I know a few is a bit vague but let’s assume at least 3. Some guy was selling a motorcycle and claimed it can be heard from a few miles away, I hope this is the right place to find out if it is probable or even possible, I will paste the seller’s post below as it contains information on what bike it is:
“Selling 1997 Harley Davidson 1340cc dyna 5 speed 30 thousand on the clocks bike has been well looked after a lot has been spent on it. Always been kept inside has custom paint job on it lots of chrome recently had seat re-covered. The bike is very loud can hear it from a few mile away but gives of a nice roar. Mot is out on the 31st August this year. In no rush to sell but would also maybe do a deal for an old British bike. Price is firm at £6550 have videos of the bike ticking over if you would like to know more feel free to message me.“
If you dismiss this to be true automatically, do you know how far you can hear one of these things?
Thanks for taking your time to to read.
Best Answer
The answer is “it depends”.
With certain atmospheric conditions, the air acts as a “lens” that guides sound over tremendous distances. This happens for example at night, over water. The water causes the air close to the ground to be colder, and this denser. This makes sound travel more slowly near the ground, and in turn sound waves that are starting out pointing “up” slightly are bent down again. See for example this answer , this, and links therein.
The result is that sound can travel really long distances.
Even if we forget that for a moment, we can instead ask about the normal inverse square law. Low frequency sounds are not absorbed much in air; if we assume attenuation can be neglected, it’s the inverse square law that determines how far away you can hear a sound.
Every time you double the distance, you lose 3dB of sound pressure (without attenuation or wave guiding). 3 miles is a little more than 4096 meters which would be 12 doublings of 1 m. So you would lose 36 dB of sound pressure if attenuation is insignificant. According to this article, the sound pressure can be 90 - 100 dB (these things are usually measured at 1 m from the exhaust although that was not specified in this article). Take away 40 dB and you can still hear it on a quiet day.
To take account of the attenuation in air you can consult this graph from https://en.m.wikibooks.org/wiki/Engineering_Acoustics/Outdoor_Sound_Propagation#/media/File%3AAtmospheric_sound_absorption_coefficient_2.svg
It shows very low attenuation at low frequencies, especially in dry air. With 0.1 dB per 100 m, you would lose another 5 dB at 5 km )roughly 3 miles), which is not very much. So the roar of the Harley, with lots of low frequencies, really does carry.
So yes, I can believe it. Of course after you have been riding a beast like that for a while you may no longer be able to hear so well...