other meeting reports
2/12/02 Meeting Highlights
by Bob Zurek
A High Frequency Acoustical Element for Multi-band Articulated Line Arrays AND The EVO powered DSP intelligent loudspeaker system.
On February 12th over 40 members of the AES Chicago Section were treated to not one but two presentations by Doug Button of JBL Professional. The first portion of Mr. Button's was the paper he presented at the 111TH AES Convention entitled "A High Frequency Acoustic Element for Multi-BAnd Articulated Line Arrays." Mr Button began by explaining that the purpose of his work was to predict frequency response at any point in the vertical plane. This work was an extension of work previously done by Ureda. The ultimate goal was to show a comparison between flat and slightly curved line arrays. Mr. Button showed that in the near field, the line array produces an interference field and thus does not drop off as 1/r. In order to predict the response, Doug had to ensure that the curved wavefronts emanating from his 3 element compression horn enclosures produced a directivity response close to the response of an equivalent ribbon. This requirement led to the design of a high frequency element that had a wave front with curvature of less than 1/4 wavelength at 14 kHz. In order to calculate the frequency response Doug utilized a brute force summing routine in a spreadsheet. The examples Mr. Button showed involved three high frequency cabinets which each had three compression horns in them. Cases were studied in which the faces of all of the cabinets were in line, as well as 1 degree and 5 degrees off axis from the center cabinet. Mr. Button showed through simulation and measurement how as the face of the array is curved, the high frequencies drop off. He also showed that as the array is articulated, the frequency response remains nearly identical versus distance, which leads to a system that can be equalized at one distance and have a good response with that single equalization independent of distance.
After a short intermission Mr. Button proceeded with the second portion of his presentation which concerned the JBL EVO DSP powered intelligent speaker system. Doug started off by saying that whenever something goes wrong with an installed public audio system people usually say its the speakers' fault. In light of this Doug's group was given the task of developing a system that would adapt itself to its environment. The goal was to have a true plug and play type system where a customer could plug the system in and let it make corrections itself. The complete Evo system or "happy meal" as Doug put comes with every thing to set up an address system for a 500-1000 person space. Included in the system are microphones, a mixing board, an EVO hub, and powered speakers with internal Digital Signal Processing. Each speaker unit comes with an A/D converter, a DSP section for processing, two low frequency 14 inch transducers driven by two 600 watt class D amplifiers, and a 2 inch compression horn driven by a 100 watt linear amplifier. The system is set up to send analog signals to each speaker, which results in no system latency, no drop outs, and no jitter. The only latency occurs in the speakers themselves. All of the system settings are sent from the EVO hub to each transducer where they are saved. Since the DSP settings are saved both on the hub and in the transducer there is little worry that the settings will be permanently lost. communication between the hub and the transducer is via an ultrasonic frequency shift keying at 190 and 210 kHz. The attainable transfer rate is 9600 Baud, which will allow download of new software in minutes. Options available in the DSP are crossovers, equalization, delay settings, limiters, and power compression compensation. The power compression is handled via a cascaded R-C model with frequency dependent cooling factors for each channel. This model predicts the voice coil temperature in real time and adds appropriate compression compensation. The model also gradually reduces the gain to the loudspeaker if one of the transducers is in trouble. Doug and the local JBL representative Jeff Phillips from Sound Marketing then demonstrated the system in the meeting room for the audience. They showed the auto equalization and delay operation as well as the systems fixed and active feedback suppression. Doug concluded the presentation by stating that with the EVO system when the system sounds good it IS the speaker's fault. The meeting concluded with a question and answer session. The AES Chicago section would like to thank Doug and Jeff for an interesting and entertaining presentation.