<ul><li>Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies. </li></ul>
<ul><li>William Derham produced the earliest reasonably accurate estimate of the speed of sound </li></ul><ul><li>Telescope at the top of a church </li></ul><ul><li>An assistant fired a shotgun at a predeterminated point, miles away. </li></ul><ul><li>Mesured the interval beetween seeing gunshot with the telescope and arrival of the noise </li></ul><ul><li>Simple division time/distance </li></ul><ul><li>provided velocity </li></ul>1657
<ul><li> In dry air at 20 °C, the speed of sound is 343 meters per second. </li></ul>
<ul><li>For humans , hearing is normally limited to frequencies between about 20 Hz and 20 kHz , although these limits are not definite. The upper limit generally decreases with age. Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz. </li></ul>
Medical sonography ( ultrasonography ) is an ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, to capture their size, structure and any pathological lesions. Ultrasound has been used by radiologists and sonographers to image the human body for at least 50 years and has become one of the most widely used diagnostic tools in modern medicine. The technology is relatively inexpensive and portable. The creation of an image from sound is done in three steps - producing a sound wave, receiving echoes, and interpreting those echoes.
An inventor from Peterborough created a clever device to keep people safe from dog attacks. Richard Wylie (64) hopes the electronic ‘Dazer’ device will be used to safeguard people from dangerous animal attacks. The device emits an ultrasonic blast that is inaudible to humans but makes animals very uncomfortable– acting as a perfect defence if they become aggressive. “ The device helped to save my wife and son Dax from getting attacked by monkeys. “ We were up on the rock of Gibraltar and my wife was holding a banana. The monkeys crowded around them and were going wild. Luckily I had the Dazer and that sent them away.” f(dazer)=23,5 kHz,
<ul><li>Now, let’s make a little experiment with you . </li></ul><ul><li>I am going to make you hear sonor waves with different frequencies. </li></ul><ul><li>Some people may hear, some not. </li></ul><ul><li>We are going to see who </li></ul><ul><li>has the best hearing ? </li></ul>