One more round of follow-ups:
Joe--
Thanks again for your answers to my follow-up questions. I really appreciate your taking the time to help me understand this stuff.
The only thing I'm still not quite clear on here is the issue of new sound (specifically new bass) being produced along the whole length of the ultrasonic column. Since the audible sound is produced as a byproduct of the ultrasonic signals (it's the distortion that's produced naturally as sound propagates through the air), it would seem that as the ultrasonic signal falls off, so would the production of new audible sound in the ultrasonic column. After some distance (I don't know how far), you'd only hear the audible sound that had been produced earlier. How much distance do you get before new sound produced in the ultrasonic column is too soft to hear?
Also, if new sound is being produced all along the column, wouldn't you get phasing problems as the different audible signals hit your ear at different times? I know I've read that your ear only tends to localize the first arrival of a particular sound, and that additional arrivals (from room eachos and the like) just make the sound "thicker" and "warmer". Is this what happens here?
And a really dopey question: When I'm listening to one of these things, where does the sound sound like it's coming from? If the transducer is pointed straight at my ears, does the sound appear to be coming from the transducer, from inside my head like with headphones, or from somewhere in between?
Thanks again,
--Rich
***
Rich,
As the ultrasonic signal falls off, so does the production of difference tone, whatever the frequency.
The ultrasonic signals travel at virtually the same speed as the difference tone so the already-made difference tones are strengthened as they go. As long as the ultrasonic wave has the amplitude required, the harmonics and the difference tone components become more pronounced, but proper phasing is maintained. After a distance, only the audio portion exists.
The loudest audio is produced near the transducer, but it does not fall off as rapid as the ultrasonic does because the air does not attenuate lower frequencies as fast, and because the audio is being reinforced downstream (more weakly as it goes, but reinforced nonetheless).
Since the all of the frequencies in question travel at virtually the same speed, there is no phasing problems.
When you listen to a prototype demo, the following happens: When the transducer is pointed at your ear, the sound seems to be coming from the direction of the transducer. When your head it outside of the beam, the sound seems to come from the first surface it is reflected from, because that is the loudest apparent source.
Just like when you point a laser beam at a wall, the spot you see appears on the wall, even though it isn't created there. If you look into the laser cavity, it seems to come from there. Same thing here.
Thanks for the idea, and the genuine interest.
Gotta get back to work...
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Joe Norris |
