Wave Changes

Properties Reflection Refraction Diffraction Resonance Resonance in String Instruments Resonance in Wind Instruments Interference: Beats Properties

REFLECTION -the turning back of a wave or at least a portion of it when it encounters an obstacle or where the waves bounce off When waves are reflected, the process of reflection has certain properties. If a wave hits an obstacle at a right angle to the surface then the wave is reflected directly backwards.

If the wave strikes the obstacle at some other angle then it is not reflected directly backwards. The angle that the waves arrives at is the same as the angle that the reflected waves leaves at. The angle that waves arrives at or is incident at equals the angle the waves leaves at or is reflected at. Angle of incidence equals angle of reflection.

Echo is produced when the sound waves are reflected from a wall or an certain object to your ears. Echoes are the reflection of sound waves

REFRACTION – the change in direction of a wave as it crosses a boundary or another medium -accompanied by a change in speed and wavelength of the waves If sound waves travel slower in the second medium, the waves will be refracted toward the normal. NORMAL -an imaginary line perpendicular to the boundary between the mediums If sound travels faster in the second medium, the waves will be refracted away from the normal.

Sound waves can also be refracted if the speed of sound changes according to their position in a medium. The waves bend toward the region of slower speed. Sounds carry farther at night than during a sunny day.

Diffraction Spreading out of waves as they pass by the edge of an obstacle or through an opening is called diffraction. - Occurs whenever a sound wave encounters an obstacle or opening - the most evident when the wave length of the sound waves is obstacle or opening. Diffraction enables you to hear a sound from around a corner, even though no straight path exists from the source of the sound to your ears.

Resonance -the reinforcing of sound -occurs when a small repeated force produces larger and larger vibrations in an object -whenever an object is able to vibrate is acted upon by a series of periodic waves having a frequency that is equal to its natural frequency of vibration -The natural frequency of vibration

The repeated force must be applied with the same frequency as the resonance frequency of the object. Resonance Frequency – the frequency which an object would vibrate naturally if disturbed in some way. The original waves and the reflected waves combine and form wave patterns that appear to stand still. Standing Waves - also known as a stationary wave - a wave that remains in a constant position.

Resonance in String Instruments Resonance increases the loudness of the sounds produced by many musical instruments. Standing waves are set up in the column of air inside the instrument. Fundamental Frequency – the lowest natural frequency of the string’s sound The wavelength of the fundamental frequency is equal to twice the length of the string.

EXAMPLE Calculate the fundamental frequency of vibration of the A-string on a guitar. Its mass is 0.30 g, 33 cm long, and is stretched with a tension of 77 N. Given: Mass of string or m = 0.30 g = 3.0 x 10 -4 kg Length of string or L = 33 cm = 0.33 m Tension (Force) or F = 77 N = 77 kg-m/s2

Resonance in Wind Instruments Wind instruments use vibrations in air columns to produce notes. The end that air is blown into is always considered an open end that must have an antinode in a standing wave. Some instruments, like flute or saxophone, have both ends of their air column open, so the only natural frequencies picked are those that allow an antinode at each end.

Interference: Beats When two sound waves of slightly different frequency are added together, the resultant sound wave will have periodically changing loudness, known as the beat. The frequency of the beat is the difference in the two frequencies, and the frequency of the resultant of the sound is the average of the two frequencies.

Beats – periodic variations of loudness - produced because the sound waves of the two tones overlap and interfere with each other. Constructive – interference of the combined waves Destructive – interference between waves Beat Frequency – number of beats per second