Understanding Sound: Amplitude, Frequency, and Wave Behavior
Bigger Amplitude = More Energy. More Energy = Louder.
Sound dissipates (lessens) as it travels away from the source. This intensity can be measured. Intensity has units of watts/meter2 or the more commonly used decibels (dB).
Frequency = How fast the medium is vibrating. Measured in the number of waves per time period. More generally, frequency (or pitch) is measured in Hertz (Hz). Human ears can detect pitches from 20 Hz to 20,000 Hz.
Period is measured in seconds and is related to frequency: T = 1/f (or f = 1/T) where T = Period.
Wavelength is how long one wave is, measured in meters (because it’s a length!).
Doppler Effect: Frequency shift due to the movement of an observer or source of sound.
When sound (or other waves) hits boundaries, interference can happen. Two or more sound waves in the same medium will interfere (interact). When the interfering waves have different frequencies, interesting patterns can form when adding the waves.
The beat (or “wah wah”) can be found by subtracting the two frequencies being played.
When sound waves interfere in a perfectly constructive/destructive way, a “standing wave” can be created.
Where perfect destructive interference happens, there is a node (no motion).
Where perfect constructive interference happens, there is an antinode (max motion).
Certain frequencies create standing waves (others won’t). Each frequency that creates a standing wave is known as a harmonic; it’s all based on the fundamental frequency.
Standing Waves and Resonance occur when sound waves are interfering.
In most cases, the medium the sound wave is traveling through behaves in one of two ways:
- Open Pipe: Antinodes at both ends.
- Closed Pipe: Antinode at the open end, Node at the closed end.
Waves don’t just have to travel along a straight line; they can interfere in many directions.
Threshold of Hearing: 1*10^-12 w/m^2 or 0 dB (10^0)
Threshold of Pain: 1*10^1 w/m^2 or 130 dB (10^13)
IPoE: 1*10^4 w/m^2 or 160 dB (10^16)
λ = wavelength
Finding Wavelength in an open pipe: multiply the length of the pipe by two.
Finding wavelength in a closed pipe: multiply the length by four.
When a wave’s amplitude increases, the wave is carrying more energy. Waves carry energy.
A sound wave is a longitudinal wave caused by compressions of particles.
As sound travels further away from the source, the amplitude decreases.
Sound travels fastest in solids because particles are very close together in the medium.
If sound intensity increases from 10 dB to 20 dB, it increases by a factor of 10.
In a given medium, as the sound’s frequency increases, the wavelength decreases.
Echoes demonstrate the reflection of sound waves.
The Doppler effect describes how sound’s frequency changes due to the movement of a sound source.
Beats are the concept used by musicians to tune their instruments.
Closed pipes allow for resonance with a node at one end and an antinode at the other.
Chladni plates are used to demonstrate locations of nodes and antinodes, resonance of sound, and to demonstrate how frequency affects standing waves.
Acoustic levitation occurs when two sounds are being played at each other at the same time, which creates the perfect frequency, which then creates a standing wave, and in that standing wave, there are nodes, and if placed at the nodes, the small balls can ‘levitate’.