Acoustics The study of how sound is affected by location, for example, how sound bounces around a concert hall.
Amplitude The height of a wave. Sound waves with high amplitudes make loud sounds and sound waves with low amplitudes make quiet sounds.
Decibel The unit used to measure the volume, or intensity, of sounds. A very quiet sound, such as the rustling of leaves is about 20 decibels (dB). A very loud sound, such as a rocket taking off, is about 120 dB. Sounds over 80 dB can damage your ears.
Echo The sound we hear when sound waves bounce off a surface.
Echolocation A type of sonar used by animals such as bats and whales. The animals emit sounds and listen to their echoes in order to locate solid objects. Echolocation allows animals to hunt in dark or murky conditions where it is difficult to see.
Frequency The number of vibrations, or sound waves, per second. Sound waves with high frequencies make high-pitched sounds. Sound waves with low frequencies make low-pitched sounds.
Hertz The unit used to measure frequency. One hertz (Hz) is one vibration per second.
Infrasonic Sounds that are too low-pitched for humans to hear. It is thought that some animals are able to hear them.
Intensity The amount of energy that sound waves carry. The intensity of sound waves determines their volume.
Music A form of art using pleasant sounds.
Musical instrument An object that is used to make musical sounds. In a cello, a bow rubs over the cello’s strings to make them vibrate. The vibrations pass into the air and also to the cello’s hollow body, making the sound louder and richer.
Noise Any kind of sound. Noise is often used as a word for unwanted sounds.
Pitch How high or low a note is. High-pitched sounds have high frequencies and low-pitched sounds have low frequencies.
Resonance The frequency at which an object will vibrate if you hit it. This is sometimes called the natural frequency of an object.
Reverberation The sound heard when sound waves continue to travel after the original source of a sound has gone.
Sonar (Sound Navigation And Ranging) A technique that uses sound waves to detect solid objects. The sound waves bounce off them and reveal their position. Ships use sonar to find shoals of fish or other ships.
Sound A form of energy made when objects vibrate (move to and fro rapidly). As an object vibrates it sets the air around it vibrating too: molecules of the gases in the air press close together and then pull apart. These regions of higher and lower air pressure are called sound waves. Sounds also travel as vibrations through liquids and solids. The molecules are closer together in liquids and solids than in gases, so sounds travel through them much faster.
Speed of sound The speed at which sound travels. Sound travels through air at about 343 metres (1125 feet) per second. The speed of sound varies depending on the substance it travels through. Loud and soft sounds all travel at the same speed.
Supersonic Faster than the speed of sound. When objects travel faster than the speed of sound the they make a loud noise called a sonic boom.
Ultrasound Sounds that are too high-pitched for humans to hear. Some animals, such as dogs and bats, can hear them. An ultrasound scanner is used to make images of inside the human body. It beams high-pitched sound waves into the body. A computer transforms the echoes into an image.
Volume The loudness of a sound. Volume is measured in decibels (dB). Some sounds are so quiet that we can only just hear them. Others are so loud that they may damage our ears. See Intensity.
Wavelength The distance between one sound wave and the next. Sound waves with short wavelengths have high frequencies and make high-pitched sounds. Sound waves with long wavelengths have low frequencies and make low-pitched sounds.
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