Technical Report


What is sound level meter?


6. Sensory measures of sound

6.1 Loudness

Sound loudness is a sensory quantity related to sound intensity, and is one of the most fundamental properties of auditory sense that also include pitch and timbre. The unit of measurement for loudness is sone, with 1 sone corresponding to a pure tone (1 kHz) with a sound pressure level of 40 dB. A sound which a person with normal hearing judges as twice as loud as 1 sone is rated at 2 sone.

Figure 6-1 shows the relationship between sound pressure level (dB) and loudness (sone) for each octave band. In the case of 1 kHz, a 10 dB increase in sound pressure doubles loudness, increasing it by 1 sone. This also applies to other frequency bands, although a 10 dB increase does not precisely double the loudness. This shows that loudness is largely a function of sound intensity and frequency. Loudness is also a function of the duration of sound, with people perceiving a sound as steady when it lasts 150 to 300 ms.

 

Figure 6-1: Relationship between sound pressure level and loudness

 

6-2 Pitch

As described earlier in this guide, pitch is a sensory concept related to the frequency of sound. With a pure tone with a constant intensity, pitch is proportional to frequency: the higher the frequency, the higher the pitch. The unit of measurement for pitch is mel, with 1000 mel corresponding to a pure tone with a frequency of 1 kHz and a sound pressure level of 40 dB.

Again, pitch is largely determined by the frequency of the sound. The pitch of compound sound, however, is not that simple, influenced by the composition of frequencies and other factors. It is also influenced by sound intensity and whether there is preceding and/or succeeding sound.

6-3 Timbre

We can distinguish different musical instruments even when they are emitting sound of the same loudness and pitch because their sounds feel different from one another. These differences come from the specific characteristics of their timbre.

Although it has been studied for years, timbre is not fully understood. According to recent studies, timbre is determined by three factors, namely aesthetic, metallic and powerful factors.

6-4 Loudness level

Loudness is largely determined by the intensity of the sound. It also depends on the frequency and duration of the sound, with people perceiving a sound as steady when it lasts 150 to 300 ms.

Based on the above, the sound pressure level of a 1 kHz pure tone that people with normal hearing determine as having the same loudness as the steady sound in question is defined as the sound’s loudness level P (phon).

Measurement of frequency-caused changes in human auditory sensation was first undertaken by Fletcher-Munson and in 1957 was studied again by Robinson, F et al. Figure 6-2 shows the relationship between the sound pressure levels of pure tones that people with normal hearing perceive as having the same loudness and frequencies, as measured by Robinson et al. These curves are called equal loudness curves or equal-loudness-level contours. Later, it was found that the equal loudness curves determined by Robinson, F et al. contained significant errors. In 2003, new equal-loudness-level contours were adopted as ISO 226. In Figure 6-2, the old equal-loudness-level contours are shown in blue and the new contours are shown in red.

 

Old and new equal-loudness-level contours

Figure 6-2: Old and new equal-loudness-level contours

 

Figure 6-2 indicates that human auditory sense has the following characteristics.

(1) Sensitivity of the ears drops significantly in the low frequency range. Let’s look at the contour for 40 phon. For a 40 dB, 1 kHz pure tone, the same level of loudness requires around 10 dB more at 250 Hz and around 35 dB more at 63 Hz.

(2) The ears are most sensitive at around 4 kHz, several dB more sensitive than at 1 kHz.

(3) In the frequency range above 4 kHz, sensitivity of the ears less sensitive toward higher frequencies.

(4) Depending on the intensity of sound, the audible frequencies for humans range from 15 to 20 Hz up to around 20 kHz.

 

The equal-loudness-level contours can be interpreted as the average values for many young people although there are huge differences between individuals. Ageing takes hold once a person turns 20 and the upper audible frequency limit continues to drop thereafter.

Again, the loudness of a pure tone can be determined using equal-loudness-level contours. With compound sound, a number of methods were proposed for predicting loudness level based on sound pressure, such as one in 1937 by Fletcher-Munson. The methods developed by Stevens, S and Zwicker, E were adopted as ISO 532.

6-5 A-weighted sound pressure level

Noise is undesirable sound to humans. Any sound that is perceived as unpleasant or disturbing by those who hear it is considered noise. Noise is a sensory quantity based on human auditory sense and, as such, must be measured using a unit based on auditory sense not on physical loudness. For this reason, A-weighted sound pressure level based on the equal-loudness-level contours explained in Section 6-4 is used as the unit of measurement (dB) for noise level LA.

A-weighted sound pressure levels are measured using sound level meters. A sound level meter uses a microphone (normally a capacitor type) to generate electric signals proportional to the sound pressure and, via a frequency correction circuit, displays A-weighted sound pressure levels.

 

7. Overview of sound level meters

7.1 What is a sound level meter?

Sound level meters are a device for measuring A-weighted sound pressure level (LA) and sound pressure level (Lp), and are designated as specified measuring instruments under the Measurement Act. Specifications and other details are specified in IEC 61672-1 (which corresponds to "precision sound level meter" in the Measurement Act, JIS C 1509-1 Class1) or IEC 61672-1 (which corresponds to "sound level meter" in the Measurement Act, JIS C 1509-1 Class2) depending on measuring accuracy. In addition, there are integrating sound level meters for integrating operations such as for percentile sound pressure level (Lx) equivalent continuous A-weighted sound pressure level (LAeq) and sound exposure level (LAE).

 

NOTE:

IEC 61672-1 (JIS C 1509-1:2005) classifies sound level meters into Class 1 and Class 2 depending on their performance. Class 1 and Class 2 roughly correspond to "Type1" and "Type2" sound level meters in IEC 60651 (JIS C 1505 / JIS C 1502).

 

Methods for measuring and evaluating various types of noise (environmental, work environment, factory/traffic, Shinkansen, aircraft, etc.) are specified by relevant laws. Noise measurement is an important component of environmental assessments.

(特定計量器に指定されている主な環境計測器)

Figure 7-1: Major environment measuring instruments designated as specified measuring instruments Vibration level meter

7-2 Differences between sound level meters (Class 2) and precision sound level meters (Class 1)

Sound level meters are designed for measuring environment noise outdoors and at places like factories and offices. They are lower-priced models and can be used easily on site. Precision sound level meters accommodate every user requirement for various studies on noise and assessment of machine noise. Basic specifications for both classes are based on the Measurement Act and JIS, which have the following major differences.

 

IEC / JIS

Table 7-1: Differences between Classes (IEC (JIS))

Class 2 (standard) Class 1 (precision)
Level linearity error (dB)
(in input level change of 10 dB or less)
±0.8 (±0.5) ±0.6 (±0.3)
Level linearity error (dB)
(Other than the above)
±1.4 (±1.1) ±1.1 (±0.8)
Frequency range (Hz) 20 to 8 kHz 16 to 16 kHz

 

NOTE:

1.The figures in parentheses take into account expanded measurement uncertainty (which corresponds to the maximum permissible measuring error in IEC, JIS).

2.The Measurement Act specifies as follows the difference between the minimum value in the measurement range and the self-noise.

Sound level meters: +6 dB Precision sound level meters: +8 dB

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