Sound and vision

The pressure disturbances that we call sound normally have the form of waves. The human ear can detect sound waves in the frequency range 20Hz (20 oscillations per second) to 20 kHz (20,000 oscillations per second). Some animals can hear much higher frequencies than humans e.g. bats and dolphins can hear frequencies up to 100 kHz. Sound levels are measured in decibels (dB) using a calibrated sound level meter: 0 dB is set to the lowest level the ear can detect and 120 dB represents the threshold of pain.

In terms of fundamental properties the relationship between visual and aural perception is very strong, primarily because both are characterised by wave-like phenomena. Both Pythagoras and Aristotle postulated that there must be correlations between the musical scale and the rainbow spectrum of hues. The term “rainbow of sound” immediately conjures up a picture of the spectral relationship between sound and colour. Light can be split up into a spectrum of component colours using a prism and in the same way sound can be decomposed into different frequency bands using a spectrum analyser. Although both sound and light can be considered as wave phenomena, these wave frequencies occupy very different band widths. Audible sound frequencies for humans lie approximately in the range 20 Hz to 20 kHz, a bandwidth of almost 10 octaves, whereas visible light waves only cover a range of about 4 – 7 x 1014 Hz, a bandwidth of less than one octave.

The way in which we perceive panoramas and soundscapes relates to the fact that humans have two eyes and two ears on the horizontal plane. Just as the slightly different images transmitted to the brain by each eye allow us to build up a stereoscopic image of the landscape, so the different signals picked up by each ear allow us to identify the location of the source. For low frequencies the time difference is the dominant cue whereas volume differences are dominant at high frequencies. In binaural recording two miniature microphones are used, one placed in each ear or in the ears of a dummy head. The recorded sounds are then listened to through headphones to obtain realistic spatial effects.

Sound clips

Pure tones (sine waves) of 100Hz and 1000Hz sounded first separately and then together

White noise, low pass filtered noise and narrow band filtered noise at 4000Hz

Image gallery

Last updated on Tuesday, 26 April 2011 - 10:05am

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