# Is sound effected by air resistance?

i was thinking... sound fades off over distance... does this happen because of air resistance? i know there are a ton of factors involved in this... but is this one of the more important ones in it?

if anyone has any ideas please post them :D

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#### chroot

Staff Emeritus
Gold Member
Nope. A source of spherical waves radiates its energy over all 4 pi steradians. A receiver captures only a fraction of that energy -- the ratio of the solid angle subtended over 4 pi. It has nothing to do with air resistance, which is a drag force experienced by objects moving through all fluids.

And finally, this isn't quantum mechanics, so I'm moving the thread to General Physics.

- Warren

heh, sorry, i got confused about where it went
so then what causes sound to not be heard over a certian distance?

#### chroot

Staff Emeritus
Gold Member
heh, sorry, i got confused about where it went
so then what causes sound to not be heard over a certian distance?
The fact that a receiver subtends a smaller solid angle when further from the source, and thus receives less energy.

- Warren

#### Ambitwistor

so then what causes sound to not be heard over a certian distance?
The fact that the waves spread out. The further away you are from the source, the smaller fraction of the total radiated power is intercepted by your ear.

(Imagine putting your ear one centimeter away from a sound source; it might intercept half of the sound. One kilometer away, and most of the sound is going to places other than where you are at.)

If you focused sound like a laser beam, then you would be able to hear it over a much longer distance, because all of it would be focused toward where your ear is (assuming you're standing in the beam). Of course, any beam will spread out eventually, just maybe slowly.

Originally posted by Ambitwistor
The fact that the waves spread out. The further away you are from the source, the smaller fraction of the total radiated power is intercepted by your ear.

(Imagine putting your ear one centimeter away from a sound source; it might intercept half of the sound. One kilometer away, and most of the sound is going to places other than where you are at.)

If you focused sound like a laser beam, then you would be able to hear it over a much longer distance, because all of it would be focused toward where your ear is (assuming you're standing in the beam). Of course, any beam will spread out eventually, just maybe slowly.
but wouldnt gravity pull it down and air slow it? i mean if it were like a beam?

#### Ambitwistor

but wouldnt gravity pull it down and air slow it? i mean if it were like a beam?

? You can form waves into a beam ... I'm not sure what your objection is. A beam is just sound waves pointed in a specific direction.

In fact, if you just talk, the sound coming out of your mouth is directional; it's just not a very tight beam.

I'm also not sure what the operation of radios in particular has to do with sound being a wave ...

#### Artman

Air resistance as such does not have an effect on sound, but wind direction seems to.

I live down the street from a football stadium. When a game is being played and the wind is coming toward me or there is no wind, I can hear some of the sound from the stadium, such as cheering crowds, the band, even words from the announcers. If the wind is blowing the other way, the sound becomes distorted or lost so I can't even hear the band.

#### chroot

Staff Emeritus
Gold Member
In fact, you could say that air resistance is required for sound to exist at all. Sound waves (pressure waves) take advantage of both an elastic and an inertial property of the medium through which they travel. The elastic property is the bulk modulus, and the inertial property is the density of the particles. In essence, the fact that the particles have mass and therefore have to have foces applied to make them move is part of the reason why sound waves exist at all.

- Warren

#### russ_watters

Mentor
I'm not sure air resistance is the best choice of words, but there is another effect: since in real life there is no such thing as a perfectly elastic collision and there are viscous effects on air molecules moving past each other, there will be some sound energy lost as heat.

#### birdus

I think what is meant by air resistance is impedence. All waves have 3 elements: Amplitude, frequency, and impedence. Impedence is due the resistance of the particles to change state/position which is dependant on mass and electrical attraction and stuff.
It is like you have a perfectly still water in a swimming pool and start a wave. The waves bounce off the sides for a long time but eventuallly fade away and the water becomes still again. However, if you had a really really sensitive thermometer, you would observe a very very slight increase in the waters temperature.

This should answer the question if you read throught it... It was enough for me...

This is an elementary subject. ok first let me correct a statement made earlier sound wave aren't how radios work. Radio work by using radio waves not sound waves although eventually the radio waves are converted into sound waves sound isn't the whole process. How the frequency of sound changes as an object gets farther and farther away was described by Christian Doppler an Austrian physicist in 1842...
____________________________________________________________

The speed of sound depends on the medium through which the sound waves travel. The properties of a medium that determine the speed of sound are density and compressibility. Density is the amount of material in a unit volume of a substance. Compressibility measures how easily a substance can be crushed into a smaller volume. The denser a medium is and the more compressible it is, the slower the speed of sound is.

In general, liquids and solids are denser than air. But they are also far less compressible. Therefore, sound travels faster through liquids and solids than it does through air. Compared with its speed through air, sound travels about 4 times faster through water and about 15 times faster through steel. The speed of sound through air is commonly measured at sea level at 59 °F. (15 °C). At that temperature, sound travels 1,116 feet (340 meters) per second. However, the speed of sound increases as the air temperature rises. For instance, sound travels 1,268 feet (386 meters) per second through air at 212 °F. (100 °C).

The speed of sound is much slower than the speed of light. In a vacuum, light travels 186,282 miles (299,792 kilometers) per second--almost a million times faster than sound. As a result, we see the flash of lightning during a storm before we hear the thunder. If you watch a carpenter hammering on a distant building, you will see the hammer strike before you hear the sound of the blow.

You may have noticed that the pitch of a train whistle seems higher as the train approaches and lower after the train passes and moves away. The sound waves produced by the whistle travel through the air at a constant speed, regardless of the speed of the train. But as the train approaches, each successive wave produced by the whistle travels a shorter distance to your ears. The waves arrive more frequently, and the pitch of the whistle appears higher. As the train moves away, each successive wave travels a longer distance to your ears. The waves arrive less frequently, producing a lower apparent pitch. This apparent change in pitch produced by moving objects is called the Doppler effect. To a listener on the train, the pitch of the whistle does not change.

Jet airplanes sometimes fly at supersonic speeds. A plane flying faster than the speed of sound creates shock waves, strong pressure disturbances that build up around the aircraft. People on the ground hear a loud noise, known as a sonic boom, when the shock waves from the plane sweep over them.

If you shout toward a large brick wall at least 30 feet (9 meters) away, you will hear an echo. The echo is produced when the sound waves are reflected from the wall to your ears. Generally, when sound waves in one medium strike a large object of another medium--such as the waves in air hitting the brick wall--some of the sound is reflected. The remainder is sent into the new medium. The speed of sound in the two mediums and the densities of the mediums help determine the amount of reflection. If sound travels at about the same speed in both materials and both have about the same density, little sound will be reflected. Instead, most of the sound will be transmitted into the new medium. If the speed differs greatly in the two mediums and their densities are greatly different, most of the sound will be reflected. Sound waves travel much more slowly through air than through brick, and brick is much denser than air. Thus when you shout at the brick wall, most of the sound is reflected.

When sound waves leave one medium and enter another in which the speed of sound differs, the direction of the waves is altered. This change in direction results from a change in the speed of the waves and is called refraction. If sound waves travel slower in the second medium, the waves will be refracted toward the normal. The normal is an imaginary line perpendicular to the boundary between the mediums. If sound travels faster in the second medium, the waves will be refracted away from the normal.

Sound waves can also be refracted if the speed of sound changes according to their position in a medium. The waves bend toward the region of slower speed. You may have noticed that sounds carry farther at night than during a sunny day. During the day, air near the ground is warmer than the air above. Sound waves in the air are bent away from the ground into the cooler air above, where their speed is slower. This bending of the waves results in weaker sound near the ground. At night, air near the ground becomes cooler than the air above. Sound waves are bent toward the ground, enabling sound near the ground to be heard over longer distances.
- World Book Encyclopedia (very descriptive)

I shouldn't be such a space thief

#### SIMON BEL

Sound waves are most affected by Air Resistance at 70 degrees F. At other temperatures the other variables that govern the speed of sound out weigh resistance.

#### russ_watters

Mentor
Heh - I read the first few posts without seeing the date and thought of another issue....then I saw I had already posted about it!

Anyway, Simon, the thread is almost five years old! There really isn't any good reason to have dug it up.

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