On September 4, 2011 the Baltimore Grand Prix was staged in downtown Baltimore. I live approximately 9 miles from the track as the crow flies. Around 8 or 9am my girlfriend claims to have heard the cars’ engines. This time coincides with a scheduled warm-up period for the race.
Is it possible she heard them?
You tell us where you live, but not where your girlfriend was at the time. 
Technically, of course, sound waves travels forever. The question is whether you can hear them or not. Low frequencies are attenuated over space less than higher frequencies. And if the wind is blowing strongly from the source, that can affect the travel, too. Not to mention the effect of the acoustics of the surrounding terrain. However, at 8 or 9 in the morning, there are going to be lots of sources of noise (depending, of course where you live). It’s certainly possible, but it would be hard to prove with the info you gave us.
Reportedly the 1883 Eruption of Krakatoa was heard 2200-3000 miles away.
duplicate post
I’m willing to concede that she heard the sound of car engines. But what makes her think that those cars were at the Grand Prix and not much closer?
Specifically I was wondering about the effect of an urban environment
on sound waves. I know being inside a city with concrete and tall buildings everywhere sound seems amplified.
By terrain do you mean the geological composition, topography, or both?
This is why I’m asking if it’s possible.
Yes, both, but I was thinking mainly the latter. If you live in an urban environment, though, sound from outside that environment is unlikely to be amplified. More than likely, the opposite would be the case.
Sound propagation is affected not only by distance, but atmospheric conditions. I remember hearing the high school’s pep band – the bass drum was quite prominent – playing for homecoming once, a distance as the seagull flies of at least 8 miles from my house one fall day. Never happened again.
I live thirteen miles from Sea World, in San Diego, and I can easily hear their fireworks events. Loud sounds (fireworks, race-car engines, etc.) carry remarkable distances.
There was a SD thread about whether people in London actually heard WWI artillery fire from the trenches…but I don’t recall which was the consensus fell.
I live 18 miles from Stennis Space Center in MS. Given the right atmospheric conditions most everyone around can clearly hear the engine tests. And the center is surrounded by several miles of buffer zone where no one can live or work because of the noise. Of course a large rocket engine is louder than a car race, but it does give an upper bound.
As for detectability, I believe labs all up the east coast as far as Boston could detect a space shuttle launch at infrasonic frequencies.
I’ve heard that they did, and I find it quite believable. I was woken up by the Buncefield explosion, and I was about 70 miles away at the time.
Loud, low frequency sounds. Think elephants and whales.
Here’s a handy site that gives the attenuation based on pressure, temp, humidity and frequency. It’s been too long since I studied this for me to remember the formula. Maybe one of our currently practicing, resident physicists will pop in with the answer.
On that site, I changed the frequency to 500 Hz to reflect what you are more likely to hear from a car engine and left everything else at the defaults. This gave a result of about 2.7 dB per km.
9 miles is about 14.5 km, so you’d end up with a dB loss of about 40 dB.
Figuring race cars are going to be somewhere up around 100 dB, you’d end up with around 60 dB where you are at, which is easily audible. This would assume a straight, unobstructed path between you and the race cars though. You could still lose another 30 dB or so from the sound bounding off of buildings and not having a clear straight path and such and still have it be audible, though it wouldn’t be easy to hear.
So yeah, sounds plausible to me, especially if your girlfriend happens to have fairly good hearing and you are in a quiet neighborhood at the time. It’s also a lot more likely if you are somewhere near the water so that there wouldn’t be much terrain between you and the sound. However, if she was standing next to the beltway in Towson at the time, it’s not so believable. Too much terrain to block the sound and too much ambient noise. If you are over by Martin State Airport where I used to live, it’s very believable. There’s not much ambient noise there and the terrain is nice and flat between there and downtown.