Anechoic driver measurements with different measuring techniques

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#41
speaker dave said:
I have absolutely no axe to grind with regard to MarkAudio, but I also feel that those curves are unusual to the point of being suspect.

I have been measuring speaker professionally for over 30 years and used a number of anechoic chambers as well as ground plane and impulse test approaches to anechoic measurement.

Although most chambers have significant low frequency flaws, essentially the standing waves of a typical room show through below the effective range of their wedges, they also usually exhibit a clean smoothness that no in-room curve can ever show. The curve shown in the OP looks more similar to an in-room with frequency smoothing (say 1/6th Octave) type of curve. It has the randomness of a noise measurement, although that randomness would typically increase for lowest frequencies.

It doesn't help that the curve uses a 120 dB range (50 is more typical) so it will look way flatter than otherwise. Still, the 10dB narrow band variation is considerable for an anechoic measurement.

I also wonder about IEC measurements. I don't know if that is and open back IEC baffle or an IEC dimensioned cabinet, but either is likely to put some response signature into the curve due to diffraction effects (1 m measurements will help/reduce that, but should also knock down the random wiggles).

There is a concept in measurements called "quefrency". This is an intentional flipping of the word "frequency". Basically it is to look at the wiggles of a response curve and interpret them as a wiggle frequency. (A log magnitude conversion of the frequency response curve as if it were a time waveform, which it isn't.) The understanding with quefrency is that a reflection close in time to the sound's origin can only give low quefrency wiggles. High quefrency wiggles require much later reflections.

As the curve shown has high quefrency wiggles all across the curve it implies that many diverse and late arriving reflections are contained in the measurement and at a level sufficient to give 10dB variation. That is unlike any anechoic measurement that I have ever seen. Even a bad chamber with a heavy floor grate will show a single periodic ripple, (Scrunching together at high frequencies on a log f plot) rather than the random variation.

Anyhow, the curves look suspicious to me. (The Zaph Audio curves mentioned look quite respectable.) I make no comment/have no opinion on the product itself.

David
Click to expand...

Agreed. I can't help but wonder if this measurement was taken with a broken mic cable or something as ridiculous as that.
 
#44
I do not read the single driver forum and do not know the relationship/history between the OP, moderators and Markaudio. If the plot in the opening post is being used as part of the marketing by a company then querying it would seem to be not only reasonable but also natural for those with an interest in the measurement of loudspeakers. Like others I could have a guess at what has happened but given the markaudio poster is now active asking him if he would like to respond seems the most straightforward way to make progress.
 
#45
The graph shown is definitely suspect. Looks like a systematic error in the measuring procedure. There's a uniform width of the spikes which doesn't make sense.

Has the manufacturer been informed? What is his response?
 
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