Sunday, October 22, 2006

A Drum, a capsule, and a banjo - a multipart tale of Tizz - Part Two

This snare drum is an example of what you can do with a Tympanic Membrane. Tympanic membranes are found in drums, your own eardrum, banjos, dynamic and condenser microphones, and of course, loudpeakers.

Here the randomly picked snare is observed by the Naked Eye microphone, same as used in the shakuhachi sessions.

Tympanic membranes are found singly, or as shown here, as a parallel pair offset and acoustically coupled, aided by a surrounding, fairly air tight seal at the periphery.

The drum, when struck, produces a note, but it is not a single, narrowband note. Instead, it is a very broadband emission with a sharp attack and a lengthy (compared to the excitation of the stick) decay. That decay almost always flattens, notewise, over its period. That's due to a phenomenon known as frequency dependent attenuation, which simply means that the higher frequencies decay, or die out, faster than the lower frequencies.

Tuning the drum by tightening the skin raises the average note, which we can readily hear and appreciate, (or not when badly tuned) but has little effect on the lateral modes, decay, and broadening. I've mentioned the work of Chladni over and over and now would be an appropriate time to revisit him to get further insight into this important acoustic phenomenon.

Not only did Chladni show how the shape of an object affects the propagation of sound across it, but he also seems to have discovered the connection between standing waves, those lateral modes that appear to be still and sometimes bear his name, and what we know today as quantum theory. Chladni also showed that meteorites are of extraterrestrial origin. Quite a guy. Harry Olsen, that famous designer of the RCA ribbon (and other) mics, knew and wrote about Chladni, and did his own diagram of Chladni waves that is quite pertinent here.

Many people imagine the drum and its tympanic cousins as a thing that moves up and down, or in and out, more or less in a uniform fashion. This is not the case at all. In fact, it is the lateral modes, the energy that propagates across the skin, then gets reflected back from the rim, and back and forth again and again until it is radiated or turned into heat, that produce the characteristic sound of the drum.

The snare drum is especially important to music. The snare is a nonlinear coupled mechanical load that generates noise when driven by the drum. That produces a very characteristic and generally high pitched, broad spectrum tshhhhh!! sound that we the children of the late 20th Century love about the snare.

Noise is sometimes hard to define, though it is often thought of as any sound energy that is undesired. The tshhhh!! of the snare is similar sounding to white noise, but it isn't noise (except in muzak) as we think of it.

Still it is important to remember:

1. The drum has a tympanic membrane that produces lateral modes subject to reflection, lengthy decay, and frequency dependent attenuation.

2. The snare drum produces a characteristic, added tshhhh!! as a result of interaction of the tympanic membrane and an adjacent structure.

You can probably see where all this is going. Don't get alarmed. Yet.


Anonymous said...

Do you do your own photography Bob? I like that one of Hugh Tripp and the telefunken microphone too. This is very interesting and informative and I hear good things about your products.

Wendi Henning

Bob Crowley said...

Hi Wendi

That was me that honked at you and waved in Framingham.