صدا و ارتعاش در صنعت جلسه سوم محمد رضا منظم

Sound waves Sound: A vibratory motion of displaced molecules in an elastic medium

Plane and spherical waves Plane wave: It needs only one spatial dimension to be described. A simple model for a plane wave travelling in the + direction is: Spherical Wave: It is propagated from a small source in free space with no boundaries. It can be modelled similarly by:

Different kinds of Sound waves Harmonic, 300 Hz Harmonic, 500 Hz Complex (periodic) Summation A Complex (periodic) wave can be simplified to harmonic waves using Fourier series

Different kinds of Sound waves Complex (none periodic, or noise) They are mostly unpleasant, but they can be sometimes useful.

Noise White noise: It (like the colour white) contains every frequency within the range of human hearing (generally from 20 Hz to 20 kHz) in equal amounts, which some times is used for: 1. Masking and blocking sounds, especially when trying to fall asleep 2. Helps improve concentration Provides sound masking benefits ( for those with tinnitus and so on) 3. Comforts crying babies and infants with colic ( for example no need to drag out the vacuum cleaner or burn out the hair dryer and …) Pink noise: Pink noise is white noise that has been filtered to reduce the volume at each octave. This is done to compensate for the increase in the number of frequencies per octave. Each octave is reduced by 6 dB, resulting in a noise sound wave that has equal energy at every octave (Pink noise is especially appropriate for audio applications). …..

Noise Noise can be divided to the following three groups: 1.Narrow band Noise: Sound energy is distributed within a narrow frequency bandwidth 1.Wide band Noise: Sound energy is distributed within a wide frequency bandwidth 1.Impulse Noise: Noise is repeated in less than a second

Effective Sound Pressure فشار صوتی موثر When a wave is complex for the amplitude, pressure and so on, their Root Mean Square (rms) is calculated. (Rms or ریشه دوم متوسط مربعات ) for the pressure for example we can say: We can find that:

Speed of sound (celerity) Speed of sound depends upon the temperature, air pressure and air density. Speed of sound is independent from frequency, amplitude and the wave movements. B: (Bulk elasticity modulus or مدول کشسانی )Bulk elasticity modulus ρ: (Density of medium or چگالی محیط ) The speed of sound in the air: R: ثابت گازها M: جرم مولکولی گاز T: دما بر حسب کلوین γ: نسبت گرمای ویژه در فشار ثابت به گرمای ویژه در حجم ثابت

Speed of sound (continue …) Example: Find the speed of sound in Hydrogen (t=0) and Nitrogen (t=20) Speed of soundElementSpeed of soundElement 5200 m/sAL340 m/sAir 3100 m/sConcrete1440 m/sPure water 3400 m/sWood5050 m/sFe

Speed of sound (continue …) When the temperature is not much greater than 20 degrees Celsius, the speed of sound in air: Changing the air temperature can produce wave refraction (Snell Law).

Impedance Three different impedances are used in acoustics: Mechanical Impedance: It is used to describe the radiation load produced by a medium to a vibration surface. Specific Acoustics Impedance: It describes the propagation of sound in free field, continues at junction between media (reflection, transmission,..) Acoustic impedance: It is used in reactive device (plane wave..)

Impedance (continue) If sound is propagated in the free field the specific acoustic impedance is: Impedance is a complex factor and its real part is usually called resistance and its imaginary part is called reactance. Admittance is the reverse of impedance

Impedance (continue) If the equation for displacement is: The particle velocity will be: And the acoustic pressure in air:

Example (1) تمرین : بیستونی در یک لوله طویل توسط یک موتور حرکت سینوسی انجام میدهد. دامنه جابجائی بیستون 0.1 سانتیمتر و سرعت موتور 6000 دور دردقیقه است. دامنه و متوسط مربعات فشار صوت ایجاد شده در این لوله چقدر است؟

Reflection When a wave front impinge a surface (hard,..) the advancing wave front can be considered as point source of secondary wavelets.

Reflection (continue…) The wave reflected from a rigid surface is some times assume as a secondary wave produce from an imaginary source. Source Image of the Source Incident wave Reflected wave

Refraction When sound wave passes from one medium to another the direction of the advancing wave front is bent or refracted.

Diffraction Change in the direction of the advancing front wave as a result of steps, edges and so on is called diffraction.

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