AES Section Meeting Reports

San Francisco - June 14, 2011

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Christopher J. Struck of CJS Labs gave a presentation to the San Francisco AES Chapter at the Dolby Laboratories Screening Room.

Mr. Struck is CEO and Chief Scientist of CJS Labs, a consulting firm in San Francisco specializing in Audio and Electroacoustic Design and Consulting services.

The presentation revolved around a hands-on demonstration of his simulated free field test rig, automated turntable and custom designed free field "point source" set up for microphone measurement. Several attendees also had the opportunity to have their microphones measured, including frequency response, sensitivity and polar pattern.

Mr. Struck presented the different elements used in his measurement rig, including hardware and software and then elaborated on each component describing other options and methodologies.

The rig was based on using simulated free field techniques with a substitution measurement method.

The microphone substitution measurement method relies on the following factors to be successful:
Correct calibration of the reference microphone.
Accurate substitution process for the microphone under test (MUT), requiring repeatable positioning and a stable and adequate sound source.

The rig presented in the meeting included a custom coherent audio source developed by Mr. Struck with a coincident source driver where the tweeter is located at the acoustic center of the woofer. Other aspects of the design included a very defined off axis response over frequency as well as the capability of producing adequate levels for measurement.

Mr. Struck explained the different methods used for equalization of the sound source/sound field the MUT is exposed as well as the process required to obtain the simulated free field conditions and the signals used for the measurement using log sweep time delay spectrometry methods.

Other options for free field measurements include the use of an anechoic chamber. A brief explanation of differences and limiting factors as well as pros and cons between the methods was also exposed by Mr. Struck.

The presentation finalized with a description of other microphone measurements.

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