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I've been working with a PAM8302A analog audio amplifier from Adafruit and added a second order low pass filter (2 x 470Ohm resistors and 2 x 2.2uF capacitors) to get rid of some of the audio feedback that occurs when the analog mic (Electret condenser mic with MAX9814 mic amp) is near to the speaker.

Breadboard with Pam8302A amplifier and second order low pass filter

I've noticed that when I am testing it out and have the speaker near my ear (less than a foot away) for even less than a minute then my ear starts to bother me even when the audio from the speaker is not very loud. I do have reasonably sensitive ears and pay attention to when they start to hurt and don't like loud concerts etc because of that. I would classify my hearing as pretty normal.

I've used an app on my phone (an instrument tuning app, Tunable) to detect the frequencies from the speaker and I've noticed that when I turn the amplifier on and off by connecting or disconnecting the power (and not changing the SD signal prior to shutdown) that there is a 10kHz burst of noise. I sometimes hear a little crackle but the audible noise doesn't seem like it could cause a problem because it is not loud.

Here is a screenshot from a spectrum analyzer. There is a spike over 22kHz (I'm able to tap on the far right and it displays 22165Hz and that is to the left of the rightmost spike) Spectrum analyzer screenshot

I am considering getting an oscilloscope to search for frequencies from the audio out.

What could be the cause of the ear issue or how can I troubleshoot it?

*** Edit to add spectrum analyzer screenshot. ***

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    \$\begingroup\$ Try running a spectrum analyser app on your phone. Post a cropped screengrab. \$\endgroup\$
    – Transistor
    Mar 14, 2021 at 18:16
  • \$\begingroup\$ That kind of speaker can not produce loud ultrasound, neither infrasound. So it is not physical damage. Some low volume but intolerable sound , like scratching nail on glass. \$\endgroup\$
    – user263983
    Mar 14, 2021 at 19:21
  • \$\begingroup\$ Microphones and speakers may not play well together. Are your sensitive ears bothered when you apply a signal source to this amplifier that cannot feed-back, like a CD or DVD music source? \$\endgroup\$
    – glen_geek
    Mar 14, 2021 at 19:22
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    \$\begingroup\$ Related... electronics.stackexchange.com/questions/149687/… \$\endgroup\$
    – user16324
    Mar 14, 2021 at 19:44
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    \$\begingroup\$ @bitrock remember the speaker is an R, usually 8 ohms \$\endgroup\$
    – user253751
    Mar 15, 2021 at 10:03

2 Answers 2

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If it bothers your ears, definitely spend some time to investigate the cause. It's possible there is an ultrasonic tone or noise being generated. This could be from acoustic feedback (speaker to air to microphone), an electronic feedback path within your breadboard circuitry, or other source of noise (power supply switching, etc). Your need to insert a low-pass filter to reduce obvious feedback makes it even more likely you have a high-frequency feedback problem.

High-intensity ultrasound is known to damage hearing. At lower intensities it could cause auditory fatigue, and longer-term could cause hearing damage as well.

Get that oscilloscope connected to the amplifier output and see what's there. Don't count on a smartphone app spectrum analyzer or sound intensity meter -- the smartphone microphone and circuitry have an upper cutoff that may be below the frequency of your problem sound.

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  • \$\begingroup\$ Are that kind of speaker on pic can emmit ultrasound? \$\endgroup\$
    – user263983
    Mar 14, 2021 at 19:12
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    \$\begingroup\$ @user263983 Yes they can. Even cheap 1" speakers often specify a frequency range up to 20,000 Hz -- and that can hurtz. \$\endgroup\$
    – Mark Leavitt
    Mar 14, 2021 at 19:48
  • \$\begingroup\$ How hurt? Can you be more specific. Amplitude in high frequency is weak. Piezo used in that range. \$\endgroup\$
    – user263983
    Mar 14, 2021 at 20:01
  • \$\begingroup\$ @user263983 Slightly sore, as in not as bad as having gone to a concert but sometimes I'll wake up even the next day and my ear will still be uncomfortable. It's the same sort of feeling after having listened to music that was too loud. \$\endgroup\$
    – bitrock
    Mar 14, 2021 at 20:17
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    \$\begingroup\$ Trust your ears. Try adding stronger filtering between the amplifier and speaker. LC better than RC. Higher order better. Lower cutoff freq better. \$\endgroup\$
    – Mark Leavitt
    Mar 15, 2021 at 1:35
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PAM8302A is a D-class amplifier with ~200kHz oscilator frequency.

The datasheed says it is filterless, so it expects the speaker coil to filter out these 200kHz out. Probably works for most speakers, but some of it is emitted as ultrasound.

200kHz is pretty much able to cause some ear discomfort. Speaker behaviour at frequencies outside of the audible range is simply not controlled. Your speaker may as well be more efficient at 200kHz than it is at 1kHz.

On the other hand, your phone will rather not pick it (it is probably limited to 20kHz or 24kHz).

What you may want to do is to add some low pass LC filter at the speaker output.

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  • \$\begingroup\$ Interested. Can you provide link to some information which proves what electromagnetic speaker can emmit 200kHz. \$\endgroup\$
    – user263983
    Mar 14, 2021 at 23:12
  • \$\begingroup\$ Why it wouldn't? The audible range is limited by the human ear abilities and not by some general acoustic waves property. \$\endgroup\$
    – fraxinus
    Mar 15, 2021 at 6:48
  • \$\begingroup\$ Simply physical. Coil has some inductance, membrane has mass and restricted flexibility. It can not move with high speed. Human ear membrane is not so flexible and refusing flexibility by age. Your guess can not be proven scientifically. \$\endgroup\$
    – user263983
    Mar 15, 2021 at 11:18
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    \$\begingroup\$ Errr, ... google.com/search?q=ultrasound+ear+damage . From personal experience, ~100kHz, 5W pushed into usual coil-and-paper-membrane tweeter feels like a pressure or weight in the ears. Found out when an analog (as if we had anything else back in 1980s) microphone saturated its level meter for no aparent reason. Coil inductance and membrane mass all good, but speaker efficiency doesn't rapidly drop at 20kHz just because no one listens there. \$\endgroup\$
    – fraxinus
    Mar 15, 2021 at 11:29
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    \$\begingroup\$ @MarkLeavitt You trying to prove that ultrasound can cause damage to human. I believe it is true. But my point is that small speaker on picture can not provide high level sound at high frequency. Even small level at 200kHz. \$\endgroup\$
    – user263983
    Mar 15, 2021 at 14:33

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