About Sound Waves

Diffraction, refraction and interference are key parts of sound waves and must be understood in order to learn about sound waves. Go through the blurbs about each key part and if you need to understand a subject more- go find the video!

Difraction

The bending of waves around small obstacles and the spreading out of waves beyond small openings. Sound diffraction has a longer wavelength implying that you can hear low frequencies around obstacles better than high frequencies.

As you can see from the image, the sound waves coming from the guitar continue to propagate around until it hits a table where it bends around the table and continues on.

Refraction

Refraction of waves involves a change in the direction of waves as they pass from one medium to another. Refraction, or bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves.

The image here shows a sound wave refracting as well as the change in the wave's wavelength and speed in the lines representing the wave.

Diffraction and Slit Width

If the slit size is much smaller than the wave it acts as a single point and the waves propagate radically in all direction. The smaller the slit width the grater the amount of diffraction.

y=mλD (distance)/a(slit width)

In this image you can see that the sound waves are propagating around where it runs into a small slit, the sound waves then diffracts and begins to propgate around again.

Interference (Constructive)

When two waves have the same sign, they add together to form a wave with a larger amplitude.

As seen in the image, the two waves have the same sign and are going the same direction forming a wave with a larger amplitude- making it constructive interference.

Refraction - Mediums

A medium is a substance that carries a wave (or disturbance) from one location to another - this can be air, water, or any other liquid or solid matter. For the speed of sound, the medium transmitting the sound is usually the air.

This image shows that the sound wave going from air to water which has different property than the air medium causing the wave to diffract. With the formula (v= λf) The speed of the wave as it goes through a different medium depends on the wave's length and frequency which will determine it's amount of refraction

Standing Waves

Standing waves is the combination of two waves moving in opposite directions, each having the same amplitude and frequency. The phenomenon is the result of interference-that is, when waves are superimposed, their energies are either added together or cancelled out.

This shows the beginning set up of a standing wave. The waves are interfering where there are points of no displacement produced at the same positions along the medium. These points along the medium are known as nodes (a point at which the amplitude of vibration in a standing wave system is zero). There are also points along the medium that vibrate back and forth between points of large positive displacement and points of large negative displacement. These points are known as antinodes (the position of maximum displacement in a standing wave system).

Interference (Destructive)

If the two waves have opposite signs then they will subtract to get a lower amplitude.

The image portrays two waves with different signs, thus they cancel each other out and get a lower amplitude.