Why is my subwoofer demodulating the treble?

[testtube44]

I have a speaker set that's doing something weird. I don't know if it's a feature or not, but it's amplitude demodulating the treble frequencies and playing them. https://www.dropbox.com/s/subbisj1omxj6vv/DR0000_0168.wav?dl=0. In the recording you can hear that the sub responds when 2 high tones are played together, and is the difference of their frequencies. If this is a feature, what's it called?

[dmills]

It is called intermodulation distortion, and is a sign that someone fucked up, something somewhere is non linear.

Regards, Dan.

[testtube44]

It is called intermodulation distortion, and is a sign that someone fucked up, something somewhere is non linear.

Regards, Dan.

Is there anyway to find out what, and how to fix it?

[Vovk_Z]

I can say only that we have to determine first is it woofer or amplifier doing it. Both can do it more or less.

[magic]

No expert but I imagine that there ought to be a crossover somewhere, to keep treble out of the sub. Speaker transducers as such are nonlinear so I think they shouldn't even be allowed opportunity to create such problems in the first place

[testtube44]

No expert but I imagine that there ought to be a crossover somewhere, to keep treble out of the sub. Speaker transducers as such are nonlinear so I think they shouldn't even be allowed opportunity to create such problems in the first place

I think it's definitely the amplifier. If the speaker was causing distortion it would be obvious, that only requires a single tone to sound bad, this takes 2. I've also had this thing accept microwave frequencies and produce pulsing sounds. That was when I held the wifi card of an xbox close to the headphone jack bypass and volume knob, perhaps more evidence that it's something "nonlinear".

[paulca]

Surely if you play two high frequencies together they are modulated and thus contain harmonics which may be in the subwoofers range?

[testtube44]

Surely if you play two high frequencies together they are modulated and thus contain harmonics which may be in the subwoofers range?

I think he's right about the intermodulation distortion. The bass tone is always the difference of the 2 tones.

[testtube44]

thanks for the insight into the fact that my speakerset is shit, therefore ive concluded my speakerset is shit, so heres some examples just for the hell of it.

 

the end

[mikerj]

Surely if you play two high frequencies together they are modulated and thus contain harmonics which may be in the subwoofers range?

Harmonics are always at higher (integer multiple) frequencies than the fundamental, not lower.  Non-linear mixing of signals creates sum and difference signals, the difference signal will be lower than the fundamentals.

[CaptDon]

If you have a 2Khz tone at 0Vu reference and you simply add a second tone of 2.080KHz also at 0Vu you can count on the fact that your subwoofer will be emitting an 80Hz 'beat note'!! This would be expected and normal, not a failure. It is the 'difference note' of the two tones and NOT an IMD artifact.

[magic]

Maybe you can count on the fact that subwoofers have IMD, but an actual difference tone is not the same as beating and it is IMD...

It should be possible to reduce this problem by lowpass filtering the input signal.

[mag_therm]

With Capt Don's example the 80 Hz may be present but hopefully at low levels (-60dB) or so, controlled to acceptable levels by low IMD in the amplifiers, and by crossover filters as magic mentioned.

[radiolistener]

this is due to non linear distortion in some component, for example your amplifier can do it or subwoofer. In order to make sure if it's subwoofer or not, try to load amplifier with the same linear load as your subwoofer, put two tone test signal on the input and check the output spectrum, if its intermodulation distortions is low enough, then the issue is caused by subwoofer. If IMD is too high, then root of cause is your amplifier.

It is possible, that both - amplifier and subwoofer can add intermods together.

Also make sure, that you don't overload subwoofer with high power and your amplifier supports your subwoofer impedance. If you use them outside specification, it can produce high IMD

[CaptDon]

Wouldn't we assume the O.P.'s setup would already include a miriad of active / passive crossover components so the IMD would have to occur prior to the crossover networks. Hopefully no 'treble' is present at the subwoofer to begin with. Maybe a bit of low mid is possible.

[ejeffrey]

If you have a 2Khz tone at 0Vu reference and you simply add a second tone of 2.080KHz also at 0Vu you can count on the fact that your subwoofer will be emitting an 80Hz 'beat note'!! This would be expected and normal, not a failure. It is the 'difference note' of the two tones and NOT an IMD artifact.

That's the definition of intermodulation distortion.  If the subwoofer is actually prododucing 80 Hz in this situation  then almost certainly the amplifier is to blame, although it could potentially be the crossover.

Your hearing is non-linear so you may "hear" and 80 Hz beat note even if the equipment is perfect, but if the woofer cone is moving it's a bad or overloaded amplifier.

[CaptDon]

I think my brain was stuck in the R.F. Mixer mode. Yes indeed, IMD is done with a two tone test. It is how my Potomac Instruments AA / AG units make the test.

[testtube44]

i noticed this problem in early 2018 when i first started using this speakerset, so i thought it was cheap shit immediately. i dont overload the amplifier, this is happening at reasonable levels and its very audible. the speakerset is one of those all-in-one setups, the amplifer (and presumibly alot of other circuitry for it) is all inside the bass housing (its a mono bass), and the left and right treble speakers r separate and connect to it, the volume knob is on the right treble speaker. entire setup connects to the sound input by a normal headphone connector (3.5mm i believe?), this is just stereo, no funky surround sound or any of that crap. it does have an amplifier, because it can get very loud, and the bass housing has a 120v ac line attached.

i did have a speakerset like this for many years before 2018, it was a diff one, and it didnt have IMD problems, instead its cables started to break inside (i think) and it became super unreliable so i eventually ended up with a new one.

theyre z323 logitech s-00075a, r these bad?

[magic]

If everything is integrated in one box then definitely the box is to blame. Cheap shit indeed.

If it were mine, I would probably take it apart and try to figure out what exactly they managed to screw up.

[tggzzz]

If you have a 2Khz tone at 0Vu reference and you simply add a second tone of 2.080KHz also at 0Vu you can count on the fact that your subwoofer will be emitting an 80Hz 'beat note'!! This would be expected and normal, not a failure. It is the 'difference note' of the two tones and NOT an IMD artifact.

Er, no. Such "difference notes" are the visible consequences of IMD.

If you have a linear system, put two tones in, you will get the same two tones out. If you have a non-linear system, put two tones in, you will also get sum and difference tones out.

Intermodulation distortion arises because of non-linearities. The degree of non-linearity determines whether you get second-order, third-order (or higher) intermodulation effects. Those effects materialise as sum and difference tones, including sum and differences of the sum and difference tones. Yes, you get all sorts of spurious tones, all over the spectrum at all sorts of amplitudes. Changing the frequency of one tone makes the frequency of the spurious tones vary "wildly".

Perhaps you are confusing the spectral components resulting from IMD with the spectral components resulting from amplitude modulation of a tone? Or maybe an amplifier slew-rate limitation?

Simple tests...

Measure apply two tones of equal known amplitude, measure the amplitude of the difference tones.

Reduce the two tones amplitude by 1dB. If the difference tone's amplitude decreases by 2dB then it is second harmonic IMD. If by 3dB then third harmonic IMD.

[tggzzz]

No expert but I imagine that there ought to be a crossover somewhere, to keep treble out of the sub. Speaker transducers as such are nonlinear so I think they shouldn't even be allowed opportunity to create such problems in the first place

I think it's definitely the amplifier. If the speaker was causing distortion it would be obvious, that only requires a single tone to sound bad, this takes 2. I've also had this thing accept microwave frequencies and produce pulsing sounds. That was when I held the wifi card of an xbox close to the headphone jack bypass and volume knob, perhaps more evidence that it's something "nonlinear".

That is a completely different non-linear effect: EMI/EMC.

The front-end semiconductor junctions are rectifying the RF power, and that causes a DC voltage shift. If the variations in RF power occur at audio frequencies, the DC shift will change at audio frequencies, and will be amplified.

Classic example is with a GSM cellphone. When a call starts you hear the ba-da-dah ba-da-dah ba-da-dah (call setup messaging) followed by the brrrrrrrrrr (voice call) of the 217Hz TDM signal.

The only solution is to prevent the RF entering the front end, which requires EMI/EMC shielding/filtering.

[testtube44]

clearly they didnt rf shield it either, but is that suprising considering the obvious quality problems elsewhere in this mess? just the other day my iphone was creating audible interference from 4ft away!

[tggzzz]

clearly they didnt rf shield it either, but is that suprising considering the obvious quality problems elsewhere in this mess? just the other day my iphone was creating audible interference from 4ft away!

Consider whether it is worth removing the input cables, and seeing whether the EMI/EMC persists. If the EMI/EMC is no longer present, then it may be entering the cables, in which case external filtering may help.

I wouldn't hold my breath, though!