By Luke Harper

There’s a particular kind of confidence in a show that doesn’t feel the need to impress you, and in many ways, that confidence mirrors the characters in Apple TV+’s Slow Horses, which has been streaming its fifth season. The series sounds exactly like it looks: grubby, pragmatic, deeply British and quietly meticulous. Guns sound like guns. Offices sound like offices. London sounds like London — not the glossy version, but the slightly wet, traffic-choked one. And yet, somehow, it’s also cinematic as hell.

For those who haven’t seen the show yet — and we absolutely suggest you do — it follows a dysfunctional group of disgraced MI5 agents who end up at the spy agency’s last refuge: Slough House. The office building reflects the team itself — tired, falling apart and in a constant state of remodeling. Its leader, Jackson Lamb (Gary Oldman), is, in a sense, the living embodiment of Slough House, but just a bit gassier.

L-R: Martin Jensen, Joe Beal and Ben Tisdall

To talk about how that balancing act actually happens, I sat down with the core sound team at Boom Post in Soho. Present were supervising sound editor Joe Beal and re-recording mixers Martin Jensen and Ben Tisdall.

A Long Game Played Under One Roof
Boom Post has been around since 1996, and over the years, it has expanded from a single floor to four. Foley, editorial, sound design, mixing and post production are packed tight into a slightly dysfunctional family in a lovely Soho building.
Jensen joined in 2003, straight out of the UK’s National Film and Television School. And that continuity matters.

“We’ve had the same core team from the beginning,” Jensen explains. “Having that stability lets us bring people in season by season, develop them, and still maintain consistency in the output.”

That consistency is profound. Slow Horses doesn’t drift tonally from season to season, even as the scope grows. It sounds like one long, confident sentence.

Two Worlds, One Philosophy
From the very first season, director James Hawes set the tone: Lean into reality but push it just enough. “The show lives in two worlds,” Jensen says. “The Park, which is sleek, polished and powerful, and Slough House, which is mundane, creaky and, frankly, a bit crap. The sound needed to contrast those spaces very clearly.”

That contrast drives everything. Slough House isn’t quiet. It’s annoyingly alive. Chairs squeak louder than they should. Doors slam harder than expected. Exterior London atmospheres hang on a beat too long. The details are amplified, not smoothed. They remind everyone, both inhabitants and viewers, that life can’t be perfect.

In that exaggeration lies comedy. “It’s not about subverting expectations by going quiet,” Jensen says. “It’s about amplifying the mundane. That’s where the surprise lives.”

One of the show’s great tricks is that nothing ever sounds “designed,” even when it absolutely is. Gunfire is a good example. Some weapons are meticulously accurate, especially when the source recording offers something distinctive.

Others take a little creative license because, as Jensen puts it, “some guns are just boring.”

The goal isn’t forensic realism. It’s felt realism. Perception is reality when curated thusly. “It still lives in the real world,” he says. “You’re never pulled out of it thinking, ‘That’s weird.’”

That restraint is deliberate and hard-won.

Dialogue is where the team draws one of its hardest lines. Slow Horses relies heavily on nuance: muttered asides, micro-pauses, Gary Oldman’s feral eating noises. Over-cleaning any of that would kill the joke. “There’s a real danger now,” Jensen says, “that everything starts to sound like it was recorded in a perfect booth.”

Modern AI noise-reduction tools are powerful and seductive. Used carelessly, they erase the very artifacts that make performances human. I have absolutely been guilty of this. Have you heard of dxRevive?

“Those tiny mouth noises, the breaths, the imperfections — that’s where the performance lives,” he explains. “We’re always asking: Are we solving a problem or creating one?”

As a result, ADR is used sparingly. Production dialogue is protected aggressively. The mix is where final judgment happens, not in the cutting room.

Part of the show’s tonal stability comes from something unglamorous but crucial: Everything gets mixed in the same rooms on the same systems by the same people. Boom’s Atmos stages are matched. The templates don’t change. The reverbs are familiar and unapologetically old school.

“We still love the TC Electronic stuff,” Jensen admits. “It just sits. You don’t hear it as reverb.”

Not broken. Not fixed. That’s not to say the sound team doesn’t explore the cutting edge, but it has to be pretty spectacular to make an appearance!

Teaching the Next Generation in Real Time
Ben Tisdall joined Slow Horses in Season 3 as an assistant mixer and has steadily moved up to mixing effects on the show, a rare opportunity in modern post schedules. “It’s very hard to get time on a final mix,” Jensen says. “This show has allowed us to bring people through properly.” That mentorshi p is part of the show’s DNA now — another reason it feels coherent rather than manufactured.If there’s one episode that nearly broke the sound department, it’s the Season 4 finale: a 20-plus-minute intercut between a crowded King’s Cross exterior and a tightly choreographed shootout inside Slough House.

Geography mattered. Perspective mattered. Fatigue mattered. “If you go full-throttle the whole time, the audience checks out,” Jensen says. “The challenge is knowing when not to push.” that’s the kind of “seems obvious but, wow, it is NOT” wisdom that time and experience teach!

The Comedy
By Season 6, the team has earned something rare: trust from directors, producers and the studio to just do the work. “We’re given a lot of leeway, a lot of trust. And that’s why it’s such a joy,” Jensen says.

You can hear that joy — and the restraint and the confidence of people who know exactly where the line is because they’ve been walking it together for years. The show is truly a joy to watch, as the sound department is a lovely encapsulation of the care put forth by every department. If you haven’t yet, have a watch. There are thrills and laughs, and while some are cheap, they feel curated… like you’re laughing along with great company.

For those who have seen the show, you know it’s often very funny, and that was a challenge for the team as well. Explosions are technical. Comedy is personal. “The most satisfying thing,” Beal says, “is finding the exact right stupid sound at exactly the right moment. That’s why we do this.”

A gunshot is physics. A fart is timing.

Luke Harper has been an audio engineer and instructor for 25 years. He lives in Minneapolis, where he owns an Atmos mix facility called DeCoded Audio.