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Phase Interference

hmm ..i just watching internal mixing by steinberg and there topic about phase interference ..actually i don't know what he say about phase interference ..can someone share to me about phase interference that can make me understand that ..thx btw

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EricIndecisive Thu, 12/03/2009 - 07:16
from what i learned in physics this year, this is what it means. this applies to say, speakers playing in a room, but i think the same concept is also applied to microphones picking them up.

you have 2 sound waves. you know the shape of them, like this: //////

well, when the tops and bottoms of these waves match up perfectly, the sound is amplified and louder (constructive interference). however, if the top of each wave coincides with the bottom of the other, the sound actually cancels out (destructive interference).

like this:
you can see as they shift one wave the volume decreases and visa versa.

In the speaker example, due to the way that sound works, there are spaces in the room where sound actually cancels out. This was telling us about a home theater setup, and you do not want to place the seats in one of these dead spots.

I'm assuming for microphones it is the same thing - that mics that are not positioned properly pick up recordings that are out of phase and have a reduced volume / tonal properties and wouldn't be good for a recording.

Kev Thu, 12/03/2009 - 13:36
in a multi speaker system there will always be dead spots at different frequencies
in stereo the centre line is pretty safe but there will be stereo mix effects that show the dead spots
don't overcomplicate things but be aware of the technicals

in addition to the display above which is for a two single tones of near even frequency and this will produce a beat frequency
there is Comb Filtering which will occur when a single sound source is mixed together with a phase shift
you get peak and valleys in the EQ spectrum and this can sound bad and annoying
however it is the basis of some Effects where the phase shift is changed with an LFO (low frequency oscillator)

phase and polarity is a big subject
take your time and investigate the variations one at a time
as it can get all very complicated if you try for max knowledge in one hit

Kev Thu, 12/03/2009 - 23:00
try two mics on a guitar amp
one in close
one 6+ ft away

the mic 6ft will have more room/reverb but will also be slightly late compared to the close mic

set the close mic ito one side of the stereo pan and then fade in the far mic ... also to the same speaker
the tone will change and may even sound uncomfortable

better still is to get someone to walk around the room with the second far mic and listen to the tone change

Multiple mics across a drum kit will have the same issues
but on a drum kit with short bursts of sound it can be hard to recognize the issues
until you get to know the sound

second try

take a simple recorded track and set it to one speaker
take the same audio file and put it on a second track
set you DAW to bump the second track by small increments ... say 1 ms ... perhaps 5 ms

as the two tracks are played IN phase ... the same time things will sound good ... just 6db louder than a single track

but as you bump the second
listen for that tone change ... comb filtering is happening
and it changes with each bump

anonymous Fri, 12/04/2009 - 07:18
Kev wrote:

second try

take a simple recorded track and set it to one speaker
take the same audio file and put it on a second track
set you DAW to bump the second track by small increments ... say 1 ms ... perhaps 5 ms

as the two tracks are played IN phase ... the same time things will sound good ... just 6db louder than a single track

but as you bump the second
listen for that tone change ... comb filtering is happening
and it changes with each bump

it's like take 1 guitar track than copy it but use ADT to copy track pan L full in original guitar track and pan R to the copy one to become stereo guitar sound ..right ?

if phase interference like that ..i should already know .. :shock:

Kev Sat, 12/05/2009 - 00:32
as jg49 said

in mono is where you hear the problems

but in stereo it can be an interesting trick ( a widening thing )
out of phase content is valid for a stereo mix
IF you know what you are doing and can control the problem it causes

a better trick is to record a second guitar as close as possible to the first
OR
use a delay that has an LFO and you can slide the amount of delay so the Left Guitar moves a bit
then the phase error changes and so to the Comb Filtering
try not to create chorus and the stereo spread of the guitar can move a little ... I think it is more stylish sound compared to the straight 5 or 10 ... 15ms delay to left

jg49 Sat, 12/05/2009 - 04:39
Kev"use a delay that has an LFO and you can slide the amount of delay so the Left Guitar moves a bit
then the phase error changes and so to the Comb Filtering
try not to create chorus and the stereo spread of the guitar can move a little ... "

Use a delay with LFO (low frequency modulation?) is this a delay with a LFO setting on it, similar to synths, or something you have to hear? I was under the impression that LFO was near or below the range of hearing though I do understand it alters how we percieve sound. Just not sure exactly what you meant here.

Kev Sat, 12/05/2009 - 12:28
yes it is a synth trick/feature

the frequency of the LFO is often down in the 1 and 2Hz area .. perhaps lower at 0.5

you don't hear this directly ... you can't

what it does is drive the control of the delay amount inside the workings of the delay unit

soooo

if you have set the delay for 5ms
the LFO will change this setting at a rate of 1Hz by the depth you have set

the depth setting is the sort of control you would use in a chorus or flange

so depth is like More swirl ... Less swirl

we don't want to make chorus ... this would be much high delay settings

the widening effect of 5 to 10ms is changing very slightly and so when in MONO
that phase sound is not static
it helps ... slightly ... to cover up the fact you have used a cliche trick of widenning
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