Spherical Data Collection
The loudspeaker is mounted to a proprietary, custom-built rotator, capable of any angular resolution (down to fractions of a degree). Five degrees is appropriate for most loudspeakers, but higher and lower resolutions are possible for special cases. We have tested loudspeakers that weight more than 225 kg (500 lbs), with lengths up to 2 m (6.5 ft). Our proprietary mounting system does not require modification of the loudspeaker in any way. Custom mounts can be produced in our metal/woodworking shop when needed.
A single microphone is placed at a distance of 8 m. This is sufficient to be in the far field of virtually any point source loudspeaker. The spherical data (transfer function) is collected by an automated process over time, up to several hours for complex devices. The environment is hemi-anechoic, and a time window of appropriate length is used to produce anechoic data.
A little history… Our anechoic wedges are from the original Shure microphone factory, and were acquired by PST when they moved to their new home in Niles, IL. If they could only speak…
< Our stepper motor-driven custom rotator. All steel construction and multiple mounting jigs allow measurement of loudspeakers of any practical size and weight, with no modifications to the loudspeaker.
Hemispherical Data Collection
In-boundary loudspeakers pose some special problems for the measurement process. These include ceiling loudspeakers of all types, plus some types of wall-mount loudspeakers. Our hemispherical microphone array handles such devices. The loudspeaker is mounted in a motor-drive baffle mounted on a stainless steel chamber mounted in a solid boundary. This produces a true infinite boundary condition. A 19-mic array collects an arc of measurements with a single sweep. Each mic has a unique correction curve to match it to our reference microphone. The loudspeaker is automatically rotated and an additional arc is collected. The process is completed until the loudspeaker is completely characterized.
A 19-mic array collects true in-boundary data. The angular resolution is 5 deg for longitude and 1 deg for latitude. >
Ground Plane Measurements
Some loudspeakers are too large for indoor testing. These include the multi-element point sources used in theater systems. In such cases the elements are measured individually in the chamber for their respective balloons and then assembled in software (CATT-A™ or EASE™ GLL). A far-field axial reference measurement for the entire system is made outdoors on a ground plane. This is the axial sensitivity of the system, normalized to 2.83 Vrms/1 m.
Outdoor ground plane measurements have no practical limit on frequency resolution. We can measure subwoofers down to 20 Hz. Subs are sometimes measured on a 2D turntable rather than the 3D rotator since they typically exhibit little directivity control.
Maximum Voltage/Power Testing
Our proprietary maximum voltage test was developed internally and allows non-destructive testing of loudspeakers of all types. A real-time transfer function allows comparison of the low voltage response of the loudspeaker with the response at any drive level. The stimulus is band-limited noise that is appropriate for the device, such as IEC-268-1. This allows us to see the onset of power compression and terminate the test before there is any thermal damage to the loudspeaker. We determined through empirical testing of many systems that EMAX has been reached when the transfer function changes by 3 dB at any frequency within the loudspeaker’s usable bandwidth. Here is an early article I wrote on our “Toaster” test. I’ve refined some details since then, but it is the same in principle.
Once the EMAX is known, it can be used directly or converted to a power rating using the rated impedance of the loudspeaker. Our EMAX procedure is used by other test labs, and is part of AES2-2012.
^ Pro Sound Testing has developed a real-time transfer function technique for continuous response monitoring during thermal testing. Our True-RMS voltmeter is the Audio Precision APX515.
Our loudspeaker testing rig includes Monkey Forest™ PC-based software, DSPs and amplifiers, plus several custom jigs needed to measure loudspeakers. >
