Measurements on loudspeakers aimed at designing the crossover network

2slow4u

Registered
Thread Starter
Joined
Aug 12, 2018
Messages
12
I need to measure the individual speakers of an acoustic speaker, already mounted on the baffle, to be able to simulate and design the crossover network. To do this I need to measure the correct phase relationships and delays between the various speakers. I have no experience with REW, is it possible to do what I just explained? If so, what is the correct procedure for making these measurements? If I understand correctly, REW sets t = 0 on the maximum pulse peak, or on the first sample. But in these cases I lose the delay information due to the different speaker offsets, so what I would like to do is not possible. Am I wrong? Thank you!
 

2slow4u

Registered
Thread Starter
Joined
Aug 12, 2018
Messages
12
Hi there!
I don't think I understand, English is not my language.
If I use a loopback, isn't it just that all measures will have the same delay? I, on the other hand, need to grasp the delay between tweeter, midrange and woofer. I need the system to include the acoustic offsets of the various speakers in the measurements. I would need step-by-step instructions on how to do this if possible. Thanks again.
 

2slow4u

Registered
Thread Starter
Joined
Aug 12, 2018
Messages
12
Hi, I'm going back to the topic because I can't find a solution. By measuring the speakers using loopback, I get a noticeable excess of phase:

42617


It's normal?
Then trying to follow this tutorial ...


...I visualize the minimum phase in Virtuix Cad and by introducing a negative delay in the appropriate field, to show measured phase without excess delay as described in the tutorial, I cannot really copy the shape of the minimum phase curve. This is the best result I can get:

42618


Measured phase and minimum phase do not coincide. Where am I wrong?
Thanks for your patience.
 
Last edited:

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
4,460
The timing reference maintains relative timing between your measurements. There will be a delay between the loopback connection and the measurement captures mainly due to the distance from driver to microphone, as explained in the second paragraph of the help I linked:

If a loopback is selected the reference channel signal must be looped back from output to input on the interface and measurements will be relative to the loopback timing. Usually this means measurements will have a time delay that corresponds to the time it takes sound to travel from the speaker being measured to the microphone, which can be removed for subsequent measurements using the Timing offset.

REW can estimate the delay for you using the "Estimate IR delay" button in the graph controls. That would typically be used on the tweeter measurement, the delay that is found can then be applied as a timing offset for further measurements so it is removed when the measurement is made. For existing measurements the delay could be removed using the "Offset t=0" feature in the Impulse graph controls, the same delay must be removed from each measurement to preserve their relative timing. That is much the same as you were doing manually in VituixCAD, though your measurement looks to be inverted, hence the 180 degree phase difference between measured and minimum phase.
 

2slow4u

Registered
Thread Starter
Joined
Aug 12, 2018
Messages
12
For existing measurements the delay could be removed using the "Offset t=0" feature in the Impulse graph controls, the same delay must be removed from each measurement to preserve their relative timing. That is much the same as you were doing manually in VituixCAD, though your measurement looks to be inverted, hence the 180 degree phase difference between measured and minimum phase.
Forgive me if I come back to the subject but I continue to have difficulties. I have two measures, one on the tweeter and one on the midrange, using the loopback and keeping the mic in the same position. Starting with tweeter I have this situation:

42888


I then ask REW to estimate the delay, take note (2.617 ms, to be applied later to the midrange measure), Then I click on "Shift IR" to remove the delay.

42889


I get this:

42890


... but returning to the SPL screen, I find a totally altered FR curve:

42891


Please, where am I wrong?
Thank you.
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
4,460
The problem is the left IR window you have applied. Set it to Tukey 0.25 and set the width to at least 1 ms. What were your reasons for changing it and setting it so narrow?
 

2slow4u

Registered
Thread Starter
Joined
Aug 12, 2018
Messages
12
The problem is the left IR window you have applied. Set it to Tukey 0.25 and set the width to at least 1 ms. What were your reasons for changing it and setting it so narrow?
There is no particular reason for setting the left window so narrow. Rather I preferred to set it to Blackman-Harris 7 because in this way I obtained cleaner and more legible curves. Are there any problems if I use Tukey 0.25 on the left and Blackman-Harris 7 on the right? With these settings I can actually successfully apply your suggestions. Thanks again for your kind advice!
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
4,460
If you wish to use BH-7 on the left set it the same width as the right window. There isn't any problem with having different window types on left and right. The left window has little influence provided the window reference position is correct and the window it is not so narrow that it truncates the response, Tukey 0.25 is a sensible choice for it.
 

sm52

Member
Joined
Mar 14, 2019
Messages
257
John
Is it possible to see how the graphs change when changing the type of the left and right windows without a new measurement? The measurement is done, I change the window in the 'Analysis' section and I want to see what has changed. Need a new measurement?
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
4,460
The Analysis defaults only apply to new measurements. For existing measurements you can change the selections in the IR Windows dialog and use the Apply Windows button.

Choosing window types for an impulse response differs from choosing a window for signal analysis (e.g. in the RTA). Tapering of the IR window reduces the levels of reflections and also reduces the frequency resolution a little (compared to a rectangular window of the same width). The default choice of Tukey 0.25 was made to preserve the response including its reflections and their influence out to the point where noise begins to dominate, then taper it from there.
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
4,460
There have been more than 60 V5. 20 builds since beta 6, if you use a current build you will see the button on the Estimate IR Delay dialog to set the timing offset.
 

2slow4u

Registered
Thread Starter
Joined
Aug 12, 2018
Messages
12
There have been more than 60 V5. 20 builds since beta 6, if you use a current build you will see the button on the Estimate IR Delay dialog to set the timing offset.
Opsss! :greengrin:
If I install the latest build will i lose all software settings?
Thank you.
 
Last edited:

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
4,460
Modal frequencies, based on the room simulator dimensions or dimensions entered in the modal analysis panel of the EQ window. A selection in the graph controls turns them on/off.
 
Top Bottom