Detector Voltage
Suppose you are attempting to measure the intensity of sound waves reflected from a sample?
You measure detector voltage versus frequency and obtain the curve shown in Figure 7.4. Then you use a constant (reference) sound source and measure the detector response. In the data obtained from the sample, one peak represents a true sound wave and the other peak is an artifact introduced by your detector. Which peak represents a true sound wave?
A. Peak at approximately 38 kHz
B. Peak at approximately 18 kHz
I only have one shot @ this so convince me 
Link to the figure: http://twitpic.com/12pe21
You didn’t say the reference source frequency has any relationship to the true sound wave, so it’s not valid to prefer the 18 kHz just because it matches the “detector response” frequency. However I prefer the 18 kHz anyway because many times I’ve found that nonlinear electronic equipment will add a higher-frequency artifact due to waveform distortion which always generates higher harmonics. And given possible errors in frequency measurement, plotting, etc., the upper peak could quite plausibly be the 2nd harmonic of the lower peak.
Addition of a lower-frequency artifact, meaning that 38 kHz is the true wave, is less likely. You could dream up various scenarios such as beating or sample-rate aliasing that could cause it, but I still prefer the waveform distortion scenario (B) since detectors are inherently nonlinear.
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