Technical Report: What is a sound level meter (sound level meter) 3

The loudness of sound is a sensory quantity related to the strength of sound, and it is one of the most fundamental properties of sound perception, alongside pitch and timbre. The unit used is the sone, which is expressed on a ratio scale where a pure tone (1 kHz) at a sound pressure level of 40 dB is defined as 1 sone. For example, the loudness of a sound that a person with normal hearing perceives as twice the loudness of a 1 sone pure tone is 2 sones.

Figure 6-1 shows the relationship between sound pressure level (dB) and loudness (sone) determined for each octave band. Looking at the case of 1 kHz, it can be seen that an increase of 10 dB in sound pressure level results in a doubling of loudness to 2 sone. Although there are slight differences in other frequency bands, the relationship remains the same: an increase of 10 dB in sound pressure level generally leads to a doubling of loudness. Thus, loudness primarily depends on sound intensity, but it also depends on frequency. Furthermore, it is influenced by the duration of the sound, which is perceived as steady between 150 to 300 ms.

As mentioned earlier, the pitch of a sound corresponds to its frequency in terms of auditory perception, such that for a constant intensity pure tone, frequency and pitch correspond in a one-to-one ratio. In other words, higher frequency sounds are perceived as higher in pitch, while lower frequency sounds are perceived as lower in pitch. The unit used is the mel, expressed on a ratio scale where a pure tone at 1 kHz and 40 dB is defined as 1000 mel.

In this way, pitch is primarily determined by the frequency of sound, but for complex sounds, it may not be uniquely defined due to the combination of frequency components. It is also influenced by sound intensity and the presence of preceding or following sounds.

Even when two sounds are produced at the same volume and pitch, we can distinguish between different types of instruments. In this way, when two sounds are equal in size but give a different impression, the property corresponding to that difference is called timbre.

The study of timbre has been conducted for a long time, but it is still not fully understood. According to recent research, timbre is said to be determined by three factors: aesthetic factors, metallic factors, and impact factors.

As mentioned earlier, the loudness of sound primarily depends on its intensity, but it also depends on frequency and duration, with a duration of 150 to 300 ms being perceived as steady. Therefore, regarding steady sounds, the sound pressure level of a 1 kHz pure tone that a person with normal hearing judges to sound equally loud is defined as the "level of loudness" P (phon). Measurements regarding changes in human auditory perception due to frequency began with Fletcher-Munson and were re-measured by Robinson et al. in 1957. Figure 6-2 below shows the curve representing the relationship between the sound pressure level of pure tones perceived as equally loud by individuals with normal hearing and frequency, known as the equal-loudness curve or equal-sensitivity curve. It is noted that the equal-loudness curves measured by Robinson et al. contained significant errors, and a new equal-loudness curve was standardized as ISO 226 in 2003. In the figure, the old equal-loudness curve is shown in blue, while the new loudness curve is shown in red for comparison.

From this diagram, we can see that human hearing has the following characteristics.

These equal loudness contours can be regarded as average values for many young people, but in reality, there are considerable individual differences. After the age of 20, signs of aging begin to manifest, and the maximum audible limit decreases with increasing age.

As mentioned above, the level of pure tone loudness can be determined by the equal-loudness contour, but for complex sounds, several methods have been proposed since 1937 by Fletcher and Manson to predict the loudness level from the sound pressure.
Currently, the international standards adopt the Zwicker method (ISO 532-1) and the Moor Grasbarg method (ISO 532-2).

Noise refers to sounds that are undesirable to humans, and any sound that is perceived as unpleasant or disturbing by the listener is considered noise. In other words, since noise is a sensory quantity based on human hearing, to express its magnitude, one must use a quantity based on human perception rather than the physical magnitude of the sound. Therefore, the sound pressure level of noise is represented by the sound pressure level weighted by frequency according to the equal-loudness contours described in Section 4 of Chapter 6, and this is referred to as the "noise level" L _A (unit: dB).

To measure noise levels, a sound level meter is used. The sound level meter generates an electrical signal proportional to sound pressure through a microphone (usually a condenser microphone), and displays the sound pressure level with A-weighting frequency correction through a frequency correction circuit.

(Note)
The term "noise level" is a unique term in our country and is used synonymously with "A-weighted sound pressure level" as described above. Furthermore, the new JIS (JIS C 1509) defines a new term "sound level" for frequency-weighted sound pressure levels, which corresponds to "A-weighted and time-weighted sound level."

A sound level meter is a measuring instrument that measures sound level (L _A) and sound pressure level (L _p), and is designated as a specific measuring instrument under the Measurement Act. Additionally, due to differences in measurement accuracy, specifications are defined for JIS C 1509-1 Class 1 and JIS C 1509-1 Class 2. Furthermore, there are integrated average sound level meters that have the function to measure integral quantities such as equivalent continuous sound level (L _Aeq) and single event noise exposure level (L _AE).

(Note)
The new standard for sound level meter, the Sound Level Meter (sound level meter) JIS C 1509-1:2005, divides performance grades into Class 1 and Class 2. In terms of performance, Class 1 corresponds roughly to the old JIS C 1505 (Precision sound level meter), and Class 2 to the former JIS C 1502 (Ordinary sound level meter). With the revision of the Specified Measuring Instrument Inspection Regulations in 2015, the standards now refer to JIS C 1516. This content extracts items necessary for transactions and certification from JIS C 1509-1.

Furthermore, the evaluation methods for measuring different types of noise (such as environmental noise, workplace noise, factory and traffic noise, Shinkansen noise, and aircraft noise) are defined by law. Noise measurement has become an important measurement item in environmental assessment.

Ordinary sound level meters are designed for measuring environmental noise in outdoor, factory, and office settings, aiming to facilitate low-cost and easy measurements on-site. In contrast, precision sound level meters are intended to accommodate all measurement conditions for users conducting noise research in various fields or evaluating the noise of machinery. Both types of sound level meters are based on the Measurement Act and JIS standards, with their main differences outlined in the table below.

Table 7-2 Differences in Performance by Class (JIS Standards)

Note