Chapter Excerpts

Chapter 6: Audio & Acoustic Measurement
Magnitude and Phase Response

Prior to Heyser, real life data from manufacturers were usually frequency vs. level responses and rarely phase response. The magnitude response is the most familiar measurement to many. It has limited value without the accompanying phase response. The phase response requires “fine tuning” via the micro-second adjustments available in modern analyzers.The adjustment is used to bring the phase response to 0° wherever the magnitude response is uniform. Once this has been done, if the device is minimum phase the peaks and dips on the phase response will be opposite the “slopes” on the magnitude response.

Why all the emphasis on minimum phase response? It is because you cannot apply conventional inverse equalization to the magnitude response unless that portion of the magnitude response is minimum phase. Non-minimum phase usually implies a significant signal delay.

Magnitude response has a vertical decibel scale and a horizontal frequency scale. Phase has a vertical scale in plus or minus degrees and a horizontal frequency scale. In all measurements it is vital to know what the frequency resolution is. Resolution that is too broad gives optimistic smoothing whereas resolution too narrow includes, in many cases, undesired reflected information.

Measuring phase instead of magnitude provides greater sensitivity and resolution. For example, finding resonant frequencies (phase passes through zero at resonance), the phase response will typically be 10 times more sensitive than the magnitude response. Acoustic delay problems jump out in phase and can be difficult, at best, with magnitude response.