Science of Sound

The crack of thunder can exceed 120 decibels, loud enough to cause pain to the human ear.

NPS Photo.

What is Sound?
Sound is perceived by humans as an auditory sensation 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. Noise is sometimes used as a synonym for sound, but there is a subtle difference. Noise is a sound that is unwanted or inappropriate in an environment.

Frequency, sometimes referred to as pitch, is the number of times per second that a sound pressure wave repeats itself.
A drum beat has a much lower frequency than a whistle, and a bullfrog call has a lower frequency than a cricket. The units of frequency are called hertz (Hz). Humans with normal hearing can hear sounds between 20 Hz and 20,000 Hz. Frequencies above 20,000 Hz are known as ultrasound. When your dog tilts his head to listen to seemingly imaginary sounds, he is tuning in to ultrasonic frequencies, as high as 45,000 Hz. Bats can hear at among the highest frequencies of any mammal, up to 120,000 Hz. They use ultrasonic vocalizations as sonar, allowing them to pursue tiny insects in the dark without bumping into immovable objects. At the other end of the spectrum are very low-frequency sounds (below 20 Hz), known as infrasound. Elephants use infrasound for communication, making sounds too low for humans to hear. Because low frequency sounds travel farther than high frequency ones, infrasound is ideal for communicating over long distances.
Graph showing the relationship between frequency and amplitude.
This figure illustrates the concepts of frequency and amplitude. The magenta wave has one half the amplitude of the black wave, and produces a quieter sound. The green wave completes half as many cycles as the black wave, so its frequency is one half that of the black wave, and it has a lower pitch.

NPS / Damon Joyce

Amplitude is the relative strength of sound waves (transmitted vibrations), which we perceive as loudness or volume. Amplitude is measured in decibels (dB), which refer to the sound pressure level or intensity. The lower threshold of human hearing is 0 dB at 1kHz. Moderate levels of sound (a normal speaking voice, for example) are under 60 dB. Relatively loud sounds, like that of a vacuum cleaner, measure around 70 dB. When workplace sound levels reach or exceed 85 dB, employers must provide hearing protection. A rock concert, at around 125 dB, is pushing the human pain threshold.

Decibels are measured on a logarithmic scale, so an increase of 10 dB causes a doubling of perceived loudness and represents a ten-fold increase in sound level (Crocker, 1997). In other words, if the sound of one vacuum cleaner measures 70 dB, 80 dB would be the equivalent of 10 vacuum cleaners.

Because the acoustical environment is made up of many sounds, the way we experience the acoustical environment depends on interactions between the frequencies and amplitudes of all the sounds. Sound levels are often adjusted ("weighted") to match the hearing abilities of a given animal. Sound levels adjusted for human hearing are expressed as dB(A).

Defining Key Terms
Because the National Park Service works to protect and enhance both park resources and visitor experiences, the Natural Sounds Program differentiates between physical sound sources and human perceptions of those sounds.
  • Acoustic resources are physical sound sources, including both natural sounds (wind, water, wildlife, vegetation) and cultural and historic sounds (battle reenactments, tribal ceremonies, quiet reverence).
  • Soundscape can be defined as the human perception of those physical sound resources. Like beauty, soundscapes are in the mind of the beholder. The rhetorical question about the tree that falls in the forest may help illustrate this. Because no human is there to hear it, the resulting crash is not a part of the human soundscape. It is however, a pretty significant part of the soundscape of the squirrel standing in the tree's path.
  • The acoustical environment is the combination of all the acoustic resources within a given area. This includes natural sounds and cultural sounds, as well as non-natural human-caused sounds. The sound vibrations made by our imaginary falling tree are a part of the acoustical environment regardless of whether a human is there to perceive them. Bat echolocation calls, while outside of the realm of the human soundscape, are also part of the acoustical environment. It is therefore critical to take the entire acoustical environment into account when working to protect natural sounds.

Sound Level
Sound levels in national parks can vary greatly, ranging from among the quietest ever monitored to extremely loud. While the din of a typical suburban area fluctuates between 50 and 60 dBA, the crater of Haleakala National Park is intensely quiet, with levels hovering around 10 dBA. Along some remote trails in Grand Canyon National Park, sound levels, at 20 dBA, are softer than a whisper. The noise levels standing near a snowcoach in Yellowstone National Park, however, can be compared to standing three feet from a churning garbage disposal.
Decibels work on a logarithmic scale;an increase of 10 dB causes a doubling of perceived loudness and represents a ten-fold increase in sound level. Thus 20 dBA is perceived as twice as loud as 10 dBA, 30 dBA would be perceived as 4 times louder than 10 dBA, 40 dBA would be perceived as 8 times louder than 10 dBA, etc. Below are some examples of sound pressure levels measured in national parks.

dBA Sound
0 Threshold of human hearing
10 Volcano crater (Haleakala National Park)
20 Leaves rustling (Canyonlands National Park)
40 Crickets at 5 m (Zion National Park)
60 Conversational speech at 5 m (Whitman Mission National Historic Site)
80 Snowcoach at 30 m (Yellowstone National Park)
100 Thunder (Arches National Park)
120 Military jet at 100 m AGL (Yukon-Charley Rivers National Park)
126 Cannon fire at 150 m (Vicksburg National Military Park)