Why are sounds louder when you are near the source of the sound, and softer when further away? Longer waves?

Question

In simple terms, could someone explain this to me? Thanks! :) I think it has something to do with longer sound waves the further away you are - it takes longer for them to hit your ear, so the lose energy, so they are softer? So when you ring a bell and are 50 feet away the sound is softer?

Thanks!

Answer 1

It's not because of the wavelength, let's just get that out of the way right not. A change in the wavelength causes the pitch of a sound wave to change. This is what happens with the doplar effect.*

The reason they are not as loud is because of the amount of time they have had to travel, and the energy lost. As I'm sure you understand, sound waves are created molecules in the air (or water, or whatever) ramming into one another creating a wave pattern. Here's where a little geometry comes into play. Since sound waves move out in all directions, you can think of a sphere or a bubble emerging from the source of the sound. Let's say, a bell on the counter at the store. immediatly after you ring the bell, the distance the sound wave has traveled is very small, so it would be like a small bubble around the bell. since the bubble is small, it has a small surface area, that is, the sound does not have as much air to push against as it would a millisecond later, and another millisecond after that. As the bubble gets bigger, it has to push more and more air to keep moving, which takes more energy. By the time the sound wave has reached your ears on the other side of the store, the sound wave has very little energy left, and thus sounds a lot quieter than it would if you were standing right next to the bell when and listening to it when the sound wave had a lot of energy to it.

*The doplar effect is like when you are standing on the sidewalk and you hear a car coming, see it pass and then hear it leaving. As it approaches, the sound builds gradually and increases in pitch, and after it passes you, it drops quickly in pitch. This is because as the car is approaching you, each sound wave has a shorter distance to travel to your ear than the first one, and as a result they "smush" each other together, shortening the wavelength and increasing the pitch.

Answer 2

Think of sound as energy (in fact, it is). Very near your bell (let's say 1 meter), that energy has spread out over a small three-demensional area. As you get further away from the sound waves source, the same amount of energy has filled a much larger area, and so its intensity in any one pint of that area is less.

The general rule is, when comparing the amount of energy a sound has in to different point, the sound wave looses 75% of its energy every time you double the distance. Therefore, if your sound wave has x energy 10 meter away from its source, at 2o meters it will have 0.25x energy.

Answer 3

Sound energy spreads out with distance - it decreases like one over teh square of the distance from the source of the sound. If you are standing one meter away from a speaker and move two meeters away, the sound energy drops by a factor of 4 (2^2). If you move 10 meters away from the source, the sound energy drops by a factor of 100. and so on.

If the medium the sound travels through does not have frequency dependent properties, then all the frequencies (wavelengths) reach your ear in the same proportion that they were in at the source. Air is pretty much uniform so there isn't much frequency selective filtering going on. The decrease in energy with distance is the culprit.