الموجات الصوتية والسمع
Physics · Key Stage 3 · The School
A wave of squeezes
Sound travels as a LONGITUDINAL wave: the particles of the medium vibrate back and forth along the direction the wave travels, making regions squeezed together and stretched apart. Nothing is carried along — each particle moves a tiny distance and returns. What travels is the disturbance, not the air itself.
Frequency and amplitude
FREQUENCY is how many vibrations pass per second, measured in hertz, and it sets the PITCH — more per second, higher note. AMPLITUDE is how far the particles move from rest, and it sets the VOLUME — bigger movement, louder sound. Human hearing runs roughly from 20 Hz to 20,000 Hz, and the upper limit falls with age.
No medium, no sound
Because sound is a vibration of particles, it cannot travel through a vacuum: a bell ringing inside a jar goes silent as the air is pumped out, though you can still see it. Sound travels FASTER in solids than in liquids and faster in liquids than in gases, because the particles are closer and pass the disturbance on sooner.
An electric bell hangs inside a sealed glass jar, ringing loudly. Start the vacuum pump. As the air thins, the sound gets steadily quieter, though the hammer is plainly still striking — you can watch it. With the jar close to empty, almost nothing is heard. Let the air back in and the sound returns. The bell never stopped vibrating; what changed was whether there were particles to carry the vibration to the glass.