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What is a sound level meter?

1. What is sound?

Sound is generally defined as a wave (sound wave, elastic wave) that propagates through a medium such as gas, liquid, or solid. However, this booklet will primarily focus on sound waves that propagate through air.

Sound waves are wave phenomena that travel at a speed of approximately 340 meters per second. We hear these waves traveling through the air with our ears. These waves are generated by continuous pressure fluctuations (alternating pressure changes) in the atmospheric pressure of the air.

Generally, pressure fluctuations from atmospheric pressure (static pressure) are called sound pressure, but to quote from the JIS standard JIS Z 8106:2000 (Acoustic Terminology):

Instantaneous sound pressure
(instantaneous sound pressure)

The value obtained by subtracting the static pressure from the pressure present at a given point in the medium at a given moment.

Sound pressure
(sound pressure)

Unless otherwise specified, the effective value of instantaneous sound pressure within a given time period.

Figure 1-1 Basic processing flow of a sound level meter

Therefore, we clearly distinguish between "instantaneous sound pressure" and "sound pressure."
(See Figure 1-1 on the right.)

However, please note that in this booklet, we continue to use the term "sound pressure" to appropriately differentiate between the two terms mentioned above, as we have done before.

The range of sound pressure that a young person with normal hearing can hear is 20 μPa to 20 Pa. μ is pronounced "micro" and is a subunit meaning 10⁻⁶. Therefore, the ratio of the sound pressure of the loudest sound to the softest sound is as high as 10⁶. Pa (Pascal) is a unit of pressure; 1 Pa is equivalent to the pressure when a force of 1 N (approximately equivalent to a weight of 0.1 kg) is applied to an area of 1 m². Weather forecasts use the unit hPa, which is 100 times larger than atmospheric pressure. (The word "atmospheric pressure" is also used as a unit, with 1 atmosphere being approximately 10¹³ hPa.)

Similar to sound intensity, the frequency range of instantaneous sound pressure that a young person with normal hearing can hear is approximately 20 Hz to 20 kHz, a range of about 1000 times. Although not a strict distinction, the frequency range of sound can generally be divided as follows:

20 Hz or less

infrasound 

20 Hz to 20 kHz

audible sound 

20 kHz or higher

 ultrasonic sound 

When we perceive sounds with our ears, we distinguish and judge differences in sounds based on the following characteristics:

pitch

What we call high-pitched and low-pitched sounds are caused by differences in sound frequencies. Even with the same "a" sound, there are high-pitched and low-pitched "a" sounds. This is because, although the shape of the sound wave for "a" is the same, the frequencies are different. High-frequency sounds sound high-pitched, while low-frequency sounds sound low and heavy.

Sound volume

Even when the sound "a" is the same pitch, there are loud and soft "a" sounds. This is because, even with similar waveforms, a loud "a" has a larger amplitude, while a soft "a" has a smaller amplitude. A sound level meter is a measuring instrument that measures this difference in sound intensity.

Tone/Sound quality

We can distinguish between different types of instruments being played at the same volume and pitch. This is because we are able to distinguish the timbre and quality of the sound produced by each instrument. Although the exact nature of timbre and quality is not yet fully understood, it is believed to be due to subtle differences in the sound waveforms.

Furthermore, because sound has wave properties, it exhibits properties such as "reflection," "transmission," and "diffraction," and its sound attenuates with distance. Please refer to the diagram below for more information.

  • Figure 1-2 Characteristics and properties of various sounds

2. What is noise?

Noise is a general term for "unpleasant sounds that we don't want to hear" or "disruptive sounds" among a variety of sounds. According to the aforementioned acoustic terminology standard (JIS Z 8106:2000), noise is defined as "unpleasant or undesirable sounds, or other disturbances." Sounds that are perceived as unpleasant can cause harm to health and the living environment, eventually developing into pollution problems.

In noise evaluation, we focus on the main frequency range of 50 Hz to 5 kHz within the audible range of 20 Hz to 20 kHz. For normal conversation, the range of 300 Hz to 3 kHz is important for hearing.

Working for long periods in loud environments can lead to hearing loss, so laws are in place to regulate noise levels in order to protect people's lives from such noise.

Sound level meters are commonly used as measuring instruments to measure and evaluate noise. There are two main methods for quantifying sound: physical scales and sensory scales.

In terms of physical measures,
① Sound pressure level,
② Sound intensity level,
③ Sound power level,
These also include octave band and 1/3 octave band levels obtained by frequency analysis.

As a sensory scale,
④ Loudness
⑤ Pitch,
⑥ Timbre,
⑦ Loudness level
⑧ There is a weighted sound pressure level, etc.
Many of these are defined by standards such as JIS and IEC (International Electrotechnical Commission). In particular, various evaluation quantities are currently being researched and published in the field of sensory scales, and it is a field that is expected to develop even further in the future.

Environmental noise is evaluated using noise levels, while product development uses acoustic power levels, 1/3 octave analysis, and more recently, sound quality evaluation parameters. Each evaluation uses measurement items appropriate to its purpose. Please refer to Chapters 5, 6, and 11 for an overview of these methods.

Sound level meters are instruments that measure ① sound pressure level and ⑧ noise level. Thanks to recent technological advancements, models are now available that can also measure octave band and 1/3 octave band levels, as well as ④ loudness.

Now let's take a closer look at noise.

Figure 2-1 shows examples of types and magnitudes of ambient noise.

  • Figure 2-1 Various types of noise and their magnitude (level)

3. Classification of Noise

Noise changes can be broadly categorized into "variations in level over time" and "differences in spectral components on the frequency axis," and noise can be classified based on these two factors.

(Note)
This chapter references JIS Z 8733:2000 Annex F (informative), "Classification of noise by spectrum and temporal variation of level."

3-1 Classification by level of time variation

Noise can be classified as follows, based on its temporal variation in level:

  • The characteristics and typical spectral waveforms of each type are as follows:

  • steady noise

    Steady-state noise is defined as noise that is continuously present at a nearly constant level at the measurement point, with no fluctuation in the sound level meter reading, or only slight fluctuations.


    Figure 3-1

  • Fluctuating noise

    Noise that fluctuates irregularly and continuously over a considerable range at a measurement point is called fluctuating noise. For example, noise observed near roads with a certain amount of vehicular traffic is almost always fluctuating noise.


    Figure 3-2

  • Intermittent noise

    Intermittent noise refers to noise that occurs intermittently at time intervals, with each occurrence lasting several seconds or more. The time interval between occurrences can be nearly constant, or it can be irregular, such as the passage of trains or aircraft.


    Figure 3-3

  • Isolated burst of sound energy

    For example, impact noise that can be separated into individual noises, such as the sound of a pile hammer driving a pile into the ground, is called isolated impact noise. Isolated impact noise can occur as a single event or intermittently. Also, the level of each occurrence may be nearly constant or may vary over a considerable range.
    This usually refers to bursts with an interval of 0.2 seconds or longer.


    Figure 3-4

  • Quasi-steady impulsive noise

    For example, noise that occurs repeatedly at extremely short time intervals (less than approximately 0.2 seconds), such as from a bell or a rock drill, is called quasi-steady-state impact noise. Such noise is often perceived as steady-state noise.


    Figure 3-5

3-2 Classification by frequency spectrum shape

Noise can be classified into three types, as follows, from the perspective of its spectral shape on the frequency axis.


The characteristics and typical spectral waveforms of each type are as follows:

  • Broadband noise

    This type of noise has a spectrum in which acoustic energy is distributed over a relatively wide frequency range.

    Examples: the sound of a waterfall, exhaust noise from an air conditioner vent, highway noise, etc.


    Figure 3-6

  • Narrow-band noise

    This type of noise has a spectrum in which acoustic energy is concentrated within a relatively narrow frequency range (within a 1/3 octave band) and does not contain discrete sounds.

    Examples include the sound of distant thunder (low frequency), the sound of wind blowing through grasslands and valleys (medium frequency), and the sound of air leaking from a car tire (high frequency).


    Figure 3-7

  • discrete pure tone

    This type of noise has periodic sound pressure fluctuations that give a sense of pitch, and in terms of frequency, it has a line spectrum.

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


    Figure 3-8