Why do we hear better during the night?

Question

At night, we hear weak and far sounds approximately clear, while during the day we cannot.

My high school physics teacher was saying that “this is because of interference of sound waves. During the day, there are a lot of sounds and they cancel each other due to interference. But, during the night, there are few sounds and they can reach to our ears without canceling each other”.

But, this doesn’t make sense because even in silent days (according to my personal experience), we don’t hear those sounds that night are heard clearly.

As I am not familiar with waves so much, I will appreciate if someone clear me by simple explanation.

Answer 1

My high school physics teacher was saying that “this is because of interference of sound waves. During the day, there are a lot of sounds and they cancel each other due to interference. But, during the night, there are few sounds and they can reach to our ears without canceling each other”.

You need a better high school physics teacher.

Temperatures tend to decrease with altitude above ground during daytime. This acts to curves sound upward. Thus in turn means you cannot hear the sound of a nearby train (a kilometer away or so) blowing it's crossing whistle. The sounds of that train are directed upwards into the atmosphere, where they dissipate.

At night, the atmospheric boundary layer tends to develop a marked temperature inversion, up to over a kilometer high. This acts to curve sound downward. This in turn means that at night you can hear the crossing whistle of a train that is from several kilometers from you. You can hear the train's progress along its track as it blows it's whistle at one crossing, and then another, and then yet again another. Even if the train was the only noisy object in the daytime, you could not hear that remote whistle in the day. You can only hear it at night.

The reason for this upward diversion of sound in the daytime versus the downward diversion at night is the strong dependency of the speed of sound in the atmosphere on temperature. The atmosphere acts like a lens that focuses sound energy upwards during the day, but keeps it at ground level during the night.

Answer 2

I would tend to agree that background noise is a factor, but rather than reducing, adding to the sound you are trying to make sense of. So part of that may be how your brain is able to filter the information from the background noise.

But at night the temperature is lower and according to this tutorial on sound propagation (which does cite reliable references), air has an energy absorption factor that is a function of temperature: $$\alpha = 869 f^2 \left\{1.84\cdot 10^{-11} \left(\frac{T}{T_0} \right)^{1/2} \!\!+ \left(\frac{T}{T_0} \right)^{-5/2}\left[\frac{0.01275 e^{-2239.1/T} }{F_{r,0}+f^2/F_{r,0}}+\frac{0.1068e^{-3352/T}}{F_{r,N}+f^2/F_{r,N}} \right] \right\} $$

and you can see here that a reduced temperature, $T$ reduces the absorption factor by the square root of $T$ in one component and by an exponent of -5/2 in another. So by reduction in the absorption of energy (by air molecules) in the path of the sound, more energy will reach your ear in the colder temperature.

Answer 3

If we suppose that the phenomenon you describe is related with wave interference. A wave is a kind of mechanical disturbance in the medium through which it is travelling. A sound wave consists of areas of relatively high and low energy, in the form of relatively high and low pressure. To understand how sound is produced, consider a speaker. The cone or diaphragm of a speaker vibrates inwardly and outwardly in response to an electrical signal. These vibrations are typically very small, only visible with larger speakers. However, they all impart energy to the air in the same way. When the cone moves outward, it pushes the air forward that originally occupied that space. This air becomes locally compressed, forming a region of relatively high pressure. When the cone moves inward again, it recoils from the space that it occupied and leaves behind a partial vacuum, a region of relatively low pressure. The frequency and amplitude of the vibrations change the characteristics of the wave that is formed, and hence the sound that we perceive. When we hear the sound, our ears are being bombarded with air molecules of rapidly varying pressures. The signal is sent to the brain where it is interpreted.

As the sound wave progresses through the air, its energy slowly dissipates. This is why sound is louder closer to the source and quieter further from the source.

Wave interference occurs when two or more waves disturb the same air molecules. If a relatively high energy part of one wave combines with a relatively low energy part of another, the result is a region of air with an average of the two. In the most extreme case, the resulting pressure is indistinguishable from that of the undisturbed air, and is therefore undetectable by the ear. This situation is known as total destructive interference. In practice, however, interference is almost always partial. Similarly, if two high- or two low-energy parts of a wave combine, they can be summative. This opposite process is known as constructive interference.

To visualise this process, you may want to look at a video or two on-line. Note that water waves illustrate the concept extremely well, but the mechanisms by which they function are very different to the process described above.

Both constructive and destructive interference occur frequently wherever there are multiple sounds. However, their effects are generally minor in the natural world. Some of the commentaries above probably provide a more accurate explanation of the phenomenon you describe.