Introduction Study Area Helicopters Methodology Results Conclusions Suggestions References Acknowledgements Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5	Appendix 5 . Sound Propagation in the open Air   While monitoring of sound levels in the field, natural resource specialist frequently collect a variety of information on atmospheric conditions (e.g., temperature, humidity, wind speed and direction) under the assumption that these conditions affect sound propagation and either increase or decrease measured sound levels. The intend of the following discussion is to provide information for natural resource specialist on what conditions have the most (and least) affect on sound propagation in the open air. This appendix is based largely on: Piercy, J.E. and G.A. Daigle. 1998. "Sound propagation in the open air". Chapter 3 in: Handbook of acoustical measurements and noise control. C.M. Harris (ed.) Acoustical Society of America . McGraw-Hill, Inc. Melville , NY   According to Piercy & Daigle (1998) sound attenuates based on the formula: Total attenuation = Geometrical divergence + Air absorption + Ground absorption + Miscellaneous absorption (e.g. from reflection or vegetation)
Geometric divergence. Sound level is known to decrease through the atmosphere as distance increases between the source and receiver. This is the result of a loss of acoustic energy as it moves from its point source. The loss of energy is known to decrease by 6 dBs for every doubling of distance from the source. Air absorption. As acoustic energy passes through the atmosphere, it is converted into heat. How quickly this energy is converted (i.e., how quickly the sound is attenuated or absorbed) depends to some degree on the amount of existing heat and moisture in the atmosphere. The formula that predicts how much air is absorbed over a given distance in meters is: Absorption = αd/100 dB Where, α = attenuation coefficient in decibels per kilometer d = distance in meters Another way to look at the formula is that since α is dB/km, the formula can also be written as: Absorption (in air) = [(dB/1000) x d]/100   = (dB/10,000) x d (Note: units are dB/meter)   As such, it is obvious that the distance "d" has to be quite large (i.e., hundreds of meters) to cause a significant change in decibels. This is why Piercy & Daigle (1998:3.3) state that at short distances (i.e., only a few hundred meters) absorption of sound in air (or the effect of temperature and humidity) is negligible. The following table is extracted from Piercy & Daigle (1998; Table 3.1) to show some values of the attenuation coefficient "α."   Temperature (°F) Relative Humidity Frequency (Hz) 125 250 500 1000 2000 4000 86 10 0.96 1.8 3.4 8.7 29 96 50 0.35 1.3 3.6 7.0 12 25 90 0.2 0.78 2.7 7.3 14 24 50 10 0.78 1.6 4.3 14 45 109 50 0.45 1.3 2.7 4.7 9.9 29 90 0.27 0.97 2.7 5.3 9.1 20 32 10 1.3 4.0 9.3 14 17 19 50 0.41 0.82 2.1 6.8 24 71 90 0.38 0.76 1.5 3.7 12 43   Using the formula and Table above, the reader can calculate the amount of absorption of sound having 500 Hz frequency at a temperature of 50°F and a humidity of 50%. From the Table, the reader can see that the absorption of sound level is 2.7 dB/km (or 0.0027 dB/meter) at a frequency of 500 Hz. Using the formula above, one can calculate how much sound would be absorbed at (say) 100 meters (328 ft.).   Absorption (in air) = (α x d)/100   = (2.7 x 100)/100   = 2.7 dB/kilometers   = 0.27 dB/100 meters   So over a distance of 100 meters (328 feet) you can expect humidity and temperature to cause sound at 500 Hz to attenuate by only 0.27 dB. (Obviously, the converse is true that at longer distances [say hundreds of thousands of meters], the absorption of sound is more significant.) This means that for monitoring of sound levels under typical field situations, if the source of the sound is relatively close to the receiver (i.e., within a few hundred feet) – the varying atmospheric temperatures and humilities will have essentially no affect on the sound level. Ground absorption. Sound level is known to be attenuated by the ground that it reflects off of. For acoustical measurements, the ground surface is typically classified as: hard, soft, very soft, and mixed (Piercy & Daigle [1998:3.4]). "Soft" ground absorbs more acoustic energy than "hard" ground. Ground covered with grass, trees, and scrubs is considered "soft ground". "Hard" ground (asphalt or concrete pavement) and "very soft" ground (i.e., snow covered ground) has somewhat different absorption capabilities. For most forest field conditions ground will likely be considered "soft" and therefore likely be a constant. Miscellaneous absorption (reflection). In acoustics, sound can be significantly affected by being reflected off of vertical, hard structures. Under normal forest field conditions, a hard vertical feature (like a wall or building) are typically not encountered in sound monitoring and is, therefore, a non-issue. Miscellaneous absorption (vegetation). Piercy & Daigle (1998:3.9) considers miscellaneous attenuation due to vegetation as negligible except for very dense vegetation (i.e., dense enough to be a visual screen). They consider trees and shrubs to be very poor noise barriers. They consider that primary advantage of trees and shrubs is that they contribute to ground attenuation by keeping the soil porous and therefore "soft" (as described above under "Ground absorption"). Below is a Table taken directly from Piercy & Daigle (1998:Table 3.4) that shows how various sound frequencies attenuates from the point source through dense foliage to the receiver.     Octave-band center frequency (Hz)   31.5 63 125 500 1000 2000 4000 8000 Attneuation due to foilage (dB/m) 0.02 0.02 0.03 0.04 0.05 0.06 0.08 0.12 From the above table one can see that 1 meter of dense vegetation attenuates sound (of 8000 Hz) by 0.12 dB. It would take 10 meters (32.8 ft.) of dense vegetation between the point source and the receiver to reduce sound by 1.2 dB, and it would take 100 meters (328 ft) of dense vegetation to attenuate sound to the receiver by 12 db. Unless vegetation of considerable thickness screens the receiver from the source of the sound, vegetation probably does not provide significant barrier attenuation. Summary. As stated in the Introduction (Main Report), the distance from the source of the sound has the greatest impact of the level of sound received. Other variables (e.g., temperature, humidity, vegetation) have essentially no impact on the level of sound that is received if the receiver is only a "short" (i.e., only a few hundred feet) distance from the sound source. As such, a land manager should probably focus on the distance from the source variable, if he wants to reliably control the level of sound received by a sensitive target.