A sound level meter is 1

Sound is generally defined as waves (sound waves, elastic waves) that propagate through media such as gases, liquids, and solids; however, this booklet will primarily explain sound waves that travel through air.

Sound waves are a wave phenomenon that travels at a speed of approximately 340 m per second. We hear these waves as they propagate through the air via our ears. These waves are generated by continuous fluctuations in atmospheric pressure (alternating pressure changes) in the air.

Generally, the pressure variation from atmospheric pressure (static pressure) is called sound pressure, but according to the JIS standard JIS Z 8106:2000 (acoustic terminology);

・Instantaneous sound pressure (instantaneous sound pressure): The value obtained by subtracting static pressure from the pressure that exists at a certain point in the medium at the moment of interest.

• Sound pressure (sound pressure): Unless otherwise specified, the effective value of instantaneous sound pressure within a certain time period.

Figure 1-1 Basic Processing Flow of sound level meter

It clearly distinguishes between "instantaneous sound pressure" and "sound pressure."
(See Figure 1-1 on the right)

However, please be aware that in this booklet, we will continue to use the term "sound pressure" to appropriately distinguish between the two terms mentioned above.

The range of sound pressure that can be heard by young people with normal hearing is from 20 μPa to 20 Pa. μ is read as "micro" and is a supplementary unit meaning 10^-6. Therefore, the ratio of the sound pressure of the loudest sound to the quietest sound is as much as 10 ⁶. Also, "Pa" (Pascal) is a unit of pressure, where 1 Pa corresponds to the pressure when a force of 1 N (approximately equivalent to a weight of 0.1 kg) is applied over an area of 1 m ². The atmospheric pressure in weather forecasts uses the unit hPa, which is 100 times larger. (The term atmospheric pressure is also used as a unit, where 1 atmosphere is approximately equivalent to 1013 hPa.)

Similar to the strength of sound, the frequency range of instantaneous sound pressure that can be heard by normally hearing young people is approximately from 20 Hz to 20 kHz, which is about a 1000-fold range. Although it is not a strict distinction, the frequency range of sound can generally be divided as follows.

below 20 Hz: infrasound

20 Hz to 20 kHz: audible sound

above 20 kHz: ultrasound

pitch of sound: The high and low sounds we refer to arise from differences in sound frequency. Even with the same 'A' sound, there are high-pitched and low-pitched 'A's. This is because, although the wave shape of the sound as 'A' is the same, the frequencies are different; high-frequency sounds are shrill, while low-frequency sounds are deep and heavy.

the volume of sound: Even with the same pitch of the sound "A", there are loud and soft versions of "A". This is because, although they have similar waveforms, the loud "A" has a larger amplitude, while the soft "A" has a smaller amplitude. A sound level meter is a device that measures the magnitude of this sound.

Timbre and Sound Quality: We can distinguish between different types of instruments played at the same volume and pitch. This is based on our ability to discern the timbre and quality of the sound coming from the instruments. Although timbre and sound quality are still not fully understood, it can be said that they are due to subtle differences in sound waveforms.

Noise is a general term for "unpleasant sounds that one does not want to hear" and "disturbing sounds" among various sounds. According to the aforementioned acoustic terminology standard (JIS Z 8106:2000), noise is defined as "unpleasant or undesirable sounds, and other disturbances." Sounds that are perceived as unpleasant can cause harm related to health and living environments, eventually leading to pollution issues.

In the evaluation of noise, the focus is on the audible range of 20 Hz to 20 kHz, particularly the main band of approximately 50 Hz to 5 kHz. Additionally, for normal conversation, the range of 300 Hz to 3 kHz is important for hearing.

Working for long hours in loud noise can lead to hearing loss, and laws have been established to regulate noise in order to protect people's lives from such disturbances.

A sound level meter is commonly used as a measuring instrument to assess and evaluate noise. There are two methods for quantifying sound: physical scales and sensory scales.

As physical measures,
① sound pressure level,
② sound intensity level,
③ sound power level,
and octave bands and 1/3 octave band levels analyzed by frequency, etc., are available,

As sensory scales, there are
④ loudness,
⑤ pitch,
⑥ timbre,
⑦ loudness level,
⑧ noise level (A weighted sound pressure level), etc.,
many of which are defined by JIS and IEC (International Electrotechnical Commission). In particular, various evaluation metrics are currently being researched and presented in the field of sensory scales, which is expected to develop further in the future.

In environmental noise, noise levels are evaluated using measurement items suitable for the measurement purpose, such as sound power levels, 1/3 octave analysis, and more recently, sound quality evaluation parameters in product development. An overview of these is summarized in Chapters 5, 6, and 11, so please refer to them.

The sound level meter is an instrument that measures ① sound pressure levels and ⑧ noise levels, and due to recent technological advancements, models that can also measure octave band levels, 1/3 octave band levels, and ④ loudness have been released.

Well then, let's take a closer look at noise.

Figure 2-1 shows examples of the types of environmental noise and their levels.

Steady noise

Steady-state noise refers to noise at measurement points where the noise level is almost continuous, and the sound level meter indicator value shows no variation or only minor variation even if it fluctuates slightly.

Figure 3-1
Fluctuating noise

At measurement points, noise that fluctuates irregularly and continuously over a considerable range is referred to as fluctuating noise. For example, noise observed near a road with a certain level of automobile traffic is, in most cases, fluctuating noise.

Figure 3-2
Intermittent noise

Intermittent noise refers to noise that occurs at intervals over time, where the duration of each occurrence is several seconds or more. The time interval between one occurrence and the next can be relatively constant, or it can be irregular, as in the case of trains or aircraft passing by.

Figure 3-3
Isolated burst of sound energy

For example, the sound of a pile driver when driving a pile is referred to as isolated burst noise, where individual noises can be separated. Isolated burst noise can occur as a single event or intermittently. Additionally, the level of each occurrence may be nearly constant or vary significantly over a range.
Typically, it refers to bursts with an interval of 0.2 s or more.

Figure 3-4
Quasi-steady impulsive noise

For example, noise generated by bells or rock drills, where impulsive sounds occur repeatedly at a nearly constant level with very short time intervals (less than about 0.2 s), is referred to as quasi-steady impulsive noise. Such noise is often perceived sensorially as steady noise.

Figure 3-5

Broad-band noise

It is noise that has a spectrum distributed over a relatively wide frequency range.

Examples: sound of waterfalls, exhaust noise from air conditioning ducts, highway noise, etc.

Figure 3-6
Narrow-band noise

A type of noise that has a spectrum concentrated in a relatively narrow frequency range (within 1/3 octave band) and does not include discrete sounds.

Examples: sound of distant thunder (low frequency), sound of wind blowing through a meadow or valley (mid frequency), sound of air escaping from a car tire (high frequency), etc.

Figure 3-7
Discrete tone

A noise that has periodic sound pressure fluctuations that create a sense of pitch, and has a line spectrum in terms of frequency.

Examples: fan hum, beeping from digital devices, instrument sounds, etc.

Figure 3-8