RT60 Projects

Reverberation_timer_1Design and Manufacturing of a unique Reverberation System, The Reverberation Timer.

In the spring of 1979, a research project involving the Telecommunication Theory group and the Technical Acoustics group at Lund University began. STU1 financed the project.

The aim was to develop a new method, with less variance, for reverberation estimation using the so called Sabine method (or RT-60 method), developed more than one hundred years ago. The system used a pseudo-random deterministic generator, and was first of its kind. Classical systems used a white or pink noise generator for the room excitation. Many years later, MLSSA2 launched a system based on the deterministic pseudo-random generator, but using the Hadamard3 transform. In the system Lagö developed, a real-time correlator was used, making the system fast and accurate, with no need for a frequency transformation. The system also incorporated Speech Transmission Index calculations (STI) and Modulation Transfer Function (MTF) as well as automatically measured RT-60 values using predefined intervals (-5–35, -5–25 etc).

The electronics design and assembly was performed by Thomas Lagö as a research project engineer. This was a major task and an interesting experience.

The complete reverberation system, with both the sound source and the receiver system, was computerized, using a Z80 microprocessor. An AMD9511 math co-processor was used for the floating-point math operations needed to keep truncation errors low. A 12-bit high performance successive approximation ADC was used with carefully designed sample and hold and pre-amplifier system. The system was built using a modular design, making trouble-shooting easy. The software was written using Z80 assembly language. Below, an illustration of the one-and-only system is inserted.

The intent was to sell and market the new idea to, the sound- and vibration industry leaders, Bruel & Kjaer of Denmark. For many reasons, mainly due to the Swedish STU financing, it was not possible to make that happen. The technology behind the system has remained secret and Thomas Lagö has the only fully documented version of this system in his possession.