This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
When measuring sound using a sound level meter (noise meter) or FFT analyzer, there is a setting called "frequency weighting characteristics (A/C/Z)". This time, we have compiled frequently asked questions about frequency weighting characteristics related to sound level meters in a Q&A format.
What is frequency weighting characteristic A?
The A characteristic is a frequency weighting characteristic defined in JIS C1509, used when measuring noise level (noise intensity).
As introduced in Measurement Column No. 184, it was created by approximating the loudness curve of the quietest sound in the equal loudness curves (old standard) of pure tones. The C characteristic was also approximated by the loudness curve of the loudest sound.
Although it was created, subsequent research revealed that the C-weighting curve is not suitable for representing noise level, and that it is better to use the A-weighting curve regardless of the sound level when representing noise level. Therefore, the frequency weighting characteristic A represents the noise level (noise).
It is widely used when measuring levels.
Which frequency weighting characteristic should I use, A or Z?
When measuring how loud a sound sounds to a person, A-weighting is generally used. The common usage is as follows:
When using frequency weighting characteristic A
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Noise measurement at the site boundaries of factories, buildings, construction sites, etc.
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Noise measurement of roads, railways, aircraft, etc.
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Noise measurement in the work environment
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When evaluating how people perceive the loudness of operating noises from machinery and other equipment.
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When measurement standards or guidelines specify that measurements should be taken using A-weighting.
When using the frequency weighting characteristic Z (FLAT)
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When recording raw sound signals and playing them back through speakers or headphones for human evaluation, the human ear itself has auditory characteristics close to A-weighting. Therefore, if A-weighting is applied to the recording, the sound will sound different compared to hearing the sound directly. For this reason, data intended for human hearing is recorded using Z-weighting.
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When calculating loudness, noise cancellation (NC) values, etc., based on measurement results, the Z (FLAT) characteristics are used for measuring the raw data because human auditory characteristics are taken into consideration during the calculation process of loudness and NC values.
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This method is used to measure the operating noise of machinery and other equipment to understand their physical characteristics, how they change over time, and to predict failures. Since this measurement is unrelated to human hearing characteristics, A-weighting is not used.
Does the AC output of the sound level meter have A-weighting applied to it?
In the case of our sound level meters, if the sound level meter unit is set to A-weighting/C-weighting, the signal with A-weighting/C-weighting applied will be output to the analog output (AC).
When the sound level meter is set to DUAL mode or QUAD mode (a mode that simultaneously measures under 2 to 4 different conditions), the signal with the frequency weighting characteristic specified in the MAIN condition is output to the analog output (AC).
When the sound level meter is set to Real-Time Octave (RTA) mode, you can apply different frequency weighting characteristics to AP1 (All-Pass 1) and AP2 (All-Pass 2). In this case, the signal with the frequency weighting characteristic specified on AP1 will be output to the analog output (AC).
When the sound level meter is set to DUAL mode or QUAD mode (a mode that simultaneously measures under 2 to 4 different conditions), the signal with the frequency weighting characteristic specified in the MAIN condition is output to the analog output (AC).
When the sound level meter is set to Real-Time Octave (RTA) mode, you can apply different frequency weighting characteristics to AP1 (All-Pass 1) and AP2 (All-Pass 2). In this case, the signal with the frequency weighting characteristic specified on AP1 will be output to the analog output (AC).
What is the AC-Z output of a sound level meter?
When analyzing the analog output signal of a sound level meter with an FFT analyzer or similar device, you may want to input a signal that is not A-weighted or C-weighted. AC-Z output is a function that always outputs a Z-weighted signal (without A-weighting or C-weighting), regardless of the frequency weighting characteristic (A/C/Z) setting of the sound level meter itself. Several of our sound level meters are equipped with AC-Z output.
Our LA-1400/4400 series has one analog output terminal, and you can select one of the following outputs: AC output, AC-Z output, or DC output. This setting is configured in the I/O ⇒ AC/DC menu.
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Figure 1: Analog output settings screen of LA-1400/4400
Our LA-3000 series has two analog output terminals. One of these terminals (AC) is fixed to AC output. The other terminal (AC/DC) can be selected to output either AC-Z output, DC output, or Through.
Settings are configured via the menu I/F (Interface) ⇒ AC/DC-Out.
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Figure 2: Analog output settings screen for the LA-3000 series.
What are the frequency weighting characteristics of WAV files saved using the sound recording function?
Whether the signal stored in the WAV file saved by the sound recording function of a sound level meter is A-weighted/C-weighted depends on the sound level meter product.
In the case of our LA-5570/5560/2560 models, the signal stored in the WAV file will have the frequency weighting characteristic set on the sound level meter itself applied. A WAV file recorded with the setting to A-weighting will contain a signal with A-weighting applied. If you need a signal without A-weighting/C-weighting applied, set the sound level meter to FLAT (Z) characteristics and perform the recording.
In the case of our LA-3570/3560/3560 models, the signal stored in the WAV file is recorded with Z-weighting (without A-weighting/C-weighting), regardless of the settings of the sound level meter itself.
Pressing the Lp/Leq button to change the displayed value also changes the frequency weighting characteristics (A/C/Z).
If you change the frequency weighting characteristics using the A/C/Z buttons on the sound level meter, and then switch the display to Leq (time-averaged sound level), Lmax (maximum value), etc. by pressing the Lp/Leq button, the frequency weighting characteristics may change.
This is because values such as Leq (time-averaged sound level) and Lmax (maximum value) are only calculated when you press the Start button on the sound level meter, and pressing the Lp/Leq button displays the result of the most recent calculation.
For example, if you perform a calculation specifying the Z characteristic and then switch to the A characteristic using the A/C/Z button, the LA value will be displayed. However, if you then press the Lp/Leq button, the calculated value for the Z characteristic, such as LZeq, will be displayed. This is because the calculation was performed with the Z characteristic specified immediately before, and the result is being displayed.
The sound level meter retains the A-weighting setting, so simply press the Start button to begin the calculation. The display will switch to LAeq, and the calculation will be performed using A-weighting.
(Excerpt from the email newsletter issued on February 21, 2017)
