The authors present details of the effect of modulation frequency variation upon the waveform of the detected photo-acoustic signal from ferromagnetic resonance in a thin film by both chopping and sinusoidally varying the microwave power level. The deviations from the predicted omega -1 dependence are explained for sine-wave modulation by considering losses to the sample environment. For square-wave modulation, distortion of the waveform and the loss of higher harmonics must also be included. The factors determining the relevance of the application of the photo-acoustic effect to the detection of ferromagnetic resonance are also reviewed.