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location New York, United States
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visits member for 3 years, 8 months
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I am an electrical engineer who is easily distracted by physics.


2d
awarded  Nice Question
Jul
15
revised Why does my wife's skin buzz when she's using her laptop?
added 80 characters in body
Jul
12
comment Is it possible to create an audible sound source in mid air by intersecting ultrasonic sound beams?
@ChrisMueller: No, the beat frequency envelope is a pressure variation at 2 frequencies. The envelope is not a frequency component, unless you demodulate it non-linearly. Mixing them together doesn't produce any new tones at 1 kHz (unless you're using the radio engineer definition of "mixing", which is nonlinear). Try playing a tone you can't hear out of one speaker (17 kHz), and another tone you can't hear out of another speaker (18 kHz) at the same time. You still won't hear anything, because you're just summing 2 inaudible frequencies.
Jul
11
comment Is it possible to create an audible sound source in mid air by intersecting ultrasonic sound beams?
@ChrisMueller Yes, it's true. Mixing (summing) two sources produces a beat frequency envelope in the loudness of the signal, but there is no tone present at that beat frequency. Linear systems cannot produce new frequencies. If you sum 30 kHz and 30.1 kHz, there will be nothing audible in the mix. If you sum them and then distort the mixture, audible tones at 1 kHz, 2 kHz, etc. will be produced through intermodulation distortion. Same difference between a combination tone and a power chord.
Jul
2
awarded  Curious
Jun
26
comment Would wearing clothing that is black on the inside and white on the outside keep you cooler?
"would a black lining inside a white robe help in cooling the body by removing infrared radiation faster?" There's no way to tell, since the visible color has no relationship to the infrared reflectance/transmittance/absorption.
Jun
26
revised Would wearing clothing that is black on the inside and white on the outside keep you cooler?
added 20 characters in body
Jun
26
revised Would wearing clothing that is black on the inside and white on the outside keep you cooler?
added 20 characters in body
Jun
26
awarded  Nice Answer
Jun
25
revised Is there a limit to the resolving power of a mirror telescope?
correct the example and radius instead of surface area
Jun
25
revised Would wearing clothing that is black on the inside and white on the outside keep you cooler?
added 217 characters in body
Jun
25
revised Do gravitational waves slow down as they pass through matter?
clarify
May
24
revised How loud is the thermal motion of air molecules?
added 22 characters in body
May
24
revised How loud is the thermal motion of air molecules?
edited tags
May
24
revised How loud is the thermal motion of air molecules?
added 193 characters in body
May
24
answered How loud is the thermal motion of air molecules?
May
23
comment Is it possible to create an audible sound source in mid air by intersecting ultrasonic sound beams?
One thing that no one seems to be mentioning is that this depends on nonlinear effects in the air. Mixing (summing) two ultrasonic waves would not produce audible sounds in a linear medium, they would just mix. The way this supposedly works is that the constructive interference at the point of intersection raises the pressure so high that the air behaves non-linearly, and the intermodulation distortion produces audible waves at that point. I don't know if these new waves would continue in the same direction as the original waves or spread outward spherically from their creation point.
May
22
revised How loud is the thermal motion of air molecules?
oh it's well-known in water!
May
22
comment How loud is the thermal motion of air molecules?
Maybe this is more well-known in the context of underwater acoustics? Wikipedia says "At very high frequencies, above 100 kHz, thermal noise of water molecules begins to dominate. The thermal noise spectral level at 100 kHz is 25 dB re 1 μPa²/Hz. The spectral density of thermal noise increases by 20 dB per decade (approximately 6 dB per octave)." citing R. H. Mellen, The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals, J. Acoust. Soc. Am. 24, 478-480 (1952). So it has a violet spectrum?
May
22
comment How loud is the thermal motion of air molecules?
And if 0 dB SPL is really the limit, as an effect of thermal motion, how can anechoic chambers be measured at −9 or −12 dB SPL?