Created on 2011-08-24 23:05:00
Question: I currently do acoustic measurements using the MLS method on an AP 2700 Series analyzer. I’m thinking of acquiring an APx500 Series instrument, which uses Continuous Sweep (Chirp) instead. Will I get the same results with both analyzers?
Answer: Yes, you will get the same results, as you can see in the graphs below, as long as the speaker has low harmonic distortion.
If the speaker is being over-driven or has a high amount of harmonic distortion, the Continuous Sweep results will be superior. Continuous sweep only looks at the fundamental frequencies and therefore is unaffected by harmonic distortion. MLS, on the other hand, does not exclude the harmonics, so frequency response and phase measurements will be affected.
Continuous Sweep also has the advantage that it returns 14 results at once (Energy Time Curve, Impulse Response, Level, Relative Level, Deviation, Delay, Phase, Group Delay, Level and Distortion, THD Ratio, THD Level, Distortion Product Ratio, Distortion Product Level, Acquired Waveform), instead of only two (level and phase) as with MLS.
When comparing, make sure that you use the same settings for time delay (to delay the acquisition until the first sound reaches the microphone) and acquisition length (to stop the acquisition just before the first reflection arrives).
Turn off phase wrapping, as the wrapping reference point is sensitive to the pickup of extraneous subsonic noise in the room and may vary between acquisitions. In AP2700, set the Phase meter to Auto, and in APx500, choose “Absolute” instead of "Absolute Wrapped" in the phase view.
Continuous Sweep (Chirp) Level, APx500 Series.
MLS Level, 2700 Series.
Continuous Sweep (Chirp) Phase, APx500 Series.
MLS Phase, 2700 Series.