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Old 2nd January 2013, 07:00 AM   #1
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Default Dipole operation

Hi,

I have a few questions concerning dipole operation. If you use a woofer without a box, not even on a baffle ... in free air ... is that dipole cancellation or just plain cancellation?

The other thing is is dipole cancellation dependent on where you are located, or is it purely based on baffle size?
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Old 2nd January 2013, 09:10 AM   #2
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A driver in free air is a dipole, and will get dipole cancellation below a frequency given by the physical size of the driver. A 15" will typically roll off below 500 Hz.

Dipole cancellation rolloff is 6 dB/octave. It depends only on the effective path length between from and rear of the speaker cone.

The speaker driver's own low frequency response is added to that rolloff, which is a result of Fs and Qts in free air.

The resulting frequency response will depend on the room acoustics, just like any other speaker.
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Old 2nd January 2013, 10:03 AM   #3
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Thanks. I saw this explanation :

Quote:
This happens because the front wave is being cancelled out by the rear wave. It is called dipole cancellation.

One of the jobs of the enclosure is to keep the front wave and rear wave seperated.

In an open baffle subwoofer (ie - driver mounted in a piece of wood either flat or with wings, etc) the dipole frequency is dependent on the path length between the front and back of the driver.

If you put your ear real close to the driver, you will hear it producing the bass. As soon as you move away, it disappears. This is because when you are close to the driver, the front wave has not yet been cancelled out, but as you move further away the cancellation occurs.
I bolded the part at the end of the quote because I assumed cancellation was due to the baffle size or path length distance of the driver, as you mentioned. But then it doesn't make sense that simply moving away would cause cancellation. Correct me if I'm wrong.

Moving away from a woofer is simply a distance vs SPL thing so of course SPL would go down if moving further away.
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Old 2nd January 2013, 11:30 AM   #4
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Yes, very close to the driver (a few cm) the front wave is stronger than the rear wave, and there will be less cancellation.
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Old 2nd January 2013, 11:33 AM   #5
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In any practical case, there is full cancellation at any distance above 50 cm
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Old 2nd January 2013, 11:47 AM   #6
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Quote:
Originally Posted by StigErik View Post
In any practical case, there is full cancellation at any distance above 50 cm
Then their is also behavior as driver becomes directional at higher frequencies.
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Old 2nd January 2013, 12:09 PM   #7
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Yes, but that will be above the frequency where full cancellation occur anyway.
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