The Sound Reducing Qualities of Greenery: Analytical Essay

Sound: A Basic Introduction

What is Sound? Sound can be described as variations in density and pressure that expand in an elastic medium which could be a gas, liquid or solid body. The precondition for variations in pressure to be defined as sound is furthermore that in addition to the source of the sound and the transmitting medium, there should also be a receiver of the sound. What we humans experience as sound are variations of pressure in the air causing the eardrum to vibrate. In order to assess sounds, our hearing uses the parameters of level, frequency and timing. When the variations in pressure reach the ear, they create a certain sound level at the ear. This sound level is measured in decibels [dB]. The frequency is the number of cycles per second [Hz] and is experienced as the tonal pitch of the sound. The sound of a pure tone can be visualized as a sine curve and its strength can be seen in the relative distance between the top and bottom of the waves. If this distance is halved then the amplitude of the sound can be assessed. The amplitude is measured in decibel [dB] in a logarithmical scale which means that a halving of perceived sound corresponds to a decrease of 10dB. In the same way, an increase of 10dB in intensity is perceived as a doubling We also receive a large amount of qualitative information about the sound by its duration. For example, strong impulse sounds with very short duration. The three factors – level, frequency and duration are used to identify the character of the source of the sound. Other factors that affect the sound are spatial, absorption and reflection which together constitute the acoustics of the space. The physical qualities of sound can be categorized as infrasound, audible sound and ultrasound. Infrasound is sound with a frequency of less than 20 Hz and ultrasound is v sound with a frequency of over 20 000 Hz. These two frequencies; 20 and 20 000 Hz, constitute the boundaries of audible sound for human beings. Our hearing, however is most attuned to frequencies between 500 and 8 000 Hz 7 (Lagstrom, 2004).

Sound normally consists of pure tones at different frequencies. In industrial environments sound includes all frequencies with a random range of intensity, called fuzz. What we perceive as speech is a mixture of pure tones and fuzz. The difference between fuzz and noise is the perceived sound level and the two concepts, sound and noise are usually characterized through the simple definition that noise is undesirable sound for the listener. The characterization of noise is therefore dependent on a subjective evaluation of whether the sound is perceived as pleasant or unpleasant. There is a range of different kinds of sound which are perceived as noise. Certain types, such as electric motors, principally emit a constant noise where the combination of frequency and intensity does not vary. The usual kind of noise is fluctuating, where the combination of frequency and intensity vary, which is usual in the manufacturing industry. There is also impulse noise such as for example hitting noises and the clatter of machines (Lagstrom, 2004).

We can perceive and differentiate between different kinds of sound with the help of our hearing. Our sense of hearing works even when we are sleeping, which can lead to us being awoken by unexpected sounds and signals. The ear is a highly sensitive organ with functions to receive and transform sound impressions and to analyze the frequency of the impression (Lagstrom, 2004).

The primary reasons to decrease noise is to avoid direct damage, but also to increase comfort whilst sleeping, working and socializing. Good insulation is important, particularly in multi-family housing and open landscape offices, not purely to protect individual integrity, but also to avoid intrusion into the private sphere of others. The degree of isolation depends on sound insulation in the roof and between walls, but also on ambient noise levels. Insulation can be achieved in many ways; by building better walls or roof to decrease their ability to transmit noise, by increasing the amount of absorbing material in the room, which is the source of the noise and finally to increase the sound levels in the receiving room to counteract the incoming sound (Lagstrom, 2004).

The Elements of the City Soundscape

Cities are noisy. Generally speaking the greatest source of noise in a city is sourced from traffic of cars, light and heavy trucks, emergency service vehicles with sirens, light and heavy trains, ambient noise from heaters and air conditioners, and industrial sites that include light and heavy industry. Traffic noise does not structurally damage a home or building; however, it can depreciate the value of a building and can be the source of psychological stress. As rapid development and expansion of cities increase the demand for quick and rapid transportation also increases, thus increasing the noise level of the city as a whole as more cars enter the existing road space (Cook and Haverbeke, 1977). Political options to curb traffic noise have also been slow in catching up to the pace of development and have, in some cases, relaxed regulations that can effectively reduce the noise at its source (Den Boer and Schroten, 2007). An example of this political laxity comes in the EU Motor Vehicle Sound Emission Directive and the Tyre/Road Directive, whose changes in test conditions have resulted in lower passing requirements for sound emission and tire noise which, in turn, have created an even higher volume of traffic (Den Boer and Schroten, 2007). The increase in road traffic is even more of a concern for developing nations, because of the increase in middle class wealth and a desire of the emerging middle class to own cars. China, in particular, has seen an explosion in private car ownership, growing at 37.4% annually since 1985 to reach 28.81 million private cars in 2008 (China Statistical Bureau, 2009). This growth is likely to continue, considering that China had just 38 cars per 1000 people in 2008 (China Statistical Bureau, 2009) while most developed countries and even emerging economies feature over 200 cars per 1000 people (e.g. Brazil: 239; Russia: 245; Denmark: 457; United States: 809) (World Bank, 2011).

As far as decibel levels are concerned, regular speaking conversation between two people around 60 cm apart from each other is recorded at 65 decibels (dB) and if the dB level increases by 10, the sound seems twice as loud; above 85 dB extended exposure to constant noise can permanently damage one’s hearing, and sound exposure at 120 dB is extremely damaging, even for short periods of time. A recent article out of New York Magazine recorded dB, where the sound level in Central Park was measured at 54 dB, while two busy New York restaurants were recorded at around 8 dB, and Astor Place subway station was recorded at 101 dB as a train was passing (Thompson, 2015). Comfort levels of sound decibel sensitivity vary significantly from person to person, but observers have reported that noise levels exceeding 70 dB are not appropriate for daytime outdoor activities especially if conversation is to be maintained. A decibel range between 60-65 is generally acceptable for the day and 50 to 55dB for the night (Cook and Haverbeke, 1977). These numbers are common amongst the vast majority of this author’s research. Noise control and reduction for cities has previously been centered on reducing the noise level at its source; lowered speed limits in urban areas, rerouting large truck access and limiting the times when heavy trucks can enter an area, enhanced engine muffling techniques, and the European Union has been phasing in new tire standards aimed at significantly lowering the noise created by car and truck tires, a major source of traffic noise (Cook and Haverbeke, 1977; Beatley, 2013). City sounds, more generally speaking, is made up of three different components: direct sound, which travels from a sound source to the listeners ears; reflected sound energy, which builds up between the road surface and the building’s exterior walls; and diffracted sound energy, which curves around the corners of city blocks (ARUP, 2016). The materials used in construction of dense urban areas are also conducive to sound reflection and the design of city centers amplifies sound. Hard surfaced materials like concrete, brick, glass, asphalt, and metals all reflect sound and extend the range of audible noise. These materials are known as acoustically rigid materials and they provide a strong amplification of the source sound from traffic noise at street level (Renterghem, 2013). As has been discussed in previous sections, urban canyons have a tremendous effect of trapping and increasing the heat that a city produces; the canyon comparison can also apply to sound properties. It is generally accepted knowledge that canyons echo when a source of sound is at the base of the canyon. The same dynamic applies to artificial canyons, especially when the sides of the canyon are made out of highly sound reflective materials, such as those listed above.

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Heath and Psychological Implications of City Sound

People are affected by noise in a number of different ways from insignificant noise which barely affects us to directly damaging noise that is both physically and mentally disturbing. Noise can from a physiological perspective include everything from undamaging to painful and damaging. The most usual ways that noise affects people is damage to hearing, sleep disturbance, decreased capacity and tiredness but noise can also affect blood circulation and cause stress symptoms. The sense of hearing cannot be disconnected either in a waking condition or in sleep, unlike the sense of sight which can be controlled by closing one’s eyes. This means that people monitor their environment even in sleep. As good sleep is important for human health, then noise disturbance can lead to serious consequences for health. There are studies that indicate that sleep quality is reduced as a result of exposure to individual sounds of 45 dB at the most for noise sensitive people, but it should be noted that this group of noise sensitive people includes one third of the population. Results of epidemiological studies in areas around airports with high noise levels (67 – 75 dB) show that heart problems, visits to the doctor and purchase of medicines are more common than in quiet areas (46 – 55 dB). Psychosocial well-being in terms of reported depression was significantly lower amongst people living in a noisy environment. Our hearing capacity naturally lessens as we grow older, but at a significantly lower rate for those who are not exposed to noise on a regular basis (Lagstrom, 2004).

The psychological effects of sound can not be ignored because they have been proven to affect the mental well being of people. Using the methods of semantic differential (words used to describe something are rated along a positive and negative scale) when white noise was introduced to a subject while looking at a picture of a waterfall the semantic differential rating moved towards more positive adjectives used by the subject. Another example is where test subjects were shown a picture of an urban landscape while listening to a recording of birdsong, compared with looking at the picture without the birdsong. Results showed a clear positive inclination in the semantic differential towards the group who could hear the twittering of birds. Yet another study showed that visual stimuli can alter the noise perception of outside sounds. Subjects were presented with a calming picture and the perceived sound of traffic noise was reduced by 10 dB lower than when the subjects were given no picture to look at. Results from these studies demonstrate that visual inputs can greatly affect the perceived sound level, making once aggravating and irritating noise easier to handle by the receiver, making the noise seem quieter by 2.5 to 5% (Fastl, 2004). While these measurements are very hard to define quantifiably there is a growing body of research that suggests strongly that the psychological perception of sound is influenced by what a person sees. Increasing the visual attractiveness of urban areas can significantly lower the perception of volume of traffic noise in cities based on survey-based research (Renterghem, 2013). A recent survey of over 100 people living in an apartment complex facing a particularly noisy road in Belgium showed that the residents who had views of green and natural landscapes were five times less likely to complain of noise pollution than their neighbors without green views (ARUP, 2016). The color green, by itself, has also been proven to reduce the perception of loud noises when compared to other colors (Arup, 2016; Fastl, 2004). Traffic noise is recognized as a serious public health issue by the WHO, and there is a great amount of evidence that traffic noise can disturb sleep patterns, affecting cognitive functioning and it can contribute to some cardiovascular diseases, and as for the affect on blood pressure; the evidence is mounting (Den Boer and Schroten, 2007). A preliminary study has demonstrated that over 245,000 people in the European Union are affected by traffic noise related cardiovascular diseases each year and the numbers are ever increasing. Because of such a high number of people suffer in this way, around 20% of these people, approximately 50,000, suffer a lethal heart attack and die prematurely (Den Boer and Schroten, 2007). Economically speaking, the social costs of traffic noise in the European Union add up to at least 40 billion euros per year, around 0.4% of total GDP, and the bulk, 90%, of these costs are accrued by passenger cars and large trucks (Den Boer and Schroten, 2007).

Reducing Sound Through Green Architectural Applications

Most plant materials have sound reducing qualities by as much as 8 dB and more, in some circumstances (Cook and Haerbeke, 1977). In order to lower the noise level from suburban cars and light trucks to an acceptable level where the suburban home is 25 meters from the centerline of the road, planting one or two rows of continuous dense shrubs as close to the curb as possible with a row of dense trees behind the shrubs. Deciduous trees are an option but the sound mitigation factor will be more constant and present if the chosen trees are conifers (Cook and Haverbeke, 1977). While research on vertical greenery systems and their sound reducing qualities is limited, there is a great body of research exists on the sound reducing qualities of ground vegetation and green rooftops. Other bodies of research on reducing the noise from heavy trains by surrounding the tracks with berms and a mixture of trees, hedges and low-lying shrubs can attest to the sound attenuation properties of plant life. In southeastern Nebraska trees have been found to reduce the sound level of passing trains by 5-8 dB, and if the width of the tree belts is increased, the sound is further reduced, beyond 10 dB (Wong 2010). These tree belts were made up of fifteen-year-old beeches, conifers, birches, and elm trees, and the width of these belts were 50 meters. This tree belt arrangement is not feasible for urban applications because existing urban roadways are not wide enough to accommodate a 50-meter-wide forest, however such research offers a good starting point for further study. Below are two comparable figures that show the impulse response from two studies. At the top is the impulse response from the study without the green roof and at the bottom is the study with the green roof.

The figure shows there is a clear difference in the sound pressure of the two studies as seen in these two impulse responses.

The direct impulse is the first impulse from the speaker that reaches the condenser microphone. The first impulse from each study is analyzed and the frequencies are separated by the computer. The distance between the curves shows the noise reduction, which is between 5 and 20 dB, distributed between the different frequencies. At a frequency of 750 Hz the noise reduction is as much as 20 dB, but at a frequency of 1 400 Hz the reduction is just 5 dB. This is explained by the different noise reduction capacity at different frequencies of the individual materials used in the roof. Note that the highest frequency is 3 500 Hz in accordance with Swedish measurement standards.

This figure shows the results of the study for the whole impulse response when the monitoring has been going on for about one minute. The upper curve shows the study without the green roof and the lower curve shows the study with the green roof. The horizontal scale shows the frequency by octave band, which in the figure means a range from 20 to 3 500 Hz. The octave bands are furthermore presented in a linear fashion which means that the figure just shows an average of a highly varied curve. As there is a level difference between the two studies it can be concluded that green roof vegetation has a statistically significant noise reducing effect. (Lagstrom, 2006)

As for rooftop gardens, depending on the size, structure, and contents of the garden, an additional noise attenuation was found to be between 5 and 20 dB (Wong, 2010). A sufficient level of rooftop garden must be in place for significant effects can be measured, recorded, or even noticed. The acoustics performance of rooftop greenery also increases when the traffic speed of light vehicles increases on the adjoining road (Wong, 2010). With green facades, the plants themselves offer a sound insulation layer through trapping air between the leaves and the wall surface, as well as the substrate layer that insulates from sound by absorption, reflection, and deflection; the substrate with plants block both higher and lower frequency sounds (Wong, 2010). The fact that green wall systems are made of porous and lightweight elements makes them interesting sound absorbers; because of the multiple reflections of sounds that occur inside a street canyon between opposite facades the amplification rate is very high. But with sound reduction at, at least, one side of the canyon, the sound is absorbed and reflected at a lower rate than before and this dynamic happens multiple times. The rate is effectively doubled when green walls are installed at both sides of the canyon (Renterghem, 2013). Insertion loss is a term used to define the difference in decibels between two sound pressure levels at a particular testing point before and after an object is put in between the measurement point and the sound source (Wong, 2010). The insertion loss for all green facades is different (from 5 to 8 dB) but the data shows that the greater density of plant life in the green façade system the greater the sound attenuation effect of the system. This dynamic is the same for substrate levels; the larger the amount of substrate or growing medium in the system the greater the sound absorption (Wong, 2010). The substrate, a highly porous medium, is thought to exert the greatest effect on sound dampening, and this effect is even greater when the substrate is fully watered (Renterghem, 2013). Green roofing systems offer the greatest sound attenuation dynamic for high frequency sounds, especially when the building height is equal or close to equal, since it propagates at nearly parallel to roof surfaces which increases the absorption coefficient when compared to other angles of roof height (Renterghem, 2013). As far as street width is concerned, generally the narrower a street the greater sound attenuation effects by green facades was observed; for a 9-meter-wide canyon the insertion loss was 3-6 dB closest to the sound source and the sound attenuation effects continues around 150 meters past the next modeled intersection. For a 29-meter-wide street canyon the sound attenuation effects were only at a 1-3 dB insertion loss that began to slowly reduce around 50 meters past the next modeled intersection past the sound source (ARUP, 2016). Building height was found to only have a minor impact on sound reduction qualities, the common thread being that the higher a building the greater sound attenuation impact existed, but not to a statistically significant degree (ARUP, 2016). When building geometry was tested, the greatest sound attenuation levels were found to be when green facades were applied to buildings with balconies (6-10 dB insertion loss in the main canyon and 3-6 dB past the first modeled intersection), while a building with a flat façade had only a 3-6 dB insertion loss in the main canyon (ARUP, 2016). This result is probably due to the fact that horizontal façade elements hold more sound energy than that of a flat, vertical façade. When façade coverage was tested the same results were found for both full façade coverage in one example and 10 meters of façade coverage at the bottom of the building, at an insertion loss of 3-6 dB in the main canyon and 1-3dB attenuation 150 meters past the first modeled intersection. There was no recorded insertion loss with the model that had full façade coverage of the building excepting the bottom 10 meters, though this does not necessarily mean that there was no insertion loss because levels were only recorded at street level. Sound reduction may have taken place further up the building sides, but studies that record insertion losses above street level have not taken place as of yet (ARUP, 2016). There were five main observation points found in the ARUP acoustic study that bear mentioning here: “green facades could reduce sound levels from emergent and traffic noise by up to 10 dB; green facades do not have a significant impact on noise level reduction close to a noise source: green facades have a greater acoustical impact with increasing distance from the sound source up to the point where ambient noise begins to dominate; green facades are unlikely to have a noticeable acoustical impact when a neighborhood’s sound environment is dominated by distributed sound sources and; green facades are likely to have a greater impact during the night, when ambient noise levels are lower and the soundscape is dominated by emergent sound sources.” (ARUP, 2016). When comparing green systems to other, more common, building surfaces, the green system regularly outperforms the other common building surfaces; however due to the cost of installation and maintenance of the green façade systems, it is not recommended as a suitable replacement of the common building materials solely for sound reduction, as there are other specialized materials that offer better insertion loss rates than plant life. However, coupled with the myriad other benefits of green facades, the choice to include green architectural elements into a building is a very good one.