4 ms·

While I agree with you that this demo is suspect, I think your comment would be more useful if you backed it up with some of the physics/engineering principles

by tmh79 9y ago

While I agree with you that this demo is suspect, I think your comment would be more useful if you backed it up with some of the physics/engineering principles that support your point of view.

daveguy 9y ago
Here are osha safety guidelines for ultrasonic sound: https://www.osha.gov/dts/osta/otm/new_noise/appendixc.pdf https://www.osha.gov/dts/osta/otm/new_noise/appendixc.pdf tldr: Maximum 115 dB Here is a calculator to convert decibels to watts: http://www.sengpielaudio.com/calculator-soundlevel.htm http://www.sengpielaudio.com/calculator-soundlevel.htm tldr: ~0.3 watts per square meter Surface area of a google pixel: 14.3cm x 6.95cm = 0.01 square meters Therefore approximately 0.003 watts over the surface area of a phone at safe volumes. At 5v that's 0.6 mA. Barely enough to light up LEDs. It looks like they're working with larger holders, say 20cm x 20cm that's 0.04 sq meters or 2.4 mA at 5v. It takes about 10 mA to trip the "charging" indicator on an android phone: http://liesandstartuppr.blogspot.com/2017/06/what-does-it-take-to-switch-phone.html http://liesandstartuppr.blogspot.com/2017/06/what-does-it-ta... This is based on the raw power of the sound waves. There will be inefficiency converting from sound to electricity, so they are probably dealing with greater than 115 dB sound at the phone. They are probably just barely tripping the charge sensor for 100 hour charge times. If they are charging significantly then they are working with very excessive excessive sound levels (>125 dB). Given imperfect transducers for 100% conversion of sound to electricity and the above dimensions they would have to be working with > 122 dB sound at the phone for 10 mA charge current. So, it's either a scam or it's not safe. Jamming that much air pressure in a small space will cause problems and will probably be inefficient. Sound is not a laser. You can beam form it, it just won't be as tight as a laser, or even a flashlight. You can't expect clean narrow patterns within a 45 degree cone from the source. There will be high and low intensity interference patterns. You can even see it in the demo.

by tmh79 9y ago

While I agree with you that this demo is suspect, I think your comment would be more useful if you backed it up with some of the physics/engineering principles that support your point of view.

daveguy 9y ago

Here are osha safety guidelines for ultrasonic sound: https://www.osha.gov/dts/osta/otm/new_noise/appendixc.pdf https://www.osha.gov/dts/osta/otm/new_noise/appendixc.pdf tldr: Maximum 115 dB Here is a calculator to convert decibels to watts: http://www.sengpielaudio.com/calculator-soundlevel.htm http://www.sengpielaudio.com/calculator-soundlevel.htm tldr: ~0.3 watts per square meter Surface area of a google pixel: 14.3cm x 6.95cm = 0.01 square meters Therefore approximately 0.003 watts over the surface area of a phone at safe volumes. At 5v that's 0.6 mA. Barely enough to light up LEDs. It looks like they're working with larger holders, say 20cm x 20cm that's 0.04 sq meters or 2.4 mA at 5v. It takes about 10 mA to trip the "charging" indicator on an android phone: http://liesandstartuppr.blogspot.com/2017/06/what-does-it-take-to-switch-phone.html http://liesandstartuppr.blogspot.com/2017/06/what-does-it-ta... This is based on the raw power of the sound waves. There will be inefficiency converting from sound to electricity, so they are probably dealing with greater than 115 dB sound at the phone. They are probably just barely tripping the charge sensor for 100 hour charge times. If they are charging significantly then they are working with very excessive excessive sound levels (>125 dB). Given imperfect transducers for 100% conversion of sound to electricity and the above dimensions they would have to be working with > 122 dB sound at the phone for 10 mA charge current. So, it's either a scam or it's not safe. Jamming that much air pressure in a small space will cause problems and will probably be inefficient. Sound is not a laser. You can beam form it, it just won't be as tight as a laser, or even a flashlight. You can't expect clean narrow patterns within a 45 degree cone from the source. There will be high and low intensity interference patterns. You can even see it in the demo.