Mt Spokane Photography said:
People can generally not hear ultrasonic tones, but they can dectect pulse trains at much higher than ultrasonic frequencies. They sound like a chirp.
http://grouper.ieee.org/groups/scc28/sc4/Human%20Perception%20FINAL.pdf
This is a fascinating study, but it is not about detecting ultrasonic tones and doesn't really apply to the subject of ultrasonic hearing. The study is about human sensitivity to and detection of pulsed
radio frequency energy, an energy which consists of oscillating electric and magnetic fields that travel in free (empty) space without air. This is the energy that, for example lives in the Amateur radio bands, well above the FM broadcast band for example, and well into the microwave region. This is the type of energy that eminates from Television brodcast antennas, cell phone towers, cordless phones, your laptop's WiFi system, and microwave repeaters. The study is not about ultrasonic "tones" and it is not about acoustic energy.
Acoustic energy (sound) consists of
mechanical pressure waves and cannot be transmitted without a medium (such as air or water). This type of mechanical energy requires an energy source of mechanical origin -- something, like a speaker cone, clapping hands, or Placido Domingo's vocal chords, has to move the air. The energy used to clean the sensor is acoustic in nature -- this energy is vibrating the bejeebers out of somethign (the sensor) to shake dust loose. In this sense, we can think of the sensor as a really high frequency tweeter that no one can hear. In any case, if the acoustic energy emitted by the camera is within the set of frequencies to which your ears are sensitive, you will hear something but it won't be the 100Khz cleaning frequency itself. The question is -- how can a sensor cleaning device operating at six times the upper threshhold of human hearing result in an audible sound?
by far the best insight into what actually happens in sensor cleaning has been provided by
TAF . Very nice info sir!
TAF showed that the fundamental acoustic energy of the cleaning routine sweeps from 100KHz to 125HKz and back. there is no way that any human being of natural orgins and abilities can hear this. The acoustic pressure waves reaching the ear just do not produce any response from the eardrum. no way.
of course, if you blast your head with a tremendous high-energy assult of air pressure waves at 100KHz, something is bound to happen internally (to your body) that can be detected. but this is not what is happening with the tiny little speaker we know as the sensor cleaning routine. its a tiny little tweeter producing a very small amount of acoustic energy at frequencies that no one can hear.
what is far more likely, as
TAF has rightly pointed out, is that there is some sympathetic or natural mechanical resonance that occurs within the camera structure itself -- much like the vibrations you hear from your dashboard when driving on certain roads... The structures themselves are responding at some subharmonic (a sub multiple of the 100KHz fundamental) that is within the range of human hearing. Note that if this sub-harmonic were present naturally in the sensor cleaning function itself you would hear a high-pitched tone that would sweep up and then back down, perhaps at the 7th or even 8th subharmonic (something between 12Khz and 18 KHz) The fact that no one has reported hearing a smooth frequency sweep is clear evidence to me that TAF is right -- there are fixed frequency mechanical resonances in the camera that will respond, some more than others, when the cleaning "sweep" reaches the right point in its sweep. Note that if there are two or more natural structural resonances within the camera, then you will hear two or more distinct tones or perhaps they will sound like chirps. in any case, you are not hearing the sensor cleaning function, you are hearing an artifact or side-effect of it.
what is clear to me , based on the above, is that (1) there is a very wide variation from camera to camera structural resonances, and owner to owner hearing abilities, with respect to what is actually heard during the cleaning routine, and (2) human hearing cannot possibly be used to measure the presence of or effectiveness of the cleaning routine because the cleaning routine itself cannot be heard!. If you hear something, then your ears are sensitive to whatever frequencies your camera's internal structures are vibrating at. Thats about it
. Moreover, when you compare your audible results with someone else, then you are not comparing your respective cleaning systems, you are comparing your own hearing sensitivies and the natural structural resonances present in your respective camera bodies
