work for airplanes for example.

You then get any mixture of these two where the resultant combined waveform is a bit louder or a bit quieter than just one of the waves and the phase of the overall combined wave is also different from each of the single waves that made it.

At a crossover point the theory of a crossover relies of each sound source being 'in phase' with the other. So for the sound to sum correctly and your music to sound right it is very important to get the phase right.

Now there is a difference between a phase shift and a time delay. But don't get too bogged down in that. You can either alter the phase of the sub via its controls to match that of the main speakers (you can tell when that is because you will get the loudest tone at the crossing frequency when they are adding in phase), or use a time delay to correct for the difference in distance from the listener. In an idea world you want to use a combination of the two since all speakers don't exhibit the same phase behaviour and then they are probably also not the same distance from the listener. For simply integrating a sub with the main speakers this is really a bit anal and is more the sort of stuff you pay attention to when designing a speaker from scratch, but it does all make a difference. The best idea is to place the sub the same distance from the listener as the main speakers and play with the phase control to get the loudest tone at the Xover frequency.

I hope that answers your questions.

 

Nice one T. Explained it well, Although, my simple take on it is : think of it as waves.
You have one wave going up&down. If you have 2 different waves going up&down at the same rate then they're in phase.
If you have one going up(eg main speakers)& one going down(eg subwoofer) then they're gonna cancel each other out.
Same goes for sound.

Hope this helps

 

Yeah, get in the bath and start splashing

 

Or just don't use subs at all

 

Hey Thanks for that informative posting. At around the crossover frequency of two speakers, if each driver is in perfect phase then that would would that cause wave addition problems, similarly a 70hz tone from a sub and also from the typical woofer in the stereo would cause this tone to be amplified and sound unnatural. I'm getting out of my depth here I fear. If the length of a room is a multiple of the frequency being listened to then the wave retuning from the end wall could cancel or add to the original wave.

Anyway what I was thinking is that phase issues are probably best dealt with in speakers with one drive unit. Other phase anomalies could be heard as effects of the domestic environment and dealt with electronically.

One bit I am really hazy on - what is "absolute phase"

 

Cheeky :JOEL:

 

I'm not quite sure what you are saying but in a speaker and sub situation yes you will get a tone from one and the other but the crossover should, at the Xover point, half the power at that frequency so that when the two add together you are back at what you should be.

With a room it is a little harder because while you can attenuate the signal from the speaker so that when the direct and reflected add you have the right level, you then have to consider the time domain as well - the reflection and the direct sound occur at different times, and from different directions! How does you mind actually hear and sum these signals, let alone acoustically? This is one area where it gets complex.

A single driver speaker is the easy way to deal with phase issues in crossovers (don’t have them!) but not necessarily the best. If a crossover is done correctly and for example the drivers are coincident (one inside the other) it would be challenging to hear the crossover. IMO many, many speakers have the crossover to the tweeter from the midrange or woofer too high in frequency. The tweeter is too far from the midrange and the sound from each does not sum in phase over the whole crossover range which causes a fair few issues like the sound not propagating from the speaker equally in all directions. That was one of the things I liked about the AML1 – it crossed to the tweeter at about 1.4KHz and as such acted almost like a point source and gave brilliant imaging and phase response.

Absolute phase is usually a phase inversion, like if you switched the + & - speaker cables on BOTH speakers from red to black to black to red. Taking a single driver speaker as an example, when the speaker is 'meant' to move forward it moves backwards and vice-versa. The audibility of this 'absolute phase reversal' is questionable and most research shows it is only audible with asymmetric waveforms. But it is very hard to say what is correct or wrong here, its just 'different'.

 

The reason I was interested in the two drivers - wave addition - was in relation to connecting a sub to the pre and the amps for the other speakers to the pre as well. This is the way I have my present speakers set up. When I connected them through the subs crossover the music "died"!

I used to have a pair of JR 150 and I own a pair of VAF DC7s (in Australia) both these designs use two woofers connected in parallel and 1 tweeter.

 

What killed the music is probably the circuitry converting the signal from high level speaker outputs to line level for the subs own amp to boost it up again.

In speakers with two woofers and a tweeter where the tweeter is on top... the lower woofer will often only work at a lower frequency, helping to boost the bass up. The lower in frequency you go, the further apart you can have the two sound sources and the sound will still integrate.

Other speakers have a woofer tweeter woofer arrangement and this controls the vertical directivity of the tweeter at the Xover point. The phase differences between the sound from the woofers and tweeter can be used to ‘steer’ the sound in a forward direction. Good or bad thing? Up to you.

 

The inputs to the Sub are at Line level and as far as I understand it all that is done to the sound at the sub level is through its active cross over system - guess its this that kills the dynamics of the sound (and everything else).

 

if you want to hear it get a copy of Chetsky's Audiophile test disc vol1 its got some great tracks on it as well as examples of in and out of phase instruments.