This is the first installment in a series that will explain how various types of audio analyzers function, how they differ in capability and usage, and the way that the test and measurement industry has evolved over the past five decades.
Way, way back in time, graphical acoustic measurements were acquired by means of chart recorders. A chart recorder looks a lot like the lie detector seen in old-time movies and is analogous to a mechanical version of a CRT (cathode ray tube). Chart recorders were widely used before CRTs (and LEDs for that matter) had been invented.
But unlike a CRT that displays pixels on a screen in real time, a chart recorder uses an ink pen attached to, and put in motion by, a coil of wire that works against an opposing magnetic field. To acquire an acoustical measurement, the coil is driven by the output of a pre-amplified measurement microphone. The result is a visual response trace displayed on a roll of paper that continually unwinds as the pen forever inscribes its mark on the paper media.
When using a chart recorder, the stimulus to the DUT (device under test) can be any signal that the user wishes, but useful results only occur when the stimulus is a swept sine wave (for frequency response measurements) or a fixed sine wave (for distortion measurements).
The mechanical design of a chart recorder has much in common with a loudspeaker driver. But instead of producing sonic energy from the oscillation of an LF cone or an HF diaphragm, the result is a visual response plot inscribed by a moving pen.
Chart recorders were manufactured by numerous companies, but those most often used for audio measurement work were the product of Bruel & Kjaer (B&K), based in Denmark. Some notable loudspeaker designers still rely on them today.
As long as no time-domain information is of interest, and the measurement environment is anechoic (that is without echoic reflections), a chart recorder will provide meaningful, high-resolution data…if the pen can accelerate and decelerate rapidly enough so that it doesn’t skew the results.
A chart recorder (also called a pen-plotter) is a mechanical beast that behaves a lot like a loudspeaker; some work well, others do not. They possess no intelligence. Rather, they simply create a graph on paper that reflects the drive voltage, at any given instant, that’s applied to the mechanism which moves the pen.
As a group, chart recorders are categorically unsuitable for measuring sound systems in rooms with the intent of using the resultant chart to apply corrective EQ. Because of their slow speed, one would still be measuring and adjusting the EQ at a gig in Las Vegas while the tour bus has moved on to the next show in Los Angeles.
The next big development in measurement technology was quickly termed a “real time analyzer” or “RTA.” The name came from the fact that it could provide results that didn’t require a paper printout.
An RTA is akin to a collection of separate chart recorders, each filtered to respond only to one part of the audible sound spectrum. But instead of needing a roll of paper, ink pens, and a lot of time for transport and setup, the RTA seemed like an amazing panacea in the world of sound measurement.
Numerous RTA products were built, many units were sold, and to this day there exists an entire culture of sound techs that still use RTAs regularly, but perhaps do not understand precisely what the display – now sometimes even in smart phone form – is actually telling them. It’s also worth noting that without the invention of LEDs as a display medium, RTAs would scarcely exist