}}}
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== How Our Hearing Works ==
Our sense of hearing plays a critical role in our ability to perform daily activities. Before our ears can detect sound, acoustic energy begins in the form of sound waves traveling through the air. These sound waves strike our eardrum, which sends them into our inner ear. Inside the inner ear are nerve impulses that are then transmitted to and perceived by the brain.
=== Measuring Sound ===
Sound waves are measured in two ways: frequency and intensity.
'''Frequency''' is the number of cycles of pressure change occurring in one second and is measure in hertz (Hz). We perceive frequency as pitch. Humans can hear sound wave frequencies ranging from 20 to 20,000 Hz. When designing for the general population, expect them to detect frequencies from 1,000 to 4,000 Hz.
'''Intensity''' is determined by the amount of pressure with which a sound wave strikes the eardrum and is measured in decibels (dB). We perceive intensity as loudness. Frequency ranges from 1,000 to 8,000 Hz require the least intensity to be heard. And tones at lower frequencies must have larger intensities to be heard. As people age, their ability to hear higher frequencies is greatly reduced. By the time most people reach age 65, very few can still detect frequencies greater than 10,000 Hz.
Decibels are a logarithmic measure, not a linear one. Note that sound, power, and voltage decibels are all different, and many of those use varying reference levels. For sound pressure levels, a 20 dB difference is a 10-to-1 SPL change; a 40 dB change is a 100-fold increase. For sound pressure, the common reference level of 0 dB is about the threshold of human hearing, at 1,000 Hz.
One decibel is, conveniently, around the smallest perceptible difference to human hearing. It is also interesting that sound is perceived differently than its actual pressure values. As a rule of thumb, an increase of 10 dB in measured sound pressure is perceived to be only about twice as “loud.” A 20 dB increase sounds about four times as loud (instead of 10 times) and a 40 dB increase is perceived to be about 16 times as loud, instead of the 100 times it actually is.
Here are some typical sound pressure levels, and their perceived levels:
||'''Event'''||'''Sound Pressure Level'''||'''Relative Perceived "Loudness"'''||
||Rustling leaves||10 dB||1/32nd||
||Whispered conversation||20 dB||1/16th||
||Quiet office interior||30 dB||1/8th||
||Quiet rural area||40 dB||1/4||
||Dishwasher in next room||50 dB||1/2||
||Normal conversation||60 dB||Baseline||
||Dial tone||70 dB||2 times||
||Car passing nearby||80 dB||4 times||
||Truck or bus passing nearby||90 dB||8 times||
||Passing subway train||100 dB||16 times||
||Loud night club||110 dB||32 times||
||Threshold of pain||120 dB||64 times||
*Hearing damage begins to be possible at 140 dB.
Understanding how our auditory sense works can give us greater insight into designing audio alters and notifications. Be mindful of how increased age affects sound wave detection, as well as what levels of loudness are appropriate in our devices.
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Next: [[Brightness, Luminance, and Contrast]]
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