(1) Longitudinal wave, wavelength, frequency
Last time, as the first installment of a new series, we started by discussing the definition of sound.
To reiterate, sound is defined as "sound waves or the auditory sensations they produce," and its attributes are classified into three categories: social, cultural, and informational. From now on, we will mainly discuss physical sound, but first, let's start with its basic properties as a wave.
We know that sound is a wave, as it is called a sound wave. However, since sound waves cannot be seen like waves on the surface of water, it is difficult to grasp them intuitively. Even if we are told that "sound is a longitudinal wave, a compression wave caused by the vibration of air," it is not easy for non-experts to understand.
To digress slightly, before explaining sound as a wave, I'd like to briefly touch upon "seismic waves" as a type of wave that is more familiar to us.
As many of you may know, seismic waves consist of P-waves and S-waves. P stands for Primary wave (the first wave), and S stands for Secondary wave (the second wave). Recently, emergency earthquake information has begun to be provided that uses information on the weaker P-waves near the epicenter to announce the arrival times of the stronger S-waves in various locations. Please see Figure 1.
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Figure 1: Seismic waves P-waves and S-waves
This diagram shows that P-waves and S-waves have different directions of vibration. P-waves are longitudinal waves (compression waves) in which the ground vibrates in the same direction as the wave propagation, while S-waves are transverse waves that vibrate perpendicularly.
P-waves travel through the Earth's crust at a speed of approximately 6.5 km/s, causing initial tremors on the ground. When they propagate from directly below, they are felt as vertical shaking. S-waves, on the other hand, travel through the Earth's crust at a speed of approximately 3.5 km/s, and are felt as large horizontal shaking on the ground. There are also other waves called surface waves that arrive even later than S-waves, but we will omit them here.
P-waves in earthquakes are longitudinal waves in which the medium (ground) vibrates in the same direction as the wave's propagation, and can be said to propagate in the same way as sound. Sound travels not only through air, but also through solids and liquids.
Bone conduction sound is sound that travels through solids, and you've probably all experienced hearing sound even underwater.
I think so.
Figure 2 schematically illustrates how sound propagates through the air as a sound wave. This figure shows the propagation of sound waves.
Using the sound produced by a siphon as an example, an extreme depiction is used to illustrate the density variations in air.
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Figure 2 Sound waves emitted from a speaker
When the speaker diaphragm moves forward, the density of the air close to the diaphragm increases, and conversely, vibration occurs.
When the board moves backward from its original position, the density decreases. This process repeats, causing air to flow.
When the density of the air becomes uneven, sound propagates. When the movement of the speaker's diaphragm slows down, the air
The intervals between dense and sparse energy widen, and conversely, as the movement speeds up, the intervals between dense and sparse energy narrow.
Sound waves are generated in two ways: firstly, when the air in contact with a vibrating object vibrates, generating sound waves (as in the case of a speaker's diaphragm); and secondly, when an airflow is ejected into still air, directly creating vortices and turbulence in the air, generating sound waves. Most sounds generated by machines around us are due to the former reason. Vibrations caused by various driving factors such as impact, collision, rotation, friction, and electromagnetic force are transmitted to the air and generate sound. Examples of the latter include explosions, voices, and engine exhaust noise.
Sound vibrations in the air cause pressure changes around atmospheric pressure. These minute pressure fluctuations are called sound pressure. Figure 3 shows a waveform that represents the temporal and spatial changes in sound pressure. Around atmospheric pressure, areas of higher density become peaks in the wave, and areas of lower density become troughs.
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Figure 3 Waveforms showing the time and positional changes in sound pressure.
If we represent the axis as distance, the distance between these peaks is the wavelength, and if we represent it as time, as in the diagram below, it becomes the period. In other words, the upper diagram shows a waveform that represents how sound propagates spatially over a certain period of time, and the lower diagram shows a waveform that represents how sound changes over time at a certain location.
If the number of reciprocating motions per second is the frequency f (Hz) and the wavelength of the sound is λ (m), then the relationship with the speed of sound (the distance sound travels per second) c (m/s) is:
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c: approx. 340 m/s (temperature 20°C)
This is the relationship between wavelength and frequency. This relationship is fundamental when dealing with sound in noise reduction work and in the design process. Based on this relationship, for example, the position and dimensions of sound-absorbing materials and the shape of the room are determined. I will explain specific examples when I have the opportunity to discuss this topic further.
○ References
- “Wave” Newton January 2009
(Excerpt from the email newsletter issued on May 28, 2009)
