Noise: nature and measurement Noise is often defined as unwanted sound or sounds that have an adverse effect on humans. What is sweet music for one person may be noise to someone else. Thus, noise is a psychological construct influenced both by physical and psychosocial properties. Sound is created by the rapidly changing pressure of air molecules at the eardrum. A single tone, such as that from a tuning fork, can be depicted as a fixed wavelength sinusoidal pressure distribution across time. The number of pressure cycles per second, measured in hertz (Hz), is the basis for the sensation of pitch. A healthy young ear is sensitive to sounds between approximately 20 Hz and up to 20 kHz. The amplitude of the sine wave is perceived as loudness. To accommodate the wide dynamic power range of the human ear a logarithmic magnitude scale for sounds has been introduced. Its unit is the decibel (dB). Adding two independent sound sources of the same dB-level will yield a sum that is ≈3 dB higher than one of them alone. The subjective effect of a change in 3 dB amounts to a just perceptible change. A change of around 10 dB is needed to experience the sound as twice as loud. The hearing threshold for pure tones is lowest in the frequency range 500–4000 Hz, which also is the range where human speech has its maximum energy content.