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Author Topic: Tell me what you see in these phase response graphs  (Read 9235 times)

Mike Caldwell

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Re: Tell me what you see in these phase response graphs
« Reply #10 on: January 27, 2018, 08:52:40 pm »

Mike,

If you open up the measurement again, in the upper right, there is a button for Controls.  Open that, and press Estimate IR Delay.  After you adjust the Impulse Response, you will have the phase plot you expect.

What you are looking at is the phase wrap of the delay caused by the travel time through the air between your speaker and microphone (as well as any measurement latency).

Marc

That got me something that looks more like it.
I'll take some fresh measurements and post a couple a screen shots.

Is that something I need to apply after each measurement or is there a way to set it to automatically apply the delay at each measurement?

Thanks
Mike C.
« Last Edit: January 29, 2018, 08:35:23 pm by Mike Caldwell »
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Erik Jerde

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Re: Tell me what you see in these phase response graphs
« Reply #11 on: January 28, 2018, 12:51:51 am »

That got me something that looks more like it.
I'll take some fresh measurements and post a couple a screen shots.

Is that something I need to apply after each measurement or is there a way to set it to automatically apply the delay at each measurement?

Thanks
Mike C.

Others more knowledgeable than me may correct this, but you need to find and set the delay each time.  Physically moving a component (even a little) will change it.  If youíre working with a DSP that has fixed latency for each processing block then changes there wonít effect the measurement latency.  Of course this doesnít hold true for a delay block!  As to messing with analog circuits there Iím unsure.  A little empirical testing should lead you in the right direction.
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Mike Caldwell

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Re: Tell me what you see in these phase response graphs
« Reply #12 on: January 28, 2018, 01:10:24 am »

Others more knowledgeable than me may correct this, but you need to find and set the delay each time.  Physically moving a component (even a little) will change it.  If youíre working with a DSP that has fixed latency for each processing block then changes there wonít effect the measurement latency.  Of course this doesnít hold true for a delay block!  As to messing with analog circuits there Iím unsure.  A little empirical testing should lead you in the right direction.

After the initial set up nothing moves or changes, other than adjustments made in the DSP.

I was wondering if I need to manually apply the delay offset each time I take a measurement or if there is a way to have it automatically applied with each measurement.

REW will not display phase response in real time, you take a measurement, adjust DSP, take another measurement and so on.

Ivan Beaver

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Re: Tell me what you see in these phase response graphs
« Reply #13 on: January 28, 2018, 09:16:51 am »

That got me something that looks more like it.
I'll take some fresh measurements and post a couple a screen shots.

Is that something I need to apply after each measurement or is there a way to set it to automatically apply the delay at each measurement?

Thanks
Mike C.
ANYTHING that changes the phase (time) will require a new setting.

Moving the mic or the speaker is obvious.  Not so obvious is adjust the bandpass filters, eqs etc.

As a "general rule" you want the phase trace to "fall" at the same point the HF response is falling.

This is not always practical, or actually done.  But it should be the goal.

A "perfect phase trace" for a loudspeaker would be sloping down on the low freq, flat throughout most of the passband, and then falling on the top end.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

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Marc Sibilia

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Re: Tell me what you see in these phase response graphs
« Reply #14 on: January 28, 2018, 10:19:47 am »

Is that something I need to apply after each measurement or is there a way to set it to automatically apply the delay at each measurement?

You can do this on every measurement, but you don't really want to.  Under <Preferences><Analysis>, if you turn off Use Loopback as a Timing Reference, and check <set t=0 at IR peak>, this will be done automatically on every measurement.  Using Estimate IR delay works the same way using the peak of the impulse response to set the delay.

Moving the mic or the speaker is obvious.  Not so obvious is adjust the bandpass filters, eqs etc.

Changing the EQ can change the timing of the peak of the impulse, so you don't really want to use the automatic delay setting, because the phase measurements won't be relative to the same point in time on different plots.  You can manually adjust for the time of flight by going from the <SPL & Phase> to <Impulse>, then under Controls, manually add the same offset to all your measurements <t=0 offset>. 

This is especially important when you are setting up the crossovers and delays and measuring different drivers on separate graphs to try to decide on adjustments to delay.  Without the Loopback timing, there is nothing to relate one graph to another.

I typically work by measuring the HF driver first, using the Impulse Response to set the fixed offset for time of flight, and then using that same offset for midbass measurements and sub measurements.

I don't know of a way to automatically apply that same offset automatically to all subsequent measurements.  But hey, it is an amazingly powerful program worth so much more than we paid for it (free)!

Marc

« Last Edit: January 28, 2018, 10:27:03 am by Marc Sibilia »
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Mike Caldwell

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Ok it was a nice day here in Ohio this afternoon to do some speaker measurements.

Things are looking better with the delay applied to the proper setting!

All EQ was by passed for measurements, the lower frequency stuff
I'm sure was affected by my outdoor surroundings.

Let me know
Thanks
Mike C.

« Last Edit: January 29, 2018, 08:36:21 pm by Mike Caldwell »
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Art Welter

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Re: Tell me what you see in these phase response graphs. Updated measurments
« Reply #16 on: February 06, 2018, 07:40:14 pm »

All EQ was by passed for measurements, the lower frequency stuff
I'm sure was affected by my outdoor surroundings.

Let me know
Mike,

The phase traces look more "normal" now, typical of phase wrap due to crossover filters, but the deep dip in combined response SPL around the crossover range could still be indicative that reversing the polarity, different crossover slopes, or delay time, or a combination may be desirable to flatten both phase and amplitude response.

The combined, non-equalized response of the LF and HF ideally should be greater than either LF or HF measured separately, and can be checked against the response when either are muted (while everything else remains the same) to verify that proper on axis summation is observed.

Cheers,
Art
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Mike Caldwell

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Re: Tell me what you see in these phase response graphs. Updated measurments
« Reply #17 on: February 06, 2018, 11:45:30 pm »

Mike,

The phase traces look more "normal" now, typical of phase wrap due to crossover filters, but the deep dip in combined response SPL around the crossover range could still be indicative that reversing the polarity, different crossover slopes, or delay time, or a combination may be desirable to flatten both phase and amplitude response.

The combined, non-equalized response of the LF and HF ideally should be greater than either LF or HF measured separately, and can be checked against the response when either are muted (while everything else remains the same) to verify that proper on axis summation is observed.

Cheers,
Art

Thanks Art

Here's a few I did a couple days after the others I posted, my fingers were frozen numb after I was done!

I normally flip the polarity on the high freq. and tweak the mid/woofer time delay to find the deepest null point.
After I flip the high freq back to normal I go back and forth a little with the mid/woofer delay watching the amplitude and phase response.
Next time out I'll see what happens trying some different crossover slopes and even some asymmetrical slopes.

Posted is a screen shot of the 15X2 flipped and normal and also shot of a double 12X2 box.

Xandy Whitesel

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Re: Tell me what you see in these phase response graphs
« Reply #18 on: February 07, 2018, 02:33:01 am »


A "perfect phase trace" for a loudspeaker would be sloping down on the low freq, flat throughout most of the passband, and then falling on the top end.

I'd love to hear a more in depth explanation of this statement.  By my understanding both electronic and acoustical manipulations will cause a delay of low end, represented by an upward sloping phase plot towards the low frequency.  How does one achieve a downward slope on the low freq?  Please set me straight.
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Xandy Whitesel
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Phil Graham

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Re: Tell me what you see in these phase response graphs
« Reply #19 on: February 07, 2018, 10:57:45 am »

I'd love to hear a more in depth explanation of this statement.  By my understanding both electronic and acoustical manipulations will cause a delay of low end, represented by an upward sloping phase plot towards the low frequency.  How does one achieve a downward slope on the low freq?  Please set me straight.

Xandy,

You and Ivan are saying the same thing. He's starting at frequency zero and looking to the "right," while you are starting from higher frequencies and looking "left." Because the group delay is defined as -d(theta)/d(frequency), the slope will be down for causal systems (i.e. output proceeds after input) when starting at low frequencies and moving towards higher.

A loudspeaker is inherently a bandpass device (i.e. high pass and low pass in series). So the best case scenario for a loudspeaker, before any additional phase manipulation before the box in a dsp, will be the idealized phase response of an ideal bandpass filter.

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