When Jurassic Park was released in 1993, moviegoers marveled at how the dinosaurs looked, but many of them never thought to marvel at what the dinosaurs sounded like. The mournful song of the Brachiosaurus, the barks and snarls of the Velociraptors, and the thunderous roar of the T. rex: somehow they all just felt inevitable.

“Of course, we don’t really know what a dinosaur sounds like,” says sound designer Al Nelson, who developed a range of new dinosaur sounds for the new film Jurassic World. “But after Jurassic Park, it felt like we did. When you see the T. rex open its mouth and do that bellow, you are convinced.”

Reptiles not being especially renowned for their vocal expressiveness, Nelson is the first to admit that dinosaurs probably didn’t sound like they do in any of the Jurassic Park movies. But he and the team at Lucasfilm’s Skywalker Sound weren’t striving for scientific accuracy, and weren’t on the first film, either. “To be honest, that might have been very interesting, but much of what we’re trying to do is to create something that is believable but also has emotional context and personality. In our case, authenticity comes down to whether you believe that the creature you see sounds like it does.”

The key to believable dinosaur sounds, it turns out, is rooted in the actual sounds of the animal kingdom. The sounds of the Brachiosaurus, Raptors, T. rex, and the rest are in fact the meticulously mixed, matched-and-mashed-up sounds of creatures much closer at hand (and much less extinct). It’s an old Hollywood trick, used on everything from King Kong to Star Wars, but it’s everywhere in Jurassic Park, from the braying donkey that lent its voice to the Brachiosaurus to the slowed-down baby elephant’s trumpet that became the T. rex’s roar.

“The trick is combining unlikely candidates and creating something new without really manipulating it a whole lot,” says Nelson. “You might place a plaintive sound of one animal next to the aggressive sound of another, creating a whole new language and a new creature.”

This painstaking slice-and-dice approach is particularly important when it comes to the cunning and communicative Velociraptors, who are chattier than ever in Jurassic World. In the first film, their extensive vocabulary is composed of the diverse vocalizations of African cranes, hissing geese, dogs, dolphins, horses, and mating tortoises. These higher sounds—which give the creature its personality—are often paired with deeper, lower sounds, like those of a walrus or tiger, to give the overall sound size and weight.

Though Nelson was able to draw on the library of sounds that had already been compiled by Jurassic Park sound designer Gary Rydstrom—and record elements of the same animal sounds once again—the expanded role they play in Jurassic World required him to go in search of brand new animals and sounds to be integrated into the language.

Some of the more expressive Velociraptor sounds were sourced from remarkably vocal species, including macaque monkeys, baby orangutans, and penguins. Even from Velociraptor to Velociraptor, efforts were made to distinguish the different vocal personalities of the individual animals: a bit of baboon in this one, a little otter in that one.

Obtaining usable sounds from wild animals requires careful preparation, Nelson says. He and his team embarked on its sound safari to zoos, farms, animal sanctuaries, Disney’s Animal Kingdom, and SeaWorld only after extensive research and communication with experts.

“You can’t just show up with a microphone and say, ‘O.K., lion. Roar.’ You have to do the research and understand—why does the lion roar? When does the lion roar? And also, does this lion love or hate this particular caretaker? Or this particular veterinarian? Research is vital so you get the most interesting things.”