In the previous introductory column, we discussed "why we measure sound and vibration."
Following our discussion of vibrations, this time we'll introduce sensors (detectors) used to measure sound.
The first thing that comes to mind as a sound detector is a microphone.
The term "microphone" encompasses a wide variety of types, including those for karaoke, conferences, smartphones, and noise measurement. This time, we will explain using a measurement microphone, which is used for measuring sound, as an example.
Measurement microphones used for measuring sound require accuracy to measure sounds from very quiet to very loud. They also need to have flat response across a wide range of frequencies, from low to high.
How a measuring microphone works
A measuring microphone has a microphone cap called a grid, which protects the microphone's diaphragm (vibrating membrane). This diaphragm vibrates when it receives sound pressure, thereby detecting sound. The amplitude of this diaphragm is converted into an electrical signal through a preamplifier and output. The output electrical signal is then converted into Pascals (Pa), the unit of sound pressure.
The unit dB (decibel), which is often used to indicate noise levels, will be explained later.
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Units of loudness and pitch
The units used to express the loudness of sound are Pa (Pascal) or dB (Decibel). The range of sound loudness that humans can hear is generally said to be 0.00002 Pa to 20 Pa. In other words, we can hear a range of sounds that is 1 million times wider. Concerns about misreading or copying digits led to the use of the decibel (dB) unit, which uses a logarithmic scale to prevent errors (0.00002 Pa to 20 Pa → 0 dB to 120 dB). Another reason why dB units are frequently used is that human hearing is said to be close to a logarithmic sense. This means that, in terms of how the human ear hears, while we can easily perceive slight differences in volume for quiet sounds, for sounds that are somewhat loud, we cannot recognize the difference in loudness unless the difference in volume is clearly defined. The unit used to express pitch is the frequency unit Hz (Hertz). The range of pitches that humans can hear is called the "audible range," and its frequency band is said to be between 20 Hz and 20,000 Hz.
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Types of microphones used for measurement
We will explain the different types of microphones used as sound sensors, using our products as examples. Our product lineup is categorized by microphone performance and intended use.
- A low-noise, high-sensitivity microphone (MI-1281) used for measuring faint sounds.
- A microphone (MI-1531) that can also be used to measure high-pitched sounds.
- A microphone (MI-1271) that delivers high performance even in harsh environments with low or high temperatures.
- Cost-effective general-purpose microphones (MI-1235/MI-1433)
In addition to measurement microphones, there are also microphones used for specific purposes. - Ultraminiature Microphone (MB-2200M10) that is effective for measuring sound in confined spaces.
- A microphone with directional properties that is effective for identifying sound sources (MI-8100)
- A sound source visualization microphone (MI-5420) calibrated with four microphones and one camera.
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Sound level meter
Many people are familiar with the term "sound level meter" as a sound measuring instrument. A sound level meter is a device that integrates a measuring microphone (microphone, microphone preamplifier) and a display unit, and displays the sound level in dB (decibels) as mentioned above. Sound level meters are also called sound level meters. The term "sound level meter" is defined in the JIS standard (IEC standard). In particular, the term "sound level meter" is used in the "Measurement Law (Law)" and is defined as a specified measuring instrument.
Sound measurement has evolved from simply measuring noise levels to applications such as identifying sound sources for noise reduction measures and evaluating sound quality. In recent years, sound level meters with these capabilities (such as our LA-7000 series) have become widely used. The LA-7000 series comes standard with a listening function that allows you to actually hear the sound being measured, making it suitable for various applications such as on-site sound source exploration and listening to the effects of adjusting the pitch of specific sounds before implementing countermeasures.
If you're interested, please take a look at the website linked at the end of this text.
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Specification (performance)
In sound measurement methods and measurement manuals, the standard for measuring instruments is specified as JIS C1509-1 (IEC61672-1). JIS C1509-1 (Sound Level Meter) divides and specifies sound level meters into Class 1 and Class 2. It also specifies various items, such as permissible error within a frequency range. JIS C1509-1 is a standard issued in Japan based on IEC61672-1 (International Standard).
Furthermore, the performance of measurement microphones is sometimes described as Class 1 or Class 2, which usually refers to Class 1 and Class 2 according to JIS C 1509-1 (IEC 61672-1). There are also other standards, such as IEC 61094-4 (for measurement microphones).
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This time, we mainly talked about sensors that measure sound, but there are several other things to be careful about when measuring sound, such as the measurement environment, the nature of the sound, and the measurement settings of the measuring instrument used.
I will explain these in more detail on another occasion.
MI Series Microphone and Preamplifier Related Products
LA-7000 Series High-Performance Sound Level Meter
(Excerpt from the email newsletter issued on October 20, 2021)
