Some RTAs have selectable time-weighting curves that will speed up their response time when only viewing higher frequencies.
While that can be a useful feature, all brands and models share the same inherent limitation: they must account for the crest factor of the applied noise by slowing down their speed of response whenever low frequency measurements are made.
The dynamic weighting of the filters in the LF range require quite a few seconds before the LF damping can settle in, eventually producing a stable and readable response curve.
The basic design concept of early analog RTAs was intended to dovetail with the basic design concept of the first generation of graphic equalizers. A one-third octave “graphic,” along with its less expensive sibling, the one-octave “graphic,” were the only type of room equalizers available back in the day.
Graphic equalizers have persisted for many years and are still commonly in use because they’re readily understood and provide an easy means of shaping the sonic signature of a sound system by ear (or by using an RTA). Once a response trace is acquired with an RTA, the technician then adjusts the various bands on the graphic equalizer until the display on the RTA reads flat, or whatever preference-curve the technician is seeking.
The basic theory seems to be sound (pardon the pun), but is actually laden with significant limitations. However, before explaining that, it should first be noted that the concept of precisely tuning a sound system by means of parametric equalization emerged much later, and in large part due to the introduction of higher resolution measurement systems.
An RTA is an “amplitude versus frequency” device confined to only the frequency domain; it has no way of measuring the time domain or “phase versus frequency.” It therefore depicts only a small portion, almost a shadow if you will, of the true system response.
I once replaced a large sound system in a very large and highly reverberant modern cathedral. The previous system designer had spent many weeks “walking the room” with a handheld RTA, logging the frequency response at almost every individual seat. Wherever he saw a deficiency in the response curve he would then install a loudspeaker, or sometimes only an HF horn and driver, in the ceiling above wherever the deficiency was logged.
He would then apply EQ until the RTA response would read perfectly flat. By the time the installation was complete there were about 100 loudspeakers in the ceiling, plus about 3,000 pew-back loudspeakers, plus three main clusters flown over the stage in an LCR configuration. A contractor’s dream come true! The cost was more than 1 million dollars.
Unfortunately, the system produced an unintelligible mess of sound because nothing in the time domain had been accounted for – only the measured response in the frequency domain. It was eventually replaced with 11 small, 2-way trapezoidal loudspeakers and two smaller fill loudspeakers. This approach provided the intelligibility that was utterly absent in the first system design, and remained in service for 15 years before being upgraded because of a desire for higher power.
So how did I measure the new, simplified system to adjust the all-important EQ?