Loudspeaker Phasing.

by convention, the 'positive' labelled terminal on a loud speaker is the one that when connected to a DC source pushes the cone outwards when positive.

Just occasionally you come across one that is reverse wired.

Using a small battery, if need be with something in series to limit the current, such as a torch bulb.

1. Place one speaker wire to the positive (+) terminal of the battery.
2. Briefly place the other speaker wire to the negative (-) terminal of the battery. 
3. Notice which way the speaker moves as you make this connection.
   1. If the speaker moves outward, then the speaker wire connected to the positive (+) terminal of the battery is the positive (+) wire.
   2. If the speaker moves inward, then the speaker wire connected to the negative (-) terminal of the battery is the positive (+) wire.
      
      
      Fancy phase testing boxes are available but that is an easy way if you can see the cone move.
      
      If y9ou get it wrong, and stand between 2 speakers driven by the same signal, one if which is mis-phased and listen to some music with a wide spectral range, and then reverse the connection, the one that sounds 'right' is right. Nothing is damaged if it is wrong, apart from the musical experience,

Generally labelling of connectors is standardised on all modern amps in the way that a positive pulse in the inputs will come out of the amplifier wth the positive speaker  terminal positive. An asymmetric waveform has to be used to verify this properly as a time delay and inversion look the same on a symmetrical waveform.

Mike.

Thanks Mike.

Z.

Go on then I`ll ask.

Mike, how does noise cancelling work, does it alter phasing?

If you are in a noisy environment and you have equipment to pick it up and send it out to counteract the effect, will it silence/reduce it or will it enhance the effect to the point of squashing your brain thru your eardrums?

how does noise cancelling work, does it alter phasing?

In short its all about the phase, and the intention is to obtain an electronic copy of the unwanted noise via microphone, and gently adjust its amplitude and phase so that when you  play it back via loudspeakers, then  at the place your ears are it is exactly equal and opposite so the air is both pushed by the acoustic noise and pulled by the speaker and in the middle is an  oasis of calm where the air does not move so you hear neither. It works well for headphones where the position of the ear and therefore where the level of cancellation is needed is pretty much fixed. It is much harder in free space, and then really only works with low frequencies where the long wavelength means that the cancellation zone occupies a significant volume and you can tolerate a fair amount of spatial inaccuracy.

In theory cancellation can be perfect at all frequencies at one pin point in space, but we usually settle for a few cubic feet such as the cockpit of a plane, and a reduction that removes the top 10-20 dB or so to reduce pilot fatigue.

I variation on the technique is 'equalising' a building to compensate for frequency dependent acoustic reflections and absorption by deliberately abusing  the wave-forms  to loudspeakers to boost missing trebles or whatever, and give a good listening experience over as much of the audience as possible. In that case the area of near cancellation may be much bigger, but the set up is more complex.
There is no free lunch - this takes extra power, and adds extra noise in other places where there was none before - it will cancel in one place, though it may well add somewhere else usually on the other side of the speakers for a start.
Mike