# Characteristics of sound waves

### Characteristics of sound waves

This animation explains the most important characteristics of waves through sound waves.

Physics

Keywords

wave, sound wave, waves, frequency, amplitude, fázis, mechanical wave, wavelength, sound, hertz, electromagnetic wave, speed of sound, high wave, vibration, simple harmonic motion, spread velocity, boiling, ultrasound, infrasound, sound intensity, periodicity, state, physics, physical, mechanics

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### Questions

• Is it true that the louder we shout, the further sound travels?
• Is it true that the wave speed depends on the frequency?
• What is the unit of frequency?
• What describes the number of vibrations in one second?
• What is the relationship between frequency and wavelength?
• What is the relationship between frequency and amplitude?
• What is the unit of amplitude?
• Where does sound travel faster: in water or in air?
• How fast does sound travel in air at 15°C?
• What is an ultrasound?
• What is the phase difference between two vibrating particles that are in phase?
• Is it true that sound waves traveling in gases are always longitudinal?
• What is a wave front?
• What is the wavelength of a 20,000 Hz sound wave traveling in air at sea level? (c=f*λ)

### Wave formation

• wavefront - A surface made up of particles vibrating in the same phase.
• wavelength (λ) - The distance between two successive wavefronts. Its symbol is λ (lambda). In reality, the wavelength of sound audible for most humans is between 16 mm and 16 m and its wave speed is ca. 340 m/s.
• particle - The particles vibrate.
• wave speed - A characteristic of the medium itself; it is not equivalent to the velocity of the particles.
• Frequency - It is the number of vibrations per second.
• Amplitude - The maximum value of one of the vibration's parameters. In the case of sound, the amplitude is usually identified with the maximum value of air pressure or displacement. Amplitude is directly related to the intensity of a sound: the higher the amplitude, the louder the sound. It is independent of frequency and the speed of sound.
• amplitude
• speaker - The speaker emits longitudinal sound waves. Sound, like any other wave, is characterised by wavelength, frequency, wave speed and amplitude.

## Wave formation

A wave is a disturbance that travels through a medium. Waves can be very diverse depending on the characteristics of the medium and the source of the disturbance.

The simplest type of waves are mechanical waves that travel through various gases, such as sound waves that travel through air. The source of the sound causes air molecules to start moving. Then the already vibrating molecules make their neighboring molecules vibrate too. This process is repeated and this is how the vibration propagates.

If the direction of the movement of the particles is parallel to the direction of wave propagation, the wave is a longitudinal wave. In air, sound always propagates as longitudinal waves. Air is compressed then rarefied, thus wave fronts are formed. The distance between the wave fronts is called wavelength. If the wave speed of sound increases, its wavelength increases too.

### Wave speed

• wavefront - A surface made up of particles vibrating in the same phase.
• wavelength (λ) - The distance between two successive wavefronts. Its symbol is λ (lambda). In reality, the wavelength of sound audible for most humans is between 16 mm and 16 m and its wave speed is ca. 340 m/s.
• particle - The particles vibrate.
• wave speed - A characteristic of the medium itself; it is not equivalent to the velocity of the particles.
• Frequency - It is the number of vibrations per second.
• Amplitude - The maximum value of one of the vibration's parameters. In the case of sound, the amplitude is usually identified with the maximum value of air pressure or displacement. Amplitude is directly related to the intensity of a sound: the higher the amplitude, the louder the sound. It is independent of frequency and the speed of sound.
• amplitude
• speaker - The speaker emits longitudinal sound waves. Sound, like any other wave, is characterised by wavelength, frequency, wave speed and amplitude.

## Wave speed

Wave speed is the speed at which wave fronts travel in a medium. This is not equivalent to the speed at which the particles themselves move; wave speed rather characterizes the medium. However, it also depends on several other parametres, such as temperature of the medium or the frequency of the wave.

The wave speed of sound in air at a low altitude is about 1,200 km/h (3,937 mph). At a high altitude, it is only about 1,000 km/h (621.4 mph) as the air is colder and less dense here. However, in a very dense, solid medium, the speed of sound can be much higher than that.

### Frequency

• wavefront - A surface made up of particles vibrating in the same phase.
• wavelength (λ) - The distance between two successive wavefronts. Its symbol is λ (lambda). In reality, the wavelength of sound audible for most humans is between 16 mm and 16 m and its wave speed is ca. 340 m/s.
• particle - The particles vibrate.
• wave speed - A characteristic of the medium itself; it is not equivalent to the velocity of the particles.
• Frequency - It is the number of vibrations per second.
• Amplitude - The maximum value of one of the vibration's parameters. In the case of sound, the amplitude is usually identified with the maximum value of air pressure or displacement. Amplitude is directly related to the intensity of a sound: the higher the amplitude, the louder the sound. It is independent of frequency and the speed of sound.
• amplitude
• speaker - The speaker emits longitudinal sound waves. Sound, like any other wave, is characterised by wavelength, frequency, wave speed and amplitude.

## Frequency

The number of wavefronts that reach the observer in one second is called frequency.

In the case of sound waves, the higher the frequency, the higher-pitched the sound we perceive. Deeper sounds have a lower frequency.

The standard unit for measuring frequency is the hertz (Hz). For example, if we perceive a 1,000 Hz sound, it means that 1,000 wavefronts reach our ears in one second.

The higher the wave speed and the smaller the wavelength, the higher the frequency of a wave.

The relationship between these three parameters can be described with the following formula:
c = f * λ
(where c is the wave speed, f is the frequency and λ is the wavelength).

The human ear can detect sound waves with frequencies between 20 Hz and 20,000 Hz. Very deep sounds with a frequency lower than 20 Hz are called infrasound. Elephants, for example, use infrasound to communicate.
Sounds with a frequency higher than 20,000 Hz are called ultrasound. Bats and dolphins use ultrasound for orientation.

When the sound source and the observer are moving relative to each other, the pitch of the perceived sound changes. The perceived pitch is higher when the source approaches and lower when the source recedes. This phenomenon is called the Doppler effect.

### Amplitude

• wavefront - A surface made up of particles vibrating in the same phase.
• wavelength (λ) - The distance between two successive wavefronts. Its symbol is λ (lambda). In reality, the wavelength of sound audible for most humans is between 16 mm and 16 m and its wave speed is ca. 340 m/s.
• particle - The particles vibrate.
• wave speed - A characteristic of the medium itself; it is not equivalent to the velocity of the particles.
• Frequency - It is the number of vibrations per second.
• Amplitude - The maximum value of one of the vibration's parameters. In the case of sound, the amplitude is usually identified with the maximum value of air pressure or displacement. Amplitude is directly related to the intensity of a sound: the higher the amplitude, the louder the sound. It is independent of frequency and the speed of sound.
• amplitude
• speaker - The speaker emits longitudinal sound waves. Sound, like any other wave, is characterised by wavelength, frequency, wave speed and amplitude.

## Amplitude

The intensity of sound is determined by the amplitude of the vibration, that is, by the maximum displacement of the particles of the medium at a given place.

In the case of non-mechanical waves, the amplitude cannot be interpreted as maximum displacement, so it refers to the maximum value of another parameter. In the case of electromagnetic waves, the amplitude can be interpreted as maximum field strength or maximum voltage.

If the wavefronts form a straight line or a plane, the amplitude of the vibration will not decrease during propagation. In general, however, wavefronts are spherical so the energy carried by the wave spreads over a larger area in space and its amplitude decreases as it moves away from the source of the disturbance.

### Phase

• Wave phases - The phase difference between two vibrating particles that are in phase is 0, and 180° between particles that are in antiphase.

## Phase

Since vibration is a cyclic process, therefore the current position of a particle within a cycle can be characterized with a 0-360° angle. The displacement of a particle from the equilibrum position depends on its current phase.

Particles that are in phase move in the same direction at all times so the phase difference between them is 0. If the phase difference between the vibrating particles is 180°, the particles always move in the opposite direction (they are in antiphase). If the phase difference is 90°, the particles are in phase quadrature.

## Wave formation

Waves are formed when a disturbance propagates through a medium. Waves can be very diverse because of the characteristics of the medium and the source of the disturbance.

The simplest types of waves are mechanical waves that travel through gases, such as sound waves that travel through air. The source of the sound causes air molecules to vibrate. Then the already vibrating molecules make their neighboring molecules vibrate too. This process is repeated and this is how vibration propagates.

If the direction of the movement of the particles is parallel to the direction of wave propagation, longitudinal waves occur. Sound is always longitudinal in air.
Air is first compressed and then rarefied thus wave fronts are formed. The distance between neighboring wave fronts is called wavelength. If the wave speed of sound increases, its wavelength increases too.

## Wave speed

Wave speed is the speed at which wave fronts travel. This is not the same as the speed at which the particles themselves travel. wave speed rather characterizes the medium. However, it also depends on several other parameters, such as temperature or the frequency of the wave.

The wave speed of sound in air at a low altitude is about 1,200 km/h (3,937 mph). At a high altitude, it is only about 1,000 km/h (621.4 mph) as the air is colder and less dense here.

However, in a dense and solid medium, the speed of sound can be much higher than that.

## Frequency

The number of wave fronts that reach the observer in one second is called frequency. The higher the frequency of a sound, the higher the sound we perceive. Deeper sounds have a lower frequency.

Frequency is measured in hertz (Hz). For example, if we perceive a 1,000 Hz sound, it means that 1,000 wave fronts reach our ears in one second.

The higher the wave speed and the smaller the wavelength, the higher the frequency of a wave.

The human ear can detect sound waves with frequencies between 20 Hz and 20,000 Hz. Very deep sounds with a frequency lower than 20 Hz are called infrasound. Very high sounds with a frequency higher than 20,000 Hz are called ultrasound. Bats and dolphins use ultrasound for navigation.

## Amplitude

The intensity of a sound perceived by the human ear does not depend on frequency or wave speed.

The intensity of sound is determined by the amplitude of the vibration that reaches the ear. In other words, it is determined by the maximum displacement of the vibrating particles of the medium.

In the case of non-mechanical waves, the amplitude cannot be interpreted as maximum displacement, so it refers to the maximum value of another parameter. In the case of electromagnetic waves, the amplitude can be interpreted as maximum field strength or maximum voltage.

If the wave fronts form a straight line or a plane, the amplitude of the vibration will not decrease during propagation. In general, however, wave fronts are spherical so the energy carried by the wave spreads over a larger area in space and its amplitude decreases as it moves away from the source of the disturbance.

## Phase

Since vibration is a cyclic process, the current position of a particle within a cycle can be characterized by a 0-360° angle. The displacement of a particle from equilibrum position depends on its current phase.

Particles that are 'in phase' move in the same direction at all times so the phase difference between them is 0. If the phase difference between the vibrating particles is 180°, the particles always move in opposite directions (they are in antiphase). If the phase difference is 90°, the particles are in phase quadrature.

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