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Frequency Perception

Audible Frequency Range​

Human hearing is limited to a specific range of sound frequencies. This range typically spans from 20 Hz to 20,000 Hz. Sounds with frequencies below 20 Hz are known as infrasound frequencies, whereas sounds with frequencies above 20,000 Hz are known as ultrasound frequencies. Unlike humans, other animals can hear infrasound and ultrasound frequencies. For example, elephants can hear down to 16 Hz, while dogs can hear sounds up to 45,000 Hz.

Although we can hear sounds at various frequencies, our sensitivity to those sounds vary significantly. The sensitivity of the human ear is greatest between 2,000 Hz and 5,000 Hz, where even faint sounds are detectable. This range overlaps with the frequencies of many important sounds, such as speech and environmental noises, which is why our ears are more attuned to these frequencies.

A phon is a unit of measure that takes into account the perceived volume of a sound, regardless of the actual amplitude of the sound wave.

Module 1 shows audio at various frequencies with and without volume boosting to match the ear's frequency response.

Module 1
The tones 50 Hz, 1000 Hz, and 12,000 Hz in sequence. The perceived volume varies when going from one tone to the next.
The tones 50 Hz, 1000 Hz, and 12,000 Hz in sequence, but with the 50 Hz and 12,000 Hz tones modified by a 3.162x amplitude multiplier. The perceived volume is more level from going from one tone to the next.

Hearing Loss​

The ability to hear high frequencies deteriorates over time due to the natural aging process. This condition is known as presbycusis, or old-age hearing loss (presbyteros = elder, akousis = hear). Presbycusis typically affects high-frequency hearing more than low-frequency hearing, although this is not always the case. There are several underlying causes of presbycusis.

  • High-frequency sounds are processed by hair cells located at the base of the cochlea, which are more prone to damage since the base of the cochlea receives the initial impact of the sound wave energy. This is why presbycusis typically affects high-frequency perception more than low-frequency perception.

  • Over a longer period of time, you are more likely at some point to experience very loud sounds that will damage your hearing.

  • Over a longer period of time, you are more likely at some point to take a medication that is considered ototoxic, which means that the medication can damage your hearing (usually as a side effect when treating another condition).

Relative Frequencies​

The perceived increase in pitch is logarithmically related to increase in frequency of the sound wave. This is known as the Weber-Fechner Law, and it's not necessarily intuitive: each time you go one one octave on the piano, you exponentially increase the frequency. Module 2 shows three pairs of frequencies to demonstrate this relationship.

  • The first pair of waves differ in frequency by 100 and have a ratio of frequencies of 200.

  • The second pair of waves differ in frequency by 1000 and have a ratio of frequencies of 1100.

  • The third pair of waves differ in frequency by 1000 and have a radio of frequencies of 2000.

Module 2
Two waves, one with an frequency of 100, the other with an frequency of 200.
Two waves, one with an frequency of 1000, the other with an frequency of 1100.
Two waves, one with an frequency of 1000, the other with an frequency of 2000.
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