Hemispherical Loudspeaker Balloon Data Using an 1/2-space Mic Array

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Pat Brown shows a technique for collecting fast, accurate hemispherical loudspeaker balloon data.

I frequently get requests to measure hemispherical loudspeaker balloon data for ceiling and wall-mount loudspeakers. This information must be collected in half-space, so in 2016 I built a mic array for this purpose.

The array consists of 19 microphones at 5 degree angular resolution. The concrete floor was excavated to house a 1 x 1 x 1 meter stainless steel-lined “pit” that houses a motor-driven turntable (LinearX LT360).

The rig allows the measurement of boundary-loaded loudspeakers with any acoustic symmetry – polar, 1/4, 1/2, or none at a 2 meter microphone distance. The time window of 60 ms provides a frequency resolution to about 50 Hz.  

The data can be used to produce GLL files that are compatible with EASE, EASE Address, EASE Evac, as well as the CLF format used by CATT-Acoustic, Odeon, Bose Modeler, and JBL DDA.

The photos below shows pictures of the array, the steel-lined pit, turntable and an example of the balloon data. pb

A photo of a 19-microphone 2 meter array

Above – A 19-microphone 2 m array. Each microphone has an individual compensation curve to match its response to my reference microphone.

Photo of the steel-lined pit and turntable

Above – Under construction. 

The steel frame tubing had to be drilled for each microphone jack.

Above – The steel frame tubing had to be drilled for each microphone jack. All wiring is internal to the frame, delivered to each hole via a fish tape.

Hemispherical Loudspeaker Balloon Data

Above – The SPL distribution over the surface of a hemisphere for a 1/3-octave band.

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