Vibration
Basics
The simplest kind of vibration is the repeated motion of a body from one position to another and back around an equilibrium point at a regular rate. The pendulum of a grandfather clock is a good example: it will continue to oscillate from one side to the other for as long as there is tension in the spring.
The graph depicting the simplest vibration is similar to the graph traced by a point on the circumference of a rotating circle progressing at a steady rate. See the adjacent figure.
The graph depicting the simplest vibration is similar to the graph traced by a point on the circumference of a rotating circle progressing at a steady rate. See the adjacent figure.
Properties of a sine wave, the ‘atom’ of sound
Amplitude (a) is the power, or height of the wave, generally the distance between 0 ... +Vmax. The unit is usually a tenth of a Bel, or a decibel, dB.
Period (T) is the duration of a wavelength, the time elapsed between two points at similar locations on the graph. Wavelength can be measured as actual length (m, cm).
The figure shows one period of a sine wave.
Frequency (f) equates to the pitch of a sound: how many times the period is repeated per second. With the pitch A4, the period is repeated 442 times per second. The unit is Herz, defined as periods per second, so the frequency, f of A4 is 442 Hz.
On older devices, you may find the term ‘cycles per second’ (cps), which means the same thing.
Note that frequency is inversely proportional to wavelength (λ): the greater the frequency (f), the smaller the wavelength (λ).
Phase (φ) is a specific location in a period, indicated in degrees related to the rotation of the imaginary circle creating the graph.
Siniaalto = sine wave
Amplitudi = amplitude
Vaiheita = phases
Periodi T tai yksi värähdys f = Period T or one vibration f
Period (T) is the duration of a wavelength, the time elapsed between two points at similar locations on the graph. Wavelength can be measured as actual length (m, cm).
The figure shows one period of a sine wave.
Frequency (f) equates to the pitch of a sound: how many times the period is repeated per second. With the pitch A4, the period is repeated 442 times per second. The unit is Herz, defined as periods per second, so the frequency, f of A4 is 442 Hz.
On older devices, you may find the term ‘cycles per second’ (cps), which means the same thing.
Note that frequency is inversely proportional to wavelength (λ): the greater the frequency (f), the smaller the wavelength (λ).
Phase (φ) is a specific location in a period, indicated in degrees related to the rotation of the imaginary circle creating the graph.
Siniaalto = sine wave
Amplitudi = amplitude
Vaiheita = phases
Periodi T tai yksi värähdys f = Period T or one vibration f
Air pressure
Sound is variations in air pressure.
At the top, red dots depict individual air molecules. The piston vibrates, and the vibration is translated into variations in air pressure.
At the bottom, the central line represents normal air pressure, with higher air pressure above and lower below.
The piston moves at a steady rate, and the air pressure graph shows the pressure variation, which traces a sine wave.
The speed of a sound wave in normal temperature, pressure and humidity is 343 metres per second, meaning that sound takes about 3 seconds to travel 1 km. This is why, when you see a lightning strike, you can estimate its distance in kilometers by taking the time in seconds between the flash and the roll of thunder and dividing it by 3 (ex. 15 seconds is about 5 km).
Human hearing extends from a wave length of 17 m (20 Hz) to 17 mm (20 kHz).
At the top, red dots depict individual air molecules. The piston vibrates, and the vibration is translated into variations in air pressure.
At the bottom, the central line represents normal air pressure, with higher air pressure above and lower below.
The piston moves at a steady rate, and the air pressure graph shows the pressure variation, which traces a sine wave.
The speed of a sound wave in normal temperature, pressure and humidity is 343 metres per second, meaning that sound takes about 3 seconds to travel 1 km. This is why, when you see a lightning strike, you can estimate its distance in kilometers by taking the time in seconds between the flash and the roll of thunder and dividing it by 3 (ex. 15 seconds is about 5 km).
Human hearing extends from a wave length of 17 m (20 Hz) to 17 mm (20 kHz).
Pressure variations, or sound waves, proceed spherically in all directions from a sound source. Note that the individual air molecules do not travel.
In practice, low frequencies spread out evenly, while the higher the frequency, the more directional the sound is. If you are seated behind a pillar in a concert hall, low pitches can still be easily heard but high pitches will be masked.
In practice, low frequencies spread out evenly, while the higher the frequency, the more directional the sound is. If you are seated behind a pillar in a concert hall, low pitches can still be easily heard but high pitches will be masked.
The adjacent diagram shows some wavelengths and the corresponding frequencies.
Facts
- The simplest regular vibration traces a graph in the shape of a sine wave
- Sound is air pressure variation, or vibration of air molecules
- Sound waves basically proceed equally in all directions from a sound source
- Amplitude is the height or power of a sound wave, measured in dB
- Frequency is the density of the sound wave or speed of vibration, measured in Hz
version 29.6.2022