Technical Report: What is a Sound Level Meter? (Part 4)

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

Sound calibrators include acoustic calibrators that generate a reference sound pressure using mechanical vibrations of a constant period, known as pistonphones, and simple acoustic calibrators that use piezoelectric ceramics as a vibration sound source. We offer the SC-2500A (JIS C 1515:2020 Class 1) and SC-2120A (JIS C 1515:2004 Class 2), respectively. The SC-2500A has a reference sound pressure of 114 dB (1 kHz), and the SC-2120A has a reference sound pressure of 94 dB (1 kHz). Before starting measurements, input the reference sound pressure of the sound calibrator, set the frequency weighting of the sound level meter to FLAT or C-weighting, and check if the indicated value is the same as the reference sound pressure. If there is a difference in the indication, adjust it using the adjustment volume until they match. If the difference is 2 dB or more, there may be some kind of malfunction, so we recommend having it inspected by the manufacturer.

Figure 7-1 Typical sound calibrator

Additionally, some sound level meters have an internal reference signal output function (REF). This function generates a stable reference signal at 1000 Hz, which corresponds to 1/2 of the full scale (-6 dB), with the sound level meter's level range set to full scale (100%), and is used to verify the operation of the electrical circuit. It is used for simple daily checks and as a calibration signal for externally connected devices such as level recorders and FFT analyzers.

 (Note) 

Products in supermarkets and other stores are labeled with a weight in grams, and the scales used to measure this weight have been certified. Measuring instruments used for trade or certification are required to be publicly guaranteed, and the Measurement Act designates "specified measuring instruments" as "in order to ensure proper measurement, it is necessary to impose regulations under the Measurement Act."

Sound level meters are designated as specified measuring instruments, and when using sound level values for trade or certification, measurements must be taken using a sound level meter that has passed calibration and is within its validity period. To simplify individual calibrations, a type approval system has been established within this calibration system. For specific types of sound level meters, rigorous tests are conducted in advance on fundamentally important matters, and models that pass are given a type approval number. Calibration is then determined by a simpler test, mainly consisting of instrument error testing. Our LA series sound level meters have obtained type approval and are therefore eligible for this calibration. Calibration of sound level meters is conducted by the Japan Quality Assurance Organization (JQA), and sound level meters that pass calibration are affixed with a certificate of conformity that includes a validity period of 5 years (revised to 5 years from April 1998).

 (Note) 

The Measurement Law includes a designated manufacturer system that allows manufacturers with superior quality control capabilities to conduct self-inspections of specified measuring instruments based on the standards of the Verification and Inspection Regulations and affix a "Certificate of Conformity" mark to their products at the time of shipment. This "Certificate of Conformity" can substitute for verification. Our company has obtained this designation, so we can perform verification on behalf of new products and affix a certificate of conformity. A sample of the "Certificate of Conformity" mark is shown on the next page. Verification of sound level meters after shipment is carried out by JQA after inspection and adjustment by the manufacturer.

Furthermore, for internal experiments and research used regardless of transactions or certifications, there is no difference in performance between our sound level meters with and without certification; they are completely identical in terms of performance. Therefore, you can use them with confidence even if they are not certified.

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

The regulations concerning noise include the statement, "Measurements shall be carried out using a sound level meter that meets the conditions of Article 71 of the Measurement Law..." Article 71 of the Measurement Law is as follows: (partially omitted)

Article 71 of the Measurement Law

Based on the above provisions, a "sound level meter that complies with Article 71 of the Measurement Law" refers to a "sound level meter with a verification certificate." It is important to note that "sound level meters without verification" do not have a certificate proving compliance with the verification tolerances specified in Article 71, Paragraph 1, Item 2.

The level linearity deviation of a sound level meter conforming to the new JIS (JIS C 1509) is specified as follows for each frequency of the sound pressure level according to each frequency weighting characteristic (A, C, Z characteristics) (taking into account the expanded uncertainty of the measurement).

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

If a signal exceeding this tolerance for linearity is input to the sound level meter, it will display an over- or under-level indicator, warning that the signal is outside the measurable range.

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

The values listed in the catalog are usually based on the linear operating range at 1 kHz, as the measurable range varies depending on the conditions described above. The instruction manual also includes a table showing the nominal upper (Over) and lower (Under) limits for each frequency weighting characteristic at the following frequencies.

Windscreens are used to reduce noise generated near microphones by wind. An ideal windscreen is one that does not significantly alter the acoustics of the microphone when attached, while also providing a substantial reduction in wind noise.

General windscreens can be broadly categorized into two types: general-purpose types (mostly made of spherical urethane foam) that can be easily attached and detached during noise measurements, protecting microphones from dust and debris and reducing physical impact; and all-weather types that are used for long-term continuous measurements of aircraft noise, factory noise, etc., and have a significant effect in reducing wind noise, and can withstand rain and snow. When taking measurements outdoors, it is desirable to attach a windscreen whenever possible.

Windscreens for general measurement are almost always attached to handheld sound level meters. In this case, the windscreen must maintain a certain level of wind noise reduction, be easy to attach to and detach from the sound level meter, and maintain a design balance with the sound level meter.

The wind noise reduction effect of a windscreen is expressed as the difference between the noise level generated near the microphone when wind directly hits it and the noise level generated when the windscreen is attached to the microphone. It is known that the wind noise reduction effect depends on the size of the windscreen, and the effect increases by approximately 6 dB when the diameter of the screen doubles.
Our sound level meters come with one of two types of windscreens, depending on the model. The LA-1400A series comes with a φ70 mm windscreen, and the LA-7000 series comes with a φ90 mm windscreen. Their respective performance characteristics are shown in Figure 7-4. For measuring outdoor ambient noise (60 dB or higher), the φ70 mm screen is practical for wind speeds up to approximately 3 m/s, and the φ90 mm screen is practical for wind speeds up to approximately 5 m/s.

For measurements taken outdoors in adverse weather conditions such as snow or rain, we recommend using an all-weather windbreak screen. For reference, an assembly diagram for the all-weather windbreak screen, which is available as an option for our sound level meters, is shown below. Please note that when using the all-weather windbreak screen, an extension cable will be required separately to connect the sound level meter body to the detached microphone unit.