Acoustics - the Science of Sound

Sound Waves in Various Forms Acoustic vibration in the early universe In the beginning was the Word, and the Word was with God, and the Word was God.

Acoustic Modes in Stars... The complex “vibrational face” of the sun...

What is sound? Review of sound as an example of a longitudinal, mechanical wave The sound spectrum: –infra-sonic (< 20 Hz) –sonic (20 Hz - 20 kHz) –ultra-sonic (> 20 kHz) How much energy does a sound wave carry? - a simple demo

So, how good is YOUR hearing? Let’s take another look at the “hearing a pin drop” activity and try to estimate the power that our ears responded to... Power how can we estimate  t?

The Decibel (dB) Scale Our ears are able to accommodate an enormous range of sound intensities. To express this in a convenient, mathematical form the Decibel Scale was developed: where  is the sound intensity level or dB level. I 0 is the threshold for acute hearing and is set as: I 0 = 1.0 x W/m 2.

Anatomy of a Sound Wave What is the amplitude of a typical sound wave? How much of a pressure variation is produced by a sound wave? How does sound intensity vary with distance How does motion affect sound?

Learning to Untangle Sounds What happens when two similar waves combine? - You get the phenomenon of beats : simulate thissimulate this Beat frequency is the difference between parent frequencies “Other” frequency is the average of the parent frequencies

Fourier Analysis Works Because... Any “regular” sound or waveform can be built out of “proper” combinations of simple harmonic waves (Fourier’s Theorem) see (and hear) this in action... see (and hear) this in action...

Fourier Analysis… the key to deciphering complex waveforms (sounds)

Wave Interference in 2D... Concept of path length and path difference Nodes and Anti-Nodes in 2D –an applet simulationan applet simulation –demo An application - hot spots in your living room

The Doppler Effect Motion affects received wavelength: –moving source –moving listener Application of this: Biology - “Bat” Sonar Meteorology - Doppler radar Astronomy - measuring motion

Medical Applications Ultrasound –Low intensity 0.01 W/m 2 –Frequency range: 3.5 MHz – 7.0 MHz –Penetration depth ≈500λ Diathermy –I ≈ 10 3 – 10 4 W/m 2 –0.8 – 1.0 MHz Angiodynography –Typical frequency 2.5 MHz

Shocks... Conditions for the formation of shocks –Mach Angle –Shock Heating –Porpoise “DeathRay”? –Bull Whips and Towel Snaps!