How they made the spooky audio immersive version of Doctor Who: Knock Knock

The latest episode of the sci-fi series features a special “binaural” sound mix that puts you in the middle of the terrifying action – and here’s how the BBC pulled it off

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This week’s Doctor Who introduces a special sound technique to the sci-fi series, with spooky episode Knock Knock (penned by Doctor Foster’s Mike Bartlett) releasing a special “binaural” version on BBC iPlayer after its airing that creates the illusion that the noises of the story are actually happening all around the viewer (assuming they’re wearing headphones – the effect doesn’t work without them)

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Also known as 3D audio, this essentially consists of a sound mix that fools the ears into thinking noises can be heard from behind, beside or above the listener, with the end result a surprisingly creepy variant of an episode filled with mysterious creaks and scuttling creatures. 

RadioTimes.com caught up with the BBC Research & Development team behind the immersive version of the episode, with the sound specialists filling us in on exactly how they included the effect (which is around 50-60 years old in different forms).

An example of the binaural sound in this week’s Doctor Who – wear headphones for the full effect

“If you have a certain sound somewhere in the room, then obviously it’s picked up by two ears, but it’s slightly different when it comes to these two ears,” Lead Technologist Frank Melchior said.

“The sound maybe arrives earlier in one ear than it does in the other ear. And this difference is between the two signals.”

Accordingly, Melchior then explained, the brain then decodes where in the immediate area the signals are coming from. The job of binaural sound engineers is basically to simulate this process in an audio recording using new software developed by BBC R&D, which “artificially mimic[s] the subtle effect the shape of the viewer’s head and ears have on what they hear,” according to a release.

“We simulate this by digital signal processing, and then we get the impression that something’s coming from behind or the side,” Melchior continued.

The process of recording on set, however, is largely the same as usual, with the majority of work taking place in post production.

“What we do is we take ordinary mono-recordings, and we add the binaural algorithm in the computer, to give the illusion that it’s coming from different directions,” BBC Wales’ Audio Supervisor Cathy Robinson told us.

“We take each individual layer in the TV post-production mix, and binauralise those elements separately. We’ve got total control, and we can fabricate it to sound really palatable and to work with the pictures as well.”

Going into a little more detail, Melchior added: “So there are three elements.

“One element is you have a certain difference in timing, so a delay. Then you have a certain difference in level, so gain, essentially. And then you have a very complex equaliser on top of this, which models the outer ear and all the things that happen to the ear canal and so on. 

“So these are the three signal processing elements the system has – and usually it does them by measuring these kind of characteristics, and then having a convolution process which has all those three in there, and processes on the mono signal.

“So you have the mono signal, the convolution for all these kind of characteristics, and then we have to two signals for the left and right ear.

“And now I’ve probably lost you – but that’s the answer!”

So there you have it – the binaural effect is all down to a complex series of computer programmes and algorithms that actually modelled people’s ears to create the effect of scary monsters scuttling all around you. Not exactly the simplest thing in the world, but we’d say it was well worth the effort

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Doctor Who continues on BBC1 on Saturdays at 7.20pm