| Open Laboratory |
| Liquid and Amorphous Semiconductor Laboratory Natural Sciences Research Division, School of Integrated Arts and Sciences Hiroshima University |
| Waves |
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Waves gather and return on the coast, and ripples spread when stones are dropped on the pond. Waves are a phenomenon we often see every day. Waves have interesting properties different from objects. For example, if two pulse waves collide with each other, they will overlap each other and then slip through as if nothing had happened (Superposition of pulse waves). Also, upward and downward pulse waves collide, the waves disappear at the moment when they overlap (Superposition of upward and downward pulses). The waves that hit the quay are reflected without being able to proceed any further. The movement of the sea surface at that time is the same as the reflection of waves at the boundary where the edges can move freely (Free end reflection). Sometimes it is reflected at a fixed boundary like a fixed rope or a guitar string, but at this time, an upward pulse that hits the end is reflected as a downward pulse (Fixed end reflection). In the vibration of the string, the reflected wave is superimposed on the incident wave, and only the wave of a specific wavelength continues to vibrate stably. Such waves are called standing waves. Shows a video of the standing wave observed in the vibration of the string. On the other hand, even when one end is a free end, the incident wave and the reflected wave overlap to form a standing wave as tthis (video). This is the same as the vibration of the air in a pipe (whistle) with the left end closed and the right end open. When there is an edge (boundary) at a vibrating substance (the substance is called a medium in the phenomenon of waves) like this, the wave is reflected at that point and the incident wave and the reflected wave overlap. To make a boundary free condition in a wave motion demonstrator, attach an attenuator to the end. This video shows that even if the wave is sent while changing the wavelength, the propagating waves from left to right can always be observed. In the same way, the propagating waves from left to right can always be observed even when returning from a short wavelength to a long wavelength as this video. |
| Description of wave motion demonstrator | The principle of sound velocity measurements & photos of the experimental device |
| Sound and ultrasonic sound speed measurements |
| Sound is a phenomenon in which vibrations, such as sparse or dense air, are transmitted through the air as a medium. The sound that people can hear is said to be several tens to 20,000 vibrations of air per second (the unit of frequency is Hz), but it becomes difficult to hear the sound with high frequency with aging. Actually, the sound handled in music generates a sound that vibrates about 30 Hz to 4000 Hz in the scale of a piano, based on the sound of "La" of 440 Hz, and actually mixing high-frequency sounds called harmonics or higher harmonics creates a beautiful timbre. By the way, ultrasonic waves are higher frequency sound waves that cannot be heard by the human ear. You may have heard that bats that fly around in the dark use ultrasonic waves to keep track of their surroundings. In the health checkup, you can use ultrasound to observe the images of the internal organs and the fetus. |
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Here, we will introduce the sound velocity measurement of liquids and solids using ultrasonic waves. If you want to understand in more detail, please see the explanation of the ultrasonic experimental device. In the ultrasonic experimental device, by generating a pulse wave, passing through the substance whose sound velocity is desired to be measured, and measuring the time until it is reflected at the end and returns. Use the formula "speed = distance/time" to determine the speed of sound. Here, we will introduce how the sound velocity of water is measured. Fortunately in Japan, water is a common liquid that comes out when you turn on a tap, but there are many areas on the earth where water is extremely valuable due to drying and desertification. In addition to that, water whose molecular formula is represented by H2O is also a liquid with very unique properties. You may take it for granted that the volume of water expands when it becomes a solid at 0oC, and the ice floats on the water (when the liquid solidifies, the density becomes low). Many people may know that the density of water reaches a maximum at 4oC. However, most substances, when melted, expand in volume and decrease in density, so solids sink to the bottom in the liquid. The speed of sound transmitted through water is about 1500 m/s, which is almost 5 times faster than the speed of sound transmitted through the air, 340 m/s, and sound waves are used in fish finder. Now, the speed of sound of distilled water also shows a temperature change that is different from ordinary liquids. The higher the temperature, the softer most substances are, and the slower the sound speed. However, in the case of water, the sound speed increases as the temperature increases in the temperature range of 0oC to 70oC, and decreases as the temperature increases in the temperature range of 70oC to 100oC, like ordinary liquids. That is, the speed of sound in water exhibits a maximum around 70oC. Let's measure this phenomenon using ultrasonic waves. |
| Address：Kagamiyama 1-7-1 Higashi-Hiroshima City, Hiroshima Pref., postal code 739-8521 |