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Technical Report: What is a Sound Level Meter? (Part 8)

9. Sound level meter reading

9-5 Maximum value (L max) and minimum value (L min) of time-weighted sound levels

The maximum time-weighted sound level within a given measurement period is represented as L max, and the time-weighted sound level is represented as L min.

L max is defined by JIS C 1509-1.

  • Figure 9-4: Varying sound level and maximum/minimum values

9-6 Peak Sound Level (L peak)

The peak level value is obtained by converting the maximum absolute value of the instantaneous sound pressure (AC output waveform) within a certain time period into a level. In the case of a single burst signal, as shown in Figure 9-11, there is a considerable difference between L Peak and the maximum noise level (L max). In the case of a steady-state sine wave signal, L Peak represents the amplitude value (peak value) of the sine wave, and L max represents its RMS value, so the difference is approximately 3 dB.

L Peak is calculated using the following formula:

Equation 9-7


However, f0 is the AC output value when the reference sound pressure (Pa) = 2 × 10⁻⁵, and fA (t) max is the maximum value of the AC output (instantaneous sound pressure) waveform.

  • Figure 9-5 Changes in waveform after A-weighting for the input signal
    f(t): Input waveform of a single burst signal
    f A (t): AC output waveform after passing through an A-weighted filter of f(t)
  • Figure 9-6 Peak values for input signals: "Difference between L max and L peak"
    L max: Maximum value of the DC output waveform
    L peak: The maximum value of the waveform obtained by performing a logarithmic (absolute value) operation on the waveform of f A (t).

(Note)

The difference between maximum value and peak value

Up until now, we've referred to the maximum value of the time-weighted level (L max) at 9.5 as the peak noise level. Let's revisit this distinction and summarize it clearly.

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

    This represents the maximum instantaneous noise level within a given time period, corresponding to the maximum value in the bottom graph of Figure 1-1 (the time trend of the level). It depends on frequency weighting and time weighting. It is the maximum value of the DC output.

  2. Peak sound level (L peak)

    This is a level representation of the maximum instantaneous sound pressure (AC signal). It's calculated by taking the absolute value of the second graph (instantaneous sound pressure) in Figure 1-1 and then leveling the maximum peak value. It depends solely on frequency weighting; time weighting is irrelevant. It represents the level representation of the maximum value of the AC output.

Of course, L peak > L max. For a steady sinusoidal signal, this difference is about 3 dB, but for typical impact noise, the difference is 10 dB or more. Peak sound level (L peak) is increasingly being used to evaluate the impactfulness of noise, replacing time weight I (impulse) (see JIS Z 8737-1 Annex A <Reference>).

  • The example above shows the change in the waveform after filtering with frequency weighting A characteristics, but the same approach can be applied to other frequency weightings such as C characteristics.

  • Due to European CE marking regulations, it is often necessary to calculate the C-weighted peak sound level (LCpeak) with frequency weighting "C".

  • The JIS Z 8737 series, "Method for Measuring the Radiated Sound Pressure Level of Machine Noise at the Working Position and Other Designated Positions," which specifies the terms peak radiated sound pressure level L p, peak. Furthermore, the C-weighted peak sound pressure level L pCpeak is specified as a basic measurement quantity.