From: brian@calusa.eel.ufl.edu (Brian Gentry)
Subject: Re: RTAs (was call for Papers/Posts)
Organization: UF EE Department (student)
Date: Thu, 19 Nov 92 23:41:33 GMT

In article <75382@hydra.gatech.EDU>, gt0869a@prism.gatech.EDU (WATERS,CLYDE GORDON) writes:

|> 5) Use analyzers to set the system but in the end, USE YOUR EARS! 
|> The ear can detect problems that the RTA cannot, like speakers in
|> incorrect phase, distortion, etc. Get the curve as close to flat
|> as possible, but stay away from radically tilted curves. Usually
|> a maximum of 6 dB drop, uniformly distributed, from 20 to 20k 
|> will result in pleasing balance without sacrificing accuracy and
|> resolution. This, of course, varies to taste. 

I have to agree with you that your ear is a MUCH better judge than a Real
Time Analyzer (RTA).  I would even go so far as to say that they are 
useless in a car environment for detecting tonal balance.  The reflections
from the multiple surfaces in a car cause the RTA to show wildly variant
readings from position to position.  You can get almost any curve you'd
like to see just by moving the mic a little.

Ah hah, you say:  but that's really what it sounds like at that point
right?  Well yes, but your ear/brain "knows" what things sound like in
acoustical environments that it is used to.  For example, you know 
what your friends sound like in your car.  The "frequency response" of 
their voice in a car is very different as compared to a large room, but 
you still recognize their voice because your brain knows what the car's 
environment does to people's voices.

With this in mind, the goal should be to have flat ANECHOIC response
from your system.  That is, the strongest arrival (in time) from the
speakers should give you a flat response.  To do this, you either
need a REALLY good ear and a lot of experience, or you need a TIME
domain measurement device (a device that can discriminate time, energy,
and frequency).  There are currently three products that I know of that
fit the bill.  One is the TEF system 20 by Techron (a division of Crown
International).  Richard Clark made extensive use of the TEF analyzer in
setting up his infamous Buick Regal Grand National system.  The TEF
uses a swept sine wave (sometimes called a gated sine wave).  DRA labs
makes a product called the MLSSA (pronounced "Melissa").  This device
uses a variant of the Fast Fourier Transform in which a Maximum Lengh
Sequence is the source of sound.  This MLS sounds a lot like a "click".
The final product is made by the same people who make the computer 
program LEAP (Loudspeaker Enclosure Analysis Program).  Their product,
like the current TEF model, is a plug in card for the IBM PC.  It also
uses gated sine waves to make it's measurements.  All of these products
can do a lot more than just frequency response; they are serious tools.

In short, don't take any RTA measurement seriously unless in was made in 
an anechoic chamber.  This includes the "in car frequency response" curves
that Car Audio and Electronics publishes in their reviews.  When I see
these plots in their magazine, it's really hard for me to take them 
seriously anymore.  Just look at the plots in this month's (December)
issue in the review of 6 1/2" component systems.  All of the "in-car"
plots look virtually identical.  Even if the car was anechoic, the
resolution of the measurement is limited to 1/3 octave intervals, which
means there are only 3 points between 10 kHz and 20 kHz.  This is 
obviously hideous resolution.

Enough RTA bashing for one day.


			Brian L. Gentry
		    (brian@sioux.eel.ufl.edu)