Tech/maintenance Saxophone Pad Acoustics Study

[aldevis]

I believe you are correct that a sine wave sweeping through all the frequencies was used. Actually the measurement was of the "acoustic impedance" of a brass tube with a loudspeaker on one end and the pad at the other. The "acoustic absorption ratio" is then deduced from the impedance measurements as far as I can tell.

Will there be a follow up with more complex waves?

 

[abadcliche]

a sine wave? Why not just play sax into a mic and put that into the loudspeaker? That would surely give a more informative result.

This is why I get annoyed at acoustics studies. I used to be a believer until I went to a leading acoustics lab and saw that the horns they had used for famous studies were junkers full of leaks.

 

[jbtsax]

Perhaps these images will help to clarify the concepts. The first is a simple diagram of the type of apparatus used to measure the acoustic absorption.



The second is the comparable graph in Dr. Eveno's first study showing the impact of a pad cover (resonator) on acoustic absorption.

 

[aldevis]

The second is the comparable graph in Dr. Eveno's first study showing the impact of a pad cover (resonator) on acoustic absorption.

This is pretty impressive too: it seems that a plastic pad dampens the 3600hz while a metal the 3000
The point about a sweeping sine wave vs. a complex wave is that it would be great to see if that dampening occurs anyway (in theory yes, but I am not sure at all).

 

[jbtsax]

This is not surprising to me. It matches with my experience, though people have been telling me for years I'm crazy. You can even change the response of a horn just by coating the pads with a layer of lacquer! Pad porosity is a leak. You might like the "darkening" of the sound that comes as a result (though personally I'd call it "deadening") and that's your choice, but it IS a leak.

As far as why frequencies above the cutoff are also responding, I would assume its because they are harmonics and the fundamental that created them is being drained of energy. Cut the bottom 2 inches off a ladder, the top rung is also 2 inches shorter.

I wonder about the pressure conversion from decibels to inches of water. If a low C# can be vibrated open easily on almost any leak-free horn, then the pressure must be enough to do so, and I'd wager that is more than the weight of a 1/4" column of water the size of the tonehole. And its easier to vibrate open a key with a subtone, which isn't as loud, so perhaps decibels aren't the do-all measurement for what pads undergo during playing.

Whether pad porosity is the same as a direct leak through a pad to the outside atmospheric pressure is a question that this study is trying to address. The frequencies above cutoff are not a part of the "regime of oscillation" as Benade calls it and do not share energy with the fundamental and lower harmonics. In this respect Benade says they are a drain on the system. Benade's Fundamentals of Musical Acoustics is a great source of information. Furthermore these frequencies above cutoff "do not see" the open toneholes and travel straight out the bell. That is why the classical "donut mute" in the bell produces a darker tone because it acts as a filter. It does not take energy from the standing wave.

Well, the graph above shows a *decrease* in the richness of the tone the more absorptive the pads got.

Old saxes typically came out of the factory with pads that had been treated- most had been sprayed with lacquer, some even had metal foil behind the pad skins, and by and large the skins were not very porous (even after all these years, especially ones that had a lacquer coating).

Unless you want to sound like you are playing from behind a closed door, you don't want porous pads.

The majority of pad from vintage saxophones I have restored have had untreated pads. It is easy to check by putting a drop of water on the surface and wait a few minutes. If it remains in a bead, the pad has been treated. If it turns into a "wet spot" then it is untreated leather. It is also significant to note that the earlier saxophone had no resos at all, just a rivet in the center. I have played on white roos over 10 years in concert settings and on gigs and no one has ever told me I sound like I am playing behind a closed door. 🙂

And cool study, JBT. Thanks for sharing. Maybe people will start believing that pad porosity matters now.

It might be best not to "jump to conclusions" on this topic just yet. There are still a lot of questions to be answered since this study is a work in progress. I would like to do what Dr. Eveno did in her resonator study and get two saxophones one serial number apart and have them repadded one with roo pads, and one with completely non-porous pads both with identical resonators and have them played by 6 or more accomplished players in a double blind study.

a sine wave? Why not just play sax into a mic and put that into the loudspeaker? That would surely give a more informative result.

This is why I get annoyed at acoustics studies. I used to be a believer until I went to a leading acoustics lab and saw that the horns they had used for famous studies were junkers full of leaks.

Play a sax into a microphone and you have the variable of the player, the resonance of the room, and many other factors that cannot be controlled. This is why scientific studies are conducted the way they are---to remove and control variables so that the outcomes are accurate and repeatable. Surely you are not dismissing all scientific acoustic research based upon one experience, that would be regrettable in my view.

 

[aldevis]

The frequencies above cutoff are not a part of the "regime of oscillation" as Benade calls it and do not share energy with the fundamental and lower harmonics. In this respect Benade says they are a drain on the system. Benade's Fundamentals of Musical Acoustics is a great source of information. Furthermore these frequencies above cutoff "do not see" the open toneholes and travel straight out the bell. That is why the classical "donut mute" in the bell produces a darker tone because it acts as a filter. It does not take energy from the standing wave.

But they are not separate entities: they are part of the standing wave.
While it can work analytically, I see this assumption debatable in the light of this new experiment.

 

[abadcliche]

Water drop on a pad tells you whether it absorbs water or not. A lacquer sprayed or otherwise treated pad that is 70 years old will have cracks and missing pieces of the treatment, and will certainly absorb water. A pad with a lead foil backing (or more recently, plastic) will still absorb water. Your neoprene pads, depending on what type of neoprene they are, will absorb some water but are still airtight. Many leather pads that are untreated will absorb water but will not let air through. I can't blow air through a sheet of paper, but it will absorb water.

My experience has been different from yours. I see original pads horns an awful lot compared to most folks in this business- typically 10-20 per year from the 1920s-1960s- often still in good playing condition, and many of those pads where the leather is intact are 1) airtight and 2) treated, usually with lacquer or sometimes what looks like some sort of thinned out wax. I have about 50 square feet of pad skin leather from the Conn factory in the 50s sitting here (alongside about 500 or so original reso-pads) and though paper thin and very delicate and unusable for pads, most of the parts of it I've tested are airtight.

 

[jbtsax]

Hmmm. . . Are you referring to original pads from the factory or pads that were treated by someone else who repadded the saxophone later? I have a closet Conn 10M with the original pads that is my go to tenor. It would be easy to check what you are saying and report back. The "neoprene" test pad was just a disk cut out of a 1/4" sheet of neoprene. I don't think it will absorb water. 🙂

But they are not separate entities: they are part of the standing wave. While it can work analytically, I see this assumption debatable in the light of this new experiment.

I stand corrected. The frequencies above cutoff are in fact part of the "standing wave" and are present in the overall sound. My research also found that the the "peaks" of these harmonics are much weaker to begin with because they don't share energy with the "regime of oscillation" and due to the fact that they have more viscous energy losses from the walls of the instrument.

 

[abadcliche]

If its a pre-war 10M and it is truly original, you'll see something on those pads. But if its your "go-to" tenor, I highly doubt its original. Even though original pads horns play if you get one out of a closet, they don't continue to do so for very long.

Anyhow, I'm a grump and I'll leave you to it before I get too argumentative because though I have some disagreements, its the only study on the subject I've seen and I don't want to detract by nitpicking (though unanswered questions regarding suction and pressure measurements would be nice to see answered).

 

[thomsax]

One of the reasons Conn had Rolled toneholes was that pads were sealing better against the rolled toneholes. Martin was also saying so in their information in the early 60's. Martin saxes are friendly to the pads!

Off topic.
@jbtsax : Thanks for posting. Very interesting.

 

[jbtsax]

My experience has been different from yours. I see original pads horns an awful lot compared to most folks in this business- typically 10-20 per year from the 1920s-1960s- often still in good playing condition, and many of those pads where the leather is intact are 1) airtight and 2) treated, usually with lacquer or sometimes what looks like some sort of thinned out wax. I have about 50 square feet of pad skin leather from the Conn factory in the 50s sitting here (alongside about 500 or so original reso-pads) and though paper thin and very delicate and unusable for pads, most of the parts of it I've tested are airtight.

We are getting a bit off topic here. Let's just say I know the history of the saxophone I bought from my repair instructor and it plays quite well with the original res-o-ring pads. These may clear up some of your questions about pressure.
Convert db to pascals

Convert Pascals to in H2O

 

[Dibbs]

...
Actually the measurement was of the "acoustic impedance" of a brass tube with a loudspeaker on one end and the pad at the other. The "acoustic absorption ratio" is then deduced from the impedance measurements as far as I can tell.

OK, that accounts for the resonances then. There must be a microphone involved. Which side of the pad is the microphone? Sorry to be a pain. I just want to try to understand what I'm looking at.

 

[kevgermany]

OK, that accounts for the resonances then. There must be a microphone involved. Which side of the pad is the microphone? Sorry to be a pain. I just want to try to understand what I'm looking at.

So you're saying that the resonances would occur for the peak at about 500Hz, and the subsequent ones. This works if we assume end effect and some of the pad thickness becoming part of the tube length. I think you've answered this one!

 

[aldevis]

So you're saying that the resonances would occur for the peak at about 500Hz, and the subsequent ones. This works if we assume end effect and some of the pad thickness becoming part of the tube length. I think you've answered this one!

Should I guess that hitting the tube will give a Bb?

 

[kevgermany]

Should I guess that hitting the tube will give a Bb?

somewhere betwen B and C.

 

[jbtsax]

Pauline just sent me an email stating that the resonance frequency of the tube used for my tests was 670 hz so chew on that for a while. I'm off to give a repair lesson. 🙂

 

[Saxstudio]

hmm wondering how the tests went.. the geeky side of me (if I really have one) is curious.
personally I like the roo pads and am of the opinion (thoughts only) that the size of the resonator and material may well have some effect on final sound. never tested it in an analytical way though so no comment I guess