How sound travels?

[steve-in-kville]

As I understand it, the more humid the air, the faster sound travels. But the drier the air, the further it travels.

As dedicated railfan, I've proven this wrong on a few occasions. Wind direction, as well as terrain are huge factors as well.

Any expert opinions on this?

[JimFoxvog]

I looked it up. Here's the abstact of a paper:

Measurements have been made of the effects of humidity on the velocity of sound in air at 20.00°C and normal atmospheric pressure in the audio frequency range. The velocity of sound was determined from measurements of the frequency of the decay of normal frequencies of a spherical chamber of very high Q. Contrary to the often‐made assumption that the addition of moisture increases the velocity of sound in air, these results show that as moisture is added to dry air the velocity of sound at first decreases. The velocity is minimum at a relative humidity (RH) of about 14%; then it rises. Above about 30% RH, the velocity increases linearly with increasing moisture content.

https://www.researchgate.net/publicatio ... und_in_Air

[MaxPC]

As I understand it, the more humid the air, the faster sound travels. But the drier the air, the further it travels.

As dedicated railfan, I've proven this wrong on a few occasions. Wind direction, as well as terrain are huge factors as well.

Any expert opinions on this?

I am no “expert” but a colleague of mine is a retired physics professor who says the vectors involved in any phenomena are complex. The only way to isolate the impact of any vector is through controlled conditions usually found in a lab. Even after isolating individual vectors, the combined outcome is almost never summative: you cannot just add up the numbers of each individual effect to get correct data of the combined total outcome. In layman’s terms this can be simply concluded as:
Your Mileage May Vary.

[ohio jones]

The only way to isolate the impact of any vector is through controlled conditions usually found in a lab. Even after isolating individual vectors, the combined outcome is almost never summative: you cannot just add up the numbers of each individual effect to get correct data of the combined total outcome.

For some reason I'm picturing lab workers Roger and Clarence questioning their buddy Vic about the design of the experiment.

[MaxPC]

The only way to isolate the impact of any vector is through controlled conditions usually found in a lab. Even after isolating individual vectors, the combined outcome is almost never summative: you cannot just add up the numbers of each individual effect to get correct data of the combined total outcome.

For some reason I'm picturing lab workers Roger and Clarence questioning their buddy Vic about the design of the experiment.

[ohio jones]

The only way to isolate the impact of any vector is through controlled conditions usually found in a lab. Even after isolating individual vectors, the combined outcome is almost never summative: you cannot just add up the numbers of each individual effect to get correct data of the combined total outcome.

For some reason I'm picturing lab workers Roger and Clarence questioning their buddy Vic about the design of the experiment.

Sometimes I engage in a bit of "airplane humor" that goes over people's heads. In this case it's Airplane! humor, from the silly 1980 movie.

Tower voice: L.A. departure frequency, 123 point 9'er.
Captain Oveur: Roger!
Roger Murdock: Huh?
Victor Basta: Request vector, over.
Captain Oveur: What?
Tower voice: Flight 2-0-9'er cleared for vector 324.
Roger Murdock: We have clearance, Clarence.
Captain Oveur: Roger, Roger. What's our vector, Victor?
Tower voice: Tower's radio clearance, over!
Captain Oveur: That's Clarence Oveur. Over.

[Soloist]

Everything is predictable by math. We just don't have the capability to calculate it yet with all the variables.

Everybody knows the speed of sound is directly correlated to the user's length of ear.

[RZehr]

My observation is that the speed of sound is determined directly by the amount of interest in the listener. The speed of sound slows incredibly when the situation is a parent telling a child to clean up, for example.

[Ken]

As I understand it, the more humid the air, the faster sound travels. But the drier the air, the further it travels.

As dedicated railfan, I've proven this wrong on a few occasions. Wind direction, as well as terrain are huge factors as well.

Any expert opinions on this?

I teach HS physics.

You are correct. It has to do with density. humid air is less dense than dry air because more of the air molecules are tied up in the water droplets which are suspended in the air but don't act as a part of it when it comes to sound wave propagation. Also H2O water molecules are less dense than O2 or N2 molecules that make up dry air. Colder air is also less dense so the same phenomenon applies. As is higher altitude air. But we are talking about small percentages of differences that you would not detect by ear but could only measure with instruments. And, of course there are complex wave interactions such as reflection, refraction, and diffraction as sound passes through different layers of the atmosphere and interacts with the terrain. And the ambient noise level is also a huge factor. It's like tossing a pebble into a glassy smooth pond vs a wind-whipped turbulent lake. In the first instance the ripples may cross the entire pond in an observable manner while in the second they will be absorbed into the existing chaos of other waves and ripples coming in all directions and be lost to detection. So something you could measure in controlled laboratory experiments doesn't necessarily really apply in real life.

[steve-in-kville]

There's times when the air is cold that I can hear the train horns miles out, almost to the point I can guess check crossing they are at. Other times, I can hear a low hum of the locomotives, but cannot hear the horns. My imagination?