FAQ-L.2: How are test instruments calibrated and adjusted using air-borne sensors?

Are the acoustic level values displayed by analog ultrasonic testers in dBμV comparable / convertable to the SONAPHONE® level in dB?

When testing with air-borne sensors, the sound pressure is usually indicated by a level value in dB or dBμV. Using the level in dB the measured values are converted to a logarithmic ratio based on a reference value. Thus, measured values can be represented in a large dynamic range. The level output is determined by the type of instrument and the sensor used.

SONAPHONE

The air-born sensors BS10 and BS30 of the SONAPHONE use a broadband-sensitive microphone capsule that converts the sound pressure into an AC voltage. The sound pressure level is based on the acoustic physical comparison variable of 20 μPa. 20 μPa correspond to a sound pressure of 0 dB, further examples are:

20 µPa = 0 dB

1 Pa =   94 dB

2 Pa = 100 dB

The calibration and adjustment of the SONAPHONE microphone capsule is done by comparing it with a reference microphone (calibration of the microphone voltages) using a 40 kHz reference signal (*1). Based on the reference signal the characteristic curve of the broadband-sensitive microphone capsule is shifted in the entire measuring range of 20 ... 100 kHz.
In the SONAPHONE, the level values are finally calculated from the broadband sound pressure between 20 ... 100 kHz. The meaning of the different level values is explained in a seperate FAQ.

SONAPHONE Pocket (and comparable analog instruments)

For analog ultrasonic instruments that are available in the market (for example, SONAPHONE Pocket) such a signal calibration is not meaningful. The reason for this is the use of an ultrasonic capsule (Murata capsule), which is sensitive to ultrasonic frequencies in a relatively narrow frequency range around 40 kHz. The level values displayed on these devices must be referenced differently. The reference value is 0 dBμV at a voltage level of 1 μV (rms value). The value varies depending on the characteristics of the individual ultrasonic capsule. On the device display the microphone voltage is displayed in dBμV, for negative values the voltage is below 1 μV.

1 µV = 0 dBµV

Due to the use of different capsules and calibration methods, different characteristic curves as well as the consideration of different frequency ranges (narrowband / broadband) the displayed values of the analog instruments in dBμV and the SONAPHONE in dB cannot be easily compared or converted. The reasons again at a glance:

Digital Instruments (SONAPHONE)

Analogue Instruments (SONAPHONE Pocket)

Calibration of the level via physical reference / sound pressure
(Reference: 20 µPa = 0 dB)

Calibration of the level via voltage signal of individual sensor
(Reference: 1 µV = 0 dBµV)

Characteristic curve of used microphone capsule (broadband)Characteristic curve of used ultrasonic capsule (around 40 kHz)C

Specific calculation of level values

Specific calculation of level values (depending on supplier)

Display of broadband levels (20 … 100 kHz)

Display of narrowband levels (around 40 kHz)

*1: In the acoustics a 1 kHz reference signal is used

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Receiver: florian.starnitzky@sonotec.de

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Florian Starnitzky

Sales Manager ǀ Preventive Maintenance
+49 345 13317-837