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Old 20th January 2009, 10:15 PM   #1
Helmuth is offline Helmuth  Netherlands
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Default bass reflex tube phase behaviour

Hi

Can any one say what the phase is of the output of the bass reflex tube.
I know it is a phase inverter.

But what is the phase shift above and under the tuning frequency exact.??

Regards, Helmuth
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Old 21st January 2009, 10:23 AM   #2
Helmuth is offline Helmuth  Netherlands
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No one here how really understand the bassreflex system?
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Old 21st January 2009, 12:37 PM   #3
bjorno is offline bjorno  Sweden
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Quote:
I know it is a phase inverter. But what is the phase shift above and under the tuning frequency exact.??

Under ideal conditions:

The box acoustical impedance phase shift above the tuning is + 90 and below - 90 degrees.

The box impedance phase shift is positive above tuning (+45 degrees) and negative below (-45 degrees).

If the SPL phase of the driver is taken as reference above the tuning frequency then the phase of the port is close to be equal in value and positive.

Lowering the phase increases the values but close to the tuning the phase starts to deviate from each other and at tuning the phase of the port shifts 180 degrees becoming negative in respect to the driver.

At tuning:

The box acoustical impedance peaks, the box impedance has a maximum where the phase is 0 and the driver SPL has a minimum (and zero phase) due to box acoustic feedback form the port that is 180 degrees out of phase and referenced to the driver.

This action force the cone almost to stop its excursion and almost all SPL is emitting from the port whose SPL peaks.

b
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Old 21st January 2009, 02:15 PM   #4
bjorno is offline bjorno  Sweden
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Quote:
The box acoustical impedance phase shift above the tuning is + 90 and below - 90 degrees.
FYI. Wrong signs! The other way around is correct :

The box acoustical impedance phase shift above the tuning is -90 and below +90 degrees.

b
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Old 21st January 2009, 04:02 PM   #5
Helmuth is offline Helmuth  Netherlands
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Thanks Bjorno,

This I concluded to day at work also, when I compare the box to a parallel LC combination. The coil will shift the phase above tuning en the capacitor below tuning.

>Fs -90
<Fs +90

This is my project. Seems to me there is a phase problem around 60Hz when I test it.?

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