Why are Sounds Louder at Night? Does Light Affect Sound?
Most recent answer: 06/30/2013
- Courtney (age 18)
Hartselle Al. US.
Light does not really affect sound. Sound is made of vibrations (aka rapid pressure fluctuations) in air, water, or solid material. Light is made of vibrations in the electric and magnetic fields. Now, the timescale (for example, the oscillation speed or the wave speed) for light is much faster than that for sound, so sound doesn't even notice when light is around.
A similar example of a "timescale argument" can be found here: http://van.physics.illinois.edu/qa/listing.php?id=21913.
Even if light weren't so fast, it's hard to see exactly how light could affect sound. The electric and magnetic fields associated with typical light beams are much smaller than those from a capacitor or a magnet, so if you wanted them to somehow interact with the vibration of particles in a sound wave, you'd probably need a huge laser pointed at the material through which sound was passing.
I should point out, though, that sound can affect light, even if it doesn't happen the other way around. For example, sound waves can cause mirrors to vibrate in sensitive optics experiments, thus messing up the data. That gives a literal meaning to the phrase "noisy signal"! In other cases, we can use this effect for engineering. For example, acousto-optic modulators (aka Bragg cells) use standing sound waves inside glass to induce index of refraction shifts. At points where the pressure from the sound wave is large, the index of refraction is increased slightly (since the medium is more dense), and where the pressure is small, the index decreases slightly. The overall effect is that the index of refraction in the glass now varies sinusoidally (like the sound wave), and light is caused to scatter (sort of like a diffraction grating).
Now, how do we explain why sounds often sound louder at night than during the day? I am pretty sure the reason for this effect is psychological. During the day, there are more other sounds that we focus on, and our minds are often distracted with whatever we happen to be doing at the moment, so we don't tend to notice the sound of, say, a train. But when we are lying in bed trying to sleep, and everything is quiet, the train seems especially disturbing.
However, I have heard qualitative arguments for why sounds could actually be louder at night or in the morning. The argument usually involves a temperature gradient. For example, if the air near the ground is cool, and the air above it is warmer, then the index of refraction decreases with height. This "gradient index" is well-known in optics, where it can be used to make a lens to bend light rays. Similarly, in the atmosphere it causes the sound waves to bend downwards, towards the earth. Since the observer is on the earth, towards which the sound rays are bending, the argument is that you will hear more of the sound than you would without the temperature gradient.
Now, I couldn't find any actual scientific papers on this topic, and most of the websites online were completely wrong (for example, the explanation here: is NOT what happens). In reality, every ray of sound in this situation bends downwards in a curved path, and rays closest to the horizontal bend the most. In real life, the temperature in the atmosphere typically falls about 5 degrees for each kilometer you go upwards. That's a rapid change; if you go skydiving, you will be a great deal colder in the air, and you can feel the temperature change in real time as you fall. With this temperature gradient, you can calculate that a ray which starts off horizontally will bend over 4 meters downwards as it travels forwards 1 kilometer. That's not huge, but it's measurable!
So, could this explain why sounds are louder at night? Honestly, I don't think so. For starters, the temperature typically decreases as you go higher, so sound waves on an average day would bend upwards. Even if the temperature gradient changes during certain times of day or seasons, I doubt the effect is noticeable. For each ray that bends downwards just enough for you to hear it, I think there would be approximately one ray bending downwards just enough so that you don't hear it.
Just to be sure, I ran a few simulations of how optical rays travel in a piece of glass with a linear gradient on the index of refraction. Since ray-tracing is basically identical for sound and for light, we can directly test the theory. If the ray density increases in a gradient index compared to a constant refractive index, then the theory will be more plausible. Here's what I found:
As you can see for yourself, the distribution of rays in both cases is almost identical. Since in my model I exaggerated the effect of the gradient much more than would be realistically found in the atmosphere, I think it's safe to say that this effect isn't noticeable in everyday life.
But maybe something else is going on... the best thing to do would be to take a decent microphone and record the sound of the train during the day and during the night. Try to get a similar sized train going a similar speed, and minimize background noises. Then, compare the amplitude of the two recordings, and let us know what you find! I suspect they won't be significantly different, but I'd love to see experimental data!
(published on 06/30/2013)
Follow-Up #1: Why can you hear farther at night?
The basic physics of sound propagation are correct in that link you sent. You're also correct that the colder night air is denser, but that's not what determines the direction in which the sound waves bend. As that site says, it's the difference in sound speed that determines the bending.
Of course you're also right that it's easier to notice sounds when the background is quiet and has been quiet for a while.
(published on 01/08/2015)
Follow-Up #2: mass of light
- Jacquelyn (age 53)
Gilbert, AZ, USA
Yes, light has some inertial mass. On a bright day, the inertial mass of light in the atmosphere is about 10-20 times smaller (less than one biliionth of one billionth) than the mass of the air. It makes no difference whatsoever.
(published on 10/02/2017)