Understanding Soundscapes and Acoustics

Natural Sound as a Resource or Value in National Parks

A "Soundscape" can be defined as the total acoustic environment associated with a given area such as a national park. In a national park setting, soundscapes may include only natural sounds, or both natural and human-caused sounds. Sound is measured in terms of amplitude and frequency. Amplitude is the relative strength of a sound wave 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.

The soundscape is often made up of many sounds, each with its own combination of frequency and amplitude. The way we experience the soundscape and the types of sounds we hear depend on interactions between the frequencies and amplitudes of all the sounds in the soundscape. The frequencies and amplitudes in a soundscape can be measured and expressed in graphical form. Figure 3 illustrates the frequencies, referred to as “octave bands,” along the X-axis, and amplitude along the Y-axis. The bars in the graph represent different frequency ranges and rise and fall as the amplitude of various frequencies rise and fall over time.

Figure 3 Click on the graph to hear and see the soundscape.

Sound levels in national parks can be very low. For example, in the crater in Haleakala National Park, minimum sound levels are between 0 and 10 dBA. In Grand Canyon National Park along some remote trails, minimum sound levels measure between 10 and 20 dBA. In contrast, 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 50 dBA would be perceived as 16 times louder than 10 dBA. Examples of sound pressure levels measured in national parks are provided below.

Sound	dBA
Threshold of human hearing	0
Haleakala NP: Volcano crater	10
Canyonlands NP: Leaves rustling	20
Zion NP: Crickets (5 m)	40
Whitman Mission: Conversational speech (5 m)	60
Yellowstone NP: Snowcoach (30 m)	80
Arches NP: Thunder (distance?)	100
Yukon-Charley Rivers NP: Military jet (100 m AGL)	120

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 on the pictures below to hear how frequencies and amplitudes interact to determine the sounds that we hear.

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?)

When we study soundscapes we are not only interested in individual sounds, but also the relationships and interactions among the sounds.  As the examples above illustrate, 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 soundscapes for current and future generations.  

The National Park Service has recognized the importance of protecting park soundscapes. Section 4.9 of the NPS Management Policies (2001) states: “The Service will take action to prevent or minimize all noise that, through frequency, magnitude, or duration, adversely affects the natural soundscape or other park resources or values, or that exceeds levels that have been identified as being acceptable to, or appropriate for, visitor uses at the sites being monitored.” Protecting soundscapes requires the collection, monitoring and analysis of acoustic data from the park and an understanding of soundscape management principles. The Natural Sounds Program Center has developed protocols for collecting and analyzing acoustic data and assists parks in interpreting and applying the data to ensure that natural and culturally appropriate sounds are left unimpaired for the enjoyment of future generations. See the Acoustic Data Collection and the Effects of Noise pages for information about the protocols and how data is used to protect park resources.