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Multi-Way Conventional loudspeakers with crossovers

Amount of hornloading in a Synergy horn
Amount of hornloading in a Synergy horn
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Old 4th June 2019, 11:34 PM   #21
weltersys is offline weltersys  United States
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Quote:
Originally Posted by mark100 View Post
Hi Chris, the 11cm measurement is a timed acoustic measurement from the 4594's mid section to the mounting flange, not simply a mechanical path length estimate.
Mark,

Looking back at measurements done on my SynTripP MEH (multiple entry horn), found that though the HF driver acoustical center is physically further from the measurement mic, it's output arrives .62ms before the BP (band pass) LF output. The combined raw output of the two 10" with the 3" diaphragm HF driver, no crossover used at all results in a relatively flat phase alignment as can be seen below.

It's even possible that Chris' MEH might eliminate or reduce the delay required by reversing the mid/high polarity- without doing the math (which I'd probably hose up ) it sounds like the path length plus the .5ms LF delay used may be close to one wavelength at the acoustic crossover point.

Art
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File Type: png HF& LF arrival times.png (323.2 KB, 139 views)
File Type: png SynTripP Raw Freq & Phase.png (171.4 KB, 167 views)

Last edited by weltersys; 4th June 2019 at 11:41 PM.
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Old 5th June 2019, 12:11 AM   #22
mark100 is online now mark100  United States
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Hey guys, with all due respect, you are making something very simple, very murky.

Diyuser, yes finding a subs acoustic center is very difficult, or better said, relatively difficult to full range / HF drivers.
But it's not difficult in the least, not for a compression driver... not at all.

Patrick, no offense but what you're saying has nothing to do with finding acoustic centers..I don't understand...

Chris, 600Hz, group delay, your plot... ALL are totally immaterial to finding the acoustic path length of your driver.. Please, Just go measure it !!!

Last edited by mark100; 5th June 2019 at 12:36 AM.
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Old 5th June 2019, 12:34 AM   #23
mark100 is online now mark100  United States
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Hi ya Art,

Ok, what I've come to be dang certain about...
...acoustic path lengths are physical geometry...they don't vary due to polarity, crossover topology or order,... etc.
Period.

I think I know this because I've found the acoustic centers sooooo many times for various multi-way drivers/sections.
Finding acoustic centers is simple and uncomplicated....just use a full range signal.

People get confused because they try to find electro-acoustical centers, with a crossover (low pass) in place.
Those findings are not acoustic centers, and lead people to all kinds of alignment errors, because they conflict/trash the cornerstone of timing....which is acoustics.
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Old 5th June 2019, 07:13 AM   #24
Cask05 is offline Cask05  United States
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There's no electrical crossover filter delay in the measurements posted for the 4592, above. No "named" crossover filters were used.

I'm honestly not sure what you're worried about.

What do you think phase response is? Group delay?

Chris

Last edited by Cask05; 5th June 2019 at 07:30 AM.
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Old 5th June 2019, 12:48 PM   #25
mark100 is online now mark100  United States
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Originally Posted by diyuser2010 View Post
AES E-Library >> Effects of Acoustic Center Position in Subwoofers

While this paper is pertaining to subwoofers, the same concept applies. The actual acoustic center position is not an arbitrary location ( like the voice coil ) or diaphragm. Only acoustic measurement along with a full duplex sound card will accurately reveal the acoustic center in the far field.

Surprisingly in the paper, all of the methods used ( BEM simulation, spreadsheet prediction, near field scanning of the transducer, and an actual 2 microphone measurement ) placed the acoustic center almost a foot in front of the cabinet.

This can lead to errors in loudspeaker polar measurements if the center of rotation is not the actual acoustic center.

Another good read:

AES E-Library >> On the Movement of a Horn's Acoustic Center
Please accept my apologies diyuser2010 and Patrick,

A bit of wine last evening had me reading your posts far too fast for fair consideration

I'd love to read the linked AES papers, both look very interesting, particularly the surprising acoustic center in front of the sub ..maybe it's about time to look into membership...???

I always use dual channel FFT to find acoustic centers. and have found once properly located, the center can be 'marked on the box' so to speak.
Subs are always a pain to get repeatable measurements, but I think that is more a function of FFT's linear data spacing, and relatively fewer data points down low trying to find peak energy, than anything else.

That said, using FIR, I have found I way to keep distance variability down to a handful of processing samples. Measure a sub's distance with no LPF in place. Then using a FIR filter with a known number of samples delay, implement whatever LPF you like. The new distance to sub's acoustic center simply becomes very accurately, the first measurement plus the FIR sample delay.

Now, because of the paper you linked I'm going to have to get out the tape measure and see where the sub actually is vs measurement !
Thx
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Old 5th June 2019, 01:32 PM   #26
mark100 is online now mark100  United States
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Originally Posted by Cask05 View Post
There's no electrical crossover filter delay in the measurements posted for the 4592, above. No "named" crossover filters were used.

I'm honestly not sure what you're worried about.

What do you think phase response is? Group delay? Chris
Hi Chris, here's my concern...

Everything I can gather from the unity/synergy patents, TD's posts, great threads like 'suitable mid-range', yours, nc535's, Patricks, Bill's, and many others..... ...all say to keep the maximum distance between the mid range ports and the HF acoustic center, within 1/4WL of the crossover frequency (due to the creation of an acoustic low pass filter near said wavelength).

Now I'm trying to do as you and a few others have done, and skip the mids, using a CD that goes low enough to mate with a woofer. But the same 1/4WL rule should apply, shouldn't it?

When I go to determining where to put the ports, by laying out the distance to the CD's acoustic center, it seems to me I need to know exactly where that distance begins (apex/HF acoustic center).

So, to even begin designing, I put a mic even with the mounting flange of my CD, and measure. I get a surprising 11cm. I repeat the measurement at greater distances, backing out tape measure mic to driver distances, and repeatedly get the same 11cm. (That's why I ask you to check yours.....)

When I look at 1/4WL for let's say a 475Hz crossover, I get 18cm port to apex distance to work with. I believe I need to subtract 11cm for the CD's path depth, plus another cm or so for a wooden mounting flange, for a total path of 12cm before even getting to the horn walls. Can this 12cm path be ignored or reduced, and in keeping with the 1/4WL rule?

Because if it can't, I'm down to 6cm of room to try and fit the woofers' ports.

It just doesn't look like it can work...and stay within the acoustic low pass design guidelines.

That's my concern.
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Old 5th June 2019, 07:35 PM   #27
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Not that I dont appreciate all th interesting discussions, I do have another question about my topic.

Would there be acoustic coupling between the 2x 15"ers? Or how does this work in a synergy?

The ports would definitely be spaced more than 1/4th wavelength apart, but could maybe be between 1/2 wavelength so coupling between the drivers could still happen within the horn itself, before there is even horn coupling added. Right?
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Old 5th June 2019, 07:46 PM   #28
weltersys is offline weltersys  United States
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Quote:
Originally Posted by mark100 View Post
Everything I can gather from the unity/synergy patents, TD's posts, great threads like 'suitable mid-range', yours, nc535's, Patricks, Bill's, and many others.....
...all say to keep the maximum distance between the mid range ports and the HF acoustic center, within 1/4WL of the crossover frequency
(due to the creation of an acoustic low pass filter near said wavelength).

When I go to determining where to put the ports, by laying out the distance to the CD's acoustic center, it seems to me I need to know exactly where that distance begins (apex/HF acoustic center).

When I look at 1/4WL for let's say a 475Hz crossover, I get 18cm port to apex distance to work with.

Can this 12cm path be ignored or reduced, and in keeping with the 1/4WL rule?

Because if it can't, I'm down to 6cm of room to try and fit the woofers' ports.

It just doesn't look like it can work...and stay within the acoustic low pass design guidelines.

That's my concern.
Mark,

The relationship of the volume of the enclosed throat chamber, length and area of the cone entry ports determine itís acoustical low pass. The distance of the cone entry ports from the throat of the horn determine a reflective null, a different concept.

There has been a lot of reference to keeping the multiple horn entry locations within 1/4 wavelength from each other at the crossover point, but the MEH output location and itís acoustic center regarding time are two different things. Different HF drivers will have different depths, those depths can be time aligned to coincide with the low pass output of the cone drivers entry points. However, if the location of the multiple entry points are not within 1/4 wavelength, off-axis lobing will start to occur due to path (time) length differences, regardless of on axis time alignment.

Consider that the HF ďplumbingĒ on MEH line arrays, or DSLís Paraline adds somewhere between 8 and 20+ centimeters path length to a HF driverís acoustical center, at much higher frequencies than you are considering, and perhaps your concern over a few centimeters will be reduced.

Art
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Old 5th June 2019, 08:05 PM   #29
weltersys is offline weltersys  United States
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Originally Posted by Droco View Post
1)Would there be acoustic coupling between the 2x 15"ers?
2)Or how does this work in a synergy?
3)The ports would definitely be spaced more than 1/4th wavelength apart, but could maybe be between 1/2 wavelength so coupling between the drivers could still happen within the horn itself, before there is even horn coupling added. Right?
Droco,

1) Yes.
2) Probably best to simulate your design in Hornresp to get a graphical representation of "how it works".
3) Axisymetric ports will result in a 6dB on axis increase over a single driver on axis, ports spaced more than 1/4 wavelength apart will reduce off axis response and smoothness.
What is limiting your port location to more than 1/4 wavelength?

Art
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Old 5th June 2019, 08:27 PM   #30
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3) What is limiting? I do mean the distance between the ports of the seperate 15" woofers. Each woofer would be on 1 side (so symmetrical axis wise), so there would be too much distance between them I think. Not sure yet. My horns arrive this friday, so this will be one of the first things I'm gonna check.
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