Sound level and pitch
Frequency, expressed in Hertz (Hz), denotes the pitch by specifying the oscillations per second: a tone with a pitch of 500 Hz oscillates 500 times per second. The higher this value, the higher the tone.
The human auditory range is between around 20 Hz and 20,000 Hz. Frequencies lower than this are called infrasound (e.g. the auditory range of elephants), frequencies above this auditory range are referred to as ultrasound (e.g. the auditory range of bats). Communication-related frequencies range from about 200 Hz to 2,000 Hz. In room acoustics, we typically take frequencies of 125 Hz to 4,000 Hz into account in order to create optimum sound conditions. The human ear perceives high frequencies particularly intensively: hearing is most sensitive in the range between 2,000 Hz and 4,000 Hz.
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Sound pressure level
The sound pressure level specifies the volume of a sound and is measured in decibels (dB). The human auditory range is between 0 dB and 130 dB (threshold of pain) and is heavily dependent on the frequency: sounds with a lower frequency need to be louder to be heard. Therefore, an increase of 10 dB is perceived as a doubling of the volume.
A sound level of 10 dB approximately corresponds to normal breathing sounds; the rustling of leaves has a sound level of about 30 dB. In a large open space, the sound level can easily reach 70–75 dB. At the workplace, hearing protection is required by law from 80 dB and over. The threshold of pain in the range of 130 dB approximately corresponds to the sound level of a jet taking off close by.
Psychoacoustic studies have shown that the sound pressure level accounts for only about 25 % of the disturbance. Therefore, it is not the sound level, but rather the quality of the disturbing sound, that is crucial for a pleasant acoustic atmosphere. Thus, conversations at the same sound level are perceived as considerably more disturbing than, for example, monotonous traffic noise (cf. noise).