Applied Acoustics Angelo Farina Dip. di Ingegneria Industriale - Università di Parma Parco Area delle Scienze 181/A, Parma – Italy 5 th October 2012Basics of Sound1

Lesson 01 Basics of Sound 5 th October 2012Basics of Sound2

SOUND Sound is generated by pressure variations in a medium (fluid or solid) that propagate without particle transport. But each particle moves back and forth around its equilibrium position with a certain particle velocity Is is characterized by fundamental measurements such as Amplitude, frequency or oscillation period, wavelength and propagation speed in the medium. 5 th October 2012Basics of Sound3

Basics of Sound: general properties Sound is characterized by the propagation of mechanical energy caused by a rapid succession of compressions and expansions in an elastic medium; this energy, which originates from a sound source, propagates through the medium in waves travelling with finite speed. In order for sound to occur it is thus necessary to have: a sound source an elastic medium 5 th October 2012Basics of Sound4

Sound source(1): Simplest case: a plane surface having an harmonic motion at one end of an infinitely-long duct filled with an elastic medium at rest. Compressions Expansions 5 th October 2012Basics of Sound5

Sound source(2): The harmonic motion of the piston is characterized by the following quantities: f = frequency, number of oscillations in a second, measured inHertz (Hz); T = period, duration of a cycle, measured in seconds (s); = angular velocity, measured in rad/s; Relationships between the quantities: f = 1/T and f = / 2 (Hz) If the frequency is between 20 and Hz, the perturbation will be perceivable to the human ear, and it will be defined as sound or noise. 5 th October 2012Basics of Sound6

Sound source (3): The piston surface make a simple harmonic motion: displacement = s = s o cos( t), velocity = v = ds/dt = - s o sen ( t), acceleration = a = dv/dt = - 2 s o cos( t), where s o is the value of maximum displacement. 5 th October 2012Basics of Sound7

Elastic medium: The elastic and mass properties of the elastic medium determine thespeed of transmission of the perturbation. Wavelength Sound speed c 5 th October 2012Basics of Sound8

Sound speed and wavelenght: The pressure perturbation propagates form the source in the medium, with a sound speed c 0 which in dry air depends just from the centigrade temperature t, following the approximate relationships: c 0 = t (m/s) the wavelenght, is related to the frequency of harmonic motion in the relationship: 5 th October 2012Basics of Sound9

Relationship between frequency and wavelenght: When frequency increases, the wavelength becomes smaller and smaller… Wavelenght frequency 5 th October 2012Basics of Sound10

Sound speed in different mediums: sound speed in water: sound speed in solids sound speed in 20°C 340 m/s 5 th October 2012Basics of Sound11