Fundamentals of Digital Measurement - Part 10: "Sound Level Meter"

The new standard for sound level meters, JIS C 1509-1:2005, is a Japanese translation (IDT: identical) of the international standard IEC 61672-1 (2002), and its title is also "Sound Level Meter (Sound Level Meter)." This implies that acoustic measuring instruments conforming to this standard are sound pressure level meters that can be used not only for noise measurement but also for a wide range of acoustic measurements.
This standard defines a new term, "sound level." Specifically, it refers to a sound pressure level weighted by a certain frequency (A or C). Furthermore, this standard specifies the following four types of sound levels:

1. Time-weighted sound levels

2. Time-averaged sound level

3. Acoustic exposure level

4. Peak Sound Level

These quantities are expressed as levels by taking a ratio against a certain baseline and performing logarithmic calculations; therefore, all are displayed in dB (decibels).

A time-weighted sound level is calculated by determining the square of an instantaneous sound pressure with a certain frequency weight, multiplying it by a time weight (through a low-pass filter), and then converting it to a level. For example, an A-weighted time-weighted sound level is:

···················(1)
Here,
τ: Time constant (s) of time-weighted characteristic F or time-weighted characteristic S
ξ: The integral variable from a certain past time to the observation time t, represented by -∞.
　　　p_A (ξ): Instantaneous A-weighted sound pressure at time ξ
　　　　p₀: Reference sound pressure

The result is as shown in equation (1). This corresponds to the (instantaneous) A-weighted sound pressure level (i.e., noise level) that we have been using. Please refer to the previous measurement column ("AC/DC and Measuring Instruments") for more information on equation (1).

Fundamentals of Digital Measurement - Part 5: "AC/DC and Measuring Instruments"

<Caution>

• Equation (1) uses 20 times the common logarithm as per the original international standard, but considering the physical implications, it is more common to define it as 10 times the common logarithm (the same applies to equation (2)).
• Modern sound level meters rapidly sample instantaneous sound pressure time signals and display the results after performing all digital calculations.

Time-averaged sound level is calculated by determining the square of the instantaneous sound pressure level weighted by a certain frequency, and then taking the power average (time average of energy values) over a specified time period to obtain a sound level. For example, A-weighted time-averaged sound level is:

·····················（2）

Here,
ξ: Integral variable from t - T to observation time t
　　　　T: average time
　　　p_A (ξ): Instantaneous A-weighted sound pressure at time ξ
　　　　p 0: Reference sound pressure

(2) This corresponds to the equivalent sound level (Leq), which is common in the field of environmental noise.
Generally, time-averaged sound pressure level (SPR) is a representative value of the sound pressure level that fluctuates over time at a specified time or during a specific operating cycle of a machine. As mentioned above, it has a clear physical meaning due to its energy-based averaging process and is a very important parameter used in many fields.
Even in recent JIS standards for noise measurement that have been harmonized with ISO standards, determining this parameter is now mandatory. The following are the main measurement fields and the terminology used in those fields.

Measurement of ambient noise (JIS Z 8731) ························ Equivalent sound level
Measurement of sound power level (JIS Z 8732/8733) ········Time-averaged sound pressure level
Sound pressure measurement of mechanical noise (JIS Z 8737 series) ······Time-averaged radiated sound pressure level

The acoustic exposure level is a level representation of the amount of acoustic exposure (the square integral of the instantaneous sound pressure over a specified time). For example, the relationship between the A-weighted acoustic exposure level (LAE) and the A-weighted time-averaged sound level (equivalent noise level, LAT) is as follows:

·····················（3）

Here,
　　　EA: A-weighted acoustic exposure expressed in square Pascal-seconds_.​
　　　E₀: (); Standard sound exposure amount ()sPsPaa212210400120−×=×μ
　　　T₀: 1s reference time
　　　T: Measurement time of acoustic exposure level and time-averaged sound level, expressed as t2 – t1 seconds.

Equation (3) is obtained. In the context of environmental noise, this quantity is called the single-event noise exposure level. Furthermore, the relationship in equation (3) is the same as the relationship between PSD and ESD, as described in the previous measurement column ("Various Time Waveforms and Spectra").

Fundamentals of Digital Measurement - Part 9: "Various Time Waveforms and Spectra"

Peak sound level is the maximum absolute value of instantaneous sound pressure weighted by a certain frequency, and is used to evaluate impact noise. Typically, according to the requirements of the EU Machinery Directive, the frequency weighting is C-weighted, i.e., C-weighted peak sound level (LCpeak) is used.

References: JIS C 1509-1 Electroacoustics - Sound level meters (noise meters) - Part 1

(Excerpt from the email newsletter issued on July 24, 2008)