Sound travels about meters per second in seawater. Sound travels much more slowly in air, at about meters per second, only 3 soccer fields a second. Unfortunately, the answer is really not quite that simple. The speed of sound in seawater is not a constant value. It varies by a small amount a few percent from place to place, season to season, morning to evening, and with water depth.
Although the variations in the speed of sound are not large, they have important effects on how sound travels in the ocean. What makes the sound speed change? It is affected by the oceanographic variables of temperature , salinity , and pressure.
We can look at the effect of each of these variables on the sound speed by focusing on one spot in the ocean. When oceanographers look at the change of an oceanographic variable with water depth, they call it a profile. Here we will examine the temperature profile, the salinity profile, and the pressure profile. Similar to the profile of your face that gives a side view of your face, an oceanographic profile gives you a side view of the ocean at that location from top to bottom.
It looks at how that characteristic of the ocean changes as you go from the sea surface straight down to the seafloor. The spot we are going to explore is in the middle of the deep ocean.
Here are basic profiles for a site in the deep, open ocean roughly half-way between the equator and the North or South pole. In these profiles, temperature decreases as the water gets deeper while salinity and pressure increase with water depth.
In general, temperature usually decreases with depth, salinity can either increase or decrease with depth, and pressure always increases with depth. Depth profiles from the open ocean of temperature, salinity and density. Copyright University of Rhode Island. On the other hand, salinity changes by only a small amount, from 34 to 35 Practical Salinity Units PSU , approximately 34 to 35 parts per thousand ppt.
Finally, pressure increases by a large amount, from 0 at the surface to atmospheres atm at the bottom. The speed of sound in water increases with increasing water temperature, increasing salinity and increasing pressure depth. The approximate change in the speed of sound with a change in each property is:.
Profile of speed of sound in water. Note the sound speed minimum at meters. The decrease in sound speed near the surface is due to decreasing temperature. The sound speed at the surface is fast because the temperature is high from the sun warming the upper layers of the ocean. As the depth increases, the temperature gets colder and colder until it reaches a nearly constant value. Since the temperature is now constant, the pressure of the water has the largest effect on sound speed.
Related: In photos: Large numbers that define the universe. It's not possible to test this theoretical top speed in the real world, because the math predicts that sound moves at its top speed in the lowest-mass atoms. The lowest-mass atom is hydrogen, but hydrogen isn't solid —— unless it's under super-duper pressure that's a million times stronger than that of Earth's atmosphere. That might happen at the core of a gas giant like Jupiter, but it doesn't happen anywhere nearby where scientific testing is possible.
So instead, Trachenko and his colleagues turned to quantum mechanics and math to calculate what would happen to sound zipping through a solid atom of hydrogen.
Imagine a whale is swimming through the ocean and calls out to its pod. The whale produces sound waves that move like ripples in the water. Once the sound waves reach the bottom of what is known as the thermocline layer, the speed of sound reaches its minimum. Another phenomenon related to the perception of time delays between two events is an echo. A person can often perceive a time delay between the production of a sound and the arrival of a reflection of that sound off a distant barrier.
If you have ever made a holler within a canyon, perhaps you have heard an echo of your holler off a distant canyon wall. The time delay between the holler and the echo corresponds to the time for the holler to travel the round-trip distance to the canyon wall and back.
A measurement of this time would allow a person to estimate the one-way distance to the canyon wall. For instance if an echo is heard 1. The canyon wall is meters away.
You might have noticed that the time of 0. Since the time delay corresponds to the time for the holler to travel the round-trip distance to the canyon wall and back, the one-way distance to the canyon wall corresponds to one-half the time delay. While an echo is of relatively minimal importance to humans, echolocation is an essential trick of the trade for bats.
Being a nocturnal creature, bats must use sound waves to navigate and hunt. They produce short bursts of ultrasonic sound waves that reflect off objects in their surroundings and return. Their detection of the time delay between the sending and receiving of the pulses allows a bat to approximate the distance to surrounding objects.
Some bats, known as Doppler bats, are capable of detecting the speed and direction of any moving objects by monitoring the changes in frequency of the reflected pulses. These bats are utilizing the physics of the Doppler effect discussed in an earlier unit and also to be discussed later in Lesson 3. This method of echolocation enables a bat to navigate and to hunt. Like any wave, a sound wave has a speed that is mathematically related to the frequency and the wavelength of the wave.
As discussed in a previous unit , the mathematical relationship between speed, frequency and wavelength is given by the following equation.
The above equation is useful for solving mathematical problems related to the speed, frequency and wavelength relationship. However, one important misconception could be conveyed by the equation. Even though wave speed is calculated using the frequency and the wavelength, the wave speed is not dependent upon these quantities. An alteration in wavelength does not affect i. Rather, an alteration in wavelength affects the frequency in an inverse manner.
A doubling of the wavelength results in a halving of the frequency; yet the wave speed is not changed. The speed of a sound wave depends on the properties of the medium through which it moves and the only way to change the speed is to change the properties of the medium. An automatic focus camera is able to focus on objects by use of an ultrasonic sound wave.
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