Re: All CD players sound the same do they?

In article <4b04bee6.156491218@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Don Pearce
<spam@xxxxxxxx> wrote:
On Tue, 17 Nov 2009 17:22:18 +0000 (GMT), Jim Lesurf
<noise@xxxxxxxxxxxxxxx> wrote:

Add in a good dose of averaging and it is all made good.

That is so - if the integration time or time constant are large
compared with 1/bandwidth of the banded noise. Alas, I tend to find
that even 1 second is not really long enough for this when trying to
deal with very low frequencies.

But of course at low frequencies, if you use pure tones, even a tiny
movement of the meter will result in huge changes in level. With a
noise signal you can move the meter about a little without ill effect.

Again, in my experience that tends to break down at low frequencies as
the actual bandwidth in Hz of the noise becomes too small if the noise
is to the same fraction of the octave as works fine at higher frequency.


I suppose it depends what you are trying to measure. If you are setting
up a subwoofer's level the LF noise signal is probably 20 to 80Hz or so.
That is enough to get a decent reading in ten seconds.

The key there is being able to average over 10 seconds. That is easy if you
or I use a way of collecting the signal and being able to do an average
over such a time. But I was referring to the problem people encounter when
just using a cheap spl meter (or even a standard B&K one) which has a much
shorter time-constant. The result is that the visible reading waggles up
and down and getting a 'ten second average' has to be done by mark one
eyeball.

A bit beside the point, but maybe illustrative is a measurement I made
the other day on my monitor speakers. They are Kef Cresta II bookshelf
speakers.

http://www.soundthoughts.co.uk/look/cresta2.png

The three curves show the output of the woofer, tweeter and reflex port.
So with care, measurements can be made at all frequencies in a pretty
convincing manner. It also shows that Kef know what they are doing; I've
always believed that this particular tweeter is worthy of speakers ten
times the price.

Yes, but the above png doesn't look like a video of the handheld spl
meter's display to me. :-)

TBH if doing this with a computer I'd be more likely to record the stimulus
in parallel with the mic output so as to also be able to do correlation and
extract phase/time info as well as aid suppressing background noises. But
again, you can't really do that just watching the display of an spl meter.

Slainte,

Jim

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