How can you be sure a horse is a horse? Usually, it is obvious, but no more so than on screen, when you often hear it neigh. And it’s driving at least one equestrian-minded ITV viewer mad.

IMDb-user sarahjgodley “thoroughly enjoyed” the recent adaptation of Vanity Fair, according to her online review that went viral last week: “EXCEPT for the usual flaw in historical dramas … Every single time a horse appears on screen, you hear a neigh, even though it’s apparent that the horse in question is not neighing – it’s like hearing the sound of a dog barking although the dog shown has its mouth shut.”

It seemed always to be “the same bloody horse, too”, sarahjgodley writes, with mounting heat: “The only time a horse makes a sound like that is when it’s a stallion which has the scent of an in-season mare.” Not only does this disregard the emotional state of the animal – whether it is feeling pensive or full of hubris, melancholic, amused or just numb – horses seldom neigh at all. “For them, it’s long-distance communication they don’t often use.”

“Somewhere in the mists of time”, she thinks, a sound engineer recorded the cry of one randy stud and used it to voice every horse on screen.

Sound designer Roland Heap, director of the London-based post-production company Sound Disposition, says that may not be far from the case. “There are only a few thousand sound-effect libraries, and however many million hours of content being created each year – it’s inevitable there’s some repetition.”

But a sound designer’s aim is not so much to reflect reality as to evoke an emotional response. In this case, Heap explains, that may mean where there’s a horse, there’s a neigh. “Cats meow all the time when they’re on screen. Dogs bark when you don’t see them barking,” he says. “It’s the nature of film sound that quite often we are a little bit ‘show and tell’. You look at the screen and go: ‘What would the audience expect to hear?’”

The broader soundscape is another factor. “Often a director will want to make the sound seem busier, and the first thing we’ll do is look at a scene: there is a horse on screen, therefore we can justify the sound of a horse.” Then, Heap says: “You want to go to a horse that sounds distinctively like a horse” – or rather, what people think of as sounding distinctively like a horse, regardless of the reality.

“Directors may ask: ‘Why isn’t that horse making an exciting noise? Why can’t I hear that horse?’,” Heap says. “It’s more about capturing the essence of what’s going on. The worlds we create in sound are far more interesting and clear than the reality, which is generally a bit of a mush.”

He gives the example of a sword being drawn: although they were historically kept in a leather scabbard lest they become blunted, viewers expect to hear metal (running a spatula along a pole can achieve the effect). “It’s all storytelling, in sound as much as anything else – everything in film sound is a lie.”

Cars seen on screen rarely correspond to the engines heard; gentle caresses land like floor-tom thuds. Animals are always making “the most exciting sound that animal can possibly make” – for birds of prey and “Hollywood frogs”, it is often the same regardless of species. “The red-tailed hawk cry is an absolute classic,” says Heap. “I’ve used that countless times. It’s lazy, but it does provoke an emotional response – and it’s only a cliche when people start noticing.”

Some sounds can even be knowingly subverted, such as with the so-called “Wilhelm scream”, which has featured in at least 380 films since 1951. “That started as a stock library sound but got a life of its own,” says Ian Waggott, of Universal Sound in Amersham, Buckinghamshire, who has worked on various blockbusters, including Quantum of Solace and Casino Royale. Sound designer Ben Burtt embraced it in his work for George Lucas and Steven Spielberg, says Waggott: “I think it started almost as an in-joke, using the same scream in every film he did.”

More often, though, budget constraints and deadlines are what drive the over-use of one particular telephone ring, big-cat roar or audience applause – but the vast majority of audiences never notice. “It’s only one or two people who say: ‘I’ve heard that red-tailed hawk’,” says Heap. “Same with canned laughter – most people are not going to be consciously saying, ‘I’ve heard that audience member before’, they’re going to be enjoying the comedy.”

The omnipotent randy stallion, he agrees, is likely to bother only those viewers who are “very horse-aware”. But sarahjgodley, in concluding her review, put the number of “people who have regular day-to-day dealings with horses in the UK alone” at well over a million – “and we’ve all noticed, so please, STOP IT!”