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Old 24th February 2010, 06:54 AM   #1
domtw is offline domtw  France
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Question driver and port phase/bass cancellation

I have been experimenting with the Ikea sphere speaker.
To join the 2 bowls tohether I made a wodden ring that goes between them.
All around that ring, I placed 8 small ports calculated with WinIsd.

Now, if I listen to the speaker from above there are some bass (relatively to the small 3" full range I am using).
BUT, if I listen to it with the driver facing me there are much less bass.

I have never came across this kind of bass cancellation.
And, as bass are supposed to be omnidirectional, the cancellation should happen from any listening position.

Is that effect due to the multiple port, to the ports position (in a ring) or what?

And how to calculate the port to have it "playing" in phase with the driver?

I would appreciate your opinion to help me understand what's happening here.
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Old 25th February 2010, 12:22 AM   #2
domtw is offline domtw  France
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Nobody ?
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Old 25th February 2010, 12:43 AM   #3
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are you sure it's not just because when you listen on-axis, you're hearing the full output of the driver, which may appear the mask the 'bass' you're hearing off axis?
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Old 25th February 2010, 12:53 AM   #4
domtw is offline domtw  France
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I did that test with some pure tones around 100Hz...

It might be the room. I have to try this elsewhere and even outside...

But, is there a reason to worry about a kind of cancellation because driver and port are out of phase?
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Old 25th February 2010, 01:09 PM   #5
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Hello,

have you got a picture showing the port and how the
speakers position is in the room ?

Best
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Old 26th February 2010, 05:51 AM   #6
domtw is offline domtw  France
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I will post pictures asap...

But, in general, is it possible to have some low frequency cancellation because the driver and the port are out of phase ?
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Old 26th February 2010, 03:14 PM   #7
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Assumed that a speaker is aligned within the usual limits i cannot
imagine that.

At fb driver and port are 90 degrees out of phase. The port
contributes the main portion at fb.

Since your speaker is small, there should not be any delay
playing a role for low frequencies.

Maybe you should estimate Qb for your enclosure, possible that
there is more friction in the port than you want.

A friend of mine has a speaker where the port is outside the box
and can be made pointing into different directions.

Bass response is not independent from where it points to, seems
to be a matter of room excitation.

According to the books only the displaced volume should matter
for the radiation of the port.

Thin ice here ... i am not quite shure whether it is in some cases
important where the velocity vector of the port points to, depending
on the speakers position.

Interestingly the velocity vector of your "port-ring" configuration
seems to be zero ...

Kind Regards
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