General notes on sound level calibrators
A sound level calibrator is an indispensable instrument for quality assurance within acoustic measurement technology.
A sound level calibrator is used to check the entire measurement chain - from the microphone to the display.
How to operate a sound level calibrator?
The sound level calibrator is plugged over the microphone and switched on. Usually such a calibrator produces a level of 94dB. In the sound level meter this value is confirmed and the measuring system calibrates itself to the new sound level. Many measurement regulations/procedures require calibration before and after a measurement.
Construction of a sound level calibrator
A sound level calibrator is a small sound source with a precisely defined sound level. Typically, we use a sound level of 94dB. 114dB is widely used in architectural acoustics. Pistonphones with 124dB are typically used in laboratories, but not for field measurements.
A sound level calibrator consists of a small pressure chamber, typically tuned to the excitation frequency.
The sound source itself is usually a small loudspeaker, but may also be a piezo element. More sophisticated sound level calibrators also contain a microphone, so that the sound level inside the pressure chamber is checked and readjusted.
This control loop in particular is very important, as it means that leaks or changes in the chamber volume do not have any effect on the sound level inside the chamber.
Calibrators in the lower price segment often do not include this important function for cost reasons (e.g. calibrators ND9 or AZ8930)
Very high-quality calibrators additionally contain temperature sensors as well as a pressure sensor - a barometer - to compensate for static air pressure.
What frequency does a sound level calibrator use?
Sound level calibrators are offered with different frequencies. Common frequencies are 1000Hz. At this frequency, the A and C weighting curves have an attenuation of 0dB. But you can also find frequencies of 125Hz, 250Hz or 500Hz. However, these devices are typically used for more specialized applications.
The typical accuracy of a sound level calibrator is between 0.2 (class 1) and 0.5dB (class 2).
How often should a sound level calibrator be verified itself?
The sound level calibrator itself should be tested at regular intervals by a laboratory. Typical intervals are 1-2years.
Important notes on the use of sound level calibrators
Always insert the microphone slowly into the sound level calibrator, as moving it too quickly will cause high pressure fluctuations that could possibly damage the membrane of the sound level calibrator or the microphone.
A quiet environment is required for accurate calibration, as external noise can affect the calibration. If the ambient noise cannot be avoided at least a higher reference level e.g. 114dB should be used to have enough distance to the ambient noise.
For measurements at higher altitudes, the enclosed correction table should be used due to the lower air pressure. The deviation between an altitude above sea level of 0m (air pressure 1013hPa) and 3000m (702hPa) is 0.5dB.
What manufacturers and products are available for sound level calibrators?
Almost every manufacturer of acoustic measurement products also offers calibrators.
The worldwide recognized reference is the 4231 from Bruel and Kjaer. This calibrator is type-approved for class 1.
Level 94dB and 114dB at 1000Hz
In the field of class 2 calibrators, the Centertek 326 or Center 326 is widely accepted for its performance. You can find the Center 326 in our Shop.
Level 94dB and 114dB at 1000Hz
In the price range below there are some calibrators from asia. However, these devices do not include a reference microphone. They do not work in a controlled loop. They achieve typical accuracies of 2dB.
The acoustic calibrator AZ8930 is a low price calibrator.
Background information for sound level calibrators
A sound level calibrator generates a sound level with a precisely defined level. Very simplified you can imagine such a device as a loudspeaker with a connected pressure chamber. An electronic circuit generates a sinusoidal signal (usually 1kHz). This signal drives the loudspeaker, which generates a defined sound pressure in the small pressure chamber. The sound pressure depends on the size of the chamber, the temperature and the static air pressure.
Since the chamber volume is quite small, even small changes in volume result in large changes in sound level. Many measurement microphones have slightly different designs. These differences can already have a significant effect on the sound level inside the pressure chamber. Therefore, we do not recommend using devices without an active control-loop. The sound level calibrators described above have a built-in microphone which is used to measure the sound level inside the pressure chamber, thus keeping the sound level constant. This control-loop behavior can be easily checked by pulling the measuring microphone a little bit out of the pressure chamber. The pressure chamber should of course still be sealed. You can now observe how the sound level drops briefly and reaches its reference value again within approx. 500ms.
In order to keep the level constant even under different environmental conditions, some devices often have sensors for static air pressure and temperature. These are especially sound level calibrators of class 0 and 1, which often also have a microprocessor built in. Furthermore, pistophones are also used. Inside a pistonphone a piston moves a defined volume of air and thus generates a defined sound level. Pistonphones are mainly used in the laboratory as reference instruments.
Frequency and sound pressure level of a calibrator
Typically, a sound level of 94dB is used. 114dB is also widely used. The reference frequency is 1000Hz, lower (250Hz) for pistophones.
The frequency of 1000Hz of a sound level calibrator has the advantage, that the weighting filter (A,B,C,D) in a measuring system has no influence on the calibration of this measuring system, because all weighting filters have a gain factor of 1.0 =0dB at 1000Hz. For other reference frequencies, the weighting filter must be taken into account accordingly.
Which microphones can be calibrated?
The measurement microphones are plugged into the sound level calibrators. The calibrators usually have openings of 1 inch for this purpose. With adapters, ½-inch or ¼-inch can also be plugged in. Adapters are available for microphones with other diameters.
Inches are sometimes misleading; they refer to the diameter of the diaphragm. A 1/2" microphone has an outer diameter of 13.2mm.
Studio microphones in general cannot be calibrated with a sound level calibrator. Ultimately, these microphones simply do not mechanically fit into a calibrator. Secondly, they are usually gradient microphones with side openings.
Measuring the transmission factor (sensitivity) of microphones
You can use a sound level meter to measure the pressure transmission factor (sensitivity) of microphones. To do this, connect a millivoltmeter to the output of the microphone. The indicated voltage divided by the sound pressure (94 dB referred to 2.10-5 Pa corresponds to a sound pressure of 1 Pa) gives the pressure transmission factor of the microphone at f=1000 Hz. Typical measuring microphones have a transmission factor of 1-100mV/Pa.
What accuracy classes are there?
According to DIN/IEC 60942, sound level calibrators are divided into different accuracy classes:
- Class 0 Reference standard
- Class 1 Laboratory instrument
- Class 2 Field measurements
- Class 3 Orientation measurement
In practice, only class 1 and class 2 are relevant. The accuracy of a calibrator is between 0.2 (class 1) and 0.5dB (class 2). For the basic accuracy of an acoustic measurement, the calibrator itself has only a minor influence. The difference at 1Khz is maximum 0.3dB. The measurement microphone has a much greater influence. For measurement microphones, the tolerance of class 1 and class 2 differs - depending on the frequency - between 0.5dB and 3.5dB (DIN61672-1:2013). However, class 1 devices are required throughout for individual measurement procedures. Furthermore, class 1 calibrators are extremely stable over the long term and are therefore reliable.
Design of a Center 326
The Centertek 326 is a widely used class 2 calibrator. It is therefore worth taking a closer look at its internal design.
The Center 326 calibrator consists of:
- Pressure chamber. In this small volume the sound pressure level is precisely controlled.
- Loudspeaker: The small loudspeaker generates the sound.
- Reference microphone: This microphone is used to measure and control the sound pressure level.
- Control board
The pressure chamber of a Center 326 with more details
If you are technically interested, we disassembled a Bruel and Kjaer 4231. This unit includes a reference microphone worth about $1000.