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Technical Report: What is a sound level meter?

Display value of the sound level meter

Maximum value of time-weighted sound level (L max) and minimum value of time-weighted sound level (L min)

The maximum time-weighted sound level during a measurement period is expressed as L max, and the time-weighted sound level is expressed as L min.

L max is defined by JIS C 1509-1.

  • Figure 9-4 Fluctuating Sound Levels and Maximum/Minimum Values

9-6 Peak Sound Level (L peak)

The peak level value is the value obtained by leveling the maximum absolute value of the instantaneous sound pressure (AC output waveform) within a certain time. In the case of a single burst signal, as shown in Figure 9-11, it can be seen that there is a considerable difference between L Peak and the maximum noise level (L max). For a steady-state sinusoidal signal, L Peak indicates the amplitude value (peak value) of the sinusoidal wave, while L max indicates its effective value, resulting in a difference of about 3 dB.

L Peak is calculated using the following formula.

Equation 9-7


However, f 0 is the value of the AC output at the reference sound pressure (Pa) = 2 × 10-5, and f A (t) max is the maximum value of the waveform of the AC output (instantaneous sound pressure).

  • Figure 9-5 Changes in waveform after A characteristic for input signal
    f (t) : Input waveform of single burst signal
    f A (t) : AC output waveform passed through A characteristic filter of f (t)
  • Figure 9-6 Difference between peak values "L max and L peak for input signals"
    L max: Maximum value of the DC out waveform
    L peak: Maximum value of the waveform of f A (t) after logarithmic operation (absolute value)

Note

The difference between maximum value and peak value.

Until now, the maximum value of the time-weighted level of 9.5 (L max) has been referred to as the peak value of the noise level. I will summarize that difference again.

  1. Maximum value of time-weighted level (L max)

    The maximum instantaneous noise level within a certain time period is the maximum value of the graph at the bottom of Figure 1-1 (time trend of levels). It depends on frequency weighting and time weighting. It is the maximum value of the DC output.

  2. Peak Sound Level (L peak)

    The maximum value of instantaneous sound pressure (AC signal) is leveled, taking the absolute value of the graph in the second row of Figure 1-1 (instantaneous sound pressure) and leveling its maximum peak value. It depends only on frequency weighting, and time weighting is not relevant. This is the leveled maximum value of the AC output.

Of course, L peak > L max. For a steady sine wave signal, the difference is about 3 dB, but for normal impact noise, it can be more than 10 dB. The peak sound level (L peak) is changing to time weighting I (impulse) and is being used to evaluate the impulsiveness of noise (see JIS Z 8737-1 Appendix A <reference>).

  • In the above example, the changes in the waveform after applying the frequency weighting A characteristic filter are noted, but similar considerations can be applied to other frequency weightings, such as the C characteristic.

  • Due to the CE marking regulations in Europe, it is common to calculate the C-weighted peak sound level LCpeak.

  • The measurement method for machine noise, JIS Z 8737 series "Measurement method for sound pressure level of machine noise at the work position and other specified positions," defines the term peak sound pressure level L p, peak. Furthermore, as a basic measurement quantity, the C-weighted peak sound pressure level L pCpeak is defined.