A sound level meter is 13

The loudness level is a method of evaluating sound that includes the auditory masking effects not considered in traditional noise levels (L _A), representing a sound magnitude that is closer to human perception.

Recent home appliances are marketed in catalogs as being 'half as noisy compared to conventional models,' and the basis for this is the value of loudness.

You can perform loudness analysis from the 1/3 real-time octave analysis data of our LA-7000 series sound level meter. In addition to "loudness," we also provide a "sound quality evaluation function" that quantifies and numerically evaluates sound using six physical quantities: "sharpness," "roughness," "fluctuation strength," "AI," and "tonality," which are psychoacoustic parameters.

When a person hears a sound, they make judgments while considering various factors as shown in Figure 11-12. The perception of sound, such as familiar sounds, varies among individuals due to psychological states. Evaluations that include psychological elements have been attempted in various ways, and it is also possible to obtain sound quality evaluation indicators using our "O-Solution Sound Quality Evaluation Function OS-0525".

Among the various quantities that represent sound quality evaluation, when looking at the auditory sensation related to loudness, it strongly depends on the physical measurement quantity of sound pressure level. The noise level (A-weighted Sound Pressure Level) that is commonly used as a measure of noise is considered as a sound pressure level that has been frequency-weighted to approximate the pure tone loudness characteristic (equal loudness contour ISO R226) of 40 phon to mimic human hearing. However, when listening to two pure tones of the same loudness but with widely spaced frequencies, they are perceived as twice as loud, while sounds with closely spaced frequencies are heard as less than twice as loud, which is known as the masking effect.

Since the noise level represents a quantity obtained by weighting the loudness curve of pure tones, it does not match the perceived loudness of sounds that contain various frequencies, like normal noise. To obtain a loudness that is more aligned with human perception, methods that consider the masking effect have been developed and standardized in ISO 532 as a method for calculating loudness related to steady noise. This standard includes two methods: Method A by Stevens, S.S. and Method B by Zwicker, E. Further explanations are provided below.

Figure 11-13 shows the equal loudness contours for pure tones (2003 ISO 226 international standard). This figure is the same as that presented in Section 4 of Chapter 6, "Loudness Level." Please refer to the explanation in Section 4 of Chapter 6 as well.

The numbers associated with the curves represent the loudness level, expressed in phons, which corresponds to the sound pressure level (dB) of a 1 kHz pure tone. The equal loudness contours connect the sound pressure levels of each frequency that sound equally loud as a 1 kHz pure tone. The equal loudness contour for 40 phons is approximated and adopted as the A-weighting filter. The value evaluated by A-weighting and energy summation becomes the noise level. However, the loudness level is not a psychological quantity; in general complex noise, a sound at 50 phons does not sound twice as loud as a sound at 40 phons, and listening to two sounds at 40 phons simultaneously does not make them sound like 50 phons. Therefore, as a unit to express loudness, a sound at 1 kHz with a sound pressure level of 40 dB is defined as 1 sone, and the loudness of noise that sounds the same is expressed as 1 sone, with twice the loudness represented as 2 sones.

Also, the sound pressure level L _p and the noise level L _A are displayed in dB, and similarly, the magnitude of sound is converted to dB and expressed in the unit of phon. The relationship between the sound level L (phon) and the sound magnitude S (sone) is expressed by the following formula when L is in the range of 40 to 120 phon.

Equation 11-9

This relationship indicates that when the level of sound intensity increases by 10 phon, the loudness doubles (S = 2 sone when L = 50 phon).
Note that when S < 1, it is calculated using the following formula.

Formula 11-10

The human ear perceives sound differently depending on the frequency, as if it is performing frequency analysis. The bandwidth corresponding to frequency analysis is called the critical band, and it can be thought that each critical band has a masking effect. For example, as shown in the lower left diagram, the combination of pure tones at 250 Hz and 1 kHz sounds twice as loud, but the combination of pure tones at 250 Hz and 500 Hz does not sound twice as loud due to the critical band, as shown in the lower right diagram, where the sound at 250 Hz masks the sound at 500 Hz.

The loudness of the sound at 250 Hz corresponds to the area of the diagonal lines, and the loudness of the sounds at 250 Hz and 500 Hz is determined from the area of the diagonal lines in the above figure. The area where the pure tone at 500 Hz is masked by the pure tone at 250 Hz (the overlapping area) is excluded, resulting in a smaller value compared to that at 1 kHz, which represents a loudness closer to perception. The characteristics of the masking effect can be understood by looking at the diagonal area at 250 Hz, where the amount of masking is minimal on the low-frequency side (approximated as having no masking), and it has characteristics that occur on the high-frequency side. Additionally, the greater the loudness, the broader the range of masking becomes.

The loudness values according to ISO 532-1 are obtained from real-time analysis of 1/3 octave bands with a FLAT frequency weighting.

Loudness is measured differently in free sound fields, such as anechoic chambers or open spaces without sound reflections, and in diffuse sound fields, such as gyms or regular rooms where sound reflections occur. The values are displayed using the initials GF for free sound fields and GD for diffuse sound fields, so please choose the one that corresponds to your measurement environment.