Waves and sound

I/WAVES

• Wave: disturbance or variation that transfers energy progressively from point to point in a medium, without a transfer of matter.
• Wave pulse: disturbance that transports energy = signal; motif
• Periodic wave: a series of identical and repeated spaced pulses
• Medium: substance through which the waves travel
• Transversal wave: a wave in which a medium moves perpendicular to the direction of wave propagation
• Longitudinal wave: moves parallel to the direction of wave propagation (sound)
• Wavelength: the shortest distance between two consecutives peaks
• Period: time between two consecutive pulses
• Frequency: Number of disturbances per second.
• Velocity of a wave:

II/SOUND

• Mechanical progressive periodic wave
• Pressure wave: the sound consists of an alternating pattern of high pressure, compressions, and low pressure, rarefactions, regions travelling through the medium.
• Pressure waves are setting the eardrum into vibration by the incoming pressure wave 🡪 mechanical signal translated into an electrical signal and sent to the brain.
• The pitch of a tone is determined from its frequency: higher/lower frequency vibrations are perceived as higher/lower pitch sounds
• The loudness of a sound is related to the intensity of the incoming sound wave. A high intensity sound is perceived as a relatively loud sound by the brain. It is associated with the amplitude. The greater the loudness of a sound the greater his amplitude. The greater the distance travelled by a sound, the lower his amplitude (the surface on which the sound propagates is larger).
• Pure sound: periodic sine wave / Complex sound: periodic wave + superposition of several pure sounds = non sine wave
• Timber: depends of the shape of the sound wave

III/DOPPLER EFFECT

• Described as the effect produced by a moving source of waves in which there is an apparent shift in frequency: the source of the wave still emits the same frequency BUT the shift is only perceived by the observer.

• Source:   motionless
• Is approaching/moving towards the observer 🡪 frequency is higher /wavelength is lower than the emitted one = observer will receive more disturbations/s
• Is moving away/receding from the observer
• If the star is receding from Earth, frequency lower/wavelength of the absorption line will be greater to the reference value, which means it is shifted toward the red end of the spectrum.

IV/DIFFRACTION

• Bending of any wave as it goes through a small opening or around an obstacle.
• The smaller the opening the greater the diffraction: if the opening is much larger than the wavelength of the wave, it is just spreading with no bending.
• The wavelength remains constant before and after the diffraction: only the direction is affected.

IV/INTERFERENCES

• Phenomenon that occurs when two waves meet while traveling along the same medium. The interferences of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium
• If the two waves arrive in phase, they reach the maximum and minimum at the same time: the amplitude will be higher: the 2 lower waves create constructive interference / out of phases: they cancel each other: destructive interferences.