Acoustic Calibration Services (IEC/ANSI)

Reasons for Performing Calibrations

Calibration may be required for the following reasons:

• New instrument
• Repair or modification
• The specified time period has elapsed
• Specified usage (operating hours) has elapsed
• Critical measurement - before and/or after
• A special event occurrence, for example
  • exposure to shock, vibration, or physical damage
  • change in environmental conditions
• Performance appears questionable or indications do not match surrogate instruments
• Requirements, e.g., customer specification or instrument manufacturer recommendation

Octave Filters

Periodic tests of Octave band and fractional octave band filters, whether as dedicated instruments, built-in sound level meters, or other instruments are performed according to standard IEC 61260 or IEC 60225.
 
When measuring the spectral contents of sound, the signal is divided into frequency bands with constant relative bandwidth. Constant relative bandwidth means that the width of the band (number of Hz included in the band) is proportional to the center frequency of the band. The most common relative bandwidths are the 1/1 and 1/3 octave bands.
 
Characteristics for the frequency bands are specified in the international standard, IEC 61260. In this standard, two classes of accuracy are stated, 1 and 2, where class 1 is the best. However, up to 1995 an old standard, IEC 60225, applied and many of those instruments specified to this standard are still in use.

Measurement Microphones

Calibration of all types of microphones according to IEC 61094-4 – 1” and other standard 1/2", 1/4", and 1/8" measurement microphones with or without preamplifiers.
 
The sensitivity of the microphone being calibrated is measured by comparison with a reference microphone. The two microphones are exposed to the same sound pressure in an active comparison coupler, which contains a built-in sound source. The measurement frequency can be set to either 250 Hz (252.2 Hz) or 1000 Hz.
 
The measured sensitivity is valid at the environmental conditions during the measurement. Correction is made for the influence of environmental pressure, temperature, and humidity on the reference microphone.
 
The frequency response is determined using an electrostatic actuator (IEC 61094-6). This is a widely used tool that can simulate a constant sound pressure on the diaphragm of the microphone over a wide frequency range. The actuator measurement is a relative measurement. The results are normalized at the sensitivity measurement frequency (reference frequency), 250 Hz (251.2 Hz), or 1000 Hz. Depending on the type and application of the microphone, free-field or diffuse-field corrections may be added to achieve the required type of frequency response.