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Elliot Thompson View Drop Down
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Points: 5365 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 29 April 2024 at 1:30pm
Originally posted by Earplug Earplug wrote:

An interesting project - but really shows that the physics is difficult and simulations obviously have some severe limitations. Measurement and experimentation is the way - but also, in the end, using a decent DSP unit with several parametric corrections may be the only solution. And generally, the simplest. 


At some point in time, you must put down the simulators and, focus primarily on measurements. It is inevitable if you want to increase your knowledge fundamentally on loudspeaker design.

Best Regards, 
Elliot Thompson
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Originally posted by Elliot Thompson Elliot Thompson wrote:

Originally posted by Earplug Earplug wrote:

An interesting project - but really shows that the physics is difficult and simulations obviously have some severe limitations. Measurement and experimentation is the way - but also, in the end, using a decent DSP unit with several parametric corrections may be the only solution. And generally, the simplest. 


At some point in time, you must put down the simulators and, focus primarily on measurements. It is inevitable if you want to increase your knowledge fundamentally on loudspeaker design.

Best Regards, 



Yes, definitely, but what I meant was that on top of that, you should also expect to have to do some correction/s with electronics! That's been part of the process since day 1!   Smile
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Elliot Thompson View Drop Down
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Points: 5365 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 29 April 2024 at 3:20pm
Originally posted by Earplug Earplug wrote:

Originally posted by Elliot Thompson Elliot Thompson wrote:

Originally posted by Earplug Earplug wrote:

An interesting project - but really shows that the physics is difficult and simulations obviously have some severe limitations. Measurement and experimentation is the way - but also, in the end, using a decent DSP unit with several parametric corrections may be the only solution. And generally, the simplest. 


At some point in time, you must put down the simulators and, focus primarily on measurements. It is inevitable if you want to increase your knowledge fundamentally on loudspeaker design.

Best Regards, 



Yes, definitely, but what I meant was that on top of that, you should also expect to have to do some correction/s with electronics! That's been part of the process since day 1!   Smile






Yes! We are in full agreement. I was just reinforcing what you said. Too many live off of Simulations and Presets. They don't realize such methods are just estimates not guaranteed. 


Best Regards,  
Elliot Thompson
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Originally posted by Elliot Thompson Elliot Thompson wrote:

Originally posted by Earplug Earplug wrote:

Originally posted by Elliot Thompson Elliot Thompson wrote:

Originally posted by Earplug Earplug wrote:

An interesting project - but really shows that the physics is difficult and simulations obviously have some severe limitations. Measurement and experimentation is the way - but also, in the end, using a decent DSP unit with several parametric corrections may be the only solution. And generally, the simplest. 


At some point in time, you must put down the simulators and, focus primarily on measurements. It is inevitable if you want to increase your knowledge fundamentally on loudspeaker design.

Best Regards, 



Yes, definitely, but what I meant was that on top of that, you should also expect to have to do some correction/s with electronics! That's been part of the process since day 1!   Smile






Yes! We are in full agreement. I was just reinforcing what you said. Too many live off of Simulations and Presets. They don't realize such methods are just estimates not guaranteed. 


Best Regards, 




Yup.

Only things guaranteed are death - & taxes!!   LOL LOL LOL
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fudge22 View Drop Down
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Points: 263 		Post Options Post Options   Thanks (0) Thanks(0)   Quote fudge22 Quote  Post ReplyReply Direct Link To This Post Posted: 01 May 2024 at 9:41pm
Quote Yes, definitely, but what I meant was that on top of that, you should also expect to have to do some correction/s with electronics! That's been part of the process since day 1!

I’m guessing that you never designed a horn whose length was an integral number of half wavelengths at the crossover frequency. That said, I agree that electronic correction is a valid method to get the response required.

Quote At some point in time, you must put down the simulators and, focus primarily on measurements. It is inevitable if you want to increase your knowledge fundamentally on loudspeaker design.

The original poster did put down the simulators, made his design, and took some measurements. Now he is stuck and remeasuring is unlikely to solve the problem or increase his knowledge.

Whilst measurements are an important part of any design process, at some point you need to understand the theory behind the simulations so that you can have any hope of understanding the measurements.

With regards the OP’s problem, it might be worth him revisiting the JBL paper that he linked to in the first post, and try to figure out the difference between the CMCD-81H and the CMCD-61H. The former exhibits the same dip in response, that his own design does, at just over 2KHz, but the latter does not.

Other things to consider, which may or may not have been accounted for in the simulation (so feel free to disregard any or all of the following), and may have a large or negligible effect on the response.

Does the model assume that all the sound passes through the nearest port/slot in the phase plug? If so, sound passing across the chamber and exiting the multiple slots will cause dips in response not predicted.
Take into consideration the mass of the air in the ports, which will act as a series inertance with some added acoustic resistance.
Treat the diaphragm as a simple rigid piston.
Consider the speed of wave propagation through the diaphragm material.
Does the frequency/depth of the notch change when you measure the response off axis?
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Elliot Thompson View Drop Down
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Points: 5365 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 02 May 2024 at 3:16am
Originally posted by fudge22 fudge22 wrote:


Whilst measurements are an important part of any design process, at some point you need to understand the theory behind the simulations so that you can have any hope of understanding the measurements.


Understanding the limitations of a Simulator will lead you to wards real-world measurements. Simulators are based on the perfect scenario. Real-World Measurements are based under real-world conditions.



Best Regards,


Elliot Thompson
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Earplug View Drop Down
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Originally posted by fudge22 fudge22 wrote:

Quote Yes, definitely, but what I meant was that on top of that, you should also expect to have to do some correction/s with electronics! That's been part of the process since day 1!

I’m guessing that you never designed a horn whose length was an integral number of half wavelengths at the crossover frequency. That said, I agree that electronic correction is a valid method to get the response required.


Haha, no - nothing that complex, but I did design a small system based roughly on the Martin F2 (1 x 18" subs & 2 x 12" + 2" mid tops). My idea being to make something lighter & easier to move. My main system uses Martin Philis. Lovely cabs, but very heavy.

I have a good carpenter friend here, now unfortunately retired, that literally had years & years of experience building cabs that helped me - and his (practical) experience was essential. Along with SMAART, of course!   LOL

That system actually turned out very good and I still use it all the time. Amazing SPL (and sound quality) for it's size.  Smile
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JulianDA View Drop Down
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Points: 156 		Post Options Post Options   Thanks (0) Thanks(0)   Quote JulianDA Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2024 at 4:17pm
I finally have time to post more of this project. I printed 2 more phaseplugs with slightly different entry points of the outer ring (shifted +2mm and -2mm) The simulations showed a massive difference, but in practice it was almost nothing.
I think the reason ist the real behaviour of the membrane in comparison to the assumed stiff piston movement in the sim.
The resulting responses are the following:


the green curve is the one with the entry shifted +2mm to the outside. I deemed it the best.
This is the same response of that pp but with added phase:


there is clearly something wonky happening at ~2kHz but with only 3 eq points i get the following response and phase:



is it fine to use it as it is now? to me it looked like destructive interference inside of the phaseplug that caused the 2kHz dip. But is it really a problem, if it is easily solvable with eq and the corresponding phase response is also good?
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Teunos View Drop Down
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At some point theory and practice start to deviate as models (i.e. simplified versions of reality) become sufficiently flawed.

Since you measured in REW, can you show the Decay plots (with sufficient spacing)?
That should show any possible resonances/standing waves as being the cause of your frequency response. Impedance and distortion measurements would be valuable as well.

Regardless, as long as you don't heavily gain up the null at 2kHz, using 3 (minimum phase IIR) EQ points that simultaneously flatten the phase response i.m.o. is perfectly acceptable and something you should always do prior to reverting to FIR filters.
However, if the cause of the null is physical and you can solve it by changing structural features instead of compensating for it electrically, you probably should. Guessing your knowledge level from your handiwork i guess you already knew that.
Best regards,
Teun.
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Teunos View Drop Down
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Originally posted by JulianDA JulianDA wrote:

Originally posted by Teunos Teunos wrote:

The project looks great, kudos for the work done.
You write the throat is 80mm 50cm^2, consider that a standing wave in the throat would have its first null at ~2140Hz, which ligns up almost exactly with the first measured null.
Thanks :) 
Yes, a standing wave at 80mm would have a null at that frequency. That could be a possibility. But i have to ask why there should be a standing wave there and not at all other parts of the horn with different dimensions? Would it just be because of the miniscule part of the throat were the outer walls change from a negative to a positive expansion and create a zero expansion segment by diong that?
Would the only effect of the plug-extension be then to block the path for this particular standing wave until the outer walls of the horn start to expand?

Your horn acts as acoustic impedance matching device. The throat will show a large discontinuity as expansion suddenly increases tenfold. Therefore the throat is by far the most sensitive to generate diffraction or excite modal resonances, regardless of their directional preference (down or across the throat).

My, and probably others' who replied the same (TimeBomb or Marjan for example) is that extending the phase plug into the horn might ease the transition and smear the effects over a bigger area. This won't alleviate the problem fully but at least lower the Q of the effects.


Edited by Teunos - 19 July 2024 at 9:34am
Best regards,
Teun.
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JulianDA View Drop Down
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Points: 156 		Post Options Post Options   Thanks (0) Thanks(0)   Quote JulianDA Quote  Post ReplyReply Direct Link To This Post Posted: 19 July 2024 at 11:13am
Thanks for your input Teunos! 

Originally posted by Teunos Teunos wrote:


Since you measured in REW, can you show the Decay plots (with sufficient spacing)?
That should show any possible resonances/standing waves as being the cause of your frequency response. Impedance and distortion measurements would be valuable as well.

Surely i can post those graphs :) what would be "sufficient spacing" for your liking? I am still kind of a novice regarding decay and waterfall plots. I mean i can tell when things are looking really bad, but thats the extend of my knowledge :D

Originally posted by Teunos Teunos wrote:

Regardless, as long as you don't heavily gain up the null at 2kHz, using 3 (minimum phase IIR) EQ points that simultaneously flatten the phase response i.m.o. is perfectly acceptable and something you should always do prior to reverting to FIR filters.

I think i have to row back on my statement regarding the eq´s and their effect on the phase... all i did yesterday was playing around with my raw measurements in REW and using the build-in eq function to simulate what can be done. 
In my opinion it is a very valuable function that is surprisingly accurate. But if my issues in frequency/phase response are from resonant effects inside of the phaseplug things will not be as easy as REW makes it seem. I will have to test my eq points again with real measurements, but i guess the result will be less than ideal.

Originally posted by Teunos Teunos wrote:


My, and probably others' who replied the same (TimeBomb or Marjan for example) is that extending the phase plug into the horn might ease the transition and smear the effects over a bigger area. This won't alleviate the problem fully but at least lower the Q of the effects.


I think i will just do a rather quick and dirty change to my phaseplug design to extend the central part while keeping the flare rates of both channels more or less equal and test that in the next measuring session :) 
My concern with expanding the middle of the plug was always that the flare rate of the plug will be lower. As far as i know a high flare rate is necessary to achieve low throat distortion.
Making the exit diameter of the whole thing bigger to account for that would also be less than optimal for the achievable high frequency dispersion characteristics.

Accoustics is complicated stuff LOL
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JulianDA View Drop Down
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Originally posted by Teunos Teunos wrote:

My, and probably others' who replied the same (TimeBomb or Marjan for example) is that extending the phase plug into the horn might ease the transition and smear the effects over a bigger area. This won't alleviate the problem fully but at least lower the Q of the effects.


I think i will just test a different centerpiece of the plug that is extended 2cm into the horn at an diameter of 2cm. This is the best i can do without changing the entire plug geometry which is a real PITA Confused



Do you think this will be enough to see if it has any effect?
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