phase reversing in monitors

[timtheenchanteruk]

I seem to remember reading somewhere that if using pairs of monitors pointing toward a mic to reduce feedback, if the signal phase is reversed on one of the monitors, this will reduce the signal at the mic, with no effect on the ears.

The physics behind this seem to be sound (forgive the pun) but it seem likly to me that you would damage the speakers, ie running it backwards,

 

Any comments??

[J Pearce]

Speakers are capable of moving equal distances in either direction from equilibrium.

 

Audio signals are ac, and go either side of 0v, so inverting it doesn't cause any problems.

[paulears]

Running it backwards? the cones go in and out no matter what you do - strictly speaking you are not reversing the phase, you are reversing the polarity. It doesn't hurt the speakers in any way. Having two monitors both pointing at the same mic does cut down feedback UNTIL you move the mic towards one or the other, and then your extra loud monitors burst into feedback. The audio also sounds very thin and weedy - all the bottom end vanishes, so although a valid technique not that useful really. It can be handy in the studio when you need a little more separation - maybe you need a loudspeaker, but don't want it in the mic channel - the twin monitors and a fixed mic do work a bit. To be honest, I've only used it in the studio just once, with a group of kids singing a 'Bradley Bear' song for a holiday centre feature for Channel 4 - I didn't have enough headphones and used a couple of monitors wired with one reversed - and a single 414 as the mic. I was surprised how much level I could get in the speakers before it became too prominent in the mic channel.

 

Live wise, reversing polarity on a single monitor often makes a worthwhile difference with any mic that's fixed - guitars, keys and drums for example.

[Carey D]

I seem to remember reading somewhere that if using pairs of monitors pointing toward a mic to reduce feedback, if the signal phase is reversed on one of the monitors, this will reduce the signal at the mic, with no effect on the ears.

The physics behind this seem to be sound (forgive the pun) but it seem likly to me that you would damage the speakers, ie running it backwards,

 

It will affect the way the peformer hears the sound. If he stands mid way between the two speakers he is likely to hear a strange and generally uncomfortable phasing effect as the sound waves from the two opposite polarity speaker cones interract. If he moves closer to one than the other the sound will return to normal. Try it yourself by reversing polarity on one of two speakers running the same mono signal. Standing mid way between two speakers and moving your head from side to side is a good way to listen for polarity or phasing errors in a PA system.

 

Reversing polarity will not damage the speakers. It just means the cone moves in the opposite direction, but over the same excursion. It is the same effect as if you switched the polarity (phase) switch on a console input channel.

Regards,

Carey

[jim b]

In scenarios where two mics are on in the same mix it sometimes helps to invert the polarity of one of the mics i.e. so that one is of opposite polarity with respect to the other BUT the pair of monitors has the same polarity. Inverted polarity on wedge pairs has never been a positive thing in my experience.

 

Jim

[Mark Payne]

I agree with Jim and Carey

 

Its a real bad bad idea.

 

The bass image will be destroyed for a start. And the HF image will comb filter itself to death in various patterns of rubbishness depending on the frequencies we are talking about and the exact position of the listeners head.

 

Not nice... try it and you will see (or hear!).

 

Cheers

 

Mark

[Camtwister]

Yup, I agree with this response.

Tim, you may wish to experiment with the angle between your two wedges.

This will alter the spill into the rear and sides of the mic. Also, the interaction of the waves from the drivers, particularly the HF units. It helps if you know the dispersion angles.

Shure usually supply recommended angles for wedge pairs with their mics. Though your acoustic environment will dictate some deviation from this, I would assume.