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Date: Wed, 3 May 1995 16:25:21 -0700 (PDT)
From: Mike Curtis
Subject: Re: timbre

> Reeds aside, the harmonica is "nothing more than a resonant cavity" either, and not even much of one of those.

Aside from the reeds, harmonicas have nothing truly "resonant", at least not at fundamental frequencies. and certainly nothing that could qualify as a "resonant cavity" at any fundamental frequency.

The flute produces sound as a resonant column of air that compresses and decompresses at the frequency (note) being played. The harmonica produces sound by vibrating one or more reeds. The only thing at resonance in the harmonica itself is the reed itself. The cover plates form a horn effective at frequencies substantially higher than the fundamental frequencies of the reeds, hence the "tinny" sound of an uncupped harp.

> If the cover is like a bell or a bell housing, it is like a bell with Quasimodo sitting on it without trousers.

Poor choice of terms. By "bell housing", I meant its shape ("housing"), and not its function. It operates more as a horn, but not a whole lot. It also acts a bit as a "resonator", but at harmonic (not fundamental)frequencies. You may hit a 500 Hertz tone (somewhere around a C), but what actually propagates from the harmonica itself (assuming no player and just a directed stream of air blown toward the hole) as measured from a distance of three feet is comprised mostly of multiples of 500, i.e. 1000 (second harmonic), 1500 (third harmonic), 2000 (4th harmonic), and every 500 Hz thereafter. As the frequencies approach the inherent resonance of the harmonica, they become stronger.

However, the cover plates don't function as "bells", even if the player is a real ding-dong (sorry - couldn't resist a bad pun :-)

> A hollow-body guitar, and violins, are designed with relatively gigantic areas of thin wood, and these do, indeed vibrate in response to the plucked string. I think to compare the tiny, and very thick substances that make up a comb to a hollow-body guitar is really stretching matters.

Yes it is. But my point was that body materials that are actually part of the _sound_producing_ mechanism make a difference in sound. The sound producing mechanism of a flute is the air within it and the openings. The column of air has characteristics quite similar in principle to an
inductor. The openings have characteristics similar in principle to a capacitor. As one and/or the other is varied, the resonant frequency changes. The fact that the enclosing material is wood or metal will have little if anything to do with the sound, because it is the air that is actually the "instrument". The flute body is incidental. Wood or metal are both infinitely hard compared to air.

With guitars, the strings are the vibratory element, but the body and closely coupled to them. Because they have more mass than the air column within a flute, they affect and in turn are affected by body materials. Hollow bodied guitars have a "sound". So do Strats, Teles, Les Pauls, and other solid body guitars. Even the way the neck is mounted affects the sound, ergo "through the body" necks on some high-end guitars.

The harmonica uses the comb-reedplate assembly as a counterpoise to the vibrating reed. Let's take the example of a vibrating reed with a measureable mass vibrating against a reedplate and comb with absolutely no mass. Because the reed has mass, it will not vibrate. Our massless
reedplate will instead do the vibrating.

Next, let's assume a massless reed and a reedplate/comb with mass. Here, our reed does ALL the vibrating, and NOTHING is transferred to the reedplate/comb.

Of course, what we have in the "real world" is something between these two theoretical "ideals". Exactly how much between them depends on reeds, reedplates, and comb mass. Ruling out the first 2 as variables, a wooden comb provides much less mass than the somewhat heavier plastic,
and ditto metal.

If you attach a contact microphone to your harmonica, it will pick up these vibrations. I believe this is the principle utilized in the new Barcus Berry harmonica pickup.

This is just one aspect. We may also consider inherent rigidity. Wood is the least rigid. Plastic is more rigid, as is metal, but metal has more mass.

Of course, we can't ignore the effects of reed mass, size, and stiffness. These will undoubtedly have an effect on our sound. My point is, they're not the _only_ thing in the harmonica itself that has an effect on sound.

Cover plates also have an effect. I've found that cover plates with larger interior volumes tend to produce a more resonant sound. However, this is empirical and far from a scientific study. To really test this, one would need to test a harp using various cover plates and a tape recorder. It also might be interesting to construct a variable cover plate, with adjustable dimensions. I theorize the cover plates (or bell housing??) of the CX-12 is largely responsible for its big, warm tone.

With harmonicas, the musician is part of the instrument, not just the "source of wind" as with flute. A harp played with no mouth attached makes a pitiful sound. But put it in a nice, resonant respiratory system and it blooms into a wonderful sound. When we cup trained hands around
it, it blooms even more. Each of these contributes sound as a function mostly of resonance.

There's a particular embouchure that causes a marked increase in sound, and this is at the resonant frequency of the note being played. This is similar to the "flute" principle insofar as being resonant. The same holds true for cupping. At a particular point, the note "blooms" and
becomes noticeably louder and fuller. This is at resonance.

> The frequencies at which the wall of a comb becomes acoustically live are, I would guess, way, way above our hearing range. Below that, the comb is just a chamber.

The resonance of the comb "enclosures" is likewise far above hearing - or at least well into the treble range. Assuming sound travels at 1129 feet per second at sea level, and that the dimensions of the longest comb column is about an inch (with most being shorter, ergo higher in
frequency), the wavelength is that of a frequency of roughly 14 kHz. This is in the "noise" register - in the upper half of what is considered to be the highest octave humans can hear. If you play in a loud band and/or are over 40, chances are you can't even hear this frequency. So for musical
purposes, the comb really isn't a chamber - or at least one of much consequence. But it becomes PART of a chamber when the player becomes part of the instrument.

> Taken together, comb and covers make a very small resonance chamber indeed, with relatively large exit holes, compared to most instruments (jaw harp has NONE!).

Actually, the jaws harp uses the player as its resonance chamber; hence its most appropriate name :-)

> >I can easily show on an oscilloscope that different signals into a "distortion" amp will produce different outputs.
>
> I'll buy that, but will my ears?

You bet'cha! We don't use oscilloscopes looking for otherwise imperceptible nothings. We use then to identify exactly what it is we're hearing (in this case.) As waveforms visibly shift, so does the sound.

Interesting thread, with a lot of interesting input!

-- mike