Understanding Sound

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 the physical sound sources and human perceptions of those sounds. The acoustical environment can be defined as combination of all the physical sound resources within a given area. Acoustic resources include both natural sounds (wind, water, wildlife, vegetation) and cultural and historic sounds (battle reenactments, tribal ceremonies, quiet reverence), and a soundscape can be defined as the human perception of those physical sound resources.

The Natural Sounds Program also likes to differentiate between the use of sound and noise, since these definitions have been used inconsistently in the literature. Although noise is sometimes incorrectly used as a synonym for sound, it is in fact sound that is negatively evaluated (undesired) or extraneous to an environment. Humans perceive sound as an auditory sensation created by pressure variations that move through a medium such as water or air and is measured in terms of amplitude and frequency (Harris, 1998; Templeton and Sacre, 1997).

Because the acoustical environment is made up of many sounds, the way people experience the acoustical environment depends on interactions between the amplitudes and frequencies of all the sounds. Amplitude is the relative strength of a sound wave (transmitted vibrations) and is described in decibels (dB). Amplitude is related to what we commonly call loudness or volume.Frequency is related to the pitch of a sound. It is defined as the number of times per second that the wave of sound repeats itself and is expressed in terms of hertz (Hz). Sound levels are often adjusted ("weighted") to match the hearing abilities of a given animal. Humans with normal hearing can hear sounds between 20 Hz and 20,000 Hz, and as low as 0 dB at 1,000 Hz. When sound levels are adjusted for human hearing they are expressed as dB(A).

Figures 1 and 2 illustrate the components of sound -  frequency and amplitude.

Sound Level

Sound levels in national parks can vary greatly. For example, Haleakala National Park is very quiet, with sound levels ranging between 0 and 10 dBA in the crater. In Grand Canyon National Park along some remote trails, minimum sound levels measure between 10 and 20 dBA. Sound levels in a typical suburban area are between 50 and 60 dBA. An increase of 10 dBA represents a perceived (to human hearing) doubling of sound pressure level; that means 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 10dBA, etc. Below are some examples of sound pressure levels measured in national parks.

Interaction of Sound

When the Natural Sounds Program studies acoustical environments and soundscapes of parks they 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 frequencies may block out or “mask” other frequencies, and some sounds may highlight or enhance our perception of others. Understanding these relationships is vital to protecting acoustical environments and soundscapes for current and future generations. Some sounds may be masked by others depending on the frequencies and amplitudes involved. Other factors such as vegetation, topography, and our individual hearing sensitivity also contribute to the soundscape experience.

Click the photo below to hear a recording of birds.  The birds were recorded at 46dB.  Can you hear the Woodpecker tapping on the tree?
Now click the photo below to hear birds and a helicopter. In this recording bird are singing at 46 dB and the helicopter is 36 dB. How many birds can you hear?
Now click the photo below 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?)