Microphone phase relationships.

Like speakers, microphones must be wired up identically, giving us 'uniform polarity.' If two mics are not wired-up the same, they will send signals to the amps that are Electrically 1800 Out-of-Phase. When a signal is applied, this causes some speakers to jump forwards and others backwards, resulting in one speaker producing a sound wave that is the exact equal, yet opposite, of the other. The speakers are both trying to 'use the same air' to set up exactly opposite sound waves! When these two 'mirror image' soundwaves meet each other, the equal yet opposite waves cancel each other out - Phase Cancellation'. [Technically, they will be in different parts of their cycle - one positive and the other negative, cancelling parts of each other out - Phase Cancellation.] This causes a major reduction in sound level and frequency response.

In real life, the sounds we hear are made up of complex waveforms and it is rare that the two are completely identical, so not all sound gets cancelled. You'll hear tell tale signs of phase problems whether they are caused electrically or acoustically. You'll hear a 'swishing' sound in the high frequencies, and the bass will sound 'thin'. You'll also notice that when moving around the stereo image 'drops out' or is 'smeared.'

Microphone Electrical Phase Testing.
To test if a mic is in phase, simply plug a mic into the mixing desk, pan it to centre and slowly bring up the volume. Take off the pop-screen (or 'windsock') and while looking at the low frequency drivers, G-E-N-T-L-Y tap the mike with your finger. When a mic is correctly wired in-phase, the speakers will jump forwards, telling us that 'positive air pressure is resulting in positive voltage. ' If the mic is wired out of phase the speakers will jump backwards. Once the phase of the 'control' mic is confirmed, check the phase of all the other mics by :

(1) Holding the in-phase mic and the mic to be tested in front of your mouth, with their diaphragms as close as possible,

(2) Put the two mics in separate channels and pan both to centre,

(3) While talking, bring up the level of the good mic, and then bring up the level of the new mic,

(4) If the mics are out-of-phase with each other a 'swishing' sound in the high frequencies, and a very 'thin' sounding bass response will be heard.

Single Microphone Acoustic Phase Relationships.
When using distant mike techniques or one mic for a group of players like a horn section, the mic is picking up both direct and reflected sound. The reflected sound will take longer to arrive at the mike and hence be 'delayed' compared to the direct sound. When the two sounds are combined, this will alter the phase relationship between the two waveforms and results in some phase cancellation, causing a slight change in the mics overall frequency response.

Multiple Microphone Acoustic Phase Relationships.
When two mics are picking-up the same sound, the problem of delayed sound again causes phase problems. These phase-irregularities are worsened when the outputs of the mics are added together electronically at the mixing desk. The problem is made worse when the stereo signal is 'summed' for a mono system. To get around phasing problems in multiple mike set-ups 'the 3:1 rule' is applied.

The 3:1 Rule states that two mics should be placed apart from each other at least three times their distance from the sound source. In this way, the sound waves that each mic receives are different enough to minimise phase cancellation.

Finishing up…
Each type of microphone has its own blend of characteristics, and selecting a microphone is as much an artistic decision as a technical one. Choosing the right microphone and putting it in the right place is at the heart of the 'black magic' of mic technique, whether it be a live-sound gig or recording project. There are only two 'rules' for micing up, reinforcing or recording sound : Rule # 1 - There are no rules, Rule # 2 - See rule number #1!!! These rules can be summed up even further by saying that our ears are the only judge of sound, how we get that sound doesn't really matter!!

The microphone is the first and most important step in the audio signal path - if the mic sounds poor, nothing you can do will fix it later. Any sound engineer will tell you in simple terms, "Shit in = shit out!!" Try different mics types in different places, and resist the temptation to reach for the Channel EQ to try to 'fix' a problem with the sound. Carving up the sound with EQ rarely produces good results and often leads to other problems like phase cancellation and increased noise in the system. EQ should be used for creatively modifying the sound, not to fix a problem. (The only exception is when 31-Band Graphic EQs are used across the Foldback Sends cut feedback…but that's for another day!)

Just remember…in our game...'the ears rool!!'

Next time we'll look at the following step in the journey from the "Stage 2 the Speakers"…. the pre-amps.

In the meantime, if you've got any audio questions give me a yell. Send me an email or post a message on roadtalkz. I'm not the font of all knowledge, but if I don't know the answer to your questions, I know enough guru's who will!!

See-ya-later.

Willy T.

willyt-downunderdesk@roadogz.com

This is extracted from the book:
"LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA Systems." © 1998 WBN Thurbon Giraffe Communications. ISBN 0 646 36408 1.
Available via email or at all good Music & Book shops.