Trumpets in miniature
Sans trumpets. Did I blog about how one of the spring's colloquim speakers essentially described a horn instrument as an acoustical laser?
Image courtesy AIP's Physics News.
The body of a horn is mostly straight tubing, or tubing whose curvature is such that refractive effects from the tubes being bent are unimportant. Valves and slides serve to change the effective length of the instrument, making it physically behave like an open-ended tube with constant diameter. This matters because the length of the tube determines the fundamental note of the the instrument and its sequence of overtones: longer, you get lower fundamentals, and shorter higher fundamentals (if I recall correctly). Adding in the flaring leading to the bell and the bell essentially gives a low-pass filter whose coupling characteristics are determined by the effective length of tube. The flaring of the tube couples a fraction of the sound to the outside world by making the diameter of the body change, and the degree of flaring (i.e., the rate of change of the cross-section) affects both the low end of the frequencies that can get out into the world at large and the effective length of the "straight" part of the instrument. The shape of the bell controls the angular spread of the sound emitted by the instrument. (The speaker pointed out that the typical PA speaker has a much wider flare, so as to make the sound it emits cover a much larger solid angle than the typical bell of a musical instrument, [ED. NOTE: making it possible to get your announcement ignored by even more people than otherwise would happen].) The combination of the bell and the flare couple a small portion of the sound energy contained inside the "straight" tubing to the outside world for us to here Satchmo's work.
Guess which one a player can control? Actually, the player can partly control both! The mouthpiece helps you couple sound waves into the resonating cavity (the "straight" tubing making up the bulk of the instrument) through some sort of feedback mechanism the speaker handwavingly explained, so picking a cup depth and diameter can make a big differece. Valves and slides change the effective length of the tube much more directly, as you can see most dramatically with a t-bone.
How is this relevant to the work in the linked article? The SASER (aka acoustic laser) replaces the resonating cavity with "an artful spacing of the lattice layer thicknesses in such a way that the layers act as an acoustic mirror". The output sound waves are generated by electrons sitting in quantum wells -- send in some phonons (the sound equivalent of photons of light) to , which cause a buildup of coherent sound waves (just like in a laser). Since there's no bell, you get a narrow "beam" of sound coming out of the device. Since the array of quantum wells is very small (the array is made of layers of semiconductors laid down using the standard semiconductor techniques, I imagine), the effective length of the tube is much much smaller and the resonating frequency is thus much much higher -- teraHertz waves have a frequency six orders of magnitude higher and a wavelength six orders of magnitude shorter than the waves humans can hear. The upshot:
Essentially ultrasound with wavelengths measured in nanometers, terahertz acoustical devices might be used in modulating light waves in optoelectronic devices.
Image courtesy AIP's Physics News.
The body of a horn is mostly straight tubing, or tubing whose curvature is such that refractive effects from the tubes being bent are unimportant. Valves and slides serve to change the effective length of the instrument, making it physically behave like an open-ended tube with constant diameter. This matters because the length of the tube determines the fundamental note of the the instrument and its sequence of overtones: longer, you get lower fundamentals, and shorter higher fundamentals (if I recall correctly). Adding in the flaring leading to the bell and the bell essentially gives a low-pass filter whose coupling characteristics are determined by the effective length of tube. The flaring of the tube couples a fraction of the sound to the outside world by making the diameter of the body change, and the degree of flaring (i.e., the rate of change of the cross-section) affects both the low end of the frequencies that can get out into the world at large and the effective length of the "straight" part of the instrument. The shape of the bell controls the angular spread of the sound emitted by the instrument. (The speaker pointed out that the typical PA speaker has a much wider flare, so as to make the sound it emits cover a much larger solid angle than the typical bell of a musical instrument, [ED. NOTE: making it possible to get your announcement ignored by even more people than otherwise would happen].) The combination of the bell and the flare couple a small portion of the sound energy contained inside the "straight" tubing to the outside world for us to here Satchmo's work.
Guess which one a player can control? Actually, the player can partly control both! The mouthpiece helps you couple sound waves into the resonating cavity (the "straight" tubing making up the bulk of the instrument) through some sort of feedback mechanism the speaker handwavingly explained, so picking a cup depth and diameter can make a big differece. Valves and slides change the effective length of the tube much more directly, as you can see most dramatically with a t-bone.
How is this relevant to the work in the linked article? The SASER (aka acoustic laser) replaces the resonating cavity with "an artful spacing of the lattice layer thicknesses in such a way that the layers act as an acoustic mirror". The output sound waves are generated by electrons sitting in quantum wells -- send in some phonons (the sound equivalent of photons of light) to , which cause a buildup of coherent sound waves (just like in a laser). Since there's no bell, you get a narrow "beam" of sound coming out of the device. Since the array of quantum wells is very small (the array is made of layers of semiconductors laid down using the standard semiconductor techniques, I imagine), the effective length of the tube is much much smaller and the resonating frequency is thus much much higher -- teraHertz waves have a frequency six orders of magnitude higher and a wavelength six orders of magnitude shorter than the waves humans can hear. The upshot:
Essentially ultrasound with wavelengths measured in nanometers, terahertz acoustical devices might be used in modulating light waves in optoelectronic devices.