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Speaker Measurements

Posted by John Atkinson (R) on October 28, 2017 at 04:44:22

John Atkinson wrote:
> > please let me know where I have been factually incorrect < <
>
> Charley Hansen wrote:
> I'm often taken aback at some of the assumptions you make about the
> reasons for and/or the significance of specific measurements.

I am starting a new thread as Charley posted this in the MQA thread that
refuses to die and his question has nothing to do with MQA.

> For example in the recent review of the KEF Reference 5, in the
> "Measurements" sidebar you state "The traces in fig.1 are free from the
> small discontinuities that would suggest the presence of panel resonances."

Yes, that's what I wrote.

> I have seen many, many cases of "small discontinuities" in the impedance
> traces of many different loudspeakers. While I suppose it may be
> *possible* that one of these could be due to a "panel resonance"(especially
> a large panel of thin material - 1/8" Masonite or similar), I have never
> seen this in my life. What led you to this belief?

A quarter century of measuring loudspeakers, Charley. In 1992 I was
investigating speaker panel resonances in order to write the article
reprinted at https://www.stereophile.com/features/806/index.html. I
noticed that the frequencies of the panel resonances correlated with small
glitches in the impedance magnitude and electrical phase angle graphs when
plotted with sufficient frequency resolution.

I mentioned this diagnostic nature of the impedance measurement in the
paper on speaker measurements I presented to a 1997 AES convention - see
linked page below. I had found that it isn't just the panel behavior that
gives rise to these discontinuities but resonances of all kinds. These
include things like a metal-dome tweeter's primary resonance and pipe
resonances in reflex ports and transmission lines.

There isn't a 100% correlation. For example, in the case of the KEF
speaker, I did find a couple of panel resonant modes with an accelerometer
that didn't affect the impedance measurement. This was perhaps due to
these resonances being low enough in both amplitude and Q to be detected
with the frequency resolution I was using. But almost always if I find
some kind of glitch in the impedance measurement, I subsequently find some
sort of resonant problem.

John Atkinson
Technical Editor, Stereophile

• https://www.stereophile.com/content/measuring-loudspeakers-p   (Open in New Window)

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