The blue whale is not the loudest animal on Earth, despite what you may have learned in school. While its calls are claimed to be louder than a jet engine at take-off, clocking in at an impressive 188 decibels (dB), the sperm whale is actually louder: its communicative clicks have been measured at 230 dB.

Looked at side by side, the numbers seem pretty conclusive, but decibels, which measure sound pressure, are not the only way to measure loudness.

In fact, loudness is subjective, dependent on how humans perceive it. This means there are many other factors to consider in any discussion of loudness.

"From a human-centred perspective, you have to consider the hearing threshold depends on frequency," says Magnus Wahlberg of the University of Southern Denmark.

To understand how humans hear, we first have to understand something about the nature of sound.

The blue whale will be perceived as less loud than the sperm whale

Hertz (Hz) is a measure of sound frequency. For instance, each note of a musical instrument comes to our ears at a different frequency: the higher the note, the higher the Hz.

Humans can hear a wide range of frequencies, from 20 Hz to 20,000 Hz (20 kHz). Many sounds lie within this range, from a blue whale's low-pitched songs to a rat's high-pitched distress calls.

But we do not hear all these sounds equally. Instead, our hearing threshold is different for sounds of different frequencies.

"Blue whales call at 20 Hz and sperm whales at around 10 kHz," says Wahlberg. For us to hear blue whale calls, they must be made at an intensity of 70 dB or more. But for sperm whale clicks, the human hearing threshold is around 15 dB.

Pistol shrimps, also known as snapping shrimps, are famous for their intense "screams"

"For loud signals, this relationship gets a bit more complicated, but in general it is true that the blue whale will be perceived as less loud than the sperm whale."

So the sperm whale is still ahead. Why then is it largely left off lists of the world's loudest animals?

According to Wahlberg, "the way we hear sound is not only related to its intensity, but also its duration." A sperm whale's click is almost over before it starts. It lasts only 100 microseconds, while blue whale calls last from 10 to 30 seconds.

Whales are not the only contenders for the loudest animals. Pistol shrimps, also known as snapping shrimps, are famous for their intense "screams".

The water boatman can produce 99 dB of sound by rubbing its penis across its abdomen

These crustaceans have a special claw that snaps shut with such speed that it creates a bubble with extremely low pressure. This means the bubble quickly bursts as it meets water outside it. When it does, it produces a shock wave measured at 200 dB.

The pistol shrimp's shot is extremely short: the bubble is formed and collapses in less than a millisecond. But it is an impressive din nonetheless for a creature so small.

However, when it comes to loudness relative to size, another tiny aquatic animal takes the title. The water boatman (Micronecta scholtzi) can produce 99 dB of sound by rubbing its penis across its abdomen.

This record raises another important point about loudness. Decibels in water are not equivalent to decibels in air.

"Water is denser than air, so sound travels through it differently – the speed of sound is different," says bio-acoustics expert James Windmill from the University of Strathclyde in the UK, who discovered the water boatman's remarkable call.

They literally make your body vibrate

"Roughly, to convert from dB in water to dB in air, you have to subtract [around] 61 dB from the reported sound level."

Underwater, M. scholtzi was producing a peak sound level of 99 dB. But that was measured on the air decibel sound level, so on the underwater decibel scale its peak is 160 dB. "People often don't take this into account when doing the comparison," says Windmill.

This means that the loudest animal calls would all need to be revised for human perception.

All the candidates so far live under water, so perhaps we should be looking for a land animal so we can get closer to the source of the sound. After all, distance also plays a role in our perception of loudness.

We just cannot hear the whole sound, as the lowest frequencies are inaudible to us

Plenty of mammals make long-distance calls. To make sure these signals carry to distant relatives, their roars, rumbles and howls have to be really loud.

Elephants make such loud rumbles "they literally make your body vibrate," says Joyce Poole, co-founder of ElephantVoices. The sound can be "deafening, something like 103 dB measured at five metres."

"People can hear almost all of the rumbles made by elephants, even the softer ones, as long as they are close enough to the elephant," adds Poole. "We just cannot hear the whole sound, as the lowest frequencies are inaudible to us."

That is impressive, but another mammal can outdo an elephant's rumble.

The greater bulldog bat (Noctilio leporinus) has been recorded crying at 140 dB as it hunts over lakes in Panama. Yet, while elephants' rumbles can be too low for us to hear, greater bulldog bats' are too high, reaching an ultrasonic 55 kHz.

A new contender for the loudest insect might one day be revealed

That is just as well, since a sound intensity of 120 dB can cause us injury. But there is a family of animals that can reach this peak, and their songs are ubiquitous in hot countries: cicadas.

They produce their calls using organs on either side of the body called timbals. These contain a series of ribs that are buckled by repetitive muscle contractions. The sound is amplified by their hollow abdomen.

They are the loudest insects in the world. In the 1990s the African cicada (Brevisana brevis) was crowned as the loudest on average, producing 106.7 dB at a distance of half a metre.

Since then, Max Moulds at the Australian Museum in Sydney has recorded local species Cyclochila australasiae, known as the greengrocer, producing 120 dB at close range.

"However, there are 3,500 described cicada species in the world, plus many more undescribed species still being documented," says fellow Australian cicada expert Lindsay Popple. Relatively few of them have had sound pressure measurements taken.

It depends on who is listening, how close they are, and what measurement is used

This means that a new contender for the loudest insect might one day be revealed.

"For instance, rather surprisingly, the largest Australian cicadas have yet to have to have their song amplitudes measured," says Popple. "They are quite deafening, so they would be obvious candidates." 

Likewise, Asia's Empress cicada (Pomponia imperatoria) – the world's biggest with a wingspan of up to 7.9 inches (20cm) – has never had its call recorded.

All in all, the loudest animal on Earth is harder to pin down than you might have thought. It depends on who is listening, how close they are, and what measurement is used.

But on decibel levels alone, the sperm whale wins.