STEP 1 - Impedance
The Process: Step-by-Step
Here's how we produce loudspeaker data files...
The loudspeaker's electrical impedance is measured. This assures that all components are passing a signal, and that the load presented to the amplifier is as expected.
- The detailed impedance curve can be used to determine the loudspeaker's "rated" impedance.
- The rated impedance is required for wire gauge calculations.
- The rated impedance is used to determine a "power rating" for the loudspeaker, based on the maximum input voltage (MIV).
- The CLF data file includes the impedance curve (1/3-octave resolution), and displays the minimum, average and rated impedance of the loudspeaker.
The impedance curve of an "8 ohm" loudspeaker.
STEP 2 - Sensitivity
The loudspeaker's sensitivity is measured. This is the on-axis frequency response of the loudspeaker in response to a known applied voltage (typically 2.83 VRMS). It is measured in the loudspeaker's far-field, and normalized to 1 m using the inverse-square law. The measurement is complex, in that it contains both magnitude and phase data (the transfer function).
- The sensitivity is the loudspeaker's "frequency response" or more correctly, the frequency response magnitude of the loudspeaker's transfer function.
- It may be averaged to get a "one number" sensitivity rating.
- This is the starting point for SPL calculations, as it can be scaled to greater distances and higher input voltages.
The sensitivity plot, showing the average and the usable bandwidth.
STEP 3 - Polars
The measurement made in Step 2 is repeated at an appropriate angular resolution (1-10 deg, typically 5 deg) over an imaginary sphere surrounding the loudspeaker. This records the loudspeaker's time/frequency response in all directions.
- The mic is placed in the loudspeaker's far-field. Our typical measurement distance is 7.4 m.
- A "robot" is used to reposition the loudspeaker for each measurement.
- The environment is anechoic, achieved by acoustical treatment and time windowing.
- The resultant data is used to produce polar plots (2D) and balloon plots (3D).
- A room modeling program is used to project the balloon data onto the model's surfaces. This "mapping" is done at an appropriate frequency resolution (typically 1/1 or 1/3-octave).
Left - Our custom-built, computer-controlled loudspeaker rotator.
Right - The 4 kHz data balloon (1/3-octave, 5-deg). This resolution is appropriate for coverage mapping in a room modeling program.
A high resolution (1/24-oct) smoothed data balloon for a 2-way loudspeaker (560 Hz). EASE GLL Viewer
STEP 4 - Maximum Input Voltage (MIV)
The maximum tolerable input voltage to the loudspeaker is measured. Response changes from increasing the drive voltage (and therefore the power) result from changes to the loudspeaker's impedance, which cause changes to the loudspeaker's frequency response (power compression). The MIV is used by room modeling programs to calculate the maximum SPL that the loudspeaker can produce.
In 2005 Pat Brown developed a non-destructive method for accurately determining the MIV of a loudspeaker. See the link for an overview. This test method is now part of an industry Standard. The "Loudspeaker Toaster" test is used to determine the MIV needed by the CLF and GLL data files. The procedure is as follows:
1. The loudspeaker's response to a 2.8 Vrms stimulus is measured and stored. This is typically IEC Noise, but other spectra are possible.
2. The ongoing real-time measurement is compared to this 3 Vrms stored response.
3. The drive voltage is increased over time, until the loudspeaker's response has changed by 3 dB at any frequency. This is the maximum input voltage (MIV) of the loudspeaker.
4. This drive level is maintained for 3 minutes, and then reduced to the original 2.8 Vrms.
5. The loudspeaker's response should settle to the original 2.8 Vrms response.
6. The "gain above reference sensitivity" is calculated. This is the maximum continuous SPL of the loudspeaker for the noise spectrum used for the test. It is compared to the SPL measured at the MIV. The MIV is modified to correlate with the measured SPL, if necessary.
Starting SPL: 96 dBZ-Slow @ 2.8 Vrms (should be in reasonable agreement with the measured sensitivity)
Measured SPL at MIV: 115 dBZ-Slow @ 30 Vrms (an increase of 19 dB)
Calculated Max SPL = 96 + 20 log (30/2.8) = 117 dB
Difference Between Measured and Calculated Max SPL = 117 - 115 = 2 dB (due to power compression)
Corrected MIV = 10^((20 log (MIV) - 2)/20) = 24 Vrms
This will produce accurate maximum SPL calculations in room modeling programs.
The test setup used to determine the Maximum Input Voltage (MIV).
STEP 5 - Data File Creation
The final step of the process is to compile the measured data into the data files used by room modeling programs. The standard data package includes a Common Loudspeaker Format (CLF) and EASE® GLL data file.
1. A wire frame drawing is generated for use in both formats.
2. The sensitivity, impedance, MIV, and balloon data are imported into proprietary software.
3. The preliminary data files are generated and submitted for review.
4. Upon approval, the release versions are created. Both are authorized and protected from modification.
5. The files are forwarded to the manufacturer for distribution. Pro Sound Testing, Inc. does not distribute data files.
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