Sound Waves

To understand ideas about human hearing and infrasound. Learning Objective To understand ideas about human hearing and infrasound. Learning Outcomes To be able to recall human hearing range and explain some uses of infrasound. Success Criteria Good if: you can recall (with appropriate units) values of the human hearing range. Excellent when: you can explain what is meant by the term Infrasound. Exceptional if: You can use formulae from previous lessons to calculate the speed of sound waves.

Sound Waves Sound waves are longitudinal waves carried by compressions and rarefactions of the air (or other medium). The speed of sound depends upon the type, pressure and temperature of the medium it is travelling through. In dry air at 20oC the speed of sound is 343m/s.

Sound Waves Loud Sound Quiet Sound High Sound Low Sound Human Voice Sounds are produced when objects vibrate. The bigger the vibration (amplitude), the louder the sound. The number of waves per second is called the frequency. The higher the frequency, the higher the pitch of the sound. Loud Sound Quiet Sound High Sound Low Sound Human Voice

Hearing Range The frequency range audible to humans is from 20Hz to 20,000Hz. Below 20Hz are infrasounds, above 20,000 are ultrasounds.

Sound vs. Noise Any sounds which are unwanted are classed as noise. The loudness of sounds is measured using the decibel scale. On this scale above 85 dB will cause some damage to hearing. 130 dB is the threshold of pain. The upper level for ‘sound’ is 194dB. Above this level the sound would be a shockwave.

Infrasound Infrasound is a sound that is below the range of human hearing (i.e. below 20 Hz). Some animals use infrasound to communicate.

Infrasound Infrasound is also produced by volcanoes and earthquakes. Some people believe that animals can sense these infrasounds before the eruptions or earthquakes start.

Infrasound Vladimir Gavreau was an infrasound pioneer. While conducting experiments in the 1950s he became aware that his lab assistants were experiencing pain due to low frequency sounds. He used this information to produce and infrasound gun. This was lethal as it rapidly induced internal bleeding when used on humans. At low power it would shake walls and ceilings to pieces. Whales, alligators, elephants and rhinos all communicate by infrasound.

To understand ideas about human hearing and infrasound. Learning Objective To understand ideas about human hearing and infrasound. Learning Outcomes To be able to recall human hearing range and explain some uses of infrasound. Success Criteria Good if: you can recall (with appropriate units) values of the human hearing range. Excellent when: you can explain what is meant by the term Infrasound. Exceptional if: You can use formulae from previous lessons to calculate the speed of sound waves.