Why Do Whale Songs Carry Hundreds of Kilometres?
― The Hidden Sound "Highway" in the Deep Sea
A human shout is doing well if it carries a few hundred metres. A whale's low call is different. It is said to reach other whales hundreds of kilometres away, and under the right conditions more than 1,000 km. It is the same kind of sound, so why does it travel so far?
In the middle of a vast ocean, a whale lets out a long, low call. The sound travels through the water and reaches another whale far away. The distance is said to be as much as from Tokyo to Osaka, or even more.
If you shout in a swimming pool, the sound becomes hard to hear after a few dozen metres. Sound moving through the same water, so why does only the whale's voice travel this far?
Two things work together
The sea is said to have a layer where sound moves especially slowly. This layer traps sound and carries it far, like a natural highway.
Low sounds (low frequencies) are absorbed by water less than high sounds. They are said to lose little energy and travel a long way.
Only when both are in place can a call reach hundreds of kilometres. Let's take them one at a time.
Reason 1: A sound "highway" hidden in the deep sea
How fast sound travels depends on the temperature and pressure of the water. Near the surface, the sun warms the water, and sound moves fast. Going deeper, the water gets steadily colder, and sound slows down.
Past a certain depth, though, the effect of water pressure takes over. Even if the temperature barely changes, sound speeds up again. In other words, if you trace the speed of sound down through the water, there is one dip, a depth where it is slowest.
When sound passes through a region where its speed changes gradually, its path bends little by little toward the slower side. Because of this, sound travelling near that "slowest depth" does not escape up or down. It swings back and forth around that depth and is carried almost straight over a long distance. This highway is called the sound channel. The same property, that a wave's path bends where its speed changes gradually, also explains why distant sounds are easier to hear at night and why mirages appear.
Reason 2: The lower the voice, the farther it goes
As sound moves through water, its energy is slowly turned into heat and it weakens. How fast this happens depends on pitch: high sounds are absorbed more strongly, and low sounds are absorbed less.
The calls of whales, especially blue whales and fin whales, are said to be so low that human ears can barely hear them (low frequency). Because they are so low, they are absorbed little by water and can travel a long distance.
The sound channel is also said to be used to detect earthquakes, volcanic activity or ship noise from far-off seas. Human instruments are borrowing the highway the whales use.
What you can check yourself
- Ask someone on the other side of a wall or door to hum in the lowest (deepest) voice they can
- Then ask them to make a sound for the same length in the highest (shrillest) voice they can
The low voice should have been easier to hear through the wall. Air and water differ in the details, but "the lower the sound, the farther it goes and the better it gets past obstacles" holds in both. A whale's low call is an extension of the same idea.
Summary
Whale calls carry far for two reasons: (1) the deep ocean has a sound channel that traps and carries sound, and (2) the calls themselves are low sounds, which are absorbed little. Both must be in place.
A whale is not simply shouting.
It is singing in the clearest range of the ocean, an enormous instrument.
The same property, that low sounds travel far, also works in air. The story of waves from a huge volcanic eruption that circled the Earth several times is told in How far does the sound of a big eruption travel?
For those who want more ― terms, formulas and links to textbooksFrom middle-school science to topics under active research, with the level of each part clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school basic physics
- High school+High-school full physics, or textbook advanced material and side columns
- UniversityUniversity-level content (ocean acoustics) not taught in high school
- ResearchNot yet settled even at university level; what researchers are studying now
Middle schoolTerms: words about whale calls
- Low frequency: a low sound with few vibrations per second (frequency).
- Refraction: the bending of sound or light as it passes through places where its speed differs.
- Sound channel: a layer in the ocean centred on the depth where sound is slowest, which traps sound and carries it far.
- Absorption: sound energy gradually turning into heat and the like as it travels, so the sound weakens.
Middle schoolHigh schoolChecking with a formula: how much farther does sound go with a channel?
When sound spreads, it either fans out in every direction (spherical) or is trapped in a single band inside the channel (cylindrical). The way its strength fades differs between the two. A calculation shows the difference clearly.
Sound strength (rough guide) = reference strength ÷ (distance to some power)
| Spreading freely in a sphere | Strength is said to fall in inverse proportion to distance squared |
| Trapped in a channel | Strength is said to fall in inverse proportion to distance to the first power (just the distance) |
Even at the same distance, the fading happens at a different "speed". The gap widens as the distance grows.
| Strength ratio, spreading freely in a sphere | 1 ÷ 1000 ÷ 1000 ＝ 0.000001 [times] |
| Strength ratio, trapped in a channel | 1 ÷ 1000 ＝ 0.001 [times] |
| How many times stronger the channel sound remains | 0.001 ÷ 0.000001 ＝ 1000 [times] |
At the same distance, sound trapped in a channel keeps 1000 times the strength of sound that spreads freely, according to the calculation.
If sound only spread freely, at 1000 times the distance its strength would drop to one millionth, far too faint to hear. Inside the channel, the same distance costs only one thousandth. "One millionth" versus "one thousandth": that difference is what carries a whale's call hundreds of kilometres.
High school+UniversityWhat decides the speed of sound?
The speed of sound in water is said to depend on three things: temperature, salinity and pressure. In the surface layer the temperature effect is large, so as the water cools with depth, the speed of sound falls. Past a certain depth (generally said to be around 600 to 1200 m), the temperature changes only slightly, and the pressure effect (the deeper the water, the faster sound moves through it) wins out, so the speed rises again. Near the depth where the temperature effect and the pressure effect trade places, a layer of minimum sound speed forms. This is the sound channel described in Reason 1.
Sound in the channel spreads cylindrically rather than spherically because it is trapped vertically and can only spread horizontally. This difference produces the gap between "first power of distance" and "second power of distance" seen in the calculation ②.
ResearchWhat is still not well understood
- The depth and properties of the sound channel are said to vary with sea area and season, and predicting them precisely remains a research problem in ocean acoustics. This is because the distribution of temperature and salinity changes greatly by place and time.
- It has been suggested that rising underwater noise from human activity, such as ship propellers, may be shrinking the effective distance whale calls reach, but the size of the effect is still hard to estimate accurately.
- What information a whale's low call carries beyond signalling a companion's position is said to be not yet well understood. Research into the meaning of the fine patterns in their songs continues.
Even whale calls that ring out in the sea every day are still being studied, both in how they travel and in what they mean. Being familiar is not the same as being understood.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of sound (frequency and pitch) | Low frequency, basics of refraction |
| High school | Basic physics: properties of waves | Calculating how strength fades |
| High school+ | Physics: how waves travel (advanced) | Spherical vs cylindrical spreading |
| University | Ocean acoustics, physical oceanography | Sound speed profile with depth, formation of the sound channel |
| Research | Ocean acoustics, behavioural ecology (unsolved) | Seasonal change in the channel, effects of ocean noise, meaning of calls |
| ― | Everyday observation | How low and high voices sound different through a wall |
- General descriptions in ocean acoustics textbooks of the sound speed profile with depth and the formation of the sound channel (SOFAR channel).
- Research reviews on marine bioacoustics (findings on how far the low-frequency calls of large whales travel).
- General descriptions in physics textbooks of how spherical and cylindrical waves fade with distance.
- Research reviews on the effects of human-made ocean noise (ship noise) on communication among marine animals.
* Figures such as the depth of the sound channel and how far sound travels vary with sea area and conditions. This article gives commonly used rough values.
* This article is a general-audience science explainer. Figures such as the speed of sound and travel distance vary by sea area and source, and are given as rough guides to help explain the mechanism.