How is noise affected in an inversion layer?

Following on from the introduction to inversion layers in a previous post, it seemed appropriate to expand the theme to explain a little better why the effect is significant for noise produced by frost fans;

The acoustical significance of such layers comes from the fact that the speed of sound is higher at higher temperatures. That means that within inversion layers the speed of sound is highest at the top of the layer and lowest at the bottom of the layer.

The consequence of this deflection of skyward-bound sound toward the ground is that the noise levels on the ground can be greater than they would otherwise be if only the direct path along the ground between the source and receptor were available to the sound. One could almost say, by way of an analogy with optics, that the source of the sound is magnified by the temperature inversion.

The reader who has never before taken note of this phenomenon might now try to see if it's 'real'. Just listen to the background noises from distant trains, distant aircraft (just during takeoff runs) and vehicles on distant roadway segments, preferably after dark or shortly before or after dawn, and note the atmospheric conditions. Disregard windy conditions, but take note of the loudness of such noises under all other atmospheric conditions. It should become apparent that the noise levels from distant sources are clearly higher on some days than on others, even though the observations were made at approximately the same time each day. With some effort it should also be apparent that the calm and clear conditions that are said here to give rise to temperature inversions are in effect when the distant sources seem loudest.

So that is all very interesting and technical, but what's the point?

Well, in order to satisfy the requirements to limit the noise produced by a frost fan, they need to be tested. If a frost fan isn't tested in frost conditions with an inversion layer, the noise measurements are incorrect. In other words, if you measure the noise of a frost fan during the day, or at night, but without a frost, those readings will not be accurate.

Therefore they shouldn't be used to demonstrate that the noise they produce is compliant.

Seems simple doesn't it? But that's not what has been happening. Resource consents have been granted for frost fans that have no accurate data on the noise they produce.

If they want to comply with the regulations, they need to be accurately tested. Anything else is either guesswork or attempted deceit.

If a resource consent IS granted for a frost fan without accurate noise measurements, this would open the Council and the applicant to an appeal to the Environment Court on the basis that there has been no accurate evidence that the devices can meet the required level.