Physics Of Sound-Classical Physics-Handouts, Lecture notes of Classical Physics

Download Physics Of Sound-Classical Physics-Handouts and more Lecture notes Classical Physics in PDF only on Docsity! PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 53 Summary of Lecture 19 – PHYSICS OF SOUND 1. Sound waves correspond to longitudinal oscillations of density. So if sound waves are moving from left to right, as you look along this direction you will find the density of air greater in -12 3 0 some places and less in others. Sound waves carry energy. The minimum energy that humans can hear is about 10 watts per cm (This is called , the threshold of hearing.) 2. To measure the I intensity of sound, we use as the unit. Decibels (db) are a relative measure to compare the intensity of different sounds with one another, relative in decibels R ≡ 10 0 tensity of sound log (decibels) Typically, on a street without traffic the sound level is about 30db, a pressure horn creates about 90db, and serious ear damage happens around 120db. 3 II I = ( ) ( ) ( ) . A sound wave moving in the direction with speed v is described by 2 , sin v where , is the density of air at a point at time . Let u m x x t x t x t x t πρ ρ λ ρ = − s understand various aspects of this formula. a) Suppose that as time increases, we move in such a way as to keep v constant. So if at 0 the value of is 0.23 (say), then at 1 t x t t x t − = = ( ) ( ) the value of would be v+0.23, etc. In other words, to keep the density , constant, we would have to move with the speed of sound , i.e. v. b) In the expression for , , repla x x t x t ρ ρ ( ) ( ) ce by . What happens? Answer: nothing, 2 2 because sin v sin v . This is why we call the "wavelength", meaning that length after which a wave repeats itself. c) In the x x x t x t λ π πλ λ λ λ + + − = − ( )expression for , , replace by where / v. What happens? Again the answer: nothing. T is called the time period of the sound wave, meaning that time after which it repeats itsel x t t t T Tρ λ+ = f. The frequency is the number of cycles per second and is obviously related to T through 1/ . d) It is also common to introduce the the and : T wavenumber k angular frequency ν ω = 2 2 and 2k T π πω πν λ = = = docsity.com