Best Woods Acoustic Health

Having established the acoustic signatures of the Arb and its average and seasonal rhythms, the acoustic health and coherence of Best Woods, can be ascertained.  Barry Truax established three characteristics that define the effectiveness of acoustic communication.  The first is variety.  A system that has difference with a multitude of dynamic audible sounds that lack redundant patterns demonstrates variability.  For example, a rain forest has numerous different sounds and thus a high-level variety.  However, a system that is homogenized with one or two sounds that may mask others, lacks variety.  Three audible components are necessary to achieve variety: 1) pressure variation (i.e. loudness level) – micro level changes in the energy of sounds exerted by an entity; 2) event characteristics – alterations in pitch, loudness, duration, and timbre; and 3) variation in long-term relationships between sounds – fluctuations in the patterns of sounds.

Based upon these standards, Best Woods is a varied system.  The study is unable to address pressure variation because the SM2+ cannot record and calculate sound pressure levels.  Nevertheless, alterations in event characteristics are apparent in audio samples and on spectrograms – visual displays of sound frequencies.  On spectrograms, sound events appear as bright colors.  The higher the color extends, the higher the frequency, which is indicated on the Y-axis.  For example, in the following audio example recorded from the SM2+, the low mass of bright color on the spectrogram is a plane flying over the Arb, the thick vertical streaks are squirrel vocalizations while the lighter vertical streaks/dots are bird vocalizations, and the long horizontal line in the middle are crickets:

Loud Jet Plane and Niches

The biophonic sounds in this example are not static, but are scattered across the spectrogram without a precise temporal pattern. The strength of the vocalizations, their duration, and their tonalities fluctuate.  Because these dynamics are audible and apparent on the spectrogram, even when the loud plane flys over (a potential sound-masking source if it had been lower and thus louder), the criterion for event characteristics is satisfied.  This display is representative of most acoustic communication from Best Woods.  More examples can be found in the Audio Archive.  Finally, the criterion for long term relationships is also satisfied because alterations in sound patterns were continuously audible, demonstrated by the graphs found under Data.  Together, these results demonstrate variation is present in Best Woods.

The second characteristic necessary for an acoustically healthy system is complexity.  An environment with sound events that are dynamic and never static is complex.  Best Woods satisfies these qualifications because it has non-redundant micro sound variations.  Although biophonic sounds, such as from squirrels and birds, have a consistent daily macro level rhythm  (Graph)(Graph)(Graph), these sounds are temporally random at the micro level – events occur in undefined patterns minute by minute.  For example, in the spectrogram above, there is no pattern to the biophonic vocalizations since they are temporally scattered across the spectrogram.  Sounds in Best Woods are never homogenized and have observable subtleties, evident on spectrograms.  For example, in these coyote vocalizations (the orange streaks) (Spectrogram), changes in the intensity and pitch are reveled by the alterations in height and brightness of their calls.  Again, there is no pattern to these calls and their non-redundant characteristics are indicative of their dynamic nature and thus revealing of complexity.

The final characteristic is balance, which controls variety and complexity.  The basis of acoustic balance is in the physical properties of sounds.  The audibility of sound is dependent upon its energy and frequency.  These factors establish an acoustic profile, the area that sound permeates and its duration, that is unique to every sound.  The more power exerted, the larger the profile and more spectral space occupied.  When a system is balanced, there is an appropriate amount of variety and complexity – sounds are not overwhelming in number or domination, and thus there is no competition for spectral space.

Based upon these principles, Best Woods is a balanced system because variety and complexity are present but not in an overabundance.  Average daily graphs demonstrate the dynamic macro-level variation of sounds from Best Woods (Data). Spectrograms provide even greater confirmation that the system is balanced because animal vocalizations are not masked by vehicle noise, as indicated above (Spectrogram).  Masking was only approached on a few occasions when loud aircrafts flew over, either due to low altitude or because their engines exerted high levels of sound power.

Bernie Krause developed the niche hypothesis that states in healthy acoustic systems, sounds have their own “niche.”  Across the audible spectrum, from 20Hz to 20kHz, each sound occupies a frequency range (niche).  In functional acoustic systems, niches do not overlap; there is no competition for spectral space.  In a homogenized and over-powered environment, such as New York City, the audible spectrum would contain broadband anthropogenic sounds across the entire frequency range that would overlap and mask niches from most diminutive sounds.

In an acoustically healthy system, each niche is audible and can be visualized on spectrograms.  Discrimination among sounds form a distinct organizational structure that allows maximal coherence.  Krause classifies such systems as being in dynamic equilibrium.  Various audio samples and their accompanying spectrograms show that Best Woods is in dynamic equilibrium.

The following two examples contain two types of cricket sounds:

Click spectrograms to enhance:

Cricket Variation


In this example, the crickets’s sound is sporadic and occurs at the precise frequency of 2,390Hz.  Thus, this frequency is the acoustic niche occupied by these crickets.



Bird and Cricket

Multiple niches are apparent in this example.  In contrast to the previous cricket example, here the crickets’s sound is consistent (represented by the solid line in the middle) and at a higher frequency – approximately 3,656Hz.  Thus, their acoustic niche is above the other crickets’s niche in the example above.  If they were heard simultaneously, their sounds would not overlap, the main principle of the niche hypothesis.  Additionally, the crickets are surrounded by bird vocalizations both underneath and above the crickets’s niche.  Squirrel chirps are also audible, indicated by the striated vertical lines.  Finally, anthropogenic sounds from the road and the unknown building motor are audible and visible as the bright orange band on the bottom of the spectrogram.  Overall, each of these sounds have a distinct frequency range, and because all are audible/visible, variety and complexity are present.

Squirrel Variation


Two types of squirrel vocalizations are apparent: squirrel “chips” and “chatter.”  The chirps are the long and brightly colored striations on the spectrogram whereas the chatter is presented as the numerous vertical lines “behind” the chirps.  The chatter originates from a squirrel farther away from the microphone than the squirrel making the less sporadic chirps.  The ability to hear and see both types of  vocalizations is revealing of the variety and complexity apparent in Best Woods.


Potential Masking


This example reveals how masking could occur and thereby eliminate or reduce biophonic niches.  Squirrels are heard in this sample and a loud jet plane flys over.  The intensity of the plane is revealed by the bright low band on the bottom of the spectrogram.  If the plane had been lower, it would have exerted more sound pressure and thus this low band would ascend the Y-axis and overlay the vertical striations from the squirrels.  If this had occurred, the squirrels would have been masked and their niche not visible.  However, this is not the case, and even though the plane is loud, the variation in the system is still present because the squirrel vocalizations are audible and visible as the plane flys over.

The variable, complex, and balanced nature of Best Woods indicates that it is acoustically healthy.  One does not have to strain to make sense of the sounds within the environment.  Although the system is predominantly anthropogenic, these human-generated sounds do not continuously outcompete more diminutive natural sounds.  Interactions between biophonic and anthropogenic sounds are audible, and the clarity of difference between these two groups indicates the system has a high level of coherence.

By studying the Arb’s soundscape, it is apparent that a fourth element should be appended to Truax’s three criteria: the potential for variety, complexity, and balance to exist.  A prairie may naturally lack variety and complexity while in a homogenized environment no variety or complexity can exist.  The distinction between these locations is that a prairie most likely has the potential for multiple sounds to be audible and dynamic; whereas, in a homogenized environment, a dominant sound would inhibit the audibility of multiple acoustic events.  The prairie would thus be acoustically healthy, but the homogenized system would have low coherence and poor acoustic health.

Most biophonic sounds have smaller profiles than anthropogenic sounds.  Due to the highways’ proximity to the Arb and the large acoustic profile from vehicles, anthrophony easily permeates the Arb.  The location of the SM2+ in a wooded area centrally located in the Arb provided maximal audibility of natural sounds.  Had the recorder been stationed in the Arb’s prairie, less of these biophonic sounds would have been heard.  Squirrel and bird populations are not as dense in open fields and vocalizations from their counterparts in neighboring forests may not propagate as far because of their smaller acoustic profiles.

Since data were obtained only from Best Woods, this study cannot qualify the acoustic health of the entire Arb.  Broad generalizations nonetheless can be made, because the Arb has similar geographic features throughout its area.  Difference in the landscape is never separated too far from similarities.  For example, the prairies are not so large that they are out of audibility range from the woods and its predominantly forest-dwelling animals.  Touring the Cowling Soundmap provides approximations of how the soundscape changes throughout the Lower Arb.   Overall, based upon the graphs, recorded audio, and time spent in the Arb listening to its soundscape, it is my conclusion that so long as the anthrophony is not overpowering, and spectral niches are present, the Arb does have the potential for a high level of coherence throughout its entirety.

The Northfield area surrounding Carleton has a relatively small population of 20,000 people, with miles of farms surrounding the town, and industrialization is minimal – except for the Malt-O-Meal factory.  Based upon this area, and the seeming unlikelihood for significantly more factories or noise-producing structures to be built around the Arb, the Arb’s soundscape should remain relatively the same.  The frequency of anthropogenic sounds, mostly from vehicles, could increase, which would cause the Arb to become more of an anthropogenic system temporally – vehicle sound would become more of a keynote.  Unless industrialization does greatly expand within the Northfield community, it is unlikely the sound pressure levels from vehicles will elevate enough to decrease the coherence of the soundscape.  Should that occur, interior parts of the Arb, such as Best Woods, should remain relatively coherent because it is sheltered enough by distance that it would be difficult for the sound power from elevated anthrophony to mask the frequencies of biophonic sounds.   However, under this scenario, the exterior of the Arb would be more susceptible to changes in anthrophony, and thus have lower coherence and acoustic health.