Physics: Sound

Following 'Physics for Scientists & Engineers with Modern Physics' by Douglas C. Giancoli

3 flashcards · Physics · by lakesare

1. What's the speed of sound in the air?

   343 m/s The speed of sound is different in different materials. In air at 0°C and 1 atm, sound travels at a speed of 331 m/s. We saw that the speed v = root(B / p) depends on the: B - elastic modulus p - density of the material. Thus for helium, whose density is much less than that of air but whose elastic modulus is not greatly different, the speed is about three times as great as in air.

2. A rule of thumb that tells how close lightning has struck is, "1 kilometer for every 3 seconds before the thunder is heard." Why does this work? Remember that the speed of light is so high (3 X 108 m/s, almost a million times faster than sound) that the time for light to travel to us is negligible compared to the time for the sound.

   The speed of sound in air is about 343 m/s, so to travel 1 km = 1000m takes about 3 seconds.

3. Ultrasonic VS supersonic?

   ultrasonic - frequencies above 20k Hz supersonic - object moving with a speed faster than the speed of sound (343 m/s) Frequencies above 20,000 Hz are called ultrasonic (do not confuse with supersonic, which is used for an object moving with a speed faster than the speed of sound).