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Fruitfly

Joined: 19/04/10
Posts: 21
Fundamental cancellation frequency
#947712 - 18/10/11 11:01 AM
Hello Sound on sound,

this is one of my few first posts. I am a student in Australia, currently studying sound engineering.

I've got a sample question here which i am trying to figure out what the formula is?

It goes like this:

A guitar amplifier is being recorded with two microphones, one 10cm from the front of the amplifier, the other 40cm from the front of the amplifier. Both microphone signals are combined in the mix. Calculate the fundamental cancellation frequency. (573.33Hz) List the next two frequencies, in ascending order, that will also be cancelled. (1720Hz, 2866.67Hz) Starting from the fundamental cancellation frequency, list the next two frequencies, in ascending order, that will be reinforced (1146.67Hz, 2293.33Hz).

The answers are in Hz and bracketed after everyone question. Anyone have any ideas?

The formulas i have for fundamental cancellation frequency is - fc = v / (2 x d). Where v is velocity and d is distance between the direct and delayed sound. in metres.

Is there anyone who knows how to solve.

I would very much appreciate it.

Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18526
Loc: Worcestershire
Re: Fundamental cancellation frequency [Re: Fruitfly]
#947719 - 18/10/11 11:22 AM
Quote Fruitfly:

I've got a sample question here which i am trying to figure out what the formula is?

Bbasically, the formula you need is frequency = speed of sound / wavelength.

Quote:

A guitar amplifier is being recorded with two microphones, one 10cm from the front of the amplifier, the other 40cm from the front of the amplifier. Both microphone signals are combined in the mix. Calculate the fundamental cancellation frequency.

Okay, so the distance between the two mics is 30cm, and you're looking for the frequency where the wave at each mic is in opposite polarity so that they cancel when mixed together. To achieve that at the lowest frequency, the wavelength has to be is twice the distance between the mics(60cm) so that the signal at the two mics is exactly 180 degrees out of phase.

Assuming the speed of sound is 344m/s, the calculation is 344/0.6 = 573.33Hz

Quote:

List the next two frequencies, in ascending order, that will also be cancelled. (1720Hz, 2866.67Hz)

These will be the higher harmonics where the same 180 degree phase shifts occur between the two mics, which will be the third and fifth (573 x 3 = 1720Hz and 573 x 5 = 2867Hz).

Quote:

Starting from the fundamental cancellation frequency, list the next two frequencies, in ascending order, that will be reinforced (1146.67Hz, 2293.33Hz).

These will be the second and fourth harmonics...

Quote:

The formulas i have for fundamental cancellation frequency is - fc = v / (2 x d). Where v is velocity and d is distance between the direct and delayed sound. in metres.

Yes... that's compatible with the basic formula I gave above.

However, rather than trying to remember different forumlae for different applications, I find it far better and easier to remember just the funamental forumla (f=v/d) that relates wavelength and frequency, and understanding how it can be applied to different situations.

Draw yourself a diagram of the two mics and the waves at different frequencies that give rise to the required cancellation and additions, and understand how to apply the basic forumla to calculating the frequencies given the speed of sound and the mic spacing (which gives you the required wavelengths).

Hope that helps

Hugh

--------------------
Technical Editor, Sound On Sound

Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18526
Loc: Worcestershire
Re: Fundamental cancellation frequency [Re: Fruitfly]
#949955 - 28/10/11 09:46 AM
It's quite all right. No need to thank me for the effort...

Students eh?

hugh

--------------------
Technical Editor, Sound On Sound

Martin Walker
Watcher Of The Skies

Joined: 28/02/01
Posts: 16477
Loc: Cornwall, UK
Re: Fundamental cancellation frequency [Re: Fruitfly]
#950072 - 28/10/11 09:00 PM
Wait until they get into the real world, and then they will realise how important it is to do some networking to get on

Martin

--------------------
YewTreeMagic

damoore

Joined: 05/07/09
Posts: 331
Loc: New Hampshire
Re: Fundamental cancellation frequency [Re: Fruitfly]
#950079 - 28/10/11 09:49 PM
Draw the diagram showing a 1Hz sound wave and understand why its wavelength is equal to the speed of sound. Then draw one with half the wavelength and see what its frequency is. You should not need a formula here - its an application of basic ideas that you need to understand intuitively in order to master this stuff. Being able to visualize this is essential to the practicing engineer. Once you understand you will be able to derive the formula for yourself.

artzmusic

Joined: 20/05/11
Posts: 115
Loc: usa
Re: Fundamental cancellation frequency [Re: Fruitfly]
#950111 - 29/10/11 09:42 AM
Apparently Hugh, the feedback did not resonate well.

Rick

Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18526
Loc: Worcestershire
Re: Fundamental cancellation frequency [Re: artzmusic]
#950114 - 29/10/11 10:01 AM
Don't think the stand-up routine is going to work...

Hugh

--------------------
Technical Editor, Sound On Sound

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