I have to give up on searching for the original device I saw there has been so much cosolidation and variation on the theme, it's possible the current products are different. The power and performance of what I saw was very limited.
In my twenties I had the bright idea to build an enclosure-less sub using two crossing modulated ultrasonic beams.
I had built 555 oscillators and two four-quadrant multipliers for driving the UT transducers.
The result at the crossing point was mosty noise and a faint amount of inteligible sound but totally distorted.
I attributed the failure to badly implemented electronics.
George
I had built 555 oscillators and two four-quadrant multipliers for driving the UT transducers.
The result at the crossing point was mosty noise and a faint amount of inteligible sound but totally distorted.
I attributed the failure to badly implemented electronics.
George
VHF Audio
At a boundary the velocity of sound is by definition low and the pressure is high. Perhaps the fact that air becomes non-linear in compression accounts for the IM at boundaries which gives the down-converted audio product.
Then again, I just helped set up a new system at a local brewery with the requisite QC duties
, and that tends to simplify thinking somewhat...GN
Howard Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org
At a boundary the velocity of sound is by definition low and the pressure is high. Perhaps the fact that air becomes non-linear in compression accounts for the IM at boundaries which gives the down-converted audio product.
Then again, I just helped set up a new system at a local brewery with the requisite QC duties
, and that tends to simplify thinking somewhat...GNHoward Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org
Spooky Sound At A Distance....
This is one of the models in my junkbox...HSS H450.
The second pic is the transducer frame....plastic with 2mm square channels moulded across, and a mylar like film with thin foil both sides (big capacitor like) is fixed to this frame.
Presumably the modulated driving voltage mechanically excites the unsupported film and produces the acoustic output.
The failure mode was arcing through the dielectric film.
The pcb is a bit industrial/agricultural, though it does incorporate a large earlier generation DSP chip.
With one of the two transducers disconnected, the performance was underwhelming, but it is a curious effect to hear sound emanating from a hard wall/surface......the output stage may not have been optimally loaded with only one of the two transducers connected.
One day I will get back to investigating the nature of the drive signal and understand it better...maybe even cough up for replacement drivers.
Elecktor magazine did a version of this idea...http://servv89pn0aj.sn.sourcedns.com/~gbpprorg/mil/mindcontrol/Ultrasonic_Directive_Speaker.pdf
Dan.
This is one of the models in my junkbox...HSS H450.
The second pic is the transducer frame....plastic with 2mm square channels moulded across, and a mylar like film with thin foil both sides (big capacitor like) is fixed to this frame.
Presumably the modulated driving voltage mechanically excites the unsupported film and produces the acoustic output.
The failure mode was arcing through the dielectric film.
The pcb is a bit industrial/agricultural, though it does incorporate a large earlier generation DSP chip.
With one of the two transducers disconnected, the performance was underwhelming, but it is a curious effect to hear sound emanating from a hard wall/surface......the output stage may not have been optimally loaded with only one of the two transducers connected.
One day I will get back to investigating the nature of the drive signal and understand it better...maybe even cough up for replacement drivers.
Elecktor magazine did a version of this idea...http://servv89pn0aj.sn.sourcedns.com/~gbpprorg/mil/mindcontrol/Ultrasonic_Directive_Speaker.pdf
Dan.
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There's someone in my head but it's not me....Pink Floyd.
https://www.google.com/patents/US3393279?dq=3393279&hl=en&sa=X&ei=bGi4UcmeHNGkkgXC-ICICg&sqi=2&pjf=1&ved=0CDQQ6AEwAA
A 1W AM transmitter attached to your skull !.
Dan.
https://www.google.com/patents/US3393279?dq=3393279&hl=en&sa=X&ei=bGi4UcmeHNGkkgXC-ICICg&sqi=2&pjf=1&ved=0CDQQ6AEwAA
A 1W AM transmitter attached to your skull !.
Dan.
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I saw no reference to U/S levels, just a simple 20 uP peak pressure no BW.
Hmmm..one has to wonder about the correlation.
jn
I would also assume that the resultant lf content would retain the planarity of the U/S wavefronts. Flat enough, and I suspect even the LF would keep a tight beam longer, until edge effects compromise it sufficiently.
Then again, I just helped set up a new system at a local brewery with the requisite QC duties
I assume your contract included the standard every 24 hour tuning and checkup regimen for the first year, right?
jn
Yes, hhoyt,jn are on a better track. The audio IS caused in thin air by the low frequency IM of a carefully computed ultrasound beam and that is why the forward planar wave is dominant. The IM is the non-linearity of the air itself, I might add part of the problem is cancelling the THD of the resultant audio. Early brute force systems had 50% THD worst case. You also can see how inefficient the process is.
hhoyt - been listening to the stream out of Chapel Hill on a regular basis (I also like the eclectic programming). Nothing like Japanese Acid Jazz at 8AM Saturday morning.
jneutron,
Perhaps this is better as far as standards go but it is just a list of places to look.
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9735
Perhaps this is better as far as standards go but it is just a list of places to look.
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9735
Thanks.
I like the "cycles per second" axis. When did we switch to Hz, 40 years ago or so?
It also stopped at 8Khz (cycles per second).
You can see why the safety guys here didn't know how to proceed..nothing to go on.
I recall charts that ran to 200 milliseconds and 144 dB impulses, that linked page only dropped to 15 minutes duration per day and 115 dB..
I also recall back in the 60's, the US Navy already recognized the issue of noise and hearing protection such that they considered the release of high pressure steam to be one of the most dangerous sounds due to the amazing high frequency content and resultant energy delivery possibility within the ear canal, well above the 8Khz regime the OSHA chart ended at.
jn
ps..I found years ago that the ACGIH was actually a very good resource for this stuff. I note that now it appears to be a more commercial site, where you have to purchase documents. I suspect that googling acgih would get others access to the information, I think I'm subscribed..
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The 8000 cps (HZ) limit is because the standard at NBS (NIST) for microphones was the ultra stable Western Electric (WE) 640A microphone. That one inch mic drops off at 8000 cps. Also, therefore, the headphones used in audiology could only be characterized to 8000 cps... so that is the highest hearing test freq.
Thx-RNMarsh
Thx-RNMarsh
As an aside, does anybody know about noise control/levels for use in the general public? We have code enforcement here armed with calibrated meters, but the legislated measurement requirements use the A weighted scale. The problem stems from the meter's rolloff at low frequency. Seems it's possible to rattle objects off shelves yet not exceed the legal noise statute at 3 in the morning.
Anybody have any knowledge or experience with regard to the use of C weighting for noise control? Or even B? If the bass were keeping me awake at 3 AM, I'd prefer C weight...but I also think it's not good to overcompensate the issue, B weight may be better...
jn
Anybody have any knowledge or experience with regard to the use of C weighting for noise control? Or even B? If the bass were keeping me awake at 3 AM, I'd prefer C weight...but I also think it's not good to overcompensate the issue, B weight may be better...
jn
The theory is that A weighting tracks hearing damage better than B or C.
Now if you look at a typical stadium sound system, some folks run them to 102 dB C weighted. Arenas usual have mandatory lower levels.
It is interesting to note some impulse noise can cause hearing damage even though it is not perceived as loud.
Now if you look at a typical stadium sound system, some folks run them to 102 dB C weighted. Arenas usual have mandatory lower levels.
It is interesting to note some impulse noise can cause hearing damage even though it is not perceived as loud.
Looks like I am the only one interested. Loading the secondary will oppose the magnetisation flux and thereby reduce it? So without a R load on the secondary when the diodes switch on and off the magnetisation flux of the transformer is modulated?
Yes a constant load does drop the distortion a bit. The advantage I was thinking of was the light load improves the regulation of the unloaded voltage on the secondary. It also provides a load to the inter winding C.
So on a low power power supply wasting a watt or two may not be too bad. In a higher power supply the regulation is better to begin with. But if you put small Cs on the diodes and R across the secondary you get the damping and less EMI.
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