Science of Sound

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, although we lose the highest frequencies as we age. Frequencies above 20,000 Hz are known as ultrasound. When your dog tilts his head to listen to some seemingly imaginary sound, he is tuning into ultrasounic 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 their own ultrasonic vocalizations as sonar, enabling them to pursue minute insects without the benefit of light and simultaneously avoid smacking into immovable objects.

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. 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. Rock concerts, at around 125 dB, (in case you needed science to confirm this) are pushing the human pain threshold.

Decibels work 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. (Nice for cleaning house, perhaps, but a bit hard on the ears!)

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. Humans with normal hearing can hear frequencies between 20 Hz and 20,000 Hz, and amplitude as low as 0 dB at 1,000 Hz. Sound levels adjusted for human hearing are expressed as dB(A).

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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 welll 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. One can understand, then, why it is critical to take the entire acoustical environment into account when working to protect natural sounds.

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Sound Level

Sound levels in national parks can vary greatly, ranging from among the quietest ever monitored, to extremely loud. While, for example, 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 ( Bell, Mace, & Benfield, 2009). The noise levels standing near a snowcoach in Yellowstone National Park, however, can be compared to standing three feet from a churning garbage disposal (California Department of Transportation, 1982).

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 would be 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 0 10 20 40 60 80 100 120 126

Sound Threshold of human hearing Volcano crater (Haleakala NP) Leaves rustling (Canyonlands NP) Crickets at 5 m (Zion NP) Conversational speech at 5 m (Whitman Mission NHS) Snowcoach at 30 m (Yellowstone NP) Thunder (Arches NP) Military jet at 100 m AGL (Yukon-Charley Rivers NP) Cannon fire at 150 m (Vicksburg NMP)

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Interaction of Sound

When the Natural Sounds Program studies acoustical environments and soundscapes of parks, we are not only interested in individual sounds but also the relationships and interactions among the sounds. Different sounds interact in interesting and sometimes surprising ways to determine what we hear in the environment. Some sounds may block out or "mask" others, depending on the frequencies and amplitudes involved, and some sounds may highlight or enhance our perception of others.

How does sound masking work?

Click the photo to the right to hear a recording of birds. The birds were recorded at 46dB. Can you hear the woodpecker tapping on the tree?
Now click the next photo to hear birds and a helicopter. In this recording birds are singing at 46 dB and the helicopter is 36 dB. How many birds can you hear?
Now click the last photo to hear the same audio clip. The birds are still singing at 46 dB but this time the helicopter is also 46 dB. Can you still hear all of the birds? (Hint: Listen for the woodpecker.)

 

Other factors such as climate, vegetation, topography, and our individual hearing sensitivity also contribute to the soundscape experience. For example, sound travels faster in warmer and more humid conditions. Sound also reflects off of very hard surfaces such as rock, water, or ice, and can travel great distances (think about the echo in a cave). Softer surfaces like leaf litter or duff tend to absorb sound (if the cave were coated in moss, would you expect an echo?). Understanding relationships between sound and the landscape is vital to protecting acoustical environments and soundscapes for current and future generations.

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References

Crocker, M. J. (1997). Encyclopedia of Acoustics. John Wiley and Sons, New York.

Bell, P. A., Mace, B. L., & Benfield, J. A. (2009). Aircraft overflights at national parks: Conflict and its potential resolution. Park Science, 26(3), 65—67.

California Department of Transportation. (1982). Caltrans Transportation Laboratory Noise Manual, Figure - Effects of Noise on People.