In article , Rob
wrote:
Jim Lesurf wrote:



One of the things I would recommend people do is to make some
'baseline' measurements of the in-room response before they start any
changes.

[snip]

Jim - or anyone. I did try this and got quite bizarre results - huge*
dips at certain frequencies I would not have expected, in fact the only
flat region was between 2kHz and 4 Kkz. That was using various frequency
steps (60) between 100Hz and 6KHz.

TBH the results you got are probably 'typical' rather than 'bizarre'. :-)

The problem is that the acoustics of most normal domestic rooms will
produce large variations in response as you change the frequency of
sustained tones. You will also find that moving the speakers, or listening
location, or other furniture, will alter the details!

This was why I was saying that any such tests are really for 'comparison'
purposes - a 'before' to compare with 'after'. Ideally keeping everything
*except* the changes of interest (e.g. use of active arrangement, etc) the
same. This means the room effects are the same 'before' and 'after' so you
can assess what you are interested in.

For a quicker cruel test - say 10 steps - what would you regard as the
key frequencies at which levels should be equal?

If you want to keep down the number of measurements then the best bet would
be something like 1/3rd octave bandlimited noise. This tends to average
over some of the room 'peaks and dips' and will give a broad-brush
impression for comparisons. This means you can cover 20Hz to 20kHz with
around 10 bands. If using sinewaves, I'd tend to recommend more than 10
frequencies unless you just want to focus on one range - e.g. just the bass
region.

FWIW I have an 'Alan Parsons' test CD which does have banded noise for
tests like these. IIRC they are 1/10th octave, but I can't find the CD at
the moment to check!

In most rooms the best thing is just to experiment a few times to see how
consistent you can get the results before moving on, and then use a set of
test signals that seem to give the most consistent results

Thanks

Rob

* well, the peak signal was 60dB (flat 1.7-3.9kHz) at the listening
place; min 20dB at 500Hz, 100Hz; and 35dB 5.4kHz and 4.5kHz

You have discovered one of the reasons we can take the responses in
magazines with some caution. :-)

Slainte,

Jim

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