It's totally unclear to me how this is working, exactly. With the valve slides gone, we're left with a fairly short, cylindrical bore. Why does that sound anything like a bassoon? Hm.
And, for those who'd like to sing along, here are the lyrics:
"I... am not an english horn. // I am not an english horn, I cannot play so high // I am not an english horn..."
DARPA (you know, those guys who actually did invent the internet) has a piece on using "acoustic fields" (ie, killer bass) to snuff out flames. Looks like magic to me!
It's totally not clear to me, from the video, what the sound spectrum being used is, or (perhaps more importantly?) what sound pressure levels are required. All I hear is a buzz. It's not so loud that there can't be a person standing next to it (perhaps wearing ear protection), but it seems to require two huge speakers on either side of the flame, attached to ginormous tubes. Also, after the flame is out, the sound switches off, accompanied by a loud click, as if a huge relay is required to maintain the "acoustic field". Here's what they say about it:
Performers also evaluated the use of acoustic fields to suppress flames.
In the video below, a flame is extinguished by an acoustic field
generated by speakers on either side of the pool of fuel. Two dynamics
are at play in this approach. First, the acoustic field increases the
air velocity. As the velocity goes up, the flame boundary layer, where
combustion occurs, thins, making it easier to disrupt the flame. Second,
by disturbing the pool surface, the acoustic field leads to higher fuel
vaporization, which widens the flame, but also drops the overall flame
temperature. Combustion is disrupted as the same amount of heat is
spread over a larger area. Essentially, in this demonstration the
performers used speakers to blast sound at specific frequencies that
extinguish the flame.
My reaction: yeah, I know lots of those words. It's still magic.
Here's a guy who carves his instruments out of ice. (Daily Mail article.)
I'd think that'd create some difficulties: ice is not exactly the easiest material to work with, even when it's cold. And I note that he tends to stay away from the wind instruments. But it works well, given that. And it reminds me that cold weather is coming here, it's already started frosting in the evening, and the days are getting shorter and nippy. I'm glad I can stay indoors while I play.
It's a little easier to see harmonics on a spectrum plot than on a sonogram. That loses how things change with time, though, as you go through different notes. So I made a movie, using a screencap on the spectrum analyzer in Live, from a chromatic scale. Ugly, but I think good enough to get the point across. (Full screen HD makes it easier to see details.) Frequencies are along the bottom, and a spike shows the intensity in each frequency bin. The bins form a comb, with a spacing set by the number of samples in each spectrum. You can see the lowest peak at the frequency of the note being played, the first harmonic an octave up (a factor of two in frequency), the second harmonic at a 12th (a factor of three in frequency), etc. Each harmonic moves up together as the note changes to the next one on the scale, but they keep their relationship to each other. To me the most interesting thing, though, is how the amplitudes of the harmonics all fit inside a single overall envelope, which is basically constant. Live keeps track of the peak hit by any momentary spectrum as a dim line, and this line shows the curve of the envelope inside of which the spectra of all the notes lie. The envelope has a broad peak around 500-600 Hz, ie around C4. So you have this funny thing where for low notes, the fundamental is weak, there will be many harmonics, and the strongest harmonic might be the third or fourth, while for high notes, you'll have only a couple relevant harmonics, with the fundamental being strongest. So the character of the sound changes dramatically in different parts of the scale, but it all makes sense, because every note is ringing inside of the same envelope. This envelope is called a formant, which refers to a broad acoustical resonance. Formants are particularly important in identifying vowels, but also help define the timbre of instruments.
Fletcher and Rossing talk about bassoon formants, quoting 440-500 Hz for the lower strong one, and a weaker one at 1220-1280 Hz. That's not so far off what I see. Saying there's a shape doesn't tell you why the shape is there, however. Fletcher and Rossing also talk about a cutoff frequency, which seems to be some kind of low pass filter coming from the tone holes, and quote 300-600 Hz for the bassoon's cutoff frequency. Another source I have, a chapter in the Springer Handbook of Acoustics, quotes Benade as measuring a cutoff of 400-500 Hz for bassoons, and stating "specifying the cut-off frequency for a woodwind instrument is tantamount to describing almost the whole of its musical personality". So it's important, however exactly it works. Fletcher identifies the lower formant with "the transition at the tone-hole frequency cutoff", and the higher one with the reed somehow. Or maybe it's the cone angle, eg the French horn has a similar bore angle, and a similar sound to the bassoon, at least during sustained notes. Regardless of how exactly the formants are formed, it's clear that they have much to do with describing the nature of tone.
Added: Joe Wolfe's delightful acoustics site has more on formants.
Thinking about tone color gives me an opportunity to geek out. I found a spectrogram generator (here's the code, and here's Wikipedia on spectrograms), and applied it to the F scale I graphed earlier. I put the frequency axis on a log scale, so that intervals are a constant length. There's tons of things visible here, you can get a direct sense of how strong the different harmonics of each note is. The fundamentals of the low notes are quite weak compared to the harmonics. And different notes can have quite different relative harmonic strengths, note to note: this reflects the changing color of each individual note. You can see the whole and half steps going up the major scale, not just in the fundamental, but also in each harmonic. The higher harmonics look a little wonky, interval wise though. The first harmonic (the octave) is basically parallel to the fundamental, the second (a twelth above the fundamental) also looks like a scale, though not perfectly smooth steps. Eg the upper D-E step (above the staff, D4-E4, so the overtone is A5-B5) looks much wider than the harmonics below. I think this is basically a departure from harmonicity, as James Kopp has written about. For that matter, the two E4's don't line up: I guess played them at different pitches. It's processing like this that allows pitch analysis and correction software to do what it does. Looking further up in frequency, I can still trace the scale up to around the sixth harmonic or so, but it becomes harder, as the harmonics become closer together. There's also things which I wouldn't expect to be there, eg there's dim bands between the fundamental and the first harmonic on the high E-F-E. It looks to be an octave below the second harmonic, so might be some kind of weird period doubling effect. Or an artifact. The fuzzy constant bands below the fundamentals are almost certainly artifacts, they change when I change things like the sampling window (these graphs done with 4096, or about 0.1 seconds).
Another view of the same data can be gotten by plotting in 3D. Here, the changes in intensity of every note are much clearer: not only the Bb3 I caught before, but a lot of other notes as well: the G3 after crossing the break, both E4's. And not just intensity (ie volume) but also color: the fundamental and the harmonic are not changing together.
I also turned the whole thing on its side, which shows all the notes put on top of each other. There's a strong but broad peak in the range 400-700 Hz, a formant, a weaker one around 2 kHz, then a cutoff around 2.3 kHz. That first formant is why the harmonics are so strong, and the fundamentals are so weak, at least until you get up high.
I had an interesting experience in the impact of weather on reeds, something I guess I've heard of, but hadn't really believed. I'd practiced yesterday morning, and my reed (G5) was behaving. But the day was hot, reaching +33 C in the afternoon. Here that often brings rain. I had a rehearsal in the evening, and we heard thunder, and rain came through. And my reeds were terrible. Fuzzy, difficult to make respond. They turned funny shapes too, and closed down. I switched to G7, which is a little lighter, and that helped a bit, but I struggled through the whole rehearsal. My sinuses felt stuffed too, whether that's due to the playing or the weather I don't know. This morning is cooler, overcast, but with a muggy, oppressive feel. I feel like I'm in Europe, London or Paris or something. Or San Diego, which often felt this way. But that's what the weather looks like, why would it affect the reed?
Here are some plots of weather conditions over the past day.
The rain began around the start of my rehearsal at 1900, right in the middle of these plots. The shifts in temperature, wind and humidity are substantial on these scales. The pressure, which seems like a natural candidate to impact acoustics, doesn't change all that much, going from 100 to 100.5 kPa, a 0.5% shift. The humidity, on the other hand, changed from 20% to 70%, which is more than a factor of 3. So is it the humidity? Dunno. It's not obvious to me how that would work. After all, the reed gets fully saturated with water when it's soaked, so why would a slight shift in external humidity change its shape or properties? Or would it be due to the humidity in the air column of the instrument bore? That air is all exhaled breath, which I'd think is generally saturated with water vapor regardless of the external humidity. Curious.
I've long wondered what exactly reeds look like in motion, and how exactly does the various scrapes affect their behavior. If you look at the reed-making literature and traditions, there are many views, most of which share little in common. You'd think the study of acoustics ought to be able to help, and maybe it can, or has for some, but I haven't figured out how yet.
One clue may come from a classic undergraduate physics demo, the Chladni pattern. The coffee cup image shows a kind of Chladni pattern, formed by shaking at 20 Hz. I've found similar patterns by dragging a filled styrofoam cup across a smooth surface, setting up a slip-stick motion.
Classically, Chladni patterns are formed by putting sand on a thin plate (say, a violin top or back) and vibrating it, by . The sand collects along lines where the plate isn't moving, which is to say the nodal lines of the mode. These patterns are actually useful to violin makers, who can assess the symmetry of their carving, and adjust the frequency of the modes by examining these. The lines separate regions which are moving up from ones which are moving down. The pic here is taken from the great acoustics site at UNSW, and corresponds to mode 7 of the violin back.
If sand on node lines is too low tech, well, you can always play with lasers. Picart and friends made some kind of fancy laser interferometry/digital holography contraption, which allows the visualization of not just the nodal lines, but also the full 3d deformation of the vibrating structure, time resolved if desired. They applied this to a clarinet reed, with the images seen here. They've also used these to study all kinds of things about clarinet reed motion, which might be very interesting if I could manage to decode their paper. One thing is clear, though. The clarinet is the best studied reed instrument, among other reasons because the reed's natural vibration frequency is very high, which reduces interactions between the reed's resonances and the air column's. The very lowest frequency in the pictures is 1880 Hz, which is about a Bb6, essentially out of the usable range of the clarinet. So yeah, even for the highest notes on the clarinet, the reed should just flap open and closed, without getting all contorted.
An acoustic laser? Well yeah, they exist, they're called a SASER. They emit in the MHz to THz, so they are acoustic, but far out of the audio range. Thinking about it, though, the standard means of sound generation is pretty much the same as a laser. A laser is a kinda nifty device. The fundamental part is something which can emit light, but doesn't really want to. This is typically a population of atoms in an excited energy state, where the transition to emit light rarely happens spontaneously, kept there by "pumping" with another frequency. The trick comes when attached to a tuned cavity. Then the light coming from the tuned cavity can exite the source, allowing it to release the pent-up energy, by emitting more photons at exactly the same frequency as the light from the cavity. This sets up a feedback loop, and you get light amplification by stimulated emission of radiation (one of the best acronyms in the history of science), resulting in very narrowband emission (a single tuned precise frequency) and coherence. The acoustic saser folks have figured out materials allowing similar effects to happen with high-frequency sound.
Actually, though, I think every musical instrument is a saser. Consider a violin. The bow, moving across the string, builds up energy as it stretches the string, attached to it with rosin. At some point it will slip, releasing the energy, and a pulse of sound into the string. Without tuning, a scratchy, high frequency squeak results. But coupled to the tuned instrument, the timing of the next slip is fixed by the returning pulse down the string. Stimulated emission. A highly tuned strong sound is emitted, with fixed phase relationships between the harmonics (coherence?). I'm no acoustics expert (though I have skimmed some acoustics literature), but this seems like a saser to me. Wind instruments are similar, with the vibration of the reed (whether cane, for reed woodwinds, or lips, for brass) being controlled by the return wave from the tuned resonator. (Flutes have a vortex shedder rather than a reed, but I think it's pretty much the same deal.)
I've long wondered what exactly reeds look like in motion, and how exactly does the various scrapes affect their behavior. If you look at the reed-making literature and traditions, there are many views, most of which share little in common. You'd think the study of acoustics ought to be able to help, and maybe it can, or has for some, but I haven't figured out how yet.
Classically, Chladni patterns are formed by putting sand on a thin plate (say, a violin top or back) and vibrating it, by . The sand collects along lines where the plate isn't moving, which is to say the nodal lines of the mode. These patterns are actually useful to violin makers, who can assess the symmetry of their carving, and adjust the frequency of the modes by examining these. The lines separate regions which are moving up from ones which are moving down. The pic here is taken from the great acoustics site at UNSW, and corresponds to mode 7 of the violin back.
