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The human range is commonly given as 20 to 20,000 Hz, [3] [4] [note 1] although there is considerable variation between individuals, especially at high frequencies, and a gradual loss of sensitivity to higher frequencies with age is considered normal. Sensitivity also varies with frequency, as shown by equal-loudness contours.
The descriptive term ultradian is used in sleep research in reference to the 90–120 minute cycling of the sleep stages during human sleep. [ 2 ] There is a circasemidian rhythm in body temperature and cognitive function which is technically ultradian.
In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. [2] It is for this reason that the ultra low frequency band of the electromagnetic spectrum between 300 and 3000 Hz is also referred to as voice frequency, being the electromagnetic energy that represents acoustic energy at baseband.
It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sleep and wakefulness. In humans, this cycle takes 70 to 110 minutes (90 ± 20 minutes). [1] Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep.
Waking is usually characterized by beta (12–30 Hz) and gamma (25–100 Hz) depending on whether there was a peaceful or stressful activity. [22] The onset of sleep involves slowing down of this frequency to the drowsiness of alpha (8–12 Hz) and finally to theta (4–10 Hz) of Stage 1 NREM sleep. [23]
Slow-wave sleep (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves. [ 2 ] Slow-wave sleep usually lasts between 70 and 90 minutes, taking place during the first hours of the night. [ 3 ]
Some non-mel auditory-frequency-scale formulas use the same form but with much lower break frequency, not necessarily mapping to 1000 at 1000 Hz; for example the ERB-rate scale of Glasberg and Moore (1990) uses a break point of 228.8 Hz, [15] and the cochlear frequency–place map of Greenwood (1990) uses 165.3 Hz.
Other frequency bands are: delta (1–4 Hz), theta (4–8 Hz), beta (13–30 Hz), low gamma (30–70 Hz), [15] and high gamma (70–150 Hz) frequency bands. Faster rhythms such as gamma activity have been linked to cognitive processing. Indeed, EEG signals change dramatically during sleep.