Sound

• Speed of sound


  330 m/s
• Sound


  • Travels in longitudinal waves
• Characteristics of musical sound

  • Pitch depends on frequency
  • Loudness depends on amplitude
  • Quality (tone/timbre) depends on the type and combination of overtones present
• Frequency limits of audibility

  • Humans can hear sounds of frequencies between 20 Hz - 20,000 Hz
  • Humans are especially sensitive to sounds of frequencies between 2,000 Hz - 4,000 Hz
• Origins of sound


  All sounds begin with an object vibrating (e.g. strings of a guitar, skin of a drum, reed of a clarinet)
• Size of vibrating object

  Determines pitch of sound (larger objects produce lower pitched sounds, smaller objects produce higher pitched sounds)
• Sound waves can travel through solids, liquids, or gases, as long as there are particles present
• Sound cannot travel through a vacuum as there are no particles to transmit the vibrations
• Echo-sounding


  • Ships use echoes (SONAR) to detect objects and measure depth of the ocean
• Refraction of sound


  Sound waves bend when they pass from one medium to another with a different density
• Sound waves have relatively long wavelengths and so readily diffract around obstacles
• High frequency, high-pitched sounds are more easily blocked by obstacles than low frequency, low-pitched sounds
• Sound waves can interfere constructively and destructively, creating patterns of sound and silence
• Acoustics

  • The science of designing buildings with the right balance of sound reflection and absorption
• Reverberation

  Multiple or repeated echoes of a sound within a space
• Reverberation time


  The time for a sound to diminish to 1/1000th of its original intensity
• Ideal reverberation time depends on the specific building and its use
• Noise pollution is any unwanted sound that can cause tiredness, lack of concentration, hearing damage, and deafness
• Noise levels can be reduced through insulation, soundproofing, and destructive interference
• Sound travels better through the ground than through the air, which is why Native American scouts put their ears to the ground to hear approaching horses
• Velocity of sound


  The speed at which sound waves travel through a medium
• The velocity of sound depends on the density of the medium it is travelling through, being fastest in solids, second fastest in liquids, and slowest in gases
• Refraction of sound


  The bending of sound waves when they pass from one medium to another with a different density
• Sound waves are refracted when they pass through non-uniform media, such as layers of air at different temperatures
• On a hot day, sound is refracted upwards, while on a cold night it is refracted downwards, affecting how far sound appears to travel
• Reflection of sound from surroundings

  Enhances the sound but too much makes it difficult to hear
• Sound will travel uniformly through air only if the air is at rest and at the same temperature throughout. These conditions are rarely fulfilled in practice.
• On a hot summer day when the ground is warm

  The air immediately above the ground is warmer than the air higher up, and as a result the sound wave is deflected upwards. This is why sound does not appear to carry long distances on a hot day.
• On a clear night the layers of air above the ground may be cooler than those higher up because the ground cools faster than the air above it

  The sound will therefore travel faster at a higher level than near the ground, and as a result the sound waves are deflected downwards. Under these conditions sound seems to travel further than normal.
• Pitch

  One of the three characteristics of sound (the others are loudness and quality/timbre)
• Loudness

  One of the three characteristics of sound (the others are pitch and quality/timbre)
• Quality/Timbre

  One of the three characteristics of sound (the others are pitch and loudness)
• A tuning fork produces a pure tone of one frequency
• Notes produced by musical instruments such as the piano and violin consist of a basic frequency and a number of other frequencies which are multiples of the basic frequency
• Fundamental frequency


  The lowest frequency in a sound
• Harmonics


  Frequencies that are multiples of the fundamental frequency
• The wave form and the sound are produced by interference between the fundamental and the overtones
• Different instruments produce different overtones and therefore different sounds
• If a sound contains a number of frequencies that are not related to each other, we do not hear a musical note; we hear noise
• An electronic synthesiser enables you to mix whatever frequencies you want to imitate various instruments or create an entirely new sound