Sound Intensity. Sound Level. Doppler Effect. Ultrasound. Applications

Transcription

1 Lecture 20 Sound Hearing Sound Intensity Sound Leel Doppler Eect Ultrasound Applications

2 Sound Waes Sound Waes (Longitudinal waes) When a gas, liquid or solid is mechanically disturbed Sound waes are produced Speed o sound in a material depends on physical properties o material (e.g. density, temperature) When sound encounters a boundary between substances, some energy is relected Relection makes ultrasound imaging possible

3 Sound Waes Speed o sound in materials Depends on Phase o the material Characteristics o the material (such as density, elasticity & temperature) In general V solids > V liquids > V gases Greater in solids because molecules interact more strongly with each other Greater in rigid materials Material Speed (ms -1 ) Air 344 Gases Helium 965 Water 1450 Liquids Blood 1570 Body Tissue 1570 Solids Copper 3750 Glass 5000 Iron 5000 Helium has a lower density than air. Resonant requencies o ocal caity increase. Spectral distribution o sounds shit to higher requencies -timbre o sound changes

4 Sound Waes Speed o sound () Depends on elasticity and density Solid bar E E Young s Modulus density Liquid B B bulk modulus Gas kt m C p speciic heat constant pressure C speciic heat constant olume m molecular mass k Boltzmann s constant T temperature (Kelin) c c p

5 Calculate the speed o sound in air at 20 o C =1.4. Boltzmann s constant =1.38x10-23 J/K Mass o air molecule = 47.97x10-27 kg V kt m ( J / K)[( ) K] kg V 343.6ms 1

6 Waes Speed o sound The speed o sound in water is 4.2 times the speed o sound in air. A whistle on land produces a sound wae with requency. 0 When this sound wae enters water, its requency is: a) b) 0 c) 0 /4.2 d) Not enough inormation gien

7 Sound Waes Hearing Sound wae enters the ear. Forces exerted on eardrum due to air pressure ariations cause it to ibrate. three small bones (hammer, anil, and stirrup) in the middle ear transmit orces to luid illed inner ear (cochlea) through the oal window (small area compared with eardrum) result pressure x 17 The motion o the luid disturbs hair cells within the inner ear, which transmit nere impulses to the brain corresponding to the sound heard. Outer ear Middle Inner ear hammer anil Oal window Cochlea sound Ear canal stirrup ear drum Ear can detect ery low intensity sounds

8 Sound Waes All waes carry energy Audible sound waes carries ery little energy Ear can detect extremely low intensity sounds Power output: Talk 10-5 W Talk 24 hours a day non-stop or 114 years 10 6 hours Total energy output is (10-5 W)(10 6 hrs) =10 Wh Equialent to quantity o energy consumed by a 100W bulb in 6 minutes

9 Intensity Waes (energy) spread out rom source Intensity (I) o a wae is deined as Energy (E) carried per unit time per unit area (A) I E/ t A thereore Sound Waes I P A Power (P) Unit o intensity: Watt per square metre (Wm -2 ) P E t Sunlight intensity at Earth 10 3 Wm -2 Human ear can detect extremely low intensities Wm -2 Maximum intensity without ear damage 1Wm -2 Large range logarithmic units useul

10 Intensity Hearing Human ear can detect extremely low intensities Wm -2 Maximum intensity without ear damage 1 Wm -2 Large range logarithmic units useul Human perception I we listen to two sounds (I 1 and I 2 ) and I 2 seems twice as loud as I 1 Measure intensities I 2 is approximately 6 to 10 times I 1 Conenient scale to measure loudness is the logarithm o the intensity

11 Sound Waes Perceied loudness is roughly Logarithmic Ear response to sound logarithmic not linear Decibel scale or intensity used Sound (Intensity) leel in decibels (b) b I0 10 I 10log10 I Wm where (threshold o hearing at 1000Hz) decibel (b) is a relatie sound leel measurement Threshold o discomort = 1 Wm -2 Aboe this pain is experienced and there is potential or long term damage

12 Hearing Sensitiity o ear Can detect sound intensity o Wm -2 Corresponds to pressure ariation o 3x10-5 Pa (Atm. Pressure 101,325 Pa) Random luctuation due to thermal motion o molecules 5x10-6 Pa Sensitiity: essentially due to mechanical layout Area ratio: ear drum to oal window 17 hammer, anil and stirrup ampliication 2 canal resonance at 3kHz pressure increase 2 Total pressure ampliication 17x2x2 = 68 Intensity ( pressure ) Intensity increases by actor o 68 2 = Brain: discriminatory role Filters unwanted noise Suppression: non-awareness o background noise ear is not equally sensitie at all requencies

13 Sound leels and Intensities Sound leel (db) Intensity (Wm -2 ) Sounds 0 1x10-12 Threshold o hearing 10 1x x x10-9 Quiet home 40 1x10-8 computer 50 1x x10-6 Normal conersation 70 1x10-5 Busy traic 80 1x10-4 Loud radio 90 1x x x10-1 Sound Waes Rock concert 140 1x10 2 Jet airplane at 30m 160 1x10 4 Bursting eardrums b I 10log10 I0 Computer 10 times louder than quiet room Does not seem so because o the logarithmic response o the ear

14 Sound leels and Intensities D a n g e r H e a r i n g l o s s Damage Threshold 5 hours/week at > 89dB damage ater 5 years > 100dB deemed hazardous 10 minutes at 120dB Temporarily changes your threshold o hearing rom 0dB to 30dB

15 Sound Waes (a) Calculate the sound leel in db o a sound intensity 10-8 Wm -2 (b) Calculate the intensity in Wm -2 o a sound leel o 80 db I (a) b 10log10 I Wm b 10log Wm b 4 10log d b (b) log I 10 I0 8 log I 10 I0 I I I Wm 10 Wm I 10 Wm 4 2

16 Sound Waes Hearing ability Loudness is a method o describing the acoustic pressure (or the intensity) o a gien sound Intensity hearing range: Wm -2 1Wm db Ability to hear is not only a unction o intensity but also requency Humans Frequency range: 20 Hz 20 khz Dogs: up to 50 khz Dolphins: up to 250 khz. Bats: up to 120 khz

17 Intensity W/m Intensity Leel db Sound Waes Pain threshold Hearing ability Hearing threshold k 10k 20k Hz requency Hearing ability as a unction o intensity and requency. The blue solid line is the pure tone threshold cure, below which the subject does not hear. Ear most sensitie at 3000 Hz Pain threshold almost requency independent

18 Waes Example A bat can hear sound requencies up to 120,000 Hz. What is the waelength o sound in the air at this requency? 1 344ms , 000Hz 3 metres =.287cms High requency short waelength Wae only disturbed by objects with dimensions similar to or greater than the waelength Smaller objects hae little eect Bats use ultrasound or naigation Can distinguish between insect and alling lea

19 Sound Waes Traeling waes transer energy rom one place to another Sound energy dissipates to thermal energy when sound traels in air. Higher requency sounds dissipate more quickly, so lower requency sounds trael urther. Examples oghorns hae a low requency Elephants communicate oer long distances (up to 4 km), requencies as low as 14 Hz

20 Sound Waes Doppler Eect Change in obsered requency depends on the relatie motion o the source and obserer. Occurs with all types o waes most notable sound waes, light waes. Christian Doppler Austrian Physicist, Mathematician Perceied pitch (or requency) o a moing sound source changes as it goes past Longer Lower stationary Shorter higher moing

21 Waes Obsered requency or a moing source obserer wae wae + sign: source moing away rom obserer - sign: source moing towards obserer source source Stationary source, moing obserer obserer wae wae obserer source +sign: obserer moing towards source - sign: obserer moing away rom source = Frequency = Speed

22 Waes Example: Moing Source A police car with a 1000 Hz siren is moing at 20 ms -1. What requency is heard by a stationary listener when the police car is: a) Moing away rom b) approaching the listener (a) obserer wae wae source source 1 344ms obserer 1000Hz 945Hz ms 20ms (b) obserer wae wae source source 1 344ms obserer 1000Hz 1062Hz ms 20ms

23 Waes Example (moing obserer) A stationary siren has a requency o 1000 Hz. What requency will be heard by driers o cars moing at 15 ms -1? a) away rom the siren? b) toward the siren? (a) w o o w ms 15ms o 1000Hz 956Hz 1 344ms s (b) o w w o s ms 15ms o 1000Hz 1044Hz 1 344ms

24 Waes Doppler eect can be used to measure speed obserer wae wae source source Radar measures Doppler shit to determine speed o car compares requency o relected wae rom car and with that emitted rom source Similarly ultrasound can measure blood speed, etal heart motion.

25 Ultrasound Frequency greater than range o human hearing Sound with requencies aboe 20 khz Normally 1 MHz 20MHz Diagnostics Therapeutic Quality control

26 Ultrasound Medical applications Ultrasound probe passed oer region o interest Relections o ultrasound pulses rom patients occur at interaces between dierent tissues Good contrast: relected rom boundaries between materials o nearly the same density Relection time proides depth inormation Image constructed rom echo and position inormation Medical ultrasound without harmul eects intensity kept low ( 10-2 Wm -2 ) to aoid tissue damage Ultrasound scanning during pregnancy Doppler eect with ultrasound can be used to detect etal heart beats pulsation o artery walls

27 Ultrasound Other uses in medicine Destructie eects Intense ultrasound produces large density and pressure changes Results Large stresses Molecules are orced to moe rapidly Heat is produced in most materials Bubbles o apour are ormed (caitation) Auto-ocusing cameras computes time taken (and hence distance o subject) or the relected ultrasonic sound wae to reach the camera lens position and then sets ocus accordingly.

28 Dental applications Ultrasound Teeth cleaning Plaque, ilm o ood and bacteria, builds up on teeth. Not remoed, it hardens into tartar. ultrasonic scalar It consists o a ultrasound probe with a small tip. The ultrasound in combination with water low eectie in plaque and tartar remoal Measurement o dental erosion Non destructie method or measuring enamel thickness, pulse-echo measurements

29 Ultrasound Why use ultrasound---not audible sound Smallest detail obserable one waelength Example Ultrasound speed =1500m/s in tissue. Using an ultrasound requency o 2MHz, calculate (a) smallest detail isible (b) time or relected wae to return to probe rom a depth o 10cm (a) 1500 m / s Hz = 0.75mm (b) time or relected wae to return to probe t s 20.10m ms sec