This analysis object allows you to determine sound levels from recorded microphone signals in a standardized way.

Calibration

To deduce the sound pressure level from the microphone output voltages recorded, the microphone sensitivity and the gain of the whole signal chain must be taken into consideration. This is done for example by generating a specified sound pressure level at the microphone using a calibrator. This analysis object supports the following working methods:

•Before the actual measurement, the calibrator is fixed to the microphone for at least four seconds and the calibration signal is measured. Without interrupting the measuring, the calibrator is then removed and the microphone is subjected to the sound event to be measured. If you would like to use this method, select the option Compute calibration from the sound signal and specify the level of the calibrator.

•You would like to carry out several measurements in succession. Before you start, you perform a calibration measurement with an attached calibrator for at least 4 seconds. For this method, select Obtain calibration from data set and then specify this calibration measurement as a calibration data set as well as the level of the calibrator. If you do not want to have the calibration value calculated every time, press the Calibrate. button. The calibration value is then determined from the data set to be specified and calibration level specified, entered in the CalibrationValue field, and the calibration mode changes to Fixed (in dB) .

•You already know the calibration value in dB required for your microphone. Select Calibration fixed (in dB)  and then specify the CalibrationValue directly in dB. A calibration value of 0 dB corresponds to a microphone sensitivity of 50 mV/Pa.

•You know the microphone sensitivity in mV/Pa from the calibration certificate. Select Calibration fixed (in mV/Pa) and specify the microphone sensitivity directly in mV/Pa.

For more details, see Calibration in Acoustics.

Time Weighting

You can choose between the time weightings FAST, SLOW, PEAK, IMPULSE and LEQ (refer to IEC 651, IEC 804). The right time weighting for you depends on the noise you have recorded and which standards you are required to observe. For measurements of time-variable noise, the time weightings SLOW or  LEQ are often chosen that have an averaging time long enough for the result to oscillate only a little. For the evaluation of level over time, FAST or LEQ with a very short averaging time are more suitable. To evaluate the damage done by noise to the human ear, such as in the work place, the PEAK time weighting is often used.

•The time weighting FAST emulates an R/C network with a rising and a falling time constant of 125 ms.

•The time weighting SLOW emulates an R/C network with a rising and a falling time constant of 1000 ms.

•The time weighting IMPULSE emulates an R/C network with a rising time constant of 25 ms and a falling time constant of 1500 ms.

•The time weighting LEQ (equivalent sound pressure level) averages the level in an averaging window without rising and falling time constants. You can choose duration of this averaging window from 20 ms onwards. An averaging duration of 0 is interpreted as averaging over the entire duration of the signal.

•The time weighting PEAK does not average and does not have any rising or falling time constants, and instead only deals with the peak value of the signal.

Frequency Weighting

You can choose between frequency weightings None (unweighted), A, B and C (refer to IEC 651). The right frequency weighting depends on which standards you are required to meet. Frequency weightings are to simulate the frequency response of the human ear. With very low volumes in particular the ear is less sensitive to very low and very high frequencies than to an average range around 1 kHz. If the measuring result is related to the noise pollution endured by human beings, an A-weighting is usually used, since this balances the frequency response of the ear for low levels fairly well. If the measuring result is related to damage to the ear, the C-weighting is usually used, since it balances the frequency response of the ear at higher levels fairly well. If noise is evaluated technically, the measurement is usually unweighted.

Since some measuring devices already generate frequency-weighted signals, you must also specify the frequency weighting of the input signal on the Data tab. The frequency weighting is only possible for signals with a sampling rate of 4 kHz or higher. From a sampling rate of the input signal of 40 kHz onwards, precision class 1 is observed, from a sampling rate of 80 kHz onwards, precision class 0 applies. Not all combinations of input signal frequency weighting and output level frequency weighting are possible:

The analysis object can process two different data structures. You can specify a time signal whose Y component contains the microphone output voltages (sound pressure values) and whose X component contains the sampling points.

As a second alternative you can specify a time signal series. Its Y component contains several microphone output voltages (sound pressure values). The X component contains the sampling points.

Result

The result of the sound level calculation can be output in various forms:

•Using level signal you receive a signal with a level signal with a time resolution of 20 ms, or a signal series with level over time if the input signal is a signal series.

•Using Level you receive the last value of the level signal or the last values, if the input signal is a signal series.

Observed Standards

The highest precision grade (grade 0) is observed for both standards.

When a frequency weighting is to be calculated, the input signal must have a sampling rate of at least 80 kHz, if precision grade 0 is to be reached.

Sampling rates between 40 kHz and 80 kHz achieve precision grade 1 for frequency weightings.

Calculating frequency ratings below a sampling rate of 40 kHz does not make sense, since the standard lists tolerances for frequencies up to 20 kHz, which would then be above half the sampling rate (Nyquist-theory).

FPScript Functions Used

SoundLevel

FrequencyWeightingA

FrequencyWeightingB

FrequencyWeightingC

AcousticCalibration

See Also

Acoustics Option

Analysis Objects

Sound Power Analysis Object

Calibration in Acoustics