From: frank@owlnet.rice.edu (Franklin Tae-Sung Kang)
Subject: EQ/spectrum analyzer tips
Organization: Rice University
Date: Thu, 9 Apr 1992 18:35:43 GMT


Here is an article I found on a BBS that I thought would be of
interest.  It is written by the sysop of a good car audio
BBS in Houston.  

Frank

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Thoughts on the use of Real Time Analyzers (RTAs)

October 24, 1991


     I have had quite a bit of experience using RTAs in setting up
both home and car stereo systems with equalizers.  The following
are points which I think are important to understand.

     1.  First of all, your ear must be the final judge.  The RTA
is just a tool to help you set the equalizer.

     2.  The quality of the microphone is very important.  Most
inexpensive microphones are not very accurate at extremely high
frequencies.  Also, the microphone responds somewhat differently
than your ear.

     3.  Microphone placement during equalizing is very important.
It is often possible to move the microphone just a few inches and
see dramatic changes in the indicated response.  It is preferable
to make measurements at several locations and then average the
response.  This diminishes the effects of standing waves which may
occur at any particular spot.  Of course this can be hard to do
and takes a lot of time.

     4.  The quality of the pink noise source is important.  When
using a tape or CD the tone controls on the head unit should be
set to flat.  If you make your own pink noise cassette, you
should use the best tape you can find and record at a level no
higher than -20 db.  This prevents tape saturation which will
completely ruin the results.

     5.  Normally equalization should be done at reasonably high
volume levels.  However, it is EXTREMELY IMPORTANT to make sure
that NOTHING in the system is clipping.  It is very easy to drive
a tweeter amp into clipping with pink noise.  If anything in the
system is clipping the results will be worthless!  Use an
oscilloscope to make sure.  (Lack of response on the RTA when you
move a slider on the EQ may indicate that you are clipping.  Try
lowering the volume.)

     6.  When you know that everything is working properly, you
should begin adjusting the EQ.  It is usually easiest to work from
the center band out to each end.  When you have adjusted both the
bass and treble go back and start again from the center!  On a
1-octave band EQ you should expect to do this several times.  On a
1/3-octave band EQ plan to do it 10 or more times!  Each time you
go through you should try to refine the curve.

     7.  Don't overadjust.  Don't move any slider more than about
20% of its available adjustment at any time.  Remember that no
equalizer has perfectly steep filters.  The slider for each band
will also affect the bands on either side of it.

     8.  Don't try for a flat curve!  Roll the high end off
gradually.  Your ear is going to be more sensitive to high
frequencies than the RTA.  Experiment with the amount of rolloff
until you find out what you like.  Most people prefer a rising
bottom end.  Don't get carried away with the lowest and highest
sliders.

     9.  It is generally better not to have too much difference
between adjacent sliders.  If a slider is at maximum while the
next one is at minimum you introduce tremendous phase shifts.  It
will normally sound better to sacrifice a little bit of flatness
to keep the sliders from being too far apart.

    10.  Remember that an EQ cannot solve all problems.  If your
speakers have large response peaks or dips which are narrower
than the bands on the EQ you will not be able to adjust them. 
Your results will only be as good as the quality of the
equipment, especially the speakers.


     If you follow these steps I think that your results will be
worthwhile.  Remember that equalization is as much an art as a
science.  Experience will allow you to achieve better and better
results.  Practice and experiment!


Guinn Unger
TDR Interactive Learning
BBS (713) 933-5006
-- 
Frank T. Kang-------------------------------------------------frank@rice.edu
"...if there was a doubling of the current engineering enrollment-the growth 
rate of the economy would rise 0.5% a year.  By contrast, if law school
enrollment doubled, the growth rate would fall 0.3% annually."  BW 4/6/92