Daqarta
Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows
Science with your Sound Card!
The following is from the Daqarta Help system:

Features:

Oscilloscope

Spectrum Analyzer

8-Channel
Signal Generator

(Absolutely FREE!)

Spectrogram

Pitch Tracker

Pitch-to-MIDI

DaqMusiq Generator
(Free Music... Forever!)

Engine Simulator

LCR Meter

Remote Operation

DC Measurements

True RMS Voltmeter

Sound Level Meter

Frequency Counter
    Period
    Event
    Spectral Event

    Temperature
    Pressure
    MHz Frequencies

Data Logger

Waveform Averager

Histogram

Post-Stimulus Time
Histogram (PSTH)

THD Meter

IMD Meter

Precision Phase Meter

Pulse Meter

Macro System

Multi-Trace Arrays

Trigger Controls

Auto-Calibration

Spectral Peak Track

Spectrum Limit Testing

Direct-to-Disk Recording

Accessibility

Applications:

Frequency response

Distortion measurement

Speech and music

Microphone calibration

Loudspeaker test

Auditory phenomena

Musical instrument tuning

Animal sound

Evoked potentials

Rotating machinery

Automotive

Product test

Contact us about
your application!

Sound Card Inharmonic (Intermodulation) Distortion

Intermodulation distortion results when two or more different frequency components interact within a nonlinear system. The output will then contain not only harmonics of the original frequencies, but also components at sum and difference frequencies that typically aren't harmonics of either input. This is also known as "intermodulation" or "IM" distortion (IMD), since the result is similar to multiplying two sinusoids together.

You can demonstrate these sum and difference products by using the Generator setup described for clipping distortion. Set the two sine waves to different frequencies and set both amplitudes over 50%. Clipping will appear on the composite waveform, but not on every cycle... only where both waves are aligned.

For example, set both Levels to 60% and set the frequencies to 500 and 700 Hz. Notice that the most-positive waveform peaks are clipped, and that this happens about every 5 msec. (The same is true for negative peaks.) Now, 5 msec "just happens" to be the period of a 200 Hz wave, which is the difference between 500 and 700 Hz, and you will indeed see a 200 Hz peak in the spectrum, as well as one at the sum frequency of 1200 Hz.

In fact, you will see peaks at all multiples of 200 Hz. If we call the higher frequency f2 and the lower f1, then all these peaks represent "difference tones" at f2 - f1, 2f1 - f2, and all integer multiples Mf1 - Nf2. (Negative and positive frequencies are equivalent here.)

IM distortion is typically a problem when you are generating two tones from the same speaker or other source at high sound levels. Since any source is nonlinear at high levels, the sound output will include not only the two tones you desire (plus harmonic distortion products from each), but also intermodulation distortion products at inharmonic sum and difference frequencies. You must particularly minimize the intermodulation products if you are studying similar products that are generated by nonlinearities in the system under test... such as an ear or a microphone.

IMD is usually measured by applying two primary tones and looking at the distortion products. There are three different standards in common use, involving different primary frequencies and levels. Daqarta's IMD_Meter macro mini-app supports all three standards.


See also Creating Low IMD Acoustic Signals, Distortion - Theory And Measurement

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