Nor1253 Calibrator, Electronic Pistonphone
The “Electronic Pistonphone”
Being a class L calibrator (“Laboratory standard” – previous class 0) the Nor1253 is often referred to as the “electronic pistonphone” since it maintains an accuracy normally associated with a pistonphone without the need for a barometer to make corrections for variations in the ambient pressure.
Standard class L/class0 version of the Nor1253 comes in two versions, producing 124dB SPL re. 20µPa at 250Hz or at 1000 Hz.
However, special versions are available. These will be of Class 1 accuracy. The output signal level is then factory preset to any level within the range 114–124dB and the frequency may be factory preset to any frequency within the range 125–1000 Hz. None of these settings are user-adjustable.
Effective Volume Corrections
Different microphone cartridges tend to have different effective front volumes. The working principle of our calibrators produces a large effective coupler volume. The variations in the sound pressure level inside the coupler due to variations in the effective front volume will therefore be insignificant for most applications.
The Nor1253 is adjusted with the ½-inch adaptor for an equivalent microphone volume of 250mm3, which corresponds to most ½-inch microphones with protection grid mounted like the Nor1220 and Nor1225.
If the cartridge under test has a different volume, the accuracy may be improved by applying a volume correction.
The typical sensitivity for the level change is -0.00002dB/mm3 (depends on calibrator frequency). Hence, if a ½-inch cartridge with an effective front volume of 150mm3 is used, the level inside the coupler will typically reduce by 0.002dB because of the volume reduction of 100mm3.
If no adaptor is used, the nominal effective front volume for no correction is 1333mm3. This corresponds to most full-inch cartridges with protection grid mounted.