Tech Note 209 Detail

Measurement in Decibels (dB)

Affected Products:

CI-6506B Sound Sensor

Problem/Symptom:
Can you calibrate the CI-6506B sound sensor to measure in dB, rather than voltage?

PASCO Solution:

Measuring Sound Level with the CI-6504(B) Sound Sensor

We recommend that users interested in measuring sound level purchase the PS-2109 Sound Level Sensor and, if needed, a PASPort interface. (Note: The PS-2002 Xplorer GLX also reports sound level; however, the range is limited to 61.3 dB to 110 dB.)

The CI-6504B consists of a microphone and preamplifier. We do not recommend it for sound level measurements because in DataStudio 1.9.x currently lacks the ability to peform FIR filtering, which is needed to mimic the various decibel frequency-weightings; however, if you are willing to accept the frequency-response of the microphone used in the CI-6504 as sufficiently close to that of the frequency-weighting of interest, then you may use the sound level sensor with a ScienceWorkshop interface.

Comparison of the two frequency responses below, shows that the CI-6506(B) Sound Sensor has a response that resembles the decibel D-weighting for high frequencies and resembles the C-weighting for low frequncies.

Typical CI-6506(B) Microphone Sensitivity Comparted to Various Frequency Weightings

If you are using a ScienceWorkshop 500 interface, set the sampling rate to the maximum (20kHz). If you are using the ScienceWorkshop 750, set the sampling rate to 40 kHz. Because DataStudio does not have the facility for performing a rolling integration with a finite time window, the calculation overhead will eventually overwhelm DataStudio after tens of seconds and data recording will stop.

Sound level is related to the standardized typical human perception of loudness. Because loudness is a function of both sound pressure and frequency, different weightings were created that are appropriate for different loudness ranges. The most widely used frequency weighting is the "A-weighting", which roughly corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve. The B and C frequency weightings are more suitable for higher sound levels.

To measure sound pressure level (SPL), we would ideally

  1. multiply by an appropriate frequency-weighting function to compensate for diffraction effects and intrinsic frequency-dependent sensitivity,
  2. take the log,
  3. multiply by 20, and
  4. integrate this instantaneous value over the time of measurement.

Due to the limitations of DataStudio, we cannot perform a frequency-weighting in software, but we can obtain an approximate Sound Pressure Level using steps 2 through 4:

SPL (in decibels) = 20 log(p/pref) = 20 log (V/Vref)

Average Sound Pressure Level = 20*(integral(log(filter(1e-6,10,v)/vref)))/timeof(v) , vref = 1.8E-5 V

where p is the given sound pressure and pref == 2 x 10-5 Nm-2 is the reference sound pressure. Although we know pref, we must determine Vref empirically. A value Vref =1.8 x 10-5 V was determined by equalizing the response to that of a commercial sound level meter for 50 dB background noise. Since the response of the sound sensor varies with frequency, you may want to calibrate the equation for the particular frequency that you will be using. To do so, you would need a stand-alone sound level meter or a calibrated sound source.

Note: The measurement of total sound intensity [W m-2] is more difficult to measure than the sound pressure level. Calculating sound intensity requires measuring both the rapidly-varying component of air pressure and the particle velocity. Instruments designed for the measurement of sound intensity consist of two or more frequency-calibrated microphones arranged perpendicularly in a tube such that there is no phase shift between the microphones.

The workbook below contains all of this configuration information for the ScienceWorkshop 750 interface. It will need to be modified slightly if you are using a ScienceWorkshop 500 interface:


File Downloads:
Sound Level.ds - ( 5 MB )
Sound Level Workbook

Creation Date: 01/1/2000
Last Modified: 01/9/2004
Mod Summary: