š In-Depth Theory
Properties of Sound Waves
SOUND WAVES are longitudinal mechanical waves ā compressions and rarefactions travel through a medium.
SOUND CANNOT TRAVEL THROUGH A VACUUM ā needs particles to vibrate.
FREQUENCY AND PITCH:
Higher frequency ā higher pitch.
Human hearing range: approximately 20 Hz to 20,000 Hz (20 kHz).
INFRASOUND: below 20 Hz ā too low for humans to hear.
ULTRASOUND: above 20,000 Hz ā too high for humans to hear.
AMPLITUDE AND LOUDNESS:
Larger amplitude ā louder sound.
Measured in decibels (dB).
SPEED OF SOUND:
In air: ~340 m/s (at room temperature).
In water: ~1500 m/s ā faster because particles closer together.
In solids: even faster ā densest medium, most efficient transmission.
Sound travels faster in denser media (unlike EM waves which slow in denser media).
ECHOES:
Sound reflects off hard surfaces ā echo.
Echo heard when reflected sound arrives >0.1 s after original ā brain distinguishes them.
Range of Human Hearing
HUMAN HEARING:
20 Hz to 20 kHz (varies with age ā upper limit decreases with age).
Most sensitive around 2ā5 kHz (conversational speech frequencies).
INFRASOUND (<20 Hz):
Natural sources: earthquakes, volcanoes, ocean waves.
Some animals detect infrasound: elephants, whales communicate over long distances.
Humans can feel vibrations from very low-frequency infrasound.
ULTRASOUND (>20 kHz):
Animals that use ultrasound: bats (echolocation), dolphins, dogs.
Bats: emit ultrasound pulses ā detect reflections ā navigate and hunt insects.
MEDICAL ULTRASOUND SCANNING:
Ultrasound sent into body ā partially reflected at tissue boundaries.
Time of reflection used to calculate depth: d = v Ć t/2.
Builds up 2D or 3D image.
Safer than X-rays ā no ionising radiation.
Used for: foetal scanning, soft tissue imaging, detecting gallstones/kidney stones.
Ultrasound Applications
SONAR (Sound Navigation And Ranging):
Used by ships and submarines to measure ocean depth and detect objects.
Send ultrasound pulse ā detect echo ā calculate distance:
d = (v Ć t) / 2 where t = time for echo to return.
INDUSTRIAL ULTRASOUND:
Detecting cracks in metal structures (non-destructive testing).
Reflection at crack boundaries ā detected ā locates defect.
Cleaning delicate objects ā high-frequency vibrations dislodge dirt.
PREGNANCY SCANNING:
Ultrasound used for foetal monitoring.
No ionising radiation ā safe for baby.
Can detect multiple pregnancies, check development, measure foetal size.
ECHOCARDIOGRAPHY:
Ultrasound scan of the heart.
Assesses heart valve function, chamber size, blood flow.
CALCULATIONS:
If ultrasound pulse reflects from a boundary at depth d:
d = v Ć t / 2
(divide by 2 because the pulse travels to the boundary AND back)
ā ļø Common Mistake
In sonar/ultrasound calculations, divide the time by 2 because the pulse travels TO the boundary AND BACK. Using the full time gives double the actual distance. Sound travels FASTER in denser media (solid > liquid > gas) ā opposite to EM waves.