Frequency Analysis from the Basics (32) - "Measurement of Impact Noise"

This time, we'll be discussing noise measurement using a sound level meter, particularly the measurement of impact noise.
The sound level meter is a commonly used instrument for measuring sound or noise. Recent sound level meters have functions to determine various measurement quantities that correspond to fluctuating sounds and impactful sounds. Here, we will mainly explain the measurement quantities related to impactful sounds.

Figure 1 shows the level conversion procedure for a recent sound level meter (the official JIS name is "sound level meter," but we will use the term "sound level meter" here).

As shown in this diagram, the basic function of a sound level meter is to convert the instantaneous sound pressure signal (AC signal) acquired by the microphone sensor into a level (DC signal) using several methods and then display it. There are three methods for converting an AC signal into a level value (DC signal), which are described below.
To clarify the terminology, the term "sound level" used below refers to the sound pressure level with frequency weighting (A, C, Z) applied. In other words, as shown in Figure 1, it is the level obtained by applying some kind of frequency weighting to an AC signal.

Time-weighted sound levels

This method involves squaring the instantaneous sound pressure signal (AC signal), averaging it with a certain time weight (root mean square), and then converting it to dB. The averaging method is an exponential mean (equivalent to a first-order low-pass filter), and the time constant can be selected from time weights F (fast, 0.125s) and S (slow, 1s). This process is a continuous process (since it is a digital process, it occurs at each sampling period of the A/D conversion), and the time variation of the sound level value is obtained, resulting in a DC output (analog output) on the sound level meter. The quantity used to evaluate impact noise using this method is the maximum sound level.

As shown in Figure 2, the maximum value of the time-weighted sound level, Lmax, is usually the value at which the sound level is highest within a given measurement time.
The time weight should be set to F (fast) to respond as closely as possible to changes. Note that in older standards, there was also a time weight called impulse, but it is no longer used.

Time-averaged sound level (equivalent sound level)

This method involves squaring the instantaneous sound pressure signal (AC signal), averaging it over the measurement time T (root mean square), and then converting it to dB. The difference from (1) above is that it uses an "equally weighted average" instead of an "exponential average," is unaffected by time weighting, and only one value remains as a representative value for the measurement time T.
Physically, it's the average energy value, and this is what's usually calculated when we talk about noise measurement.
For example, the A-weighted time-averaged sound level with frequency weight A is calculated using the following formula:

p _A (t): A-weighted instantaneous sound pressure
​_p0: Reference sound pressure 20 μPa
T: Integration time (= t ₂- t ₁)

The quantity used to evaluate impact noise using this method is the acoustic exposure level (LE).
The acoustic exposure level is the energy integral value at the measurement time T, not averaged over the measurement time T.
This is a simplified version. For example, the A-weighted acoustic exposure level (LAE) is calculated using the following formula.

p _A (t): A-weighted instantaneous sound pressure
​_p0: Reference sound pressure 20 μPa
T ₀: 1s

Furthermore, from equations (1) and (2), the relationship with the time-averaged sound level is given by equation (3).

L_AE = L_Aeq + 10 log(T )

The acoustic exposure level represents the level of sound if the accumulated acoustic energy over a certain measurement time were considered as a steady-state sound over 1 second. It is often used to evaluate intermittently occurring noises such as the sound of passing trains or pile driving.

Peak Sound Level

This method does not use the mean-squared calculation method described in (1) and (2) above, but rather converts the maximum absolute value of the instantaneous sound pressure time waveform (AC signal) within a certain time period into a level, which is calculated using the following formula. (p1 is Figure 5)

As can be understood from the measurement method, it is possible to evaluate impactful sounds as the most significant without being affected by time weighting. Also, for frequency weighting, C-weighting is often used instead of A-weighting to evaluate impactful and loud sounds. C-weighted peak sound level is used in European CE marking and JIS Z 8737 series "Method for measuring the radiated sound pressure level of mechanical noise at work positions and other designated positions".

Table 1 summarizes the three methods.

Table 1 Comparison of 3 methods

Figure 6 shows the results of measuring a single noise event for 2 seconds using Ono Sokki LA-3560 sound level meter. (For ease of comparison, all frequency weights are set to A-weighting.)
As you can see from this, the relative magnitudes of the above evaluation values are usually as follows:

L_Aeq << L_AE << L_AFmax << L_Apeak

(Note 1) The acoustic exposure level depends on the measurement time T.
(Note 2) The maximum value of the time-weighted sound level depends on the time weight.

According to Annex A (Reference) of JIS Z 8737-1, the criteria for determining the impactfulness of noise include the difference between the C-weighted peak sound level and the C-weighted time-averaged sound level (L _Cpeak- L _Ceq) and the difference between the C-weighted peak sound level and the maximum value of the C-weighted time-weighted sound level (L _Cpeak- L _CSmax).

Finally, here's a summary.

1. Recent JIS standards offer three methods for evaluating impact noise, based on the levelization of instantaneous sound pressure signals (AC signals).
2. The first is the maximum value of the time-weighted sound level at measurement time T.
3. The second is the acoustic exposure level, which is the energy integral value at measurement time T.
4. The third is the peak sound level, which is the maximum absolute value of the instantaneous sound pressure at measurement time T.

【keyword】

Sound level meter, instantaneous sound pressure, AC signal, levelization, DC signal, sound level, frequency weighting, sound pressure level, time-weighted sound level, mean square, dB, exponential mean, low-pass filter, time constant, F (fast, 0.125s), S (slow, 1s), maximum value of time-weighted sound level, impulse, time-averaged sound level, equivalent sound level, energy mean, A-weighted time-averaged sound level, acoustic exposure level, peak sound level, A-weighting, C-weighting, C-weighting peak sound level, CE marking

【reference】
Ono Sokki Technical Report: "What is a Sound Level Meter?" →
JIS C 1509-1:2005 Sound level meter (noise meter)
JIS Z 8737-1 Method for measuring the radiated sound pressure level of machine noise at work locations and other designated locations (Practical measurement method in a quasi-free sound field on a reflective surface)

(Excerpt from the email newsletter issued on March 22, 2017)