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Old 1st March 2021, 03:59 AM   #321
DonVK is offline DonVK  Canada
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Hi David

I still think it would be extremely difficult to get a consistent profile trying to blind fill a 2.5m tube of 2.5cm dia. I'm still not convinced it would help me.

I agree, there is no need for the SWR and it is still a PWT.

I don't know the details of how REW processes samples and I don't know what the error term is. I'll think about it some more because, I don't know. Do you have an experiment in mind to prove/disprove this? or alternate measurement s/w ?

Last edited by DonVK; 1st March 2021 at 04:11 AM.
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Old 1st March 2021, 04:43 AM   #322
DonVK is offline DonVK  Canada
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Default Acoustic impedance tube

Another piece of test gear I've wanted is for measuring material sound absorption coefficients (damping). It's based on ASTM E1050-10 or ISO 10534-2 and there are a few published projects to create one without needing to buy the official publications. I'm hoping it will be sufficient using these simple materials and better than guessing. End goal is for BEM models.

This one is based on a 75mm dia x 1m long tube with a 20cm sample chamber. These impedance tubes typically use 2 mics placed in 2 of 3 positions for 2 freq ranges. I currently only have one set of mic positions for the [300Hz->2500Hz] range but another mic hole will provide [80Hz->700Hz] .

The two electret mic modules are from a previous project. The local preamp gain and filters have been modified to extend the bandwidth. They run on batteries to reduce noise and prevent ground loops. The max output is ~1Vrms (line level) intended to be connected to the stereo LineIn of a sound card. A PC83-8 full range driver is a perfect fit for the 75mm tube. The total cost is ~$50 CDN if you had to buy all the parts.

At this point the sample chamber plunger still requires a seal and there is some programming (scripting) required in Octave to process the data.

These are some good free docs that describe impedance tubes at:
https://pdfs.semanticscholar.org/540...4a47b94d4e.pdf
http://web.engr.uky.edu/~dherrin/Ban...dance_Tube.pdf

.
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Old 1st March 2021, 05:11 AM   #323
Dave Zan is offline Dave Zan  Australia
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Hi Don
Quote:
Originally Posted by DonVK View Post
I still think it would be extremely difficult...to blind fill a 2.5m tube of 2.5cm dia.
I do plan to use a clear acrylic or polycarbonate tube so I won't have to do it literally blind.
I think we are not "blind" because the impulse response will tell how the absorption behaves, Time Domain Reflectometry with acoustic waves!
I am not sure how consistent the profile needs to be - provided we absorb the back wave then the profile details should make little difference.
Or that's my assumption behind why I think (hope?) it shouldn't be too difficult.
But yes, easy to propose when I haven't actually done it yet.

Quote:
I don't know the details of how REW processes samples and I don't know what the error term is. I'll think about it some more because, I don't know. Do you have an experiment in mind to prove/disprove this? or alternate measurement s/w ?
I don't know the details of REW either, luckily the author answers questions on-line so we have that option.
I believe the sample time/resolution trade-off is classic theory but it's not clear to me that it's impossible to do better - so I will try to think of an experiment.
Thank you for this reply, it has made me think too!

Best wishes
David
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Old 1st March 2021, 12:34 PM   #324
hifijim is offline hifijim  United States
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Quote:
Another piece of test gear I've wanted is for measuring material sound absorption coefficients (damping).
Data from this test will be quite informative, and quite useful. A lot of the absorption data on fiberglass, mineral wool, etc., was taken a long time ago, and the exact test methodology is unclear. It would be nice to have a consistent set of data for all the common MODERN materials we use today... at varying densities.

As always, I find your postings to be very valuable.

j.
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Old 1st March 2021, 02:07 PM   #325
DonVK is offline DonVK  Canada
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Thanks.

That's exactly the issue I've been having. I need curves that show the absorption vs density vs freq. That information is always hard to find and impossible to find for composite stacks of material.
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Old 16th March 2021, 03:43 AM   #326
DonVK is offline DonVK  Canada
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Default Acoustic impedance tube - processing the data

The scripts that generate the test signal and process the mic data are nearly finished

Pics [1,2,3] : I'm using the transfer function method, so I need a pseudo random test signal but it also needs to be bandwidth limited because the current tube and mic positions are suitable for 300-2500Hz. I used a digital FIR 255tap that attenuates ~ -20dB at the tubes corner freqs. Pics below show the filter (pic1), the random signal before/after (pic2)and the bandwidth limited FFT spectrum (pic3). I can capture sufficient data in just a 1 sec burst.

The transfer function and reflection factor is calculated based on the published paper formulas using the mic data's complex FFT, physical spacing distances and complex attenuation factor. I used the complex attenuation factor because the tube length exceeds 3x diameter and its claimed to be more accurate.

Pics [4,5] : I also made a "virtual" tube, in Octave, to test the processing scripts. This allows some ideal test cases, like no reflections (pic#4), and LP digital 2nd order IIR attenuation of reflections to test frequency dependencies (pic#5).

Pics [6,7] : The first attempt with the physical tube was just to verify the drive (via headphone) and microphone preamp levels (via Line in). I packed 10cm of poly in the sample chamber and took a few measurements to verify that I had enough dynamic range. I still need to calibrate the mic differences, and add a seal to the plunger (tube leaks), and add frequency averaging to smooth things. However, for a test flight, it appears to be working. There is a frequency dependent standing wave in the tube (pic6, FFT), and the attenuation increases with freq (pic7). Real measurements will also include the sample's mass and volume to get its density to produce material curve sets (absorption vs density vs frequency).

If there is interest, I can spin out this topic to a new thread and publish some data for damping materials.
.
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Old 16th March 2021, 03:50 AM   #327
fluid is offline fluid  Australia
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Quote:
Originally Posted by DonVK View Post
If there is interest, I can spin out this topic to a new thread and publish some data for damping materials.
Good progress I'm sure the data would be valuable, maybe an ABEC cardioid optimization will become more practical soon
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Old 16th March 2021, 04:01 AM   #328
DonVK is offline DonVK  Canada
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I have a few designs in mind that require more accurate damping models than "medium" .

Cardiods are on the list
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Old 26th March 2021, 01:33 AM   #329
DonVK is offline DonVK  Canada
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Default acoustic impedance tube - test samples

The tube was modified with a seal for the sample plunger, another port added for a 3rd mic position, and a brake for the sample plunger. Most of the damping material is "springy" if compressed, and the brake holds the plunger position (volume). The 3rd mic port is to support 2 freq ranges and provides a 3rd port to relieve static pressure when the plunger is moved. Pics#1,2. The s/w also had some frequency averaging and some smoothing added.

The tube range (on high) should theoretically be [300,2500] however I have noticed measurements are clean from [340,2300] which may be related to the mic size or their acoustic centers. Not a problem for me.

First material test run was with 13g (measured using a postal scale) of polyester fill, and varied sample volume to change the density. Pic#3 has the reflection curves and Pic#4 has the absorption curves for various densities. The curves only show magnitude but the coefficients are complex and carry phase information as well. Sufficient for modelling.

A sample of acoustic tile (USG-Radar-2120) was also measured, because it had some published data (but only 4 data points) and had fixed volume (thickness). Acoustic tiles are measured using a different ASTM spec , but the absorption figures should be "ballpark" correct in the impedance tube.
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Old 26th March 2021, 09:44 AM   #330
docali is offline docali  Germany
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Hi Don,

imho this very interesting approach definitively deserves it's own thread
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