Technical Report-FFT Analyzer II

9-6 Maximum sound level (L_max)and minimum sound level (L_min)

The maximum sound level and the minimum sound level over a given measurement time are termed L_max and L_min respectively.

Fluctuating noise level and the maximum and minimum values

Figure 9-8: Fluctuating sound level and the maximum and minimum values

9-7 Maximum value over N seconds (Tact max)

Instantaneous values such as L_p and L_A show values per second from among the sampled data whereas Tactmax shows the maximum value from among one-second, high-speed sampled data. In Japan, instantaneous values are used as sound levels whereas in Europe, 1-second or 3-second Tactmax is normally used.

NOTE:

In the old standard IEC 60651, instantaneous sound pressure levels are defined as the maximum sound pressure level per second (i.e. Tactmax 1 sec).

Sampled sound pressure level and Tactmax (1 sec)

Figure 9-9: Sampled sound pressure level and Tactmax (1 sec)

9-8 Peak Sound level (L_peak)

A peak sound level is the maximum absolute instantaneous sound pressure (AC out waveform) over a time period presented on a logarithmic scale. With isolated burst signals, there is a significant difference between L_Peak and the maximum sound level (L_max) as shown in Figure 9-11. With steady-state sine waves, L_Peak shows the amplitude of the sine waves (peak value) and L_max shows the root mean square value, resulting in a difference of around 3 dB.

(Equation 9-7)

where f₀ is the AC out value when the reference sound pressure (Pa) = 2 × 10^-5 and f_A(t) _max is the maximum value of the AC out (instantaneous sound pressure) waveforms.

Change in waveform after A-weighting of input signals

Figure 9-10: Change in waveform after A-weighting of input signals
f (t) : Input waveform of isolated burst signals
f_A (t) : AC out waveform of A-weighting filtered f (t)

Difference between peak values

Figure 9-11: Difference between peak values L_maxand L_peak of input signals
L_max : Maximum value of DC out waveform
L_peak : Maximum (absolute) logarithmic value of f_A (t) waveform

NOTE:

1. Maximum sound level and peak sound level

Maximum sound level (L_max) (refer to Section 9.6) used to be called peak sound level.
Today, they are different parameters from each other as summarized below.

(1) Maximum noise level (L_max)

This is the maximum instantaneous sound pressure level over a time period and corresponds to the maximum value in the bottom graph (change in level over a time period) in Figure 1-1. It is the maximum DC out value, which is dependent on frequency- and time-weighting. In IEC 61672-1:2002 (JIS C1509-1:2005), it is the maximum value of time-weighted sound level.

(2) Peak level value (L_peak)

This is the maximum instantaneous sound pressure (AC signals) presented on a logarithmic scale. It is the maximum peak value of the absolute value of the second graph (instantaneous sound pressure) from the top in Figure 1-1 presented on a logarithmic scale. It is dependent on frequency weighting, with no relation to time-weighting, and is called peak sound level in IEC 61672-1:2002 (JIS C1509-1:2005).

As a matter of course, L_peak > L_max . The difference is around 3 dB for steady-state sine waves and 10 dB or more for impulsive noise. Peak level value (L_peak) is gradually taking over I (Impulse)-weighting as a means of evaluation of impulsive noise. (Refer to ISO 11201 Annex A (informative), JIS Z 8737-1 attached document A.)

· The example described above shows a change in waveform after A-weighting. This is also applicable to C- and other frequency weighting.
· There has been a stronger demand for C-weighted peak sound level L_Cpeak for compliance with CE marking.
· ISO 11200 series (JIS Z8737 series) "Noise emitted by machinery and equipment – Measurement of emission sound pressure levels at a work station and at other specified positions" specifies peak emission sound pressure level L_{p, peak} and, as a basic measure, C-weighted peak sound pressure level L_pCpeak.