Technical Report: What is a sound level meter?

To perform the calibration of the sound level meter, including the microphone, a sound calibrator as specified in JIS C 1515 (IEC 60942) is used.

The sound calibrator includes an acoustic calibrator that generates a reference sound pressure through mechanical vibrations known as a pistonphone, and a simple acoustic calibrator that uses piezoelectric ceramics as a sound source. Our company offers the SC-2500A (JIS C 1515:2020 Class 1) and SC-2120A (JIS C 1515:2004 Class 2). The SC-2500A provides 114 dB (1 k Hz), while the SC-2120A provides 94 dB (1 k Hz). Before starting measurements, input the reference sound pressure of the sound calibrator and set the frequency weighting of the sound level meter to FLAT or C characteristics to check if the indicated value matches the reference sound pressure. If there is a difference in the indication, adjust it using the calibration volume to match. If the difference is 2 dB or more, there may be some abnormality, so it is recommended to have it inspected by the manufacturer.

Figure 7-1 Typical Sound Calibrator

Additionally, the sound level meter may have an internal reference signal output function (REF). This function generates a stable reference signal at 1000 Hz, corresponding to -6 dB, which is half of the full scale (100%) of the sound level meter's level range, for the purpose of verifying the operation of the electrical circuit. It is used for simple daily checks and as a calibration signal for external connected devices such as level recorders and FFT analyzers.

 Note 

Products in supermarkets are labeled with a weight in grams, and the scales used to measure this weight are certified. For measuring instruments used in transactions or for certification, public assurance is required, and the Measurement Law states that "it is necessary to impose regulations under the Measurement Law for the proper implementation of measurements," designating "specific measuring instruments."

Sound level meters are designated as specific measuring instruments, and when used for transactions or proof of noise level values, they must be measured with a sound level meter that has passed certification and is within its validity period. To facilitate individual testing under this certification system, a type approval system has been established. For specific types of sound level meters, rigorous tests on essential matters are conducted in advance, and those that pass receive a type approval number, with the certification outcome determined through simpler tests primarily focused on instrument deviation. Our LA series sound level meters have obtained type approval, allowing them to undergo this certification. The certification of sound level meters is conducted by the Japan Quality Assurance Organization (JQA), and a certificate indicating a validity period of 5 years (revised to 5 years from April 1998) is affixed to the meters that pass the certification.

 Note 

Under the Measurement Act, there is a system for designated manufacturers with excellent quality control capabilities, allowing them to conduct self-inspections based on the standards of the testing inspection regulations for specific measuring instruments at the time of new shipment and affix a "conformity mark." This "conformity mark" can replace the need for testing. Our company has obtained this designation, so we can act on behalf of the testing for new shipments and affix a certificate of compliance. A sample of the "conformity mark" will be provided on the next page. After shipment, the sound level meter will undergo testing at JQA following inspection and adjustment by the manufacturer.

In internal experiments and research, regardless of transactions or certifications, there is no difference in performance in the manufacturing of our sound level meters, so you can use them with confidence even if they are not certified.

(Note) If you require a certificate, please contact our sales office.

The regulations concerning noise state, "Measurements shall be conducted using a sound level meter that has passed the conditions of Article 71 of the Measurement Law..." Article 71 of the Measurement Law is as follows. (Some parts omitted)

Article 71 of the Measurement Law

According to the above provisions, a "sound level meter that has passed Article 71 of the Measurement Law" refers to a "certified sound level meter." Caution is necessary for a "non-certified sound level meter" as it does not have a certificate proving compliance with the tolerances specified in Article 71, paragraph 1, item 2.

The level linearity deviation of sound level meters that conform to the new JIS (JIS C 1509) is defined for each frequency of sound pressure level according to the respective frequency weighting characteristics (A, C, Z), as specified below (considering the expanded uncertainty of measurement).

Furthermore, the acceptable tolerance for changes within ±10 dB at any signal level in each level range is as follows (including measurement uncertainty).

If a signal exceeding the allowable range of linearity deviation is input into the sound level meter, it will display an over level or under level warning, indicating that it is outside the measurable range.

The measurable range of a sound level meter (linearity range) can be described as the operating range where there are no over or under indications for each frequency weighting characteristic, each range, and each frequency.

The values listed in the catalog vary in measurable range due to the conditions mentioned above, so the linear operating range at 1 kHz is typically noted as a representative value for the linearity range. Additionally, the instruction manual presents nominal values for the upper (Over) and lower (Under) limits at the frequencies below in a tabular format for each frequency weighting characteristic.

A windscreen is used to reduce noise generated by wind near the microphone. An ideal windscreen is one that, when attached to the microphone, does not cause significant acoustic changes and has a substantial effect in reducing wind noise.

General wind screens can be broadly categorized into two types: a general-purpose type that is easily detachable during noise measurements, providing protection against dust and physical shocks, usually made of spherical urethane foam, and an all-weather type designed for long-term continuous measurements in environments such as aircraft noise or factory noise, which has a significant wind noise reduction effect and can withstand rain and snow. It is recommended to use a wind screen as much as possible during outdoor measurements.

General measurement windscreen is usually attached to handheld sound level meters. In this case, the windscreen should maintain a certain degree of wind noise reduction while being easy to attach and detach from the sound level meter, and it is also necessary to consider the design balance with the sound level meter.

The wind noise reduction effect of the windscreen is represented by the difference in noise levels generated when wind directly hits the microphone and when the windscreen is attached to the microphone. It has been found that the wind noise reduction effect is influenced by the size of the windscreen, with the effect increasing by nearly 6 dB when the diameter of the screen is doubled.
Our sound level meter comes with one of two types of windscreen depending on the model. The LA-1400A series includes a φ 70 mm windscreen, while the LA-7000 series includes a φ 90 mm windscreen, with their respective performances shown in Figure 7-4. When measuring outdoor environmental noise (above 60 dB), the practical range is up to a wind speed of 3 m/s for the φ 70 mm and up to 5 m/s for the φ 90 mm.

For measurements during bad weather conditions such as outdoor snow or rain, we recommend using an all-weather windscreen. As a reference, we have provided the assembly diagram for the all-weather windscreen that is optionally available for our sound level meter below. Please note that when using the all-weather windscreen, an extension cable will be required to connect the microphone part, which is separate from the main body of the sound level meter.