Case Study: Outdoor Sound Propagation
Salford Acoustics Audio and Video has worked on the long range propagation of sound or noise over many decades. The work has included both experimental and theoretical studies.
Why are you interested in how sound propagates outdoors?
Noise is a problem to many people, causing annoyance and lowering their quality of life. Many noises that we hear, such as roads, have first propagated from the source (the cars) to the listener. Consequently, being able to measure and predict the propagation of sound outdoors is important to being able to design roads and the environment to minimise problems of noise.
What are the key scientific problems to be solved?
One of the main problems with outdoor sound propagation is dealing the metrological conditions. Wind and temperature profiles all affect how sound propagates through the atmosphere. For example, I live a few miles from Old Trafford football ground, but most of the time I cannot hear the football matches, but under some weather conditions, the chanting and cheering can clearly be heard in my garden. Being able to model sound propagation through a complex metrological condition is difficult.
Can you give an example of some experimental work on sound propagation?
A good case would be sound propagation across water. In this case the noise being investigated was blast noise. (Blast noises, from guns and quarries for instance, pose particular problems to acoustic prediction because it is non-linear, but that is another story.) Complaints were being received along the southern shoreline of an estuary due to blast noise being generated on the northern shoreline. From a research perspective, the interest was in how the propagation across water compared with propagation across land.
How did you carry out the investigation?
Two experiments were set up (i) on the range over land and (ii) off-shore over the water. On land the measurement was relatively straightforward, but there were many problems with the over the water measurement - not least how to place microphones above the water without mounting them on large boats which then would have reflected sound and affected the measurement. Furthermore, we needed to set up off-shore firing position across the water to eliminate the effect of the shoreline. The solution lay in the particular characteristics of the Estuary, which is tidal, and has sands exposed for parts of the day. The explosives were hung from scaffold at low tide, with the scaffold being on the sands. The explosives were then fired at high tide when the explosives were completely surrounded by water. The microphones utilised our own design of recording remote monitors, these were mounted on rubber rafts which were laid on the sand on low tide, and were allowed to float with anchors at high tide.
What did you find out?
Normal blast waveforms over land loose high frequencies as they propagate. But over water we discovered that the blast waveforms reminded 'peaky' over the whole distance which shows they retained considerable high frequency audible content. This happens because the ground absorbs considerable amount of sound energy, whereas water is relatively reflected, and the sound propagates with less energy loss. So blast noise is more of a problem across water which is why there were so many complaints. In addition, general meteorological effects were resulting in considerable directional effects on a few days, so for days on ends the neighbours didn't hear anything, but a change in weather made it very audible. The intermittent nature often makes noise more noticeable and potentially more annoying.
How was the problem solved?
As with many noise problems, education can go a long way to alleviate people's concerns. A series of public meetings were used to explain what was happening so that people understood that they were safe (the blasting posed no danger). Also considerable efforts are made so that blasting takes place on days and times when least annoyance is likely to occur.
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