For a start, "better" is a very subjective term. What sounds good to one person, may not sound so good to another. Also there is such a thing as context - something that sounds appropriate in blues, may not sound quite so appropriate in classical music. It is commonly stated that wood bodied instruments sound warmer than plastic bodied instruments and that metal bodied instruments sound bright and clear. However, the evidence does not show that this is the case. The few tests that have been conducted demonstrated that if you take two harps that are identical in every respect except for the comb material, listeners will not be able to distinguish between them by the sound alone. The first such public test took place at a SPAH convention in 1997, with a second one at the Buckeye Harmonica Festival in 1999. Identical reedplates were mounted on identically shaped combs made from the usual wood, aluminium alloy and ABS plastic, as well as some more extreme examples made from lead, titanium, balsa wood, concrete and styrofoam. Although this test and a subsequent one held at a later harmonica festival were flawed in many ways, the results suggested that if there were tonal differences due to comb material, they were very subtle indeed. A somewhat different test conducted by Brendan Power at SPAH 2010 attempted to discover whether players could detect any audible differences between comb materials that were not apparent to the audience. No such ability was demonstrated.
Many people reading this will be surprised, but there again, few people have really sat down and tested one material after another, taking care to eliminate all other variables. A very common piece of faulty logic runs something like this: "Lee Oskar harps sound brighter than Marine Bands; Lee Oskars have a plastic comb; Marine Bands have a wooden comb; therefore plastic combed harps sound brighter than wooden combed harps." This comparison completely ignores the fact that as well as having different comb materials, these two brands of harp have differently shaped air passages, different reeds and different covers, often with the reeds adjusted very differently - all of these factors have a much greater effect than that of the comb material. Also, if you are comparing chords rather than single notes, the tuning of the harp makes a big difference to the perceived tone of the instrument, for example the equal tempered tuning of the Lee Oskar makes the chords sound "buzzy", whereas the less tempered tuning used on Marine Bands makes the chords sound much rounder and smoother.
When I first started experimenting with harmonica construction, I assumed that the comb would play a significant part in the production of the harmonica sound. I was quite surprised to learn that it actually played an almost insignificant role, but upon learning more about the physics of the instrument, this became much less surprising. In the case of the guitar, the sound we hear is that caused by the vibration of the strings, which is in turn amplified by the soundbox of the guitar. Changing the material from which the soundbox is constructed has a noticeable impact on the volume and tone of the sound. However, when we play the harmonica, the sound we hear is not the actual vibration of the reed itself, but instead the sound of a column of air being set into motion by that reed. The comb of the harmonica does not act like the soundbox of the guitar, but is merely a support for the reedplates. The "soundbox" of the harmonica is the person playing it - their hands, mouth and upper respiratory system. The same is true for all wind instruments and many tests have been conducted with flutes, saxophones, trumpets, as well as organ pipes, which show that although the shapes of the various instrument parts affect the sound by directing the airflow in different ways, the actual materials have relatively little effect. Rather than any resonance effects, some people claim that the surface finish has an effect on the sound by reflecting certain frequencies and absorbing others. Again, tests with other wind instruments have failed to show that people can hear these alleged differences in blind tests and even if this were the case, then the materials of the room in which you play would have a far greater effect on your tone than the materials in your harmonica.
Another theory is that having a greater mass under the fixed end of the reed somehow improves the vibratory properties of the reed itself. This is an intriguing suggestion and certainly deserves further investigation. However, even if there is an effect, it is likely to be an extremely subtle one at best.
By way of demonstration, here are some audio samples. I took standard reedplates and covers from a Hohner Marine Band, a Lee Oskar and a Suzuki Promaster, all in the key of C. I tuned and adjusted each set of reedplates similarly, then mounted each of them in turn on similarly shaped combs made from wood, plastic and metal (the metal one was actually the original aluminium alloy comb from the ProMaster), played a G chord on each of them (holes 2, 3 and 4 draw) and recorded the results.
First of all, here are the Marine Band parts on the wood comb, the Lee Oskar parts on the plastic comb and the ProMaster parts on the metal comb. As they are all tuned and adjusted in the same manner, the differences between them are quite slight, but I think most people would say that the Marine Band has the warmest tone, the ProMaster is somewhat brighter and the Lee Oskar is somewhere in between.
Now let's listen to the Marine Band reedplates and covers on the wood comb, the plastic comb and the metal comb.
The Lee Oskar reedplates and covers on the wood comb, the plastic comb and the metal comb.
Finally, the ProMaster reedplates and covers on the wood comb, the plastic comb and the metal comb.
I certainly wouldn't claim that all these samples are completely identical, but I think they do show that the widespread belief that "wood = warm, plastic = bright and metal = harsh" is not particularly well-founded. The small differences that show up between the samples are more likely due to the fact that it is impossible for a player to play exactly the same thing twice and I think, at least from the listener's point of view, any tonal effect of the comb materials can be considered negligible.
However, many players claim to be able to perceive tonal differences between different comb materials, even if they are not apparent to a listener. This is hard to test objectively, as a player can usually tell the different materials by the way the feel to the mouth, or by the weight of the instrument (although the SPAH 2010 test took pains to try to minimise this). Obviously, if the player expects a certain material to sound a certain way, this will bias how they perceive it. The usual reason given for the player's ability to hear differences not readily apparent to listeners, is the direct conduction of the sound via the player's jaw bone. However, the volume of bone-conducted sound is very low when compared with the sound of the vibrating air column in the player's mouth and throat, so is likely to be drowned out. Also, bone-conducted sound has been studied by many researchers and it is well established that whilst it gives an accurate perception of pitch, it is extremely inaccurate when it comes to timbre. Thus any small differences in tone are much more likely to be obscured by bone-conduction than they are to be made more apparent. It would seem much easier to take the simple explanation that players hear a difference in tone when they expect to hear a difference.
For more on the subject of audio testing in general and the unreliability of the human ear when trying to make objective comparisons, you may wish to view this excellent video from Ethan Winer.
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