I know what you are thinking. Does it make any difference in the sound produced by the mouthpiece? Not enough to detect or worry about. But given that most players are looking for that special advantage provided by some mystical property of their instrumentation, we should look more closely at whether mouthpiece material matters "acoustically." For that examination, we can use expert testimony, existing data, test results, and common sense. If you don't have those things, don't worry, you can use mine.
With expert testimony, we need to be careful because there are many unsubstantiated claims out there. Assertions like "plastic mouthpieces make you sound like a duck because duck calls are made of plastic." We would have to resort to our other sources (existing data, test results, and common sense) to determine whether to believe that expert's statement.
I'm going to rely on two people as my material experts. Mr. Otto Link (who I have used before as an expert) and Mr. Ralph Morgan. I'm going to start with an article authored by Ralph Morgan and printed in the Saxophone Journal many years ago (Does the material used make any difference in how mouthpieces play?). Right on point and the article is a wealth of assertions. Let's see if we can answer that question with the evidence presented in the article.
The article begins by talking about wood mouthpieces, but the main take away is that wood mouthpieces are difficult to manufacture because of shrinking, cracking, etc. Wood was abandoned when other materials became available. So, materials that are more durable than wood and more uniform to machine than wood are preferable. Material matters to the manufacturing process. The article hints at the problem of there being hundreds of types of wood and that any claims of the effect of the vast variety of wooden mouthpieces would be silly. Hint: don't put a mouthpiece made of dogwood in your case with a mouthpiece made of pussy willow; they will fight. Things like that.
Most modern players are not concerned with wooden mouthpieces, so we will begin with the article's discussion of hard rubber mouthpieces. Mr. Morgan states: "Hard rubber became “the thing to use” after Harvey Firestone discovered how to vulcanize, or harden, natural gum rubber. This happened none too soon since the need for clarinets and saxophones grew rapidly in the late 1800s."
Let's look at that statement a moment. The U.S. Patent Office issued patent 3633 for the vulcanization of rubber on June 15, 1844. Harvey Firestone was born on December 20, 1868. Do you see a problem? Harvey Firestone missed the first several decades of ebonite development because he wasn't born yet. Harvey Firestone did get a patent related to vulcanized rubber. His patent was for tires on horse drawn buggies, and later, automobiles. Firestone was a contemporary of Henry Ford and became a millionaire as a result of pneumatic tires. But a claim that Firestone "discovered how to vulcanize?" It was Charles Goodyear who patented the vulcanization of latex rubber. Ummmm, I'm going to be skeptical on all further testimony by this materials expert, okay?
Mr. Morgan then talks about the hardness of rubber measured on the Shore D scale. Albert Shore developed a method to measure hardness of vulcanized rubber in the 1920s (long after ebonite had been successfully used for woodwind mouthpieces). It is basically a spring loaded pin and the measurement is how far the pin deforms the test material without penetration. Two different scales exist, Shore A and Shore D. Shore A is for softer rubber and Shore D for harder.
Ebonite is usually tested with a Shore D meter, which uses a sharper pin and 5 times more force than a Shore A durometer. Ebonite is defined as vulcanized rubber above 70 on the Shore D scale. Some of this is defined in ASTM D2240 if you are interested in the minute details. It should be noted that ASTM D2240 states "No simple relationship exists between indentation hardness determined by this test method and any fundamental property of the material tested." In other words, according to the American Society for Testing and Materials, rubber hardness means diddly about its acoustical properties. I'm going to start from that point of view.
There are problems with Shore test accuracy, of course. In effect, you are trying to measure a particular rubber's "squishiness," which (as you would imagine) is a rather "elastic" concept. Manufacturers of the Shore meters usually state that because of test sample temperatures, operator error, etc., the meters are only accurate within + or - 5. So a sample that reads 90 could later test as either 86 or 93 the next moment or the next day. Here is a YouTube video of using a durometer. The pin tends to slowly extend if the durometer is held in place, so even the quickness of the reading effects the result.
Another influence on the test results is the size of the test "puck." ASTM D 2240 defines the size and thickness required of a test piece (the test must be conducted on a flat surface greater than 6mm thick and further than 12mm from the edge). Because of this size requirement, a Shore durometer can't be used on an actual woodwind mouthpiece. In fact, it can't be used on a cylinder. Here is a video supposedly showing how to get a correct reading. You can see that the reading varies because you can't get an accurate reading on a cylinder.
Durometer measurements are important for commercial and industrial uses of rubber and plastic, although whether it tests as Shore D 85 one day and Shore D 82 the next doesn't matter. The test would be performed on a sample "puck" and the material is deemed fit for it's intended purpose when properly molded, cooked and cured. So the manufacturing process also effects the Shore D number. If the part manufactured is small and cylindrical, you have to simply guess that you have come close to the intended Shore D number based on the rubber formula and manufacturing process. It clearly is not an exact science.
But wait a minute. What if we are not interested in the the durometer reading for industrial use? What if we don't want to acknowledge the limitations of Shore D measurements? What if we are only interested in the mystical musical properties of vulcanized rubber? What if we have a theory that a Shore 85 ebonite mouthpiece produces a luscious harmonic melody and a Shore 82 mouthpiece makes a dull flatulent moan?
A "perfect Shore D mouthpiece" is basically what is claimed in the Saxophone Journal article. How do we examine our expert's testimony when he makes claims beyond known material science? Super accurate Shore D numbers aren't available, direct testing of a mouthpiece isn't available, yet it is alleged to make a huge difference in the sound of a mouthpiece.
There are inherent problems with claiming that a specific Shore D test number is required for an ebonite mouthpiece. At approximately 80C, ebonite undergoes a thermoplastic transition, i.e., it approaches it's "yield" temperature and becomes plastic again. (At approximately 200C, ebonite becomes liquid). The Shore D number for all rubber products drops with an increase in temperature. It is the nature of the material. Race car driver's understand this and use a specific Shore D tire based on track conditions, knowing that the tires will soften when warm. Even hip skate boarders know this one. It is why the garden hose feels stiff on a cold day and limp on a warm day.
We can't predict exactly how much a hard rubber mouthpiece softens with warmth (and we can't measure a mouthpiece), but we know that it drops. ASTM D2240 says that all Shore D testing must be performed at temperatures below 30C. My durometer states that it can't be used at temperatures above 30C. Based on the stated accuracy of my meter, one can assume that temperatures above 30 would cause inaccuracies even beyond the meter's claimed accuracy of + or - 5.
My durometer instructions and the American Society for Testing and Materials also state that relative humidity must be below 80%. High relative humidity causes rubber, including ebonite, to become more flexible. Maybe not enough so that you or I can tell, but enough for an inaccurate reading on a Shore D meter. Again, it could result in an error outside of the meter's + or - 5 accuracy.
Commercially available tables show that 10 degree changes in temperature would change the Shore reading of some rubber products by about 5. I couldn't find any tables on the direct effect of humidity. The initial Shore D rating, as well as different additives, tend to effect the amount that heat and humidity will reduce the Shore D number. Basically, there is no way of knowing how much the heat and humidity changes will "soften" a particular mouthpiece that we have guesstimated to be a "perfect" Shore D 85.
Since we can't actually perform the Shore test on a mouthpiece, let's do some hypothetical testing. We will start with an ebonite mouthpiece that we believe is a perfect Shore 85 (based on a sample of the material prior to molding, cooking, and curing). First, we put the ebonite mouthpiece in our mouth. Then we blow moist, warm air through it for a few minutes. We will have now raised the temperature of the ebonite to 32C (90F). It has condensation on the inside, indicating 100% humidity. Our hypothetical test has now created the conditions scientifically known to increase the softness of the rubber by greater than 5 on the Shore D scale. In fact, both temperature and humidity are now in excess of the requirements for an accurate Shore test. In case you haven't noticed, we create these conditions every time we play a hard rubber mouthpiece.
We don't need to know exactly how much blowing through an ebonite mouthpiece would change the Shore number. We do know that our durometer (which was only accurate within 5 begin with) is no longer accurate. Maybe the Shore number has changed by 6? We can't tell because the ASTM D2240 test can't be used on a warm, moist mouthpiece. So what is the Shore D number after playing an ebonite mouthpiece that started out as the "perfect Shore 85"? 82? Less? We wouldn't know because even if it dropped to 74 it is still ebonite, i.e., it is still hard rubber that holds it's shape. It appears that it is the mouthpiece shape, not some vague and changing Shore D number, that determines how ebonite plays.
And we don't need to know the Shore D number of a warm, moist ebonite mouthpiece. At this point, we can rely on our experience and common sense. Have you ever noticed that getting the mouthpiece warm and moist makes a perfect hard rubber mouthpiece unplayable after a few minutes? No. Have you ever heard the difference when an ebonite mouthpieces undergoes this dramatic change every time it is played? No. Nobody has ever noticed this because a Shore D fluctuation of >5 doesn't matter. Conclusion: the range of Shore D fluctuation in an ebonite mouthpiece does not effect acoustics.
Even Mr. Morgan agrees that Shore D doesn't matter for mouthpieces when he goes on to talk about metal mouthpieces. Shore D doesn't matter on metal pieces "since the average thickness of material used can do nothing but act with a damping effect on the reeds (sic?) vibration." I have to admit, I have no idea what that statement means. At least we can agree that Shore D claims are nonsense.
Okay, enough, enough. Let's go on to our second expert, Mr. Otto Link. I have used Mr. Link as an expert in another blog. Since this blog is getting long, I'll start another one about Otto Link agreeing with Mr. Morgan that material doesn't matter. I'll "Link" to it when it is written.