The recent controversy over poor-quality sound on the hit BBC One show Happy Valley has once again focused senior TV executives’ minds on the problem of inaudible dialogue in drama.

Last month, BBC Controller of TV Channels Charlotte Moore pledged to tackle the “big issue” of sound. She added that it was “incredibly hard to get to the bottom of where things go wrong”. She explained she had introduced a new set of best-practice guidelines to help avoid future problems with inadequate audio.

Complaints over muffled dialogue in high-profile TV dramas show that this is a regular source of irritation for viewers. Two years ago, another BBC One drama, Jamaica Inn, generated more than 2,000 complaints about mumbled conversations; its adaptor, Emma Frost, admitted that it “sounded like listening through mud”.

In 2013, Director-General Tony Hall highlighted difficulties with hearing dialogue in drama as one of the issues he wanted to confront. “Actors muttering can be testing,” he said. “I don’t want to sound like a grumpy old man but I think muttering is something we could have a look at.”

And, as far back as 2009, the then-BBC One Controller Jay Hunt launched an “audibility project”, involving a 20,000-strong panel of viewers and listeners.

Why, then, do these difficulties persist? Are they endemic, or is it a problem literally built into flat-screen TVs – something highlighted by Which? magazine. “Modern TVs might have fantastic picture quality, but their sound is often disappointing, because new, slimline TVs have limited space for built-in speakers,” it said in a consumer report.

Moore’s explanation that, after Episode 1 of Happy Valley, “we took everyone back to the edit suite to really make sure, to work very hard to make it crisper, and change these [sound] levels” appears to have irritated some sound specialists.

Chris Ashworth, 55, a veteran sound engineer, who worked on the BBC’s acclaimed War and Peace and, most recently, Netflix’s The Crown, says that the belief that sound can be fixed by simply changing the levels has astounded technicians.

He says: “I welcome the debate. It is time to get away from the blame culture. A lot of my colleagues feel undervalued.”

Audibility is not just an issue for the BBC: it is industry wide and includes documentaries as well as drama.

Another sound engineer adds: “The levels are not a problem. The whole nation can hear an actor mumbling. If you bring up the levels, the actor is still mumbling. There is an entire culture of denial at the BBC. Written guidelines, a set of training videos… are pointless.”

Ashworth is a member of the Institute of Professional Sound, which has around 600 members working in television, film and theatre (see below). The IPS view is that Moore is – in this case – making a simple problem complicated.

Ashworth concedes, in fairness, that audibility is not just an issue for the BBC: it is industry wide and includes documentaries as well as drama.

Nevertheless, filming location drama often presents a unique set of challenges for sound engineers. There is the signal-to-noise ratio – the recorded dialogue against the background noise, including the sound of generators.

Take Ashworth’s experience of working on War and Peace. The battle scenes, where lightweight equipment and short booms were used alongside hand-held cameras, presented singular problems. So, too, did Jim Broadbent’s scenes as the emotionally repressed Prince Nikolai facing the loss of his son Prince Andrei.

“We were using sets inside a very echoey Lithuanian warehouse. We contained it. We used clip-on mics,” Ashworth explains. Broadbent’s scenes were hailed as a triumph.

His method of working is to learn the cues, but not the lines, because “I want to understand what is being said and I am not over-familiar with the script.

“Actors who are cast and paid to do the job, turn their back or head, or speak quietly. Some directors like the fact that the audience is straining to hear what is said.”

If he found audibility problems, it was not his job to directly instruct actors: he marked the place in the script and told the director.

Diederick Santer, Joint Chief Executive of the drama producer Kudos, says that he sometimes has to resort to subtitles because the sound is so bad. He has poor hearing in his left ear.

A former Executive Producer of EastEnders and ITV’s Grantchester, he backs Moore’s assessment of what can go wrong.

“She is right… it is a complex combination of factors. Actors mumbling can be an issue, particularly on a show going for a naturalistic look. In real life, people do mumble.

“In real life, the people they are talking to say pardon, and it gets repeated more clearly. It is down to producers and sound recordists to police this.”

It can be, acknowledges another producer, disheartening for actors, when they are immersed in the part, to be told to do it again, enunciating their words more clearly. They need to bear in mind that the real audience, unlike in the theatre, is absent.

Jamaica Inn lost a third of its viewers following the outcry over hard-to-hear dialogue. Gabriel Byrne’s performance in Quirke was also criticised later the same year for the same fault.

Santer adds: “For sound professionals, recording on set is tricky.” The use of two or three cameras makes it challenging for boom operators who must stay out of shot: “They have to be creative to find places to put microphones, not in shot or casting shadows. Considerations of sound are almost always secondary to those of picture on set.”

He adds that radio mics, often resorted to as a solution, give the flattest of sound. This means that everyone sounds the same. Furthermore, they can pick up the rustling noises made by clothing.

Simon Bishop, Chair of the Institute of Professional Sound, agrees: “There is a huge lack of understanding by producers, younger people, about what makes a good soundtrack.

“On a TV or film set, everyone – actors, make-up, costume, lighting, camera operators, directors – are there to make the picture experience good. It may be that three out of a crew of anything from 25 to 125 are doing the sound.

“The camera is king. But, if the sound doesn’t work, you don’t have a programme. There is a complete imbalance.”

In post-production, it is down to the dialogue editor, says Santer, to make sure the best takes are used. They can swap in sound from other takes, mix and match to create the best dialogue track.

But “this is a slow and time-­consuming job. And shows with tight deadlines or low budgets may not have the resources to do it properly.” Automatic dialogue replacement is an option, where the sound recorded on the day is unusable.

“The problem is that the replacement isn’t always convincing… a mumbled or slurred line is uttered by mumbling or slurred lips, so the synch-perfect enunciation on to those lips can look and sound ridiculous.”

“This is a slow and time-­consuming job. And shows with tight deadlines or low budgets may not have the resources to do it properly.”

At the final mix, after adding in the music, “as you sit in the audio suite surrounded by big speakers, you can hear every chirp of bird song, every pluck of harp, every lip smack, it all sounds rich and lovely.

“What I do and a number of other producers and engineers do, is wheel in a crappy telly and play the whole thing in the way that replicates the experience at home… and not compel the audience to switch on subtitles, which I am so often forced to do by badly recorded and mixed shows.”

Perhaps the last word should go to Tom Harper, Director of War and Peace. He says that, while he respects the views of sound recordists, in his opinion and experience, if there are audibility problems, “they arise at the broadcast and TV reception point, as the soundtrack is played out on reduced bandwidth to two tiny speakers.”

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Dialogue that doesn’t lose the director’s ear

Simon Bishop, Chair of the Institute of Professional Sound, says: ‘A lot of this is a people problem. We need producers with balls, who will stand up to the director and actor and say: “This is incomprehensible.”

‘Most of the issues with sound recording could be solved before a frame is shot, if people made sensible decisions from the outset. I mean people taking responsibility beforehand.

‘Sound recordists know there are numerous actors who are mumblers, who have form. It is solvable. It is a British trait. We don’t like embarrassing confrontations.

‘If an actor chooses to mumble, I can make a perfectly accurate record of it. You will hear mumbling. If an actor has been told [by the director] to speak quietly, we can bring the level up.

‘But if the fundamental speech is mumbled it will be unclear, because it is unclear, and I would regard it as the duty of a sound recordist to report it up the chain. I say that my Mum only gets to watch this once. If people can’t hear, they’ll turn off.

‘But what happens if they [directors and producers] don’t listen? Or the shooting timetable is being hurried along? Directors have many plates to spin. If the sound recordist keeps going back to the director, you can end up in a situation where you have lost the ear of the director, who doesn’t want to hear negatives. There are definitely directors who choose to ignore sound technicians – the experts.

‘Modern production techniques also conspire to make the end result less good. Twenty years ago, shooting on film, rushes would be watched overnight by producers, and the sound [checked] through a loudspeaker.

‘If there was a problem, including background noise, they would pick it up. In 2016, the producer might watch on an iPad or even iPhone. That doesn’t lend a [proper] appraisal of the sound.

‘That said, 99% of all film and television sound is excellent. Programmes, from Waking the Dead to The Hollow Crown and Wolf Hall, all have beautiful sound.’

The Institute of Professional Sound changed its name in 2013 from the Institute of Broadcast Sound, which was founded in 1977 by BBC TV and Radio and ITV sound balancers.