Technical Report: What is a Sound Level Meter? (Part 3)

Loudness is a perceptual quantity related to the intensity of sound, and is one of the most fundamental properties of sound perception, along with pitch and timbre. It is expressed using the sone as its unit, as a ratio scale where a pure tone (1 kHz) with a sound pressure level of 40 dB is defined as 1 sone. For example, a sound that a person with normal hearing would perceive as twice as loud as a 1 sone pure tone is 2 sones.

Figure 6-1 below shows the relationship between sound pressure level (dB) and loudness (sone) for each octave band. Looking at the case of 1 kHz, we can see that when the sound pressure level increases by 10 dB, the loudness doubles by 2 sones. In other frequency bands, although it differs slightly from the 1 kHz case, the relationship remains the same: when the sound pressure level increases by 10 dB, the loudness approximately doubles. Thus, loudness depends mainly on the intensity of the sound, but also on the frequency. Furthermore, it also depends on the duration of the sound, with a steady state being perceived when the duration is between 150 and 300 ms.

As mentioned earlier, pitch is an auditory property related to sound frequency. For a pure tone with constant intensity, there is a one-to-one correspondence between frequency and pitch. That is, high-frequency sounds are perceived as high-pitched, and low-frequency sounds as low-pitched. The unit used is mel, which is a ratio scale where a pure tone with a frequency of 1 kHz and a sound pressure level of 40 dB is defined as 1000 mel.

Thus, while pitch is primarily determined by the frequency of the sound, the pitch of complex sounds may not be uniquely determined due to the combination of frequency components. It is also affected by the intensity of the sound and the presence or absence of preceding and succeeding sounds.

Even when two instruments produce sounds of the same volume and pitch, we can distinguish between different types of instruments. Similarly, when two sounds have the same volume but give different impressions, 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. Recent research suggests that timbre is determined by three factors: aesthetic factors, metallic factors, and impact factors.

As mentioned earlier, the loudness of a sound primarily depends on its intensity, but also on its frequency and duration, with a duration between 150 and 300 ms being perceived as steady. Therefore, for steady-state sounds, the sound pressure level of a 1 kHz pure tone that a person with normal hearing would perceive as being the same loudness is defined as the "loudness level" P (phon). Measurements of how human hearing changes with frequency began with Fletcher-Munson and were remeasured in 1957 by Robinson et al. Figure 6-2 below shows the relationship between the sound pressure level and frequency of pure tones perceived as equally loud by a person with normal hearing, as measured by Robinson et al. These are called equal-loudness curves or equal-sensitivity curves. It should be noted that the equal-loudness curves measured by Robinson et al. contained a large error, and in 2003, new equal-loudness curves were standardized internationally as ISO 226. In the diagram, the old equal loudness curves are shown in blue, and the new loudness curves are shown in red for comparison.

This diagram shows that human hearing has the following characteristics.

While these equal-loudness curves can be considered as the average for a large number of young people, there is actually considerable individual variation, and after the age of 20, aging phenomena appear, with the maximum audible limit decreasing with increasing age.

As described above, the loudness level of a pure tone can be determined using equal-loudness curves, but several methods for predicting the loudness level of a complex tone from its sound pressure have also been proposed since Fletcher Munson in 1937.
Currently, the international standards adopted are the Zwicker (E) method (ISO 532-1) and the Moor-Grasberg method (ISO 532-2).

Noise is any sound that is undesirable to humans. Any sound that is perceived as unpleasant or disruptive by the listener is considered noise. In other words, since noise is a sensory quantity based on human hearing, to express its magnitude, we must use a quantity based on human hearing rather than the physical magnitude of the sound. Therefore, we use the sound pressure level of noise, which has been frequency-weighted (A-weighted) according to the equal-loudness curves described in Section 4 of Chapter 6, as a quantity to express the magnitude of noise, and call it the "noise level" LA (unit: dB).

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

(Note)
The term "noise level" is unique to Japan and, as explained above, is used as a synonym for "A-weighted sound pressure level." Furthermore, the new JIS standard (JIS C 1509) defines a new term, "sound level," in contrast to frequency-weighted sound pressure level, and according to this standard, it corresponds to "A-weighted and time-weighted sound level."

Sound level meters are measuring instruments that measure sound level (_LA) and sound pressure level (_Lp), and are designated as specified measuring instruments under the Measurement Law. Furthermore, due to differences in measurement accuracy, specifications are defined in JIS C 1509-1 Class 1 and JIS C 1509-1 Class 2. In addition, there are integral-averaging sound level meters that have the function of measuring integrated quantities such as equivalent sound level (_LAeq) and single-event sound exposure level (_LAe).

(Note)
The new standard for sound level meters, JIS C 1509-1:2005, divides performance into two classes: Class 1 and Class 2. In terms of performance, Class 1 is roughly equivalent to the old JIS C 1505 (precision sound level meter), and Class 2 is roughly equivalent to the old JIS C 1502 (ordinary sound level meter). Following the revision of the Specific Measuring Instruments Verification and Inspection Regulations in 2015, the standard now refers to JIS C 1516. This standard extracts the items necessary for trade and certification from JIS C 1509-1.

Furthermore, the evaluation methods for measuring noise are legally defined depending on the type of noise (environmental noise, work environment noise, factory/traffic noise, Shinkansen noise, aircraft noise, etc.). Noise measurement is an important measurement item in environmental assessment.

Standard sound level meters are designed for measuring environmental noise in outdoor areas, factories, offices, etc., aiming to provide low-cost and easy on-site measurements. In contrast, precision sound level meters are designed to accommodate a wide range of measurement conditions for users conducting noise research in various fields or evaluating the noise levels of machinery and equipment. Both types of sound level meters adhere to the basic specifications of the Measurement Law and JIS standards, and their main differences are shown in the table below.

Table 7-2 Performance differences by class (JIS standard)

(Note)