In-Depth Answers to PADI Open Water Course Questions:

Question: Since it travels about four times faster in water than in air, you will have difficulty determining the origin of _______ underwater.

Possible Answers: (a) light (b) sound

The correct answer is: (b) sound

Learn Why Sound Travels Faster and Farther in Water Than in Air:

• Water Is More Dense Than Air

Density is a measurement of molecules in a given amount of space, such as a cubic foot.

Water has more molecules per a cubic foot than air does. Because water contains more molecules per cubic foot than air, water weighs more than air. The fact that water molecules are packed more closely than air molecules is important when considering sound transmission.

• Sound Is a Pressure Wave:

Sound is transmitted as a pressure wave. One molecule collides with a second molecule, setting the second molecule in motion. The second molecule collides with a third molecule, setting it in motion, and so forth. The closer the molecules are together, the shorter the distance a molecule must travel before colliding with another molecule and passing the sound wave on. The shorter the distance between molecules, the more quickly they are able to transmit a sound wave. As water molecules are closer together than air molecules, they transmit sound waves more quickly.

• Sound Travels About 4.3 Times Faster in Water Than in Air:

In air, sound waves travel at a speed of about 767 miles per hour (or 343 meters per second).

In fresh water, they travel at about 3315 miles per hour (or 1497 meters per second). Interestingly, because salt water is more dense than fresh water, sound travels slightly more quickly in salt water, at a rate of about 3355 miles per hour (or 1500 meters per second). Temperature also affects the speed of sound transmission; sound will travel more quickly through warm air or water. However, this is related to the greater energy of warm air and water molecules, not the density of the molecules.

Sound also travels about 4.3 times further in water than in air. This is why divers often hear boat engines long before they can see the boat, and why whales and other aquatic creatures can use sound waves to communicate over extremely long distances.

_{More In-Depth Answers to PADI Open Water Questionss:}

• What Is the Most Important Rule in Scuba Diving?
• What Is The Definition of a Squeeze in Scuba Diving?
• How Does a Wetsuit Keep You Warm Underwater?

The Increased Speed of Sound Transmission Makes It Difficult to Locate a Sound:

• How Do Humans Determine the Direction of Sound?

Above water, sound waves reach a person's ears at slightly different times. The time lapse between a sound wave striking the first and the second ear, as well as slight differences in intensity and shielding of the sound wave by the person's head allow him to determine a direction and distance for the origin of the sound.

As sound travels more quickly underwater, lag time between when the sound wave reaches each of a diver's ears is diminished. As humans are adapted to above-water hearing, this short lag time makes determining the direction of sound nearly impossible for a diver. Many aquatic creatures, however, are adapted to hearing underwater and can use sound waves to determine direction and location.

• Sound Waves Do Not Transmit Well Between Air and Water:

Another reason that the origin of a sound is difficult to determine underwater is that sound waves do not transmit well between air and water. In air, sound waves are transmitted from the surrounding air, through the ear drum, and to an air-filled chamber in the middle ear. Unless the sound is very, very loud, the sound waves can not be felt in other body spaces (which are fluid-filled) such as the head and chest.

However, underwater, the opposite occurs. Sound waves do not transfer well between the water and a diver's ear drum, but they do transfer well between the water and the diver's fluid-filled head. Underwater sound is absorbed through a diver's entire head, which of course makes it seem like the sound is coming from all directions at once.

The Take-Home Message About Sound Transmission Underwater:

Sound travels about 4.3 times faster in water than in air, and does not transmit well between the two. This makes it difficult to determine the origin of a sound underwater. Most divers will experience sounds underwater as coming from between their ears or from all directions at once.

More: What Is the Best Way to Keep Water From Entering Your Scuba Cylinder?