The resolution problem relates to how the RTA presents its information to the user, and isn’t so much a fundamental flaw with the tool as it is a flaw with the packaging.
The more serious flaw with an RTA measurement is that the device doing the measuring has no idea what kind of signal went into the system, or at what time, in order to see the difference between what the PA should be doing and what it is doing.
To understand why this is so critical, let me introduce the concept of a transfer function (often abbreviated TF) analyzer, which is the real measurement tool we’ve been seeking in the first place.
The transfer function measures what’s coming out of the mixing console on its way to the PA (let’s call this the source) and then uses a microphone (let’s call this the reference) to measure what that signal looks like after it’s come out of the loudspeakers.
The measurement software can then display the difference between these two signals, and the operator can see in an instant how much of the information they put into the loudspeaker system transfers back out.
In a perfectly linear system, the display would show only a horizontal line. In the real world it looks more like Figure 3.
Of course, this is a simplification, because the transfer function can be used to see the effects of any system on any signal. By measuring what gets put in and comparing it to what comes out it is possible to precisely determine the actions of equalizers, crossovers, effects processors, microphones – almost anything audio.
This tool is in no way limited to analyzing loudspeaker systems, it just happens to be used for that a lot.
There are a handful of products on the market that have transfer function capability – Smaart, TEF, SIM, EASERA, and so on. All we really need to understand is that, at their core, all of these tools do the same thing, albeit in slightly different ways.
An experienced engineer seeking to accurately measure a loudspeaker system could use any of these tools and, while some are more suited to live work than others, get usable results.
The most obvious benefit of the transfer function is this ability to show the changes a system makes to a signal, but the advantages don’t stop there.
Because the transfer function knows what’s going into the system and what’s coming out, it can measure the difference in time between the two measurements and align them. This time difference can be due to anything from signal processing delay to the distance between the loudspeaker and the reference microphone.
Once this delay is compensated for, and the two measurements aligned, advanced transfer function measurement tools then apply what are called “time windows” in order to separate the system’s response from the room’s interactions with it.