In Simple Terms
In a field experiment off Florida, blacktip sharks changed direction when researchers played loud, pulsing sounds underwater—even when the sharks were far from the speaker. The results show that sharks can respond to sounds at a distance, but they do not tell us exactly how the animals sense them, how far away they can hear, or whether sound could reliably keep sharks away from people.
A Field Experiment in Florida Waters
A loudspeaker underwater might seem like a small detail in a shark’s world. But when a free-swimming animal changes course from tens of meters away, it raises an important question about how sharks sense their surroundings. A team from Florida Atlantic University filmed blacktip sharks off Palm Beach County and recorded some of them changing direction when particular sounds began. The farthest response the researchers observed was at 74.0 meters; other responses occurred at 66.1 and 61.8 meters. These figures describe what the team saw in one specific experiment, not the maximum distance at which sharks can hear.
Caroline Sullivan, Edmund Gerstein, and Stephen Kajiura of Florida Atlantic University conducted the study, which was published in Integrative Organismal Biology. The team chose blacktip sharks that gather in shallow water near Palm Beach in winter. Because many sharks can be found in an area that is relatively easy to monitor, the researchers could film them from above as they moved through their natural habitat.
The researchers anchored a boat in water 2 to 5 meters deep, then let a loudspeaker float behind it, 13 to 19 meters away. A drone flew 40 to 50 meters above the water, recording the sharks without requiring the team to approach them in the water. The float carrying the speaker was also fitted with a light that turned on when the sound began. That allowed the researchers to match the moment the speaker switched on with the shark’s movement in the video.
A shark’s mere presence near the equipment did not count as a response. The team recorded a response when a shark changed course immediately after the sound began. To estimate the distance between the animal and the speaker, the researchers used the boat’s length—6.73 meters—as a reference in the aerial footage. This method made it possible to estimate distances in real-world conditions rather than limiting observations to a tank or other enclosed setting.
Unwelcome Sounds, Not a Dinner Bell
Each test began with a high-frequency calibration tone at 10 kilohertz, followed by 5 seconds of silence and then one of three low, pulsing sounds. The calibration tone itself lasted 5 seconds. The sharks showed no noticeable change in response to the 10-kilohertz tone, which lies outside the known range of shark hearing. The low-frequency sounds, by contrast, were associated with changes in direction.
The team chose loud sounds, not faint ones intended to imitate a potential food source. The sound level was 140 decibels, referenced to a pressure of 1 micropascal at a distance of 1 meter from the source. That was about 80 decibels above the quietest background noise at the study site. The researchers’ stated aim was to provide a signal unpleasant enough to prompt a clear reaction, even if the sharks’ motivations differed from one moment to another.
That distinction matters when interpreting the results. A turn does not necessarily mean a shark heard a familiar natural sound or was attracted to it. The experiment tested the animals’ reactions to a sound deliberately chosen to be unpleasant. Sound level also matters: a quiet signal may have a very different effect from a sudden, loud burst.
What Do the Numbers Tell Us?
The researchers recorded sharks turning away from the sound source in 165 of 209 tests, or about 79 percent. They recorded no response to the high-frequency calibration tone in any of the 209 tests. Of the responses, 118—or 71.5 percent—occurred in the area the researchers classified as the far field.
There is a small calculation discrepancy worth noting. The study’s published abstract gives a figure of 71.6 percent alongside the number 209, but 209 is the total number of tests, not the number of responses. Calculating the share of far-field responses as 118 out of 165 total responses gives about 71.5 percent. This correction does not change the broad finding, but it clarifies which total should be used when interpreting the percentage.
The farthest response differed among the three sounds: 74.0 meters for one, 66.1 meters for another, and 61.8 meters for the third. So, a response was observed at a distance of at least 61.8 meters—about 62 meters. The sounds also differed in how much louder they had to be than the surrounding noise for a response to occur. The increase was about 16 decibels for the lowest-frequency sound and about 45 decibels for the highest-frequency one.
Why Do Researchers Distinguish Between Near and Far Sound Fields?
Sound does not travel underwater in quite the way people might imagine from a loudspeaker in a room. In underwater acoustics, researchers distinguish between an area close to the source and another farther away. In this study, the boundary between the two was set at three wavelengths. Wavelength depends on sound frequency: the lower the frequency, the longer the wavelength and the wider the area close to the source.
Sound travels through water at about 1,500 meters per second. At a frequency of 100 hertz, dividing the speed by the frequency gives a wavelength of 15 meters. Using the study’s three-wavelength rule, the near field therefore extends 45 meters from the source. The researchers estimated the near-field boundaries for the three sounds as about 45 meters for the lowest-frequency sound, 22.5 meters for the middle one, and 11.3 meters for the highest-frequency one.
This distinction matters because sharks are commonly thought to lack an air-filled swim bladder. In some other fish, that structure can help detect components of sound over longer distances. As a result, it has often been assumed that sharks mainly respond to sound close to its source. But the researchers recorded turns at distances beyond the calculated near-field boundary for two of the sounds. That challenges the assumption without, by itself, explaining which sensory mechanism the sharks used.
Sharks’ Responses to Sound Are Nothing New
Scientists have known for decades that sounds can affect shark behavior. In 1963, Donald Nelson and Samuel Gruber published a study suggesting that low, pulsing sounds could attract large sharks near Miami’s reefs. The most effective pulses were generally in the 20-to-60-hertz range, which can resemble the sounds of a struggling fish. Higher-frequency pulses and continuous low tones did not have the same effect.
Later research shows that a shark’s response can change depending on sound level and the animal’s circumstances. In a 1978 study in the Florida Straits and the Bahamas, silky sharks approached a sound source but quickly moved away when the level rose by about 20 decibels, especially when the increase was sudden. Their response weakened over repeated tests, and the effect was also smaller in young sharks when freshly minced fish was present. A study of lemon sharks the following year found that the sound had to rise by at least 18 decibels above the background level to prompt retreat, according to the account of previous research in the current study.
So the relationship between sharks and sound cannot be reduced to a rule that sound always attracts them or always drives them away. The nature and level of the pulses, as well as the shark’s condition and its past exposure, may all influence the result. The Florida experiment was designed to observe a strong reaction to an unpleasant sound—not to test a guaranteed way to keep sharks away from people.
What Does the Evidence Not Show?
The clearest finding is that blacktip sharks changed direction after low-frequency sounds began, and that some of these responses occurred at distances classified as far-field under the study’s acoustic definitions. The experiment does not prove that a shark could pinpoint the loudspeaker’s location. The researchers interpreted a turn away from the source as evidence that the animal sensed some direction, but overhead footage cannot reveal how accurately it located the source or how it worked out where the sound came from.
The study also did not measure the water-particle motion that accompanied the sound, although measuring it could have helped explain what signal the sharks detected. The researchers noted that making this measurement was not practical under field conditions. The possibility that the sharks relied on particle motion therefore remains a proposed explanation, not a mechanism directly confirmed by the measurements.
The farthest figure, 74.0 meters, also needs to be read cautiously. The drone camera did not capture an unlimited area: the longest dimension of its field of view ranged from 75 to 94 meters. So 74 meters was the farthest response visible in the footage, not the distance at which shark hearing stops. There may have been responses farther away beyond the camera’s view, or there may not have been; the experiment cannot settle that question.
The lowest-frequency sound provides a more complicated kind of evidence than the other two. It produced the farthest response, but its longer wavelength also gave it a wider near field. According to the study’s calculations, only 33.9 percent of responses to that sound occurred in the far field, compared with 87.2 percent and 96.6 percent for the other two sounds. Based on the study’s counts, these percentages correspond to about 20, 41, and 57 responses, respectively.
The researchers noted that the lowest-frequency sound included some energy below 100 hertz, which extended its calculated near field. Even so, they argued that responses associated with energy between 400 and 800 hertz could not be explained simply by that extension. This matters because it cautions against treating every response as equally strong evidence that the sharks sensed a sound from far away.
Could Sound Become a Barrier Against Sharks?
This experiment cannot answer that question. It involved one shark species, at one location on the Florida coast, under specific sound conditions. And an immediate turn during a test does not show whether the effect would last after repeated exposure or prevent sharks from approaching swimming areas. In the 1978 study, silky sharks’ responses weakened after repeated exposure, illustrating how habituation can change behavior.
A 2019 study published in Scientific Reports offers another reason for caution. Great white sharks showed no significant change in their response when researchers played orca sounds, and the researchers concluded that sound alone was not an effective deterrent. That does not mean every sound has no effect. But it does mean the reaction observed in the blacktip shark experiment is not enough to establish a practical form of protection.
Where Might Their Sensitivity Come From?
The Florida team suggests that a structure in the inner ear known as the macula neglecta may be part of the explanation. It is an area containing sensitive hair cells. Close relatives of the blacktip shark have been found to possess more of these cells than some other shark species, but researchers have not yet counted them in the species used in the experiment. Any connection between these cells and the observed responses therefore remains a hypothesis that needs to be tested directly.
Kajiura says the next step is to find out how the sensory system detects these distant signals and interprets them. Sullivan, who completed the work as part of her master’s degree, points out that studying sound at sea matters because sound waves in tanks bounce off the walls and reach an animal from multiple directions. Those reflections can make an enclosed experiment resemble a soundscape full of overlapping signals rather than a simple sound that can easily be isolated.
The Bottom Line
The experiment provides controlled field evidence that blacktip sharks responded to low, pulsing sounds at distances reaching 74.0 meters in the recordings. The researchers observed 165 responses in 209 tests, including 118 classified as far-field responses. But the study does not establish the maximum range of shark hearing, prove that the animals pinpointed the sound source, or reveal exactly how they detected the sound. Nor does it show that loudspeakers could protect swimmers. For now, the strongest conclusion is that some sharks respond to distant sound; how they do it, and whether that response can be relied on, remain open questions.