What is a sound level meter? 13

Loudness level is a sound evaluation method that includes auditory masking effects, which were not considered in conventional noise level (LA) measurements, and represents sound intensity in a way that is closer to human perception.

Recent home appliances are advertised in catalogs and other materials with claims such as "half the noise level of previous models," but the basis for this claim is the loudness rating.

Our LA-7000 series sound level meter allows you to perform loudness analysis using 1/3 real-time octave analysis data. In addition to "loudness," we also offer a "sound quality evaluation function" that quantifies and values sound using six physical quantities: "sharpness," "roughness," "variability intensity," "AI," and "tonality," which are psychoacoustic parameters.

When people hear sounds, they make judgments while considering various factors, as shown in Figures 11-12 above. How people perceive sound varies from person to person depending on their psychological state; for example, they may not notice familiar sounds. Various evaluation methods that include psychological elements have been attempted, and it is also possible to obtain sound quality evaluation indicators with our "O-Solution Sound Quality Evaluation Function OS-0525".

Regarding the auditory perception of loudness, one of the various quantities used to evaluate sound quality, it strongly depends on the sound pressure level, which is a physically measured quantity. The noise level (A-weighted sound pressure level), which is commonly used as a quantity for evaluating noise, was conceived as a sound pressure level with frequency weighting that approximates the 40 phon pure tone loudness characteristic (equal tone curve ISO R226) in order to mimic hearing. However, when listening to two pure tones at the same volume, if the frequencies are far apart, they are perceived as twice as loud, but if the frequencies are close together, they are perceived as quieter rather than twice as loud; this is called the masking effect.

Since noise levels represent a quantity determined by weighting the loudness curve of a pure tone, they do not match the perceived loudness of sounds containing various frequencies, such as typical noise. To obtain a sound level that is more in line with auditory perception, methods that take masking effects into account have been devised and standardized in ISO 532 as a method for calculating loudness for steady-state noise. This standard has two methods: Method A by Stevens, SS. and Method B by Zwicker, E. Further explanation follows.

Figure 11-13 below shows the equal-loudness curves for pure tones (ISO 226 international standard, 2003). This figure is the same as the one shown in Chapter 6, Section 4, "Loudness Level." Please also refer to the explanation in Chapter 6, Section 4.

The numbers attached to the curve represent the loudness level, expressed in units of phon, which is the same value as the sound pressure level (dB) of a 1 kHz pure tone. The curves are shown as equal-sound curves, connecting the sound pressure levels of each frequency that sound the same loud as a 1 kHz pure tone. The equal-sound curve at 40 phon is used as an approximation and is adopted as an A-weighted filter. The noise level is obtained by weighting and summing the energy of the A-weighted filter. However, the loudness level is not a psychological quantity, and in general complex noise, a 50 phon sound does not sound twice as loud as a 40 phon sound, nor does listening to two 40 phon sounds simultaneously sound like 50 phon. Therefore, as a unit to express loudness, a 1 kHz sound with a 40 dB (sound pressure level) is defined as 1 sone, and the loudness of noise that sounds the same loudness is expressed as 1 sone, and twice that loudness is expressed as 2 sones.

Furthermore, just as sound pressure level Lp and noise level LA are expressed in dB, sound intensity is similarly converted to dB and expressed in units of phon. The relationship between sound intensity level L (phon) and sound intensity S (sone) is expressed by the following formula when L is in the range of 40 to 120 phon.

Equation 11-9

This relationship shows that when the loudness level increases by 10 phons, the loudness doubles (when S = 2 sones, L = 50 phons).
When S < 1, the calculation is performed using the following formula.

Equation 11-10

The human ear perceives sound at different points depending on its frequency, much like performing frequency analysis. The frequency band corresponding to the bandwidth of 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, combining pure tones of 250 Hz and 1 kHz sounds twice as loud, but combining pure tones of 250 Hz and 500 Hz does not sound twice as loud. This is because the critical band is involved, and as shown in the lower right diagram, the 250 Hz sound masks the 500 Hz sound.

The loudness of a 250 Hz sound corresponds to the shaded area, and the loudness of 250 Hz and 500 Hz sounds is determined from the area of the shaded region in the diagram above. The area where the 500 Hz pure tone is masked by the 250 Hz pure tone (overlapping area) is excluded, so the value will be smaller than that for 1 kHz, representing a loudness that is closer to what we perceive. As can be seen from the shaded area for 250 Hz, the masking effect is characterized by less masking at low frequencies (approximately considered as no masking) and occurring more at high frequencies. Also, the louder the sound, the wider the area of masking becomes.

The loudness value for ISO 532-1 is determined by real-time analysis of 1/3 octave of the FLAT (flatness) frequency weight.

Loudness is calculated differently depending on whether the measurement is taken in a free-field environment (such as an anechoic chamber or open space) or a diffuse-field environment (such as a gymnasium or a regular room) where sound reflections occur. The values are displayed using the initials GF and GD respectively, so please use the one that matches your measurement environment.