Program for measuring IMD

[percy]

I know ARTA and HolmImpulse can do full range THD measurements but it doesn't look like these programs can do IMD. Correct ? Are there any programs that would do a full range sweep IMD ?
Older version of RMAA had customizable IMD measurement but only for one set of frequencies at a time, and I'd like to see a line chart in % or -db over a range rather than a FFT spectrum of the signal.

[PLB]

Hi Percy,

See here --

PRAXIS is a sophisticated audio measurement system

Regards

Peter

[percy]

Thanks. but I suppose sophisticated = pricey ? I am not looking at spending any money or atleast not that much money. I though that was implied when the 3 programs I referred to are all free programs but here I said it loud now!

[jerome69]

Only Holmimpulse is fully free
I think if you want functionalities, you must pay to have them because the developer worked.
Arta, Soundeasy are not pricey.

[cvjoint]

I have a follow up question, somewhat related. In the context of HD testing, are IMD sweeps a lot more accurate than sine sweeps? PE costumer support seems to not know, and I couldn't find much on google or using the search on a bunch of forums.

Excuse the newbiness.

[wakibaki]

I would think you could synthesize a 2-tone test signal, record it to a CD, or play it through a DAC to the system under test from a PC and look at the output with one of the many sound-card based analysis tools.

In fact a quick google gave me this:- Using audio spectrum analysis for end to end linearity testing of a SSB transmitter / receiver which is for a transmitter/receiver pair, but the principle is the same.

w

[speaker dave]

Originally Posted by cvjoint

I have a follow up question, somewhat related. In the context of HD testing, are IMD sweeps a lot more accurate than sine sweeps?

Excuse the newbiness.

IMD tests aren't really more accurate but they might be more relelvant to certain speaker issues.

All are measuring nonlinearity. THD uses a sine wave and shows the harmonics generated. With IM tests a useful look is for measurements of amplitude modulation in the case where high excursion from a bass tone severly modulates the level of a midrange tone riding along. The usual cause is that high excursion gets the coil moving to the point where BL is dropping off at the excursion maxes. The upper tone amplitude directly falls (twice) in each cycle at the excursion extremes. This is generally very audible if the distortion level is moderatley high.

Under the same scenario, though, if the upper tone is above the crossover point then the IM distortion will drop to nothing. So in the end you may need to do a variety of distortion testing with various frequency choices to get a realistic overview of system performance.

David S.

[cvjoint]

David,

I think you are comparing an IMD test with an HD test. My question was with regards to the test signal used to capture HD only, not IMD. I have the Dayton Omnimic setup coming in the mail. That one uses sine sweeps to test HD. From what I gather SoundEasy uses an IMD sweep to perform the same HD test (2nd-5th, and THD). In this case the test is for the same thing but the test signal is of a different nature.

[weltersys]

Originally Posted by speaker dave

IMD tests aren't really more accurate but they might be more relelvant to certain speaker issues.

All are measuring nonlinearity. THD uses a sine wave and shows the harmonics generated. With IM tests a useful look is for measurements of amplitude modulation in the case where high excursion from a bass tone severly modulates the level of a midrange tone riding along. The usual cause is that high excursion gets the coil moving to the point where BL is dropping off at the excursion maxes. The upper tone amplitude directly falls (twice) in each cycle at the excursion extremes. This is generally very audible if the distortion level is moderatley high.

Under the same scenario, though, if the upper tone is above the crossover point then the IM distortion will drop to nothing. So in the end you may need to do a variety of distortion testing with various frequency choices to get a realistic overview of system performance.

David S.

"The usual cause is that high excursion gets the coil moving to the point where BL is dropping off at the excursion maxes. The upper tone amplitude directly falls (twice) in each cycle at the excursion extremes. "

You make an excellent point. IMD has been called "Doppler distortion", the raising and lowering of pitch due to the short waves being produced by a high excursion speaker. Your explanation for the garbled, frequency modulated tone of a speaker pushed hard over a wide frequency range makes much more sense than the "Doppler" explanation.

I had thought of IMD in terms of FM(frequency modulation) rather than AM (amplitude modulation), but FM should not be audible at the short distances involved in a moving speaker.

Turns out what we hear as IMD is AM modulated by the low frequency tones.

Your explanation makes me understand why the sound quality in a wide range speaker can change far more than HD, which is often musical, would indicate, once Xmax is exceeded.

Thanks!

Art