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What might cause 1 kHz sinusoidal signal to change such as seen on the screen? How is this phenomenon possibly called?

I have quite a lot of hand microphones with built-in opamp preamp and electret microphone. Some of them function ok and give ideal sinusoidal signal to the output, but quite many of them behave like seen in the picture. 1 kHz sinusoidal signal with 0.8V amplitude is fed to a small loudspeaker and from it through air to the microphone. Signal goes through the preamp and is measured using an oscilloscope.

The question is general, and at the moment I am not trying to find out component or connection level reason for this, but perhaps someone has faced the same thing, and could explain possible causes for this.

enter image description here

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  • \$\begingroup\$ That looks like a slight phase shift. As to what's causing it, my best guess is the preamplifier is exhibiting some phase instability. However, I'm not sure what the cause or fix is. Good question. \$\endgroup\$
    – JYelton
    Jan 13, 2023 at 21:26
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    \$\begingroup\$ There may be some low frequency noise present. Adding this voltage to the sine shifts it vertically on the scope display, which shifts the trigger point. Use a high pass filter to reduce the LF noise. \$\endgroup\$
    – user16324
    Jan 13, 2023 at 21:57
  • \$\begingroup\$ @user_1818839 exactly right. \$\endgroup\$
    – Andy aka
    Jan 13, 2023 at 21:59
  • \$\begingroup\$ One way to avoid this is to trigger the oscilloscope on the speaker signal instead (assuming the scope has at least two channels). \$\endgroup\$
    – jpa
    Jan 14, 2023 at 9:02

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If I understand your question, I'm pretty sure you're looking at noise on the scope trigger dorking around with where the sweep starts displaying. Try suppressing any scope memory and look at single sweeps. If there's a noise suppress option in your trigger menu, use it.

In other words, you're looking at multiple identical sweeps simply starting in different places on your scope screen.

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    \$\begingroup\$ Try switching to negative-slope-trigger ...if the falling-edge appears noise-free instead of the rising edge (as in OP's 'scope photo), the Scott has nailed it. \$\endgroup\$
    – glen_geek
    Jan 13, 2023 at 21:49
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perhaps someone has faced the same thing, and could explain possible causes for this.

This looks like 50/60 Hz pick-up from AC wiring in your building/home. Try slowing your scope time-base to 10 ms per division and look at the wobble on the waveform at 50/60 Hz. I get it on my guitars and mics sometimes.

What might cause 1 kHz sinusoidal signal to change such as seen on the screen? How is this phenomenon possibly called?

It's usually caused when you use unbalanced microphones or guitars and/or unbalanced amplifier inputs (single-ended). It's a common problem that is sometimes solved by moving the cable around a little bit.

Clearly it's not "noise".

What your oscilloscope is displaying are several 1 kHz sinewaves all superimposed on each other. Normally, without the 50/60 Hz being present, the "several waveforms" all perfectly coincide thus, giving the impression of just one waveform. But, because of the 50/60 Hz being present the "several waveforms" do not coincide when viewed on time-bases suitable for displaying 1 kHz. Slowing the oscilloscope time-base down will show this type of image: -

enter image description here

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Your microphone is adding room echoes to the speaker signal. The echoes are also reducing, increasing or cancelling some frequencies.

When I measured the frequency response of a speaker I did not have an anechoic chamber so I laid the speaker in its enclosure on the ground outdoors pointing upwards away from the building and hung the microphone above it.

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    \$\begingroup\$ Wouldn't any echo in the room be at a reduced amplitude (and probably a substantially increased phase shift)? The extra waveforms appeared to me to be at the same amplitude. \$\endgroup\$
    – JYelton
    Jan 13, 2023 at 22:29
  • \$\begingroup\$ If the mic is not aimed "dead center" to the speaker then parts of the speaker are a little farther away and are delayed than closer parts, causing phase shifts. \$\endgroup\$
    – Audioguru
    Jan 14, 2023 at 23:35
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Finally I found out the reason for this behavior and an easy fix, which has helped to get approximately from 15 to 20 mics working correctly again. The reason is as follows: the mic capsules had been "clued" inside small plastic tubes that came with them due to wrong type of plastic material for this purpose. Some kind of aging of the plastic has been taking place. The material seems to be "too plastic" and obviously has been replaced by the manufacturer after some years, now being used "more rubber" containing material. And because of this "clueing" I suppose the signal on the scope screen has some echoes. This is difficult to examine in details, but seems quite logical explanation. To fix this is easy, just push the mic out from the pipe and then push it back - that is all. Little bit difficult because of the clueing tightly. But all microphones were successfully fixed using this method. Attached photo clarifies this.To fix, just push the mic out and then back.

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