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Discussion Starter · #1 ·
Having an interest in both acoustics and saxophone repair. I conducted a study to accurately measure the "porosity" of several different brands and styles of pads. The photos below show the apparatus used, the measuring device, and the data gathered.






The chart below shows the amount of air pressure lost by each pad at both 2"H2O pressure, and 4"H2O pressure with 206 grams of weight on the key cup. The pads were pressure fit into the key cup and not glued in. The brass tube was constructed to closely simulate a saxophone tonehole.


The next measurements were done by Dr. Pauline Eveno who you may know from her study of saxophone pad resonators. They were intended to measure the acoustic losses of each of the pads.
The results were such that they defied analysis. Dr. Eveno concluded that the resonant frequency of the brass tube may have skewed the results. Before we could do further tests with a longer brass tube with a lower resonant frequency she became busy with her SYOS project making mouthpieces and was forced to withdraw from this project.



The porosity tests clearly show that untreated "roo" pads are more porous than regular pads with some type of "waterproofing" when tested at 2" H2O and 4" H2O pressure. The "acoustic" measurements failed to show that there is a clear relationship between pad porosity and acoustic losses and that more work needs to be done in this area.

Subsequent to this study I was able to purchase a Mic i436 acoustic measurement microphone and the software it uses to measure sound pressure in decibels shown in the photos below.




With this equipment I was able to measure the maximum sound pressure in an alto saxophone just below the neck tenon. That measured 130 decibels. What I found interesting is that when converted to pressure in inches of H2O, 130 decibels is equal to .25" H2O. I haven't retested the pads at .25" H2O pressure since that discovery since they are still in Paris with Dr. Eveno and I haven't replaced them.
 

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I'd be interested to see the equation and unit conversion to get from sound pressure level to vacuum.

Regarding acoustic response: I expect that the surface roughness of the pad will have a much greater effect than the porosity. Size and distribution of the surface defects will affect acoustic reflection regardless of whether the pores are closed at the back side or not. If you seal pores at the surface of the pad, you are changing the surface topology, again regardless of the porosity.
 

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Interesting. I know a tech that told me confidentially that Roo pads were very porous compared to regular tan skin pads. I guess this experiment supports that. Whether or not that makes any discernible difference in tone is the question I suppose. It seems at least plausible to me that it could effect responsiveness.
 

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Discussion Starter · #4 ·
I'd be interested to see the equation and unit conversion to get from sound pressure level to vacuum.

Regarding acoustic response: I expect that the surface roughness of the pad will have a much greater effect than the porosity. Size and distribution of the surface defects will affect acoustic reflection regardless of whether the pores are closed at the back side or not. If you seal pores at the surface of the pad, you are changing the surface topology, again regardless of the porosity.
This is the site I used to convert 130 Db to 63.25 Pascals. This unit calculator to convert 63.25 Pascals to .25" H2O.

Here is a link to Arthur Benade's comments on acoustic losses due to wall material. Most of the energy of the soundwaves whose frequencies are above the cutoff frequency (around F#3 on the saxophone) is not lost at the walls (or pads) of the instrument since those sound waves go straight out the bell and in Benade's words "don't see" the open toneholes. :bluewink:
 

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The only interest in porosity might be regarding air loss in a closed pad, which would need literal holes in it with a transfer system to vent the air through the pad to the area outside the tone hole rim before it became a problem. Obviously, this is never going to be the case so the study has no point regarding air leakage.
Acoustically, we know Tone Boosters work so common sense would tell you that a sax without them is not going to have the presence of the same sax with them, which was a huge selling point when Selmer developed them. This is because leather and fish skin pad covers absorb more of the higher-frequency sound than a hard material like plastic. Assuming you use Tone Boosters in your sax, which take up most of the area of the tone hole, the type/porosity of the pad material is most likely of no consequence to your sound.
 

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Discussion Starter · #6 ·
Just a quick update. While I am still looking for an acoustics lab willing to do "acoustic absorption" measurements of pads with different measured porosity, I have been able to start the process to perform a "non-scientific" double blind test using "nearly identical" saxophones---one with "Roo" pads installed and one with traditional leather pads. In this test the same neck will be used on each saxophone, and neither the players nor the listeners will know which sax is being played in randomly selected trials. After each trial, the player will be asked to compare that "saxophone" with the one previously played in terms of tone, response, resistance, etc. Each of the listeners will be given a similar questionnaire to compare the sound of the trail to the previous one. Any ideas or suggestions would be welcome.
 

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Hi!
The blind study you explain is also scientific!
Did you get the chance to do it? Can you share the results?
 

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I'd be interested to see the equation and unit conversion to get from sound pressure level to vacuum.

Regarding acoustic response: I expect that the surface roughness of the pad will have a much greater effect than the porosity. Size and distribution of the surface defects will affect acoustic reflection regardless of whether the pores are closed at the back side or not. If you seal pores at the surface of the pad, you are changing the surface topology, again regardless of the porosity.
I was thinking the same things. I'd think surface finish, and combined hardness or spring stiffness of the leather stretched over the hard felt, including the hardness of the felt, would be major drivers. My intuition, is that a harder, stiffer substrate would reduce lower frequency losses from the standing waves, and the hardness of the leather coating would also reduce higher frequency losses. I'd think of the lower frequencies as the note fundamentals (the frequency associated with the pitch played), and the higher frequencies as the harmonics (overtones that are multiples of the pitch). Looking at Eveno's plots, there is a healthy divergence even in the low frequencies. The way several curves having combing, I'd consider there is coupling with natural frequencies of the pad/cup itself, amplifying/cancelling at resonant frequencies.

I wonder if the difference from greatest to least as shown in the plot is that audible? I've tried to observe differences in pads with and without metal and plastic resonators from actual practice, deciding there is no detectable difference? Perhaps the plot is shifted or exaggerated in scale? I thought it was a consensus (mostly) that resonators don't significantly affect tone or response? Am I mistaken?
 

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Discussion Starter · #10 ·
Hi!
The blind study you explain is also scientific!
Did you get the chance to do it? Can you share the results?
I'm not sure as an "amateur" I would be able to recreate the "controls" required for a legitimate scientific study. However, I am going to use your research as a model to follow. I am still in the process of acquiring the saxophones and setting up the test with a local university. My work load as a repair tech has increased so it is hard to find the time to follow through on things I want to do. Maybe if I raise my prices??? :)
 

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Taking things to an extreme, just for illustration, and as a diversion from the "head hurting"...
Consider a pad whose surface is a lot more porous than porous leather, indeed, having no leather over the felt.
Many of us have played a sax where there is a pad like this at the upper end of the instrument. It has a serious "muffling" effect, especially for the low notes.
Have several pads like this and the sax becomes unplayable.
 

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Just a quick update. While I am still looking for an acoustics lab willing to do "acoustic absorption" measurements of pads with different measured porosity, I have been able to start the process to perform a "non-scientific" double blind test using "nearly identical" saxophones---one with "Roo" pads installed and one with traditional leather pads. In this test the same neck will be used on each saxophone, and neither the players nor the listeners will know which sax is being played in randomly selected trials. After each trial, the player will be asked to compare that "saxophone" with the one previously played in terms of tone, response, resistance, etc. Each of the listeners will be given a similar questionnaire to compare the sound of the trail to the previous one. Any ideas or suggestions would be welcome.
I know this is now more than two years old, but I just found it.

Did you ever finish carrying out this experiment?
 

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Yes, I've seen the Storher video that you linked. While I generally like Matt's videos, this one doesn't answer the question that @saxoclese's experiment was intended to answer (i.e., "can a blinded player reliably discriminate between the playing characteristics of pads with known porosity differences?").

In addition, note that the @saxoclese post that started this thread already contains much more concrete and quantitative information (than Matt's video) about the effects of pad porosity on air leakage when mounted in an actual pad cup.
 
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