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National Park Service In partnership with Penn State University

Physics Of Sound

Sound is created by pressure variations that move in waves through a medium such as air or water. It is measured in terms of frequency and amplitude.

Frequency

Measured in hertz (Hz)
The number of times per second that a
sound pressure wave repeats itself.
A drum beat has a lower frequency than a whistle
A cricket has a higher frequency than a bullfrog call

AMPLITUDE

Measured in decibels (dB)
The relative strength of sound waves,
which we perceive as loudness or volume.
Threshold of human hearing
Volcano
crater (Haleakala National Park)
Leaves
rustling (Canyonlands National Park)
Crickets
at 5m (Zion National Park)
Casual speech
at 5m (Whitman Mission National Historic Park)
Motorcycle
at 30m (Yellowstone National Park)
Thunder (Arches National Park)
Military jet
at 100m AGL (Yukon-Charley Rivers National Park)
Cannon fire
at 150m (Vicksburg National Military Park)

Click on the icons above to hear the sounds live

Sounds above 85dB are considered harmful if exposed for long periods of time. Exercise caution when listening to sound clips with the caution icon.

Decibels

A small change in decibels means a big change in loudness.

That’s because decibels work on a logarithmic scale. An increase of 10dB represents
a ten-fold increase in sound level, which causes perceived loudness to double.

Decibel LevelSound Level/IntensityPerceived Loudness
20 dB
100 (102)
2
30 dB
1000 (103)
4
40 dB
10000 (104)
8

If 1 vacuum cleaner measures 70dB than 80dB equals 10 vacuum cleaners.

70 dB
=
80 dB
=
How Sounds Travel

Propagation

How a sound wave travels outward from the
source, through a medium, like air or water.
Noise levels in park transportation corridors today are at 1000 times the natural level.
*For illustrated purposes only and not an accurate representation of scale.
Road noise impacts on wildlife have been shown to extend over a mile into the forest (Brumm 2004).
*For illustrated purposes only and not an accurate representation of scale.

Key Words

ACOUSTIC RESOURCES
Physical sound sources including natural sounds (wind, water, wildlife, vegetation) and cultural and historic sounds (battle reenactments, tribal ceremonies, etc).
SOUNDSCAPE
The human, or animal, perception of all combined acoustic resources.
ACOUSTIC ZONES
Areas with similar vegetation, terrain, animals, and weather likely have similar acoustic characteristics, including sound sources and sound characteristics. Acoustic zones may be based on existing management zones.
Zion National Park has different 7 unique Acoustic Zones
NOISE FREE INTERVAL
The period of time between noise events, during which no human-cause sounds are audible.
In a transportation
corridor, the noise free
interval can be as small as
seconds 30 during high traffic times!