dB = decibarks?
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
That's good: it's a reciprocity measurement. All you have to do is estimate the effective area of the receiver and the pattern of the transmitter.
Yup, conservation of energy. Might get tricky if the efficiency is low.
I've seen absolute sound level calibrators that used an oscillating piston in a cylinder to make calculable sound pressures, but obviously at low frequency.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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As I recall, volume controls for hi-fi are log taper. That's non-linear.
Try listening to it. If you don't hear it, it's working.
So it looks like spamtrap and Les could be right.
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It would be a great experiment to set up two ultrasonic sources of slightly different frequencies and listen to what happens. That is so appealing, I might look into it as soon as time permits.
John
dB is a relative measure - usually 10*lg(p2/p1) where p1 is measured sound pressure and, IMHO(!) p1 is sound pressure at auditory threshold.
I used to believe dbA refers to p1 being the auditory threshold pressure of the frequency of the musical note a (440 Hz, but I was more thinking about 1000 Hz), but it turns out that is nonsense. It seems to be more complex:
Inaudible ultrasonic sound obviously doesn't have an auditory threshold sound pressure, so you would have to define p1 if you want db.
Bernhard
Although Bernhard has a rational argument that because 0dB (SPL) is set to the threshold of audibility, it should be a function of frequency, by convention the sound pressure level reference for measurements (O dB(SPL)) is still taken to be 20 =B5Pa RMS, regardless of frequency.
For interest, the SPL at which a vacuum is pulled (From 2 atm to 0) is
194 db (SPL)."Blarp"
** ROTFL !!There will be no such note.
Classic blunder of assuming " beat notes" really exist.
.... Phil
Yes, scroll down this web page to Psychoacoustics for some related weird stuff (links); ie. silent sound, Dakota Audio ...
http:/www/mindcontrolgrid.com/applications.htm
Ken Gillmore
Which is why rockets taking off, with over 200dB SPL, are so impressive...
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Good to see that you're on top of this with your Classic breadth of knowledge.
There exist directional audio sources designed for localized listening in crowded environments like museums and trade shows. They claim to work by mixing two carefully predistorted and highly directional ultrasonic signals that mix to produce audio using the nonlinearity of the air at the point of intersection.
I once emailed a vendor asking the SPL required to produce sufficient nonlinearity in the air and how they determined it to be safe for human hearing. They chose not to respond. Maybe they couldn't HEAR ME!
I have a matched set of Earthworks piezo film omnis that shows flat response and an excellent pulse response with about 2.5 us rise time, iirc. Impact of very small particles is a good way to make delta function stimulus sources for microphones, but that generally needs vacuum and electrostatic acceleration.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
** Yep - known about them for decades.
Got SFA to do with the post I replied to.
The so called " beat note " is imaginary, the result of rapid amplitude variations and not the generation of a low frequency difference product.
** My understanding is that the non-linearity being exploited is inside the hearer's ears and as a result some hear the disembodied sound much clearer than others do.Air is pretty much linear up to about 150dB SPL.
... Phil
"Tim Williams"
** Yep, the nearby air is compressed by more than 2 atmospheres and shock waves are generated - as with explosions and gun shots.By computing a SPL figure taken at some distance back to the source ( using inverse square ) one comes up with figures like 200dB.
... Phil
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e t dHi John, The log taper just makes the resistance logarithmic with the knob angle. It doesn't make the gain nonlinear. So I've been reading ultra sonic stuff. Here's a way to make some sum and difference frequencies. Take a *non- polarized* piece of Ferro electric (Barium titinate, or PZT) and send both frequencies in. The strain developed in the X-tal depends on the square of the electric field. It's like magneto-striction. (We've all heard transformers hum at 120/100 Hz. twice the line frequency.) I don't know where to get non-polarized ferro-electrics. But if I find some I'll let you know.
George H.
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X5R, Z5U and worse dielectrics tend to be nasty in microphonics. Maybe a small value would be suitable so as not to load down the circuit too badly.
Else, the curie temp of ferroelectrics isn't high, you could easily anneal one of those brass disc types. Careful they don't come unsoldered, or oxidize, or some glue burns up, or I don't know what else..
Tim
Hi John, The log taper just makes the resistance logarithmic with the knob angle. It doesn't make the gain nonlinear. So I've been reading ultra sonic stuff. Here's a way to make some sum and difference frequencies. Take a *non- polarized* piece of Ferro electric (Barium titinate, or PZT) and send both frequencies in. The strain developed in the X-tal depends on the square of the electric field. It's like magneto-striction. (We've all heard transformers hum at 120/100 Hz. twice the line frequency.) I don't know where to get non-polarized ferro-electrics. But if I find some I'll let you know.
George H.
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Oops, that won't make sum and difference frequncies that you can hear. Only in the voltage developed across the X-tal.
George H.
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