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Topic: Distortion *undertones*--mechanism? (Read 2151 times) previous topic - next topic
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Distortion *undertones*--mechanism?

When I tone test headphones and speakers, at high frequencies I often hear / measure a distortion "harmonic" that's *lower* in frequency than the fundamental.  (Obviously, if the fundamental is so far up in frequency as to be inaudible to my old ears, any tones that do become audible are under the fundamental)

They are not IMD products in the simple sense, because only a single tone is being played.

What is the mechanism by which such undertones appear?

Are they "secondary" IMD products in that
1. Distortion harmonics appear above the fundamental
2. The fundamental frequency intermodulates with those distortion harmonics to create the distortion undertone which is heard?

But there's doubt in my mind that distortion products of one kind can themselve serve as the source for distortion products of another kind, it feels like circular logic of some sort.

Help?  :)

Re: Distortion *undertones*--mechanism?

Reply #1 –
Quote from: Joe Bloggs on
When I tone test headphones and speakers, at high frequencies I often hear / measure a distortion "harmonic" that's *lower* in frequency than the fundamental.  (Obviously, if the fundamental is so far up in frequency as to be inaudible to my old ears, any tones that do become audible are under the fundamental)
They are not IMD products in the simple sense, because only a single tone is being played.
What is the mechanism by which such undertones appear?
Are they "secondary" IMD products in that
1. Distortion harmonics appear above the fundamental
2. The fundamental frequency intermodulates with those distortion harmonics to create the distortion undertone which is heard?
But there's doubt in my mind that distortion products of one kind can themselve serve as the source for distortion products of another kind, it feels like circular logic of some sort.
Help?  :)

When you apply a high frequency pure tone, you hear the distortion under the fundamental, and the distortion over the fundamental is inaudible for you old ears.
It seems that you've posted the question and the answer IMHO  ;)

Re: Distortion *undertones*--mechanism?

Reply #2 –
Quote from: Joe Bloggs on
When I tone test headphones and speakers, at high frequencies I often hear / measure a distortion "harmonic" that's *lower* in frequency than the fundamental.  (Obviously, if the fundamental is so far up in frequency as to be inaudible to my old ears, any tones that do become audible are under the fundamental)

They are not IMD products in the simple sense, because only a single tone is being played.

What is the mechanism by which such undertones appear?

Are they "secondary" IMD products in that
1. Distortion harmonics appear above the fundamental
2. The fundamental frequency intermodulates with those distortion harmonics to create the distortion undertone which is heard?

But there's doubt in my mind that distortion products of one kind can themselves serve as the source for distortion products of another kind, it feels like circular logic of some sort.

One possible explanation is that what you are hearing is modulation noise.   The high-frequency high-intensity tone that you have applied is overloading the circuit in such a way that other potential sources of noise such as power supply noise  leaks though into the output.

Now you might think that since common power line voltages are AC at 50 or 60 Hz, any power supply related noise must be some kind of a hum, and that may be so. However, rectified line voltage is rich in harmonics so much higher frequency noise may be heard instead or as well.

Also, in these days the power line is often converted to high frequencies in switchmode power supplies. Switchmode power supplies can operate at frequencies as low as 30 KHz, so if you apply a 20 KHz test tone, you've inadvertently created a source of for an approximately  10 KHz via intermodulation of your test tone and the operational frequency of the switchmode power supply.