Could a faulty circuit emit ultrasound? - Page 1
Forum section: electronics / beginners

author: atmfjstc
author_rank: Regular Contributor
timestamp: April 10, 2019, 05:23:09 pm
content: I've noticed some curious behavior from my cat. Whenever I come back home, sit at my desk, and take the laptop out of sleep mode, the cat, if it happened to be dozing nearby, quickly gets up and leaves the room. I've done some variations to the routine and it seems the cat does not have a problem with me, the room, laptops in general, or the laptop when its screen is off, or me coming into the room without turning on the laptop screen. But it does seem to be reliably bothered by the act of me turning the laptop back on.I'm wondering, could there be something about a running laptop that's bothering the cat? Perhaps ultrasound? I've had this idea that maybe aging components in the backlight circuit might cause some cap to vibrate at ultrasound frequencies. I've definitely seen crappy SMPS buzzing at audio frequencies.So two questions:1. Has anyone heard about this thing happening, i.e. an aging circuit emitting ultrasound. Is it plausible?2. Is there some quick and dirty way I can measure the ultrasound (40-70kHz) levels in the room? I don't need super precision, of course, just an indication of whether the power in that range increases considerably vs the background level when the laptop's screen comes on.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 10, 2019, 05:31:34 pm
content: Yes, that's plausible. The circuit doesn't have to be defective anyway, it just might be its regular operating frequency.Afair, those CCFL backlights operate just above 20...30kHz, and they might either use a magnetic transformer - now there's the thing called magnetostriction that might cause the transformer to emit ultrasound, or it might use a piezo transformer, that's a part that uses ultrasound coupled into and out of a resonant piece of material - no wonder if this emits also ultrasound.One might try a microphone and an oscilloscope to find the source of the ultrasound.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: April 10, 2019, 05:48:07 pm
content: Another way to listen in is to use a spectrum analyzer application on an iPhone or something. The particular one I have tops out at 20 kHz, but perhaps others go higher.

author: iMo
author_rank: Super Contributor
timestamp: April 10, 2019, 05:49:38 pm
content: Quote from: atmfjstc on April 10, 2019, 05:23:09 pmYour cat became a cyber widow..

author: rf-loop
author_rank: Super Contributor
timestamp: April 11, 2019, 05:30:32 am
content: Also health electronics may produce audible noise from low freq to ultrasoundThis have been long time known "feature" of some components but this old thing still is surprice to some new peoples, even some new designers may meet this fact and wonder it after product is sensitive for vibration or generate vibration (yes they can produce voice ) but they can also receive! so we must not talk only about "singing capacitors", we need talk also capacitors as listeners what is some times really big problem... drop one walnut on your oscilloscope and you know and perhaps can predict designers age (I do not mean age after born but of course profession age).Of course some smps circuits inductors can sing as also capacitors. Many electronics can produce also ultrasound noise, even lot of.Some capacitors (example MLCC) can be really as sound transducers, they can send and they can receive.This is sometimes useful but many times big problem.https://micro.rohm.com/en/techweb/tech-info/engineer/3468https://product.tdk.com/en/contact/faq/31_singing_capacitors_piezoelectric_effect.pdf

author: Berni
author_rank: Super Contributor
timestamp: April 11, 2019, 05:46:36 am
content: Yes it sure can.I have a LCD monitor that does it. In a quiet enough room i can hear audible noise coming from it, certain patterns displayed on the screen can make the noise louder and more pronounced. The most pronounced pattern seam to be horizontal black and white bars. So its likely caused by ceramic capacitors on the supply rail for the LCD matrix drive circuitry.Applying the right waveform to the right kind of ceramic SMD capacitor can make it beep so loud that you can easily hear it into the next room.Where i work i even got complaints about these capacitors being too loud and distracting because they ware part of the LED backlight DC/DC and it was being PWM controlled with a few KHz. The solutions ranged from simply changing the PWM frequency, to changing to a different switchmode controller or capacitor, or implementing analog dimming of the output.