Many cities are experiencing a significant decrease of its social, recreational and economic activities. An often unnoticed facet of this change is that within the auditory landscape. Audio levels of urban and nature categories were recorded in February, pre-COVID changes, and again in April. Acoustical analyses showed distinct reduction in urban sound signatures and increased natural sounds.

Humans are the dominant sound makers in most landscapes – from machinery to cars to just being about. Urban lifestyles are often disconnected from nature; the sounds of nature can be rare for city residents. They are there, just overshadowed by human noise. In an urban context, audio data is more comprehensive than its visual counterpart; noise can be detected from kilometers away or emanating from hidden buildings.


Dr. Richard leBrasseur of the Green Infrastructure Performance Lab at Dalhousie University studies the differences between urban and rural landscapes.   As part of his ongoing research, he captured the noise dynamics of a Friday rush-hour at a 4-way intersection on February 28 and and April 3, 2020 for 80 seconds in Truro, Nova Scotia. 




What is a Lichen

Even if you don't know what a lichen is, you have probably seen one before.  They grow just about everywhere and there are over 3000 species of lichens in North America.   Lichens resemble strange-shaped moss or mold growing on trees and rocks.    Even though lichens may look like moss, they aren't plants.


Lichens are actually a combination of organisms living together in what is called a symbiotic relationship.  A lichen is a partnership between a fungus and algae or, in some cases, cyanobacteria.  The algae cells live surrounded by a network of fungus cells where they are protected (see diagram to the right).  They work together as a team to help each other survive and each brings specific benefits to the partnership.  


The Role of the Fungus

The fungus or mycobiont of the lichen relationship provides structure and support.  It is the fungus that gives the lichen its unique shape and some of its color.  The bright yellows, reds and oranges of some lichens (see image on the left) are due to pigments that the fungus produces.  Since the fungus surrounds the alga, it protects it from the environment.   Without the fungus, the alga would be exposed to the elements.  The fungal coating allows the agla to live in environments where it normally could not. 


The Role of the Algae

The algae in lichens also has an important role.  Unlike plants and algae, a fungus cannot make its own food.  The alga (or cynanobacteria) in a lichen has chlorophyll and is essentially a food factory.  As it completes photosynthesis, the alga produces a lot of sugar.  The fungus feeds off this sugar as an energy source.  The green color present in some lichens is due to the chlorophyll of the alga, as shown in the video to the right.



Although there are thousands of species of lichens, there are really three main types of lichens.  Lichens are classified by their growth form or shape of their body.  The three types of lichens include many subgroups due to the many variations of lichen body types.  There are three main types of lichens.

Crustose Lichens

The most important characteristic about Crustose lichens is that they grow right up against their surfaces.  These lichens form a "crust" on the rock or other material that they grow on and cling very tightly.  It would be very hard to pull a crustose lichen off a rock that it is growing on because, as the lichen grows, it actually grows into the rock and becomes embedded in it. Crustose lichens often look like colored scaly or flaky patches on rock.  Other times they form very small spheres that can take on a pebbly appearance when many of them are together

Foliose Lichens

The identifying characteristic of foliose lichens is that they are leaf-like.  Unlike crustose lichens, the body of a foliose lichen is not fully attached to its substrate.  While the base of the lichen is attached, most of the body is not.  A person would be able to pull most of a foliose lichen off of a rock or branch that it is growing on. The word "foliose" means leafy.  Many foliose lichens have flat, frilly sections.  They often look like tufts of odd-shaped and odd-colored lettuce growing on a branch or rock. 

Fruticose Lichens

Fruticose lichens have the most unique shapes.  While crustose lichens cling very close to their substrate and foliose lichens are mostly two-dimensional structures, fruticose lichens are very three-dimensional.  They can be bushy or hang from trees and are often described as shrub-like or mossy and the fastest growing type of lichen. Fruticose lichens are easily identified by the fact that they are not flat but actually "stick out" from their substrate.  They often grow branched structures that can sometimes be inches long. 

Situation of measuring device: iPhone XS, held 115 cm above ground, facing road intersection centre. Synchronized audio-video captured.

Sampling: Audio - 48 kHz sampling rate with 24 bits/sample.

Video: 2160p@24/30/60fps, Single Lens: 12 MP, f/1.8, 26mm (wide), 1/2.55", 1.4µm.

Calibration: 5 seconds clapping at beginning of each recording (removed from dataset for analysis).

Overall, these sounds, as a grouping, are considered emblematic of urban areas in contrast with rural or natural areas where a very different set of categories and auditory signatures would occur.

Analysing Peri-Rural Aural Events


The raw acoustic data was uploaded and analysed with the SINUS Acoustic Multichannel Universal Analysis v3.0 software and were processed within SNR (Signal-to-Noise Ratio) parameters which provided an output in relation to the surrounding acoustic level, this data output allowed for a graphic description of this environmental noise relationship.

The metrics used to describe the acoustic sound events found in the categorization process conducted over the raw acoustic data are briefly described. The duration, the LAeq, the SNR, and the timestamp, crucial for a precise real-time noise mapping, were used to describe each and all of the auditory events identified.


Duration: This was evaluated in seconds and corresponds to the time that the visualization platform would spend to show                        its values for each Peri-Rural Audial Events categorisation.


LAeq: This is also known as the time-average or equivalent sound level (IEC, 2013), and it stands for the

              equivalent of the total sound energy measured over a limited period of time.


SNR: The Signal-to-Noise Ratio is the relation between any noise event’s evaluated power surrounding the event of study.

Visualisation Approach and Procedure


Humans are adept at detecting and recognizing small deviations within a circular form (Wilkinson et al., 1998), thus a straightforward visualisation represented single sound events as concentric circular forms.  The color of each circle or 'ripple' was assigned according to the category to which the sound was associated, resulting in five different possible colors in each location.


The radius of the circle was assigned according to the calculated SNR of the sound event in dB and the various colors within the circles were set according to the LAeq value or the average auditory power (dB) as recorded during that timeframe duration of that signature; meanwhile, the SNR was represented by varying the pattern and size of signature fluctuations.


The circle became a direct informative element of the visualization instead of being purely aesthetic and was not generated randomly, but with a defined dB. However, colors and texture were chosen to deliberately enable their visualisation and abstraction such as downscaling and spacing distance to clarify relationships. 


While this procedure represents all recorded signatures, it may introduce a cognitive bias and create confusion in order to understand the impact of ambient or contextual sounds, as these are the most prominent representation. The logarithmic scale is complicated, especially in visual comparative terms; but these visualisations do represent the data within an appropriate margin of values.

Research Significance

The goal of this analysis was to develop a visualisation of Truro’s overall peri-rural sonic data.  The results serve as a specific audial spatio-temporal analysis which can be correlated against future analyses. Increases in these auditory levels indicate a change of Truro from a more peri-rural landscape to more peri-urban and eventually urban one.


Huang, N., & Elhilali, M. (2017) Auditory salience using natural soundscapes. The Journal of the Acoustical Society of America, 141(3), 2163-2176.

International Electrotechnical Commission (2013) Electroacoustics—Sound Level Meters—Part 1: Specifications (61672-1, I); International Electrotechnical Commission: Geneva, Switzerland.

Wilkinson, F.; Wilson, H.R.; Habak, C. (1998) Detection and recognition of radial frequency patterns. Vis. Res.  38, 3555–3568.


GIPL's Mission is to advance the understanding of green infrastructure planning and design to positively impact the challenges our everyday landscapes face.  GIPL seeks to build partnerships, combining research & practice which generate innovative solutions and ideas toward healthy communities.


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The Green Infrastructure Performance Laboratory

Director, Richard leBrasseur, PhD

Dalhousie University

Department of Plant, Food, and Environmental Sciences

20 Rock Garden Road, EE Building, Room 223

Truro, Nova Scotia, Canada  B2N 5E3

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