| This time around we will take a look have
at microphone phase relationships.
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.
(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.
Multiple Microphone Acoustic Phase Relationships.
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.
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!!
This is extracted from the book: