Serge Auckland wrote:

-- http://audiopages.googlepages.com "Rob"
wrote in message
...
Serge Auckland wrote:
"Rob" wrote in message
...
[snip to bit about measurement]

1. Listening. The physical swapping out is doable. I would
find/have
found level matching incredibly difficult. I tried this some
months ago
with a 20-20k hz cd, taking levels at various points. I
couldn't get
close to level matching across the range - I'd got weird
'spikes' at
points in the frequency range, around 2000hz IIRC. Even using
the same
amp! Anyway, this sort-of-obviously means I need some more
reliable
measuring equipment and techniques - any pointers here would be
appreciated.

This is quite normal:- If you are level-matching by using a SPL
meter
in-room, then what you are measuring is the sound pressure at the
point
at which you are holding the meter (so much is obvious :-) ) At
mid-high
frequencies, the measurement you make will be very influenced by
local
reflections, even from your own body. If I put my SPL meter on a
photographic tripod, then move around even only by a few
centimetres, the
reading on the meter will vary by a dB or more at mid-high
frequencies.
At low frequencies, it is much more stable, being dependent on room
reflections, and hence positioning in the room, much less
influenced by
body movements as the wavelength of the sound becomes larger than
my body
dimensions (even in my current less than sylph-like state)


Yes, I'd realised this - if not strictly taken it into account.
Quite how
I clamp the listener's head, and avoid ear twitching, is an issue
for a
later day :-)

You should level-match by measuring across the 'speaker terminals
ideally
by using a high impedance audio millivoltmeter (once called a
valve-voltmeter). I have found that my normal inexpensive
multimeter is
accurate enough at low audio frequencies. Mine actually work fine
up to
20kHz, but if you use a 100Hz tone for level matching you should
be fine.
Use your SPL meter to make sure the volume level is around 85dBC
(say
80dBA) at 100Hz and level-match with the multimeter at that
loudness. You
should fine it easy to level-match to better than 0.5dB even with a
multimeter.


Ak! I can't have that I'm afraid. I think I understand what you're
saying,
but what I'm saying is that loudspeakers are non-linear loads and
something (I know not what) might be happening between the signal
and the
sound. I suppose my thinking here comes from tests of kit I've read in
magazines, and the suggestion that different amplifiers do not do the
'wire with gain' thing in a linear way. But I'm afraid I can't
remember
the details - I don't even have a hifi magazine in the house,
having given
up on them many years ago.

Loudspeakers are *not* in any way non-linear loads. The impedance of the
'speaker system varies with frequency, but (except for some electrostatics)
there is no variation with level, consequently they *are* linear.
Amplifiers
are also linear to the limits of their distortion characteristics, which is
why amplifiers need to have low distortion, maintained into the minimum
load
presented to them.

***** That being the case, it's very likely that you're right - most
modern amplifiers sound identical. I suspect we're at odds about the
definition of linear, and variables under consideration, and hence the
maintenance of linearity across all variables. It makes *sense* to me to
measure the sound and not the voltage, but I think you're saying that
has no sense - the two things come to the same in a linear way, and it's
a waste of time measuring sound when there's a far easier method.


What you may be referring to is that loudspeakers have
impedances that drop well below (and raise well above) their nominal
figure.
Some amplifiers may change their sound at the impedance minima. This is
true, but this is why I say that two amplifiers being compared have to be
used within their design limits. If one amp is designed for 4-8 ohms, and
another amp is designed for 3-8 ohms, then both should sound identical into
a nominally 8 ohm 'speaker, but could well sound different into a nominally
4 ohm 'speaker as the minimum load could be 3.2 ohms (the minimum figure to
qualify for a 4 ohm rating).

****** That's OK - I don't have any speakers below 3.5 ohms, and I'm
pretty sure all the amps are OK to 4 ohms. And yes, I think that's where
I get the non-linearity thing from, at least in part.

Rob