frequency of an object's vibration=frequency of the sound it makes? - Page 1
Forum section: electronics / general

author: engineheat
author_rank: Frequent Contributor
timestamp: June 05, 2020, 01:36:26 am
content: If a beeper makes a 3000hz tune, does it mean the beeper is vibrating at the same frequency? I mean, I understand the frequency of the pressure wave (sound) is 3000hz, but I'm not sure if this is also equal to the vibration frequency of the sound source.In another word, can we perform audio analysis of the sound an object makes without a microphone, but through detection of its physical vibration?Thanks

author: ataradov
author_rank: Super Contributor
timestamp: June 05, 2020, 02:26:53 am
content: Yes, you can. But if vibration is in the audible range, then microphone is essentially a vibration sensor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2020, 02:38:28 am
content: Yes, that is precisely how; well, not to overemphasize the point, but entire fields of study take advantage of that!It may be noteworthy that nonlinear functions are particularly cheap in mechanics -- consider for example the tuning fork. As the tines move back and forth oppositely, they rotate, effectively the forks get very slightly shorter as they deflect. Therefore, the center of mass is moved in and out attwice the frequencythe tines move at, and this is sensible as axial vibration on the handle.In short, whack a tuning fork and touch near the base of a fork, to a table: you will hear its characteristic frequency. Then, whack it and touch the handle to the table: you will hear twice that frequency (an octave higher).Linear systems cannot modify frequencies, the frequency is always perfectly equal throughout the system. (The distribution of frequencies might vary -- filtering is definitely a thing. Butafrequency can't be modified.) This is great news for, say, audio amplifiers, radio transmission and optical systems, and some acoustics; but anywhere that assumption is broken, i.e., nonlinearities become significant, you can get weird effects (harmonic generation, mixing, parametric oscillation, chaotic oscillation..).So, if the audio analysis you're thinking of, can be done through detection of its physical vibration (sensed with some other transducer, say a high-speed camera?), if it behaves linearly, yes, we can perform that analysis.We might even be able to do it from a single snapshot, if we know how sound moves through the physical system -- for example, if that beeper is hanging off a wire, and we know the velocity of that sound on that wire, we can see its wiggly path from just a single frame -- assuming of course the wire isn't bent like that all the time, of course! This is not a change of frequency, but of wavelength -- the amount ofdistancea wave travels in a given amount of time. The time is always the same (the time period being 1/frequency). A visualization:https://www.seventransistorlabs.com/Images/Refraction.gifSee how each part of the wave goes up and down at the same rate, but the distance travelled varies.Tim

author: helius
author_rank: Super Contributor
timestamp: June 05, 2020, 03:53:34 am
content: An ideal pressure transducer is vibrating at the same frequency (or sum of frequencies) as the air it is moving. But no pressure transducer is ideal, so in reality they are not the same. When you are next near a loudspeaker with the grill removed, look closely at the cone to see if you can detect any movement. A cone with perfect coupling to the air does not move when it transmits force to the air! Motion of the cone is a result of an impedance mismatch. When the mismatch is extreme, the cone can move so far that it reaches its mechanical limit, or "bottoms out", which causes a clipped distorted sound wave. Usually there is one frequency at which the impedance is perfectly matched, which is called resonance. You don't want to drive the transducer at its resonant frequency, because the amplitude of its oscillations will rapidly increase until it buzzes itself to bits. Speakers are designed with a high-pass filter (electric or mechanical) above their resonant frequency to avoid this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2020, 07:16:22 am
content: Quote from: helius on June 05, 2020, 03:53:34 amEr, not quite; if it had zero motion but transmitted sound, it would have a mechanical impedance of zero (0 velocity / x force) and therefore transmit zero power.The mechanical impedances throughout a mechano-acoustic transducer are finite, and presumably well controlled, so that the impedance match is reasonably good, over a wide bandwidth. Which also means the displacement or velocity will be well behaved (which one depends on the transducer type), finite and nonzero.Tim