Sound Chapter 12

If we had no Sound… No Speech – Rassi’s lectures No Noise – no car horns, no buzzers for sports, no alarm clocks No Music – no radio, no bands, no MP3 players, no CDs

Sound waves – what are they? Sound waves are longitudinal waves Sound sources push against medium creating more dense and less dense pockets of the material. more dense = compression; less dense = rarefaction

Speed of Sound Sound travels at 343 m/s in the air at 20°C – it will travel faster through warmer air, slower through cooler air Sound travels faster through denser objects, air < water < rubber < iron Chart pg 410

Sound behaves like a wave Sound reflects off of hard objects, we call the reflections echoes. Bats, navy use echoes Sound waves interfere and create nodes that are ‘dead spots’ where you don’t hear anything Sound obeys the v = λf equation

Pitch – a particular musical tone Pitch is directly related to frequency – Middle C is 262 Hz, the E above middle C is 327 Hz. Pitches below 20 Hz = Infrasonic Pitches above 20000 Hz = Ultrasonic The ceiling of what we can hear drops as we get older, by 70 most people can not hear above 10000 Hz

Measuring Loudness Loudness comes from amplitude of the sound waves Intensity = Power/area Intensity = P/(4πr2) We can hear a very large scale of amplitudes ranging over 10-11 orders of magnitude

Loudness - Decibels We measure loudness on a sound scale measured in Decibels logarithmic scale - This means that a doubling of the decibel level corresponds to a 10 times greater intensity We generally perceive a 10 decibel increase in sound as twice as loud

Loudness of different sounds – pg 417 Faintest sound you can hear 0 dB Whispers 20 dB Moderate conversations 50 dB Traffic noise 80 dB Fire Engine 100 dB Painful, dangerous if continuous exposure – airplanes, heavy machinery, rock concerts 120 dB and up – ear protection

Isn’t It Resonating Resonance occurs when forced vibrations = Natural Frequency (inc. in amplitude) Not restricted to wave motion

How the Ear works Outer ear collects the sound and sends it in Middle ear has eardrum & 3 bones the anvil, hammer, and stirrup Inner ear – cochlea; fluid filled, hairs in cochlea vibrate with sound, sending nerve signals

Feel the Effect Doppler Effect Frequency change due to motion V is constant for a given medium Frequency and wavelength change Play Song with PP

Doppler Effect Moving toward – higher frequency – lower wavelength Moving away – lower frequency – higher wavelength

Doppler Effect on light - Blue Shift – Increase in frequency Red Shift – Decrease in frequency

Shift up or down? f = (vw+vr) f0 (vw+vs) vr velocity of receiver (toward source = positive) vs velocity of source (away from receiver = positive) f0 frequency produced f frequency received

Closed Pipes Rules for pipe resonance: Node@closed end/Antinode@open end Smallest fraction of standing wave that fits is: ¼=Length Largest  -> smallest frequency – called Fundamental frequency (f0) Other wave fractions that fit: ¾; (5/4); (7/4);… Frequencies for these are called harmonics H1 = 3f0; H2 = 5f0; H3 = 7f0; H4 = 9f0…

Open Pipes Same rules: Node@closed end/Antinode@open end Smallest fraction of standing wave that fits is ½ = Length Largest  -> smallest frequency – called Fundamental frequency (f0) Other wave fractions that fit: ; (3/2); 2; (5/2)… Frequencies for these are called harmonics H1 = 2f0; H2 = 3f0; H3 = 4f0; H4 = 5f0…

Instruments - types What vibrates in each? Examples of each? Percussion String Wind Brass Reed

What sounds good? Dissonance – Pitches that sound bad together Consonance – Pitches that sound good together – also called a chord in musical terms Whole number ratios of the frequencies create consonance

Chords and Octaves Frequencies are in a ratio of 1:2 are an Octave apart A ratio of 4:5 is called a major third; 3:4 is a perfect fourth; 2:3 is a perfect fifth Combine major third and perfect fifth: 4:5:6 and you have a major chord

Beats Ever heard a tone get louder, then softer repeatedly? This is called a beat – it results from the interference of 2 tones very close in frequency

Beats II The Beat has a frequency too, how often it moves from loud to soft in a second Freqbeat = |FreqA – FreqB| Beats below 7Hz we hear as a change in loudness

Bow Waves What happens when an object’s speed is faster than the speed of waves in that medium? The wave crests overlap and “pile” up Berry the bug – draw circles

Shock Waves Shock Wave – Like a bow wave, but its a 3-D cone produced in the air by a supersonic aircraft The air “piles up” – jet moves faster than sound waves Waves pile up & create a cone Extends back like a boat wake

Shockwaves Bug in the water Bow waves on ships Sonic booms Concord, Thunder Bull whip