To go back to my original example of the room that is excessively reverberant at 500 Hz, a properly set up transfer function would see the 500 Hz segment of the signal going to the mains and know based on this time alignment when to expect it at the microphone.
Once that time has passed, it ignores unrelated 500 Hz content that may arrive at the microphone, “windowing out” a large number of room reflections and reverberance. The measurement engineer can then properly equalize the system without having the measurement affected by the room or other noise sources unrelated to the PA.
Then, satisfied with the system setup, other tactics can then be used to deal with the effect of the loudspeaker system on the room. If the only tool available had been an RTA, the engineer would have been unable to distinguish between a system response bump and a reverberant room, or between air handling system noise and what’s really coming out of the loudspeakers.
There are a number of other technical things that can be seen with a transfer function measurement, like phase, that are beyond the scope of what we’re discussing here. Suffice to say that there’s a reason that advanced measurement engineers almost only use some form of transfer function, with RTA style measurements relegated to quick spot checks and analysis of room issues.
The bottom line is that RTA is a tool with limited application, and auto RTA has no value whatsoever.
An auto RTA computer doesn’t know the difference between the room and the sound system, doesn’t know what you’re putting into the system, and doesn’t know what the loudspeaker system’s response should look like in the first place. All it knows is that the microphone is seeing less of one frequency and more of another, so it adjusts them back to some arbitrary baseline.
Measured deficiencies could be because the PA simply isn’t capable of full output down there at 40 Hz, or it could be because your mid/high loudspeakers are interfering with each other.
Boosts could be anything from real system problems, to a room with long reverb decay, to having placed the measurement microphone too close to a wall.
Unlike the system user, with a human brain and ears, the auto RTA sees only numbers. It makes decisions based on far too limited information and often tries to solve problems that can’t be equalized with the only tool it has: 31-band EQ.
All measurements require human interpretation, because in live sound every part of the sound system, from the microphone through to the loudspeakers, affects the final result.
From the genre of music to the size of the venue, from weather conditions to the number of audience members, even a very well designed and exceptionally powerful computer doesn’t have enough pieces of the puzzle to do a good job.
Most importantly, no computer hears like we do, so no computer can accurately judge the effects of changes it makes upon the quality of the sound. If computers could make accurate equalization decisions I’d be out of a lot of work, and if a 31-band EQ were all one needed to correct sound system problems, I’d be living in a gutter.
Fortunately, making a sound system sound good requires a diverse skill set that no piece of electronics can master. Any measurement tool is just one small portion of an experienced system engineer’s arsenal.
Only by carefully considering a situation based on their knowledge of acoustics, skills with a good measurement tool, and past experience can any engineer make good decisions.
All of the tools discussed here should only serve to aid us in deciding how to solve a problem they face. No squiggly line on an analyzer display shows anyone how to optimize a system, but it does give more information to apply to an issue they can hear. Equalization starts and ends with the tools attached to the sides of your head.
Bennett Prescott is a frequent contributor to the ProSoundWeb Live Audio Board, and he owns a sound company in New York State and represents EONA ADRaudio in North America.
More articles by Bennett Prescott:
Clearing Up Certain Line Array and Point Source Loudspeaker Misconceptions
Managing Power To Properly Use (And Not Abuse) Professional Loudspeakers
Staying Analog In An Increasingly Digital World