# The speed of sound is the greatest in

## Physics Sexl 6 RG, textbook

2.4 Acoustics Through hearing and seeing we can orientate ourselves in the world and communicate with others. Information is constantly brought to us via light and sound. We can not only perceive sound waves, but also produce them ourselves and thus exchange information. In the following we deal with the fundamentals of acoustics, the theory of sound. a The Creation of Sound Waves Experiment: What are Sound Waves? 52.1 E 1 You need: a suitable leaf spring, possibly a vice. Clamp the leaf spring in a vice (or simply hold it) and set it vibrating. When the leaf spring vibrates quickly enough, you will hear a sound. Let us consider the process in slow motion: The deflecting spring compresses the adjoining layer of air. This increases the pressure in this layer of air. On the back of the spring leaf, the air is thinned and the pressure is reduced. The oscillation of the spring causes the air to be periodically compressed and diluted, ie there is a periodic change in the air pressure. This change in pressure spreads in space as a pressure wave. If the oscillation frequency of the spring exceeds about 20 Hz, then we perceive this pressure wave as sound. Pressure waves can propagate in any medium. Sound waves transport energy and not matter. Sound waves are periodic compressions or dilutions of a medium. The medium can be solid, liquid or gaseous. Sound waves are longitudinal waves. Investigate, consider, research: Sound sources 52.1 Which possibilities of sound generation do you know? What are the differences? S 1 a) Forms teams, one of which each team describes a certain type of sound source more precisely (eg wind instruments, street noises, animal noises, ...). E 1 b) Analyzes the noises, tones, etc. with suitable software (notes on this physikplus.oebv.at). If the sound is caused by vibrations in a solid, liquid or gaseous body, then there must be no sound in a vacuum. This is actually the case, as shown by experiments which R OBERT B OYLE carried out for the first time 300 years ago. Experiment: Sound propagation in a vacuum 52.2 You need: a vacuum pump, a vacuum bell (recipient), an alarm clock, a cell phone, a container with water, a waterproof container for the cell phone. E 1 a) Put on the alarm clock, place it under the vacuum bell and evacuate it. E 1 b) Put the cell phone under the vacuum bell and call. E 1 c) Put the cell phone in the waterproof container, put it in the water and call the cell phone. Describe and interpret your observations. Investigate, consider, research: Sound propagation 52.2 E 2 Someone doubts that sound can also propagate in solid bodies. Think of two counter arguments. 52.3 E 1 The speed of sound is different in gases, liquids and solids. What could be the reason for this? In which medium will the speed of sound be greatest? Give reasons for your answer. 52.1 A leaf spring is held and made to vibrate. 52.2 A jew's harp consists of a solid frame and a leaf spring. The jew's harp is pushed into the mouth and the leaf spring is stimulated to vibrate with a finger. Changing the shape of the oral cavity creates different tones. 52.3 R OBERT B OYLE hung an alarm clock with a powerful chime on a thin thread in a glass vessel and pumped out the air. He writes: “We were very quiet and waited for the moment when the bell should start to ring, and were satisfied when we did not hear the alarm clock at all. We only heard the bell when we were very excited and ordered that we let some air into the vessel. ”52 WELLEN For testing purposes only - property of the publisher öbv