This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
We frequently receive inquiries about how to use sound level meters, such as "I want to know the maximum value," "Does it have a peak hold function?", and "I want to know the maximum noise level during a given measurement period."
Sound level meters measure two values that represent the maximum sound level: the MAX value (maximum time-weighted sound level) and the PEAK value (peak sound level). Both of these values represent the maximum sound level generated within a given time period, but they are calculated using different methods and are used in different situations.
Since some customers seem to confuse these two values, this time we will explain the difference between the MAX value (maximum value of time-weighted sound level) and the PEAK value (peak sound level).
Equality
PEAK value
The MAX value and PEAK value have the following differences:
→MAX value… The maximum value of the time-weighted sound level within the measurement period. Time weighting is applied.
→PEAK value… A value obtained by converting the maximum absolute value of instantaneous sound pressure within the measurement time into a level.
Time weighting is not applied.
The sound level Lp can be calculated using the following formula. This type of calculation is also called "leveling". p0 is the reference sound pressure (2 × 10⁻⁵ Pa), and p is the effective value of the instantaneous sound pressure (sampled data). (In the case of peak sound level, it is the maximum value of the absolute instantaneous sound pressure.)
In other words, it's the common logarithm of the ratio to the reference sound pressure, multiplied by 20.
What is the MAX value?
When determining the maximum value, the sound level meter performs the following processing on the input sound signal:
- Apply frequency weighting (A-weighting, C-weighting, Z-weighting).
- The effective value is obtained by applying time weights (time constants: Fast, Slow).
- Calculate the sound level (time-weighted sound level)
- Determine the maximum sound level within the measurement time.
The MAX value is the maximum value of the sound level (time-weighted sound level), and therefore its magnitude may change depending on the time constant when it is converted to an RMS value.
Here, we will explain in more detail how differences in the time constant affect the maximum value.
Time-weighted sound levels apply time weighting when calculating the RMS value, so they may not be able to keep up with rapid sound fluctuations.
The figure below shows the time-weighted sound level measured for a sound pressure (burst) signal with a duration of 200 ms. Compared to the time-weighted Fast (time constant: 125 ms) value, the time-weighted Slow (time constant: 1 s) value takes a long time to rise, causing the signal to disappear before it can rise, resulting in a difference in the measured value compared to Fast.
What is the peak value?
When determining the peak value, the sound level meter performs the following processing on the input sound signal:
- Apply frequency weighting (A-weighting, C-weighting, Z-weighting).
- Calculate the absolute value of instantaneous sound pressure.
- Determine the sound level (peak sound level) of the maximum instantaneous sound pressure within the measurement time.
The PEAK value is calculated as the maximum instantaneous sound pressure value within a given measurement time, without performing RMS calculations, and therefore is unaffected by time weighting. L peak > L max, resulting in a difference of approximately 3 dB for a sine wave signal.
MAX value and PEAK value
For short-duration, impactful noises, the difference between the MAX and PEAK values becomes much larger. For impact noise, a difference of 10 dB or more is not uncommon.
The figure below compares the level obtained by converting the RMS value to a signal with a duration of approximately 250 μs, and the level obtained by converting the absolute value of the sound pressure.
Both MAX and PEAK values represent the maximum value within a given time period, but they are used in different contexts.
The MAX value is used for measuring intermittent noise in environmental noise and for measuring proximity exhaust noise, among other things.
Because the PEAK value has good responsiveness to instantaneous changes in sound pressure, it is used to evaluate the impact of noise and to measure the C-weighted peak sound level (Lcpeak) for CE marking regulations.
If you want to measure the maximum noise level, you need to determine whether PEAK or MAX is the appropriate measurement method and then select the correct one.
Please note that you cannot determine the peak value from the maximum value, or vice versa.
Make sure you understand the differences correctly and clearly define what data you need before you start measuring.
Reference: Why is there a difference between the sound level meter reading and the calculated result?
The sound level meter display updates every second, so it changes from "Display A" to "Display B," but the level data is calculated using the sound level meter's sampling time interval.
For the LA-1411/LA-1441A, calculations are performed at 31.25 μs intervals; for the LA-4441A, at 20.83 μs intervals; and for the LA-7000 series, at 15.625 μs intervals.
Since the Lmax and Lpeak values are obtained from data calculated over this sampling time interval, they may differ significantly from the values shown in the instantaneous value display.
(Excerpt from the email newsletter issued on February 17, 2021)
