Density & the Speed of Sound

Imagine you have to get across a stream by jumping from rock to rock to get to the other side. If the width of the streams below is equal,

In which situation would it be easier/quicker for you to get across?

Why?

How the medium affects the speed of wave

Recall that a medium can be a solid, liquid, or a gas.
In which medium, to you think a mechanical wave travels fastest?

​

​A wave travels more quickly through denser materials, so it travels faster through solids, than it would through a liquid or gas.


**The more tightly packed the particles, the greater the density.

Quick review on
States of Matter

​Particle arrangement and motion

Solid

In a solid, particles are packed tightly together so they don't move much, but are dense. The electrons of each particle are constantly in motion, giving the atoms a small vibration, but they are fixed in their position.

​

Solids have a definite shape, as well as mass and volume, and do not conform to the shape of the container in which they are placed. 

Liquid

In a liquid, the particles are more loosely packed than in a solid, but closely enough that it is not easily compressible.

​

The particles are able to flow around each other, giving the liquid an indefinite shape. Therefore, the liquid will conform to the shape of its container.

Gas

In a gas, the particles have a great deal of space between them, so they move easily, and have high kinetic energy (energy of movement).

Multiple Choice

In which medium does sound (mechanical wave) travel slowest?

• solid

• liquid

• gas

Explanation

Sound waves travels slowest in a gas.

Rationale: In a gas, the particles are spread out far apart so it doesn't have as many particles to support it as it does in a solid (tightly packed particles) or a liquid (looser particles).

Multiple Choice

The table to the left shows the speed of sound in 6 materials. Which substance has the greatest density?

• air

• aluminum

• steel

• water

Steel has the greatest density of the 6.

​Rationale:

The chart shows the speed that sound travels in the 6 substances.

If I know that sound travels fastest in denser materials, then I know that the material with the fastest speed must have the greatest density.

Multiple Choice

According to the chart, how does the speed of sound in water compare to the speed of sound in air?

• The speed of sound is slower in water because the molecules in water are closer to each other and more dense.

• The speed of sound is slower in water because the molecules in water are closer to each other and less dense.

• The speed of sound is faster in water because the molecules in water are closer to each other and less dense.

• The speed of sound is faster in water because because the molecules in water are closer to each other and more dense.

​According to the chart, the speed of sound in water (1498m/s) is faster than the speed of sound in air (343m/s), so you can rule out any choices that say the speed of sound in water is slower than in air. **Rule out the 1st 2 in this example.

Rule out!
**explanation above

Rule out!
**explanation above

Speed of sound faster- true

False part- water molecules are more dense- not less.

Closer particles means more dense.

Faster in water.- Y
molecules closer- Y
more dense- Y

Correct

To Summarize!

Sound Waves (Mechanical Waves)

Sound waves are mechanical waves, meaning they need a medium to travel. The more tightly packed the particles of the medium are (more dense), the faster the wave can travel.

**If sound is traveling though the air (a gas), it will slow down when it hits water (a liquid). This causes the sound wave to refract. (bend)