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Audio Breakthroughs with TEF Analysis


Lately I’ve found myself digging through the strata of the last couple of decades and recalled taking delivery of my first big test-and-measurement purchase, a new TEF-10 audio and acoustic analyzer from Crown International Tecron (later Techron) division.

The time was December 1983 and the TEF analyzer was an early Christmas present to myself, ordered the year before, after the AES convention in Anaheim where a group of fellow enthusiasts were wowed with the measurement possibilities.

With the arrival of the TEF analyzer, my consulting business was officially launched. This one dedicated piece of test equipment could perform the array of time-energy-frequency (TEF) measurements based on Richard C. Heyser’s pioneering work on time delay spectrometry (TDS) for audio.

A scientist at the Jet Propulsion Laboratory and an audio enthusiast, Heyser published his seminal paper on TDS in 1967 in the Journal of the Audio Engineering Society (AES). During the years that followed, he published other papers that expanded on the theory and further applications of TDS.

Heyser was not just a theoretician. He used these techniques himself in his loudspeaker testing and reviews for Audio magazine. Audio training company Synergetic Audio Concepts (Syn-Aud-Con), founded by Don and Carolyn Davis, picked up on this work and presented it in their classes.

LESS EXPENSIVE, MORE PORTABLE

Before the TEF analyzer came along, one would need a shelf full of different and expensive instrumentation interconnected with a custom-built interface to make even the basic measurements. While Syn-Aud-Con and a few of my friends did exactly that, for me the TEF analyzer was a more economical and practical approach, (economical, relatively speaking, that is.)

The TEF10 revolutionized audio measurement techniques when it was introduced in the early 1980s. Photo credit: AE Techron, Inc. Although the TEF analyzer wasn’t cheap by any count—at the time even the preproduction price could have been a down payment on a house—but it was orders of magnitude less than the alternatives and much more powerful and portable, albeit on the heavy side.

The TEF analyzer had another key advantage over other acoustic measurement approaches at that time. Even though it was a purpose-built machine, it was also a computer and it wasn’t long before those with programming savvy started to write software that enhanced its capabilities, (we were way ahead of our time with apps).

Those early TEF years of the 1980s were fun and exciting times. Most, if not all, of the early group of TEF enthusiasts were not academicians, just people interested in audio and acoustics, and thrilled with the process of discovery. We got together and measured everything we could—studios, control rooms, concert halls, loudspeakers—around the United States and even in some cities in Europe.

Syn-Aud-Con provided a forum for sharing ideas in the form of workshops and newsletters. As a result, one idea led to many more and new concepts emerged about acoustics and design of sound systems, loudspeakers, studios and control rooms, concert halls and performing arts spaces, materials for acoustical control, how we hear and localize sound, among other things.

TDS techniques give us control of how much space we want to consider around a sound source and measuring microphone. If we’re measuring a loudspeaker we can create a setup to make anechoic measurements in a real room by dialing out early reflections and measuring only the direct sound.

If we wanted to measure anechoically at low frequencies, we would need a larger space, but for most of the practical measurements we needed, the ability to get this information without an anechoic chamber was enormously beneficial.

For control room analysis, we could open up the measurement window to look at not only the direct sound, but early reflections as well. Early control room design often used reflective materials like wood on the front and side walls, and absorption in the rear.

Early reflections from these hard surfaces as well as the console surface itself mixed with direct sound and created nulls and peaks in the frequency response at the mixer’s position. A real-time analyzer using pink noise as a sound source might give some clues that this was happening, but the TEF analyzer with its swept sine wave signal and linear frequency scale display showed very clearly any problems.

Of course these effects were audible to a discerning listener, and for many of us, making TEF measurements helped us to calibrate our ears, as Don Davis would say, to help us hear these effects better.

Interestingly the comb filter patterns generated by these room configurations mimicked comb filter patterns created by the ear’s pinna as sound impinges on the ear from different directions. The comb filter patterns change as the location of the sound source changes. Carolyn A. “Puddie” Rodgers reported these different comb filter patterns are important cues to help the ear/brain localize sound.

LIVE-END, DEAD-END

Others soon wondered if control room design at that time inadvertently created spaces that masked or confused the ear/brain system that prevented us from perceiving directional cues correctly. Also, the comb filtering at the mixing position created a less than smooth frequency response. No wonder why some mixes sounded so different in different control rooms. Perhaps a more neutral critical listening environment was called for.

Acoustical designer Chips Davis proposed the Live-End Dead-End (LEDE) design concept where the front of a control room would be absorbent to soak up the detrimental early reflections, and the rear more reflective, opposite of what had been done. These early LEDE control rooms did indeed produce better imaging and frequency response.

About the same time scientist Peter D’Antonio entered the scene and added broadband predictable sound diffusion products to an acoustical designer’s tool kit. With a background in diffraction physics, and an interest in audio, D’Antonio founded RPG Diffusor Systems Inc. in Upper Marlboro, Md., where he’s president/CEO. Over the years the company’s offerings have expanded, covering a wide range of acoustical products and applications.

Placing diffusion on the rear wall created a more natural sounding space, with the timing of the diffuse soundfield being such that it integrated with the direct sound, as perceived by a person.

With the Energy-Time-Curve (ETC) capability of the TEF analyzer, we could look at the sound field as a function of level vs. time to see this diffuse soundfield, and also to locate any problem reflections. Often we could spot-treat surfaces of existing rooms with appropriate acoustical materials, absorption, reflection, or diffusion; or re-aim or replace loudspeakers to make significant, yet cost-effective improvements.

The TEF analyzer also provided a way to look at the build-up, or more frequently, the decay of sound in both the time and frequency domains together. This taught us a lot about room modes, loudspeaker resonance, and the effects of heavy filtering for digital electronics, among other things.

More innovations in control room design followed, and, of course, this was only just the start of our TEF adventures. Much of what we learned then are taken as “de rigeur” in good design today, but still, there’s enough “interesting” rooms out there that could benefit from our experiences.

Mary C. Gruszka is a systems design engineer, project manager, consultant and writer based in the New York metro area. She can be reached via TV Technology.