Hilbert Bode - An old topic but perhaps a new question

JMB · 18th August 2011, 11:31 AM

In performing a Hilbert Bode on Tweeters with Soundeasy (though this question really addresses these findings from all software), I am finding that I am requiring a negative slope on the highpass portion of the tweeter amplitude to match phase. I don't think that this is a setup problem as I measured 9 tweeters and found two with this issue without changing the setup and controlling for noise as much as possible in my home. I took an averaged measurement (4 measurements in sequence) to help to minimize noise. Is this reasonable (a negative or rising amplitude response in the high pass tail) or what might be the sources of error here, if not? Thanks.

AllenB · 18th August 2011, 11:41 AM

How's the physical separation? Is it possible to have phase cross just for about an octave +/- less than 45 degrees?

john k... · 18th August 2011, 04:00 PM

It sounds as if you have removed too much excess delay (phase) from the measurement. You may have place the start of the FFT window too close to the start of the impulse.

JMB · 18th August 2011, 07:38 PM

Hi John,
That is what I was thinking but like I said, the other 7 measurements made with the exact same environment (distance, mic placement, delay removal, etc.) all came out looking good. I also re-measured everything with the distance to the baffle made very carefully. The only thing that I can think of is if early reflections that I may have not measured accurately enough (i.e., perhaps the window is too large for them and so they appear) may be throwing off the phase measurement. I am traveling now, but will re try measurements and review this. If you have any other thoughts, please suggest them as I would love to explore them. Jay

parodielin · 19th September 2011, 03:28 AM

Quote:

Originally Posted by john k...

It sounds as if you have removed too much excess delay (phase) from the measurement. You may have place the start of the FFT window too close to the start of the impulse.

John,

Is there a way to "visualize" if we place the FFT window too close to the impulse? This is the acoustical distance correct?

If I'm doing a two way system, once I setup the distance in the tweeter, I should use the same distance for the woofer and figure out the delay between reference point and woofer's acoustic center using HBT, correct?

JMB · 23rd September 2011, 09:32 PM

Yes, once you set the acoustic distance, leave it the same for the woofer. That way, the relative acoustic offsets are built into your measurements. Jay

18th August 2011, 11:31 AM	#1
JMB diyAudio Member Join Date: Apr 2004 Location: Texas	Hilbert Bode - An old topic but perhaps a new question In performing a Hilbert Bode on Tweeters with Soundeasy (though this question really addresses these findings from all software), I am finding that I am requiring a negative slope on the highpass portion of the tweeter amplitude to match phase. I don't think that this is a setup problem as I measured 9 tweeters and found two with this issue without changing the setup and controlling for noise as much as possible in my home. I took an averaged measurement (4 measurements in sequence) to help to minimize noise. Is this reasonable (a negative or rising amplitude response in the high pass tail) or what might be the sources of error here, if not? Thanks.

18th August 2011, 04:00 PM	#3
john k... diyAudio Member Join Date: Aug 2004 Location: US	It sounds as if you have removed too much excess delay (phase) from the measurement. You may have place the start of the FFT window too close to the start of the impulse. __________________ John k.... Music and Design NaO Dipole Loudspeakers. "We have no right to assume that any physical laws exist, or if they have existed up to now, that will continue to exist in a similar manner in the future." Max Planck

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18th August 2011, 11:41 AM	#2
AllenB diyAudio Member Join Date: Oct 2008	How's the physical separation? Is it possible to have phase cross just for about an octave +/- less than 45 degrees?

18th August 2011, 07:38 PM	#4
JMB diyAudio Member Join Date: Apr 2004 Location: Texas	Hi John, That is what I was thinking but like I said, the other 7 measurements made with the exact same environment (distance, mic placement, delay removal, etc.) all came out looking good. I also re-measured everything with the distance to the baffle made very carefully. The only thing that I can think of is if early reflections that I may have not measured accurately enough (i.e., perhaps the window is too large for them and so they appear) may be throwing off the phase measurement. I am traveling now, but will re try measurements and review this. If you have any other thoughts, please suggest them as I would love to explore them. Jay

23rd September 2011, 09:32 PM	#6
JMB diyAudio Member Join Date: Apr 2004 Location: Texas	Yes, once you set the acoustic distance, leave it the same for the woofer. That way, the relative acoustic offsets are built into your measurements. Jay

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