What is sound level meter?
Sound is generally defined as waves (acoustic waves, elastic waves) that are propagated through media such as gases, liquids and solids. This guide will primarily look at acoustic waves that are propagated through air. Acoustic waves, traveling at around 340 m/s, are a wave phenomenon. We hear these waves as they travel through air. These waves are generated as the atmospheric pressure of the air continuously fluctuates (i.e. alternating pressure fluctuation). Pressure fluctuation away from atmospheric pressure (static pressure) is generally called sound pressure. The following are citations from IEC 60050-801 (JIS Z 8106:2000).
Instantaneous sound pressure:
The value obtained by subtracting the static pressure from the pressure at a point of the medium at a specific moment
Sound pressure:
The root mean square value of instantaneous sound pressure over a period of time unless otherwise specified
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As shown above, the IEC (JIS) standard clearly distinguishes instantaneous sound pressure from sound pressure (see Figure 1-1). Note that in this guide; the term "sound pressure refers to either the "sound pressure or the "instantaneous sound pressure as defined by IEC 60050-801 (JIS Z 8106:2000), whichever is appropriate. A person with normal hearing can hear a sound pressure range of 20 µPa to 20 Pa. The symbol µ, which means micro, is used to represent 10-6. This means that the loudest sound that can be heard is 106 times louder than the smallest sound that can be heard. Pa is a unit of pressure. 1 Pa corresponds to the pressure obtained when 1 N (weighing around 0.1 kg) is applied to an area of 1 m2. Atmospheric pressure used in weather forecasts is expressed in hPa, which is 100 times greater than 1 Pa. (1 atm is equal to around 1013 hPa.) As with intensity of sound, a young person with normal hearing can hear instantaneous sound pressure with a frequency range of around 20 Hz to that around 1000 times higher, 20 kHz. Sound can be roughly divided into the following frequency bands.
Below 20 Hz: Infrasound |
Figure 1-1: Flow of basic processing by a sound level meter |
We distinguish between different sounds based on the following characteristics of sound.
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Pitch: |
Pitch is the degree of highness or lowness of a tone, which is determined by the frequency of the tone. The higher the frequency, the higher the tone. The lower the frequency, the lower the tone. |
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Loudness: |
There is louder "a" and less loud "a" both with the same pitch. They have similar waveforms but louder "a" has a large amplitude and less loud "a" has a smaller amplitude. Sound level meters are designed to measure the loudness of sound. |
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Timbre/Tone: |
We can distinguish different musical instruments even when they are emitting sound of the same pitch and loudness. This is because we are capable of distinguishing different types of timbre and tone. While neither timbre nor tone has been fully clarified, they are considered to be related to the waveform of sound. |
As waves, sound also has related effects such as reflection, Transmission and diffraction shown below, which are attenuated in proportion to distance.
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Figure 1-2: Effects of sound |
Among various types of sound, noise collectively refers to unpleasant and annoying sound that no one wants to hear. IEC 60050-801 (JIS Z 8106:2000) mentioned earlier in this guide defines noise as unpleasant or undesirable sound or other disturbances. Unpleasant sound can damage one’s health and the living environment and develop into noise pollution.
Of the audible sound range of 20 Hz to 20 kHz, noise assessment specifically handles the range of around 50 Hz to 5 kHz. In everyday conversations, the range of 300 Hz to 3 kHz is important for being audible. Working long hours in loud noise can damage your hearing. Noise regulations are in place to protect us from noise and help ensure a normal life.
Sound level meters are normally used for measuring and assessing noise. There are two types of measure of sound: physical and sensory. Physical measures of sound include sound pressure level, sound intensity level, sound power level, and octave and 1/3 octave bands for frequency analysis. Sensory measures of sound include loudness, pitch, timbre, loudness level and A-weighted sound pressure level. Many of these measures have been defined by JIS (Japanese Industrial Standard) and the IEC (International Electrotechnical Commission). Sensory measures are a growing field of science, with numerous studies conducted in the past and yet to be conducted.
Different measures of sound are used for different assessment purposes: A-weighted sound pressure level for assessing environmental noise; sound power level and 1/3 octave band for product development; and more recently, tone quality evaluation parameters. Refer to Chapters 5, 6 and 11 for more details.
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Sound level meters have been used for measuring sound pressure level and A-weighted sound pressure level. With technological advances in recent years, there are some models on sale today that are also capable of measuring octave band and 1/3 octave band levels and loudness.
Let’s look at noise more closely. |
Figure 2-1: Typical noises and their levels |
Changes in noise can be roughly divided into level fluctuation on the time axis and spectral components difference on the frequency axis, and noise can be classified accordingly.
NOTE: This Chapter is based on citations from JIS Z 8733:2000 Appendix F "Classification of noise based on spectra and time fluctuation of levels" (ISO 12001). |
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These types of noise have the following characteristics and typical time-axis waveforms.
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Steady noise Steady noise is a noise that stays at roughly the same level at the point of measurement, with no or slight fluctuation of meter reading. |
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Fluctuating noise Fluctuating noise is a noise whose level fluctuates fairly widely and irregularly at the point of measurement. A typical example is noise measured near a road with a certain level of vehicle traffic. |
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Intermittent noise Intermittent noise is a noise occurring at time intervals, with each period of noise lasting more than several seconds. Some time intervals are fairly regular while other time intervals are irregular such as when trains and aircraft are passing by. |
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An isolated burst of sound energy Impulsive noise that can be separated is an isolated burst of sound energy, a typical example of which is a pile hammer. Some are a single burst while others occur intermittently. Some stay at roughly the same level while others fluctuate fairly widely. What are normally called isolated bursts of sound energy occur at intervals of 0.2 seconds or more. |
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Quasi-steady impulsive noise Quasi-steady impulsive noise occurs at extremely short intervals (around less than 0.2 seconds) while staying at roughly the same level, examples of which include bells and rock drilling machines. There are many cases where quasi-steady impulsive noise is perceived as steady noise. |
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3-2 Classification of noise based on frequency spectra
Noise can be classified into the following three types based on the spectral shape on the frequency axis.
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These types of noise have the following characteristics and typical spectral waveforms.
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Broad-band noise
Broad-band noise has a spectral shape with sound energy distributed across a relatively wide range of frequency.
Examples: sound of a fall, sound of air conditioning vents, highway noise, etc. |
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Narrow-band noise
Narrow-band noise has a spectral shape with sound energy concentrated in a relatively narrow range of frequency (within the 1/3 octave band) and does not have a discrete tone.
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Discrete tone
Discrete tone has periodic sound pressure fluctuation with a feel of pitch and line spectral frequencies.
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