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Multi-Way Conventional loudspeakers with crossovers

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Old 2nd December 2018, 08:15 AM   #321
Paul Vancluysen is offline Paul Vancluysen  Belgium
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Augerpro:

In your method the driver responses must have the exact extra phase difference which is caused by the AC offset that has to be determined.
But the problem I see with it, how you will obtain the individual responses with that correct phase difference caused by the AC offset. If it is not possible in the measurement equipment to set the start of the FFT at the same absolute time w.r.t. ‘time 0’ for both driver measurements, the phase difference due to AC offset will not be correct.

I think It can be done as follows. It is the same almost like I describe it in post 317, but easier.

First measure both single driver responses (mic between both drivers centers), and for each measurement, place the start of the FFT just before the impulse start to create the frequency response, SPL and phase, for both. These are responses that have a phase that is equal to the AC position phase (minimum phase to be expected).
Import in software, reverse the tweeter, calculate the sum response and find the offset of the tweeter for maximum cancellation.

Second, find the same cancellation but now by placing the drivers in parallel and shifting the microphone in vertical direction. The difference between this offset and the offset of the first measurement is the AC offset between both drivers.

I think it must be possible to find the same kind of cancellation with both measurements, also with a not so flat phase.

Last edited by Paul Vancluysen; 2nd December 2018 at 08:31 AM.
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Old 2nd December 2018, 03:57 PM   #322
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Paul Vancluysen View Post
Matthias,

About your measurement procedure, it can work, but I think it has to be done as follows.

At first you have to know how much the reversed tweeter has to be moved backwards in an AC aligned condition for maximum cancellation.

At second you measure how much the reversed tweeter has to be moved backwards in the actual AC condition for maximum cancellation. That is what you have done now.
Hmm, I don't quite understand the difference between the two things. But yes, my method is not as good as it could be. I guess we should just forget about my not-so-great attempt...

I have now added the loopback compensation (or "dual channel setup") to my setup, which completely removed the inconsistencies of the timing in the impulse response measurement. With this, I measured the impulse response of the tweeter and the mid, with the microphone exactly 1m away from the baffle and at the height of the vertical center of the tweeter and the mid. The data are shown in the attachments, and the tweeter impulse arrives at the microphone 0.029 ms later than the mid impulse (determined from the excess phase slope).

This corresponds to an offset of the acoustic centres of about 10mm. That's not too bad. Is it good enough, or should we try to further optimize that by moving the mid backwards? Sounds like a complicated baffle or a custom waveguide assembly for the VM752, and I am not sure we'd gain a lot from this.
Attached Images
File Type: png ac_IR.png (31.2 KB, 227 views)
File Type: png ac_SPL.png (31.5 KB, 172 views)
File Type: png ac_exphase.png (29.6 KB, 172 views)

Last edited by mbrennwa; 2nd December 2018 at 03:59 PM.
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Old 2nd December 2018, 04:22 PM   #323
EarlK is offline EarlK  Canada
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Open Source Monkey Box-ac_ir-png

This image clearly shows the mid-range impulse is delayed compared to the tweeter's impulse.

ie; it's Acoustic Center is behind the tweeter's AC .

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Old 2nd December 2018, 06:07 PM   #324
mbrennwa is offline mbrennwa  Switzerland
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The peak-max or the peak-center of the mid is indeed later than the tweeter. This is simply because the impulse response of the mid is wider than that of the tweeter, which reflects the fact that the mid does lower frequencies than the tweeter. What counts is the BEGINNING of the peaks (i.e., when they arrive at the microphone). The attached zoom-in shows that the mid impulse arrives BEFORE the tweeter.
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Old 2nd December 2018, 06:17 PM   #325
EarlK is offline EarlK  Canada
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Is this corroborated ( roughly supported ) by the relative locations of these driver's voice coils ?

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Old 2nd December 2018, 08:26 PM   #326
augerpro is offline augerpro  United States
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That's really really close to the same start. How deep is the waveguide in this?
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Old 2nd December 2018, 09:25 PM   #327
Juhazi is online now Juhazi  Finland
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Voice coils don't pump air molecules, the driver membrane does. And it is quite a difficult to say the plane of a dome or a cone, so acoustic measurement with loop-back timing is the only exact way to define that. At least mid and tweeter should be measured in the baffle from same mic location. A two-channel soundcard with loopback wire at channel 2 to lock timing. Looking at reverse null and summing or system's step is another way, but it can not be used for design software.

Doing simulated design is just simulation with some tolerance anyway, helps you to get on the map and avoid disasters. Final tweaking must be based on carefully performed acoustic measurements and auditioning, that's the salt and pepper of diy!
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Last edited by Juhazi; 2nd December 2018 at 09:28 PM.
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Old 3rd December 2018, 07:16 AM   #328
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Juhazi View Post
My baffle is 30cm wide with rounding radius 4cm, speaker in midline. Your test baffle looks narrower, and the dip is at same F left/right - doesn't match with baffle effect!

My guess is that the Volt's steep waveguide-like edges make it disperse like a 6" woofer. That dip is in factory datasheet too.
You are right. Assuming the SPL chart in the factory datasheet was measured on an IEC or similar "large" baffle, the dip at 2.2 kHz cannot be related to the baffle. The dip does get less off-axis, so I don't think it's a mechanical resonance within the dome/suspension. 2.2 kHz corresponds to a wavelength of 15.6 cm, which is about twice the dome diameter (7.8 cm). Is this a reflection/suck-out within the waveguide?

(P.S.: the dimensions of the Monkey Coffin prototype baffle are here. It is 36.8 cm wide, so a bit more than your example)

Quote:
Originally Posted by augerpro View Post
That's really really close to the same start. How deep is the waveguide in this?
The modified WG148 is about 19 or 20 mm deep (measured from the mouth plane to where the throat meets the surround). The VM752 waveguide is about 28 mm deep.
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Old 3rd December 2018, 09:06 AM   #329
Paul Vancluysen is offline Paul Vancluysen  Belgium
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Quote:
Originally Posted by mbrennwa View Post
Is it good enough, or should we try to further optimize that by moving the mid backwards?
It is ok now. Btw, what is the sample frequency of your measurement system?

Designing a X-over now with amplitude perfect on target, the phase of the tweeter will lag a little, being 26 degrees at 2500 Hz. And this all is only true if the drivers are perfect minimum phase, which is also not the case at the bandwidth limit of the drivers maybe.
Designing the X-over a little off target outband also can improve phase behavior.

Last edited by Paul Vancluysen; 3rd December 2018 at 09:15 AM.
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Old 3rd December 2018, 09:27 AM   #330
Juhazi is online now Juhazi  Finland
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Quote:
Originally Posted by mbrennwa View Post
You are right. Assuming the SPL chart in the factory datasheet was measured on an IEC or similar "large" baffle, the dip at 2.2 kHz cannot be related to the baffle. The dip does get less off-axis, so I don't think it's a mechanical resonance within the dome/suspension. 2.2 kHz corresponds to a wavelength of 15.6 cm, which is about twice the dome diameter (7.8 cm). Is this a reflection/suck-out within the waveguide?
---
Funny that Volt's waveguide diameter is 158mm, matches quite well with 2,2kHz wavelength 156mm! Cone diameter looks irrelevant. And because the profile is so deep without rounding, this efffct gets so bad.

The 2" Volt has diameter 109mm and dip at 3,2kHz (107mm) but for some reason there is lots of smoothing in the datasheet measurement...

Jeff Bagby tested ATC75-150S, it doesn't show dip and has rounded edges in wg.
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Last edited by Juhazi; 3rd December 2018 at 09:36 AM.
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