Loudspeaker polarity checker?

[Mr.B]

The reason being that certain kinds of crossovers produced a phase shift that amounted to 180 degrees at the crossover frequency.
So connecting the drivers anti-polarity produced phase-coherent response through the critical crossover frequency.

I have often wondered about this... Perhaps should have Googled it...
Thank you for the education Richard.

[pickle9000]

Set up another speaker (correctly wired as a reference) and mic, and record both mic's in stereo, just look at it on a scope.

[HackedFridgeMagnet]

If these speakers are in the same room, you can play a high frequency tone through the system and walk between the speakers to hear if there is a zone of cancellation.

I did this plenty of times but I looked for phase cancellation at low freq!?? Typo?

In the situation you seem to be describing, phase may not be necessary to match at all, and an out of phase signal may actually be  more correct than an in phase one.   The reason being that the wavelength of sound traveling through the air is very short..  For instance a 1 kHz tone has a wavelength of just over a foot in room temperature air.  That means the phase reverses every 6 inches or so.   Unless your listener is going to be in a specific spot, being this picky about phase isn't really worth it.

From personal experience often the best performance out of a multiple speaker system installed in a building is obtained simply by a trained ear walking the site and reversing speakers where there seems to be a phasing issue which might be removed by flipping the speaker phase.

In large live sound systems such as those used in a concert, multiple speaker locations are used, and the signal to each speaker is actually intentionally delayed such that the signal is exactly in phase with the main speakers on or near the stage. 

This is right but assuming it's all about single driver 110v line speakers, yes you may as well be sure they are all in phase.
The reason they sound shit (inaudible if it is a PA) is that the differing delays to each spot from multiple sound sources ie. multipathing.
Best, if possible to minimise the number of sources and make them louder, so less null zones.
The other one to fix inaudibles is to minimise hard surfaces and also somehow also minimise the resonances of each room.

[commongrounder]

I believe the OP is talking about a distributed sound system with a large number of tranformer coupled, ceiling mounted, speakers.  It is important that they be in phase with each other, or there will be "suck-outs" as a listener passes between the acoustic overlap points between them.  In my installation business, I used a Sencore/Terrasonde SoundPro/Audiotoolbox.  These units have a phase checker that uses a special waveform (like what EricTheNorwegian mentioned) that is triangle on the positive side and sine on the negative, with a 1khz frequency.  With these units, one just walked around the room, with the acoustic measurement microphone on a boom, and hold it in front of each speaker.  The toolbox would give you the polarity, along with the certainty of correctness, instantly.  You could do a room full of speakers in minutes.  One can still find these units on auction sites for very little money.
Another thing about 70 or 100 volt distributed systems.  They are often wired in a daisy chain fashion, so it is very difficult to electrically isolate one speaker without pulling it (What the OP is trying to avoid).  Getting close with a microphone acoustically isolates the speaker, and is usually the easier way to check out a finished install.

[LaserSteve]

MY former employer used a Rolls PT102, and the schematic is out there on the web..

Or just buy a Phase-It...

However, there is an easy way, and its what I use.

Holm Impulse is free, does more then just phase, and just needs a laptop with a sound card that can send and record at the same time. You'll of course need a appropriate mic and cabling.

http://www.holmacoustics.com/holmimpulse.php

While a cheap microphone will give you your phase vs frequency with Holm Impulse, for commercial work you usually want something known to be calibrated.

Reference mics with reasonably flat response:

Home Made:
http://www.johncon.com/john/wm61a/

Commercial :
http://www.parts-express.com/dayton-audio-emm-6-electret-measurement-microphone--390-801

http://www.guitarcenter.com/dbx/RTA-M-Reference-Microphone-for-DriveRack-PA.gc

Note that commercial reference mics need Phantom power. I  usually use a 44$ 2 channel mixer to get Phantom Power with my RTA-M

[LaserSteve]

Mobile Solutions has This:

http://www.mobilesolutions-usa.com/store/p839/PT-9A_Kit.html

Steve