LOUDSPEAKER MEASUREMENT PROCESS
Here is the workflow for a typical loudspeaker measurement project.
1. Electrical Impedance
The electrical impedance is the load presented to the amplifier. It is used to determine the rated impedance, which in turn is used to establish power ratings and select the appropriate wire gauge based on length.
< The impedance magnitude of a popular loudspeaker, showing the rated impedance (8 ohms) calculated from the data
2. Axial Sensitivity
This is the far-field on-axis sound pressure level resulting from the application of a reference voltage (e.g. 2.83 Vrms) and normalized to a reference distance (1 m). A “one number” sensitivity rating is the average of this plot over the loudspeakers useful bandwidth. Most loudspeaker sensitivities must be measured at a greater distance to be in their far field. The sensitivity is then normalized to 1 m by calculation. Our distance limit is 8 m.
The high resolution sensitivity plot can be post-processed into any frequency resolution >
3. Spherical or Hemispherical Measurements
The impulse response / transfer function is measured over the surface of a sphere at an appropriate distance (e. g. 8 m) and at an appropriate angular resolution (e.g. 5 deg). All directivity measures are derived from this data. This data set is used to produce both CLF and GLL files for the loudspeaker.
< There are several softwares that can produce a CLF balloon plot from the measured data set. Shown is Sound Bubble Maker from HX Audio Lab.
4. Maximum Input Voltage – EMAX
A real-time transfer function is used to determine the maximum linear drive voltage for 3 dB of power compression using an appropriate stimulus. This represents the thermal limit of the loudspeaker, and can be used along with the rated impedance to calculate the rated power handling.
A measurement rig for non-destructive thermal testing of loudspeakers >
5. Cabinet Wireframe
A properly scaled wireframe drawing is needed to serve as a visual reference for the loudspeaker in the room modeling program. It also shows the point-of-rotation POR used for the spherical data.
< A cabinet wireframe as shown in CATT-Acoustic
6. Data File Generation
The data described above must be compiled into a proprietary data file used by the room modeling program. Pro Sound Testing produces a Common Loudspeaker Format data file (CATT-A®, Odeon®, Modeler®) and a GLL data file (EASE®). Both data files are produced from the same data set. These data files, when properly authorized, can be directly imported into their respective room modeling platform.
