Witness for the Prosecution is a site specific production of Agatha Christie’s classic play, which opened in London’s County Hall in 2017.

The production features extensive abstract soundscapes, a large sound installation including over 120 loudspeakers, and complex sound reinforcement arrangements, using wired and wireless microphones, delayed to many different audience zones

As a long running play in the centre of London, it’s an ideal production for sound design students to study in detail. These notes detail the design decisions, that informed the creation of the soundscapes, and the sound equipment selected and installed for the production.

Sound Design Objectives.

When I began work on this project I established the following list of guiding principles that would inform my work throughout the design process.


Create an Immersive sound design that is site specific by utilising architectural features, and historical fittings and fixtures of the County Hall debating chamber.

In a conventional proscenium theatre, audiences are generally sat in conventional, tip-up theatre seating in an area separate from the stage. The audience will be looking at scenery representing the location of the scene being performed, but this will usually be confined to the stage area. The area the audience is sitting in will have décor and architectural features appropriate to the period in which the theatre was built, which often have no relationship to the play being performed or the location depicted on the stage.

Our production of Witness for the Prosecution (WFTP) is set in the debating chamber of London’s County Hall. Although not a court room it shares many of the architectural features of a court. For those in the main chamber, generous leather seating, with writing tables, are set within an opulently decorated marble hall which completely surrounds the audience. For the courtroom scenes the audience are in the same room as the actors and wherever they look they are immersed in the architecture of the chamber. The sound design of the production similarly envelops the audience, by diffusing the playback of recorded sound through multiple loudspeaker systems that completely surround audience members providing a sonic immersion that parallels the visual immersion.

The chamber has some quirky architectural features which are specific to the building and are incorporated into the sound design. The original heating and ventilation system consists of a network of huge pipes which connect with grilles at every seat, nicknamed ‘the octopus. Loudspeakers are placed in these pipes, to produce sound that appears to come from nowhere, and everywhere simultaneously. For instance, footsteps and voices echoing from the corridors beyond the courtroom.

Photo of 'The Octopus' pipework beneath the chamber

The original loudspeaker system in the chamber consists of small speakers hidden in brass fittings with a portcullis grille which hang on leather straps in between the seats. In our production we use these speakers to create the illusion of recorded reactions, appropriate to those attending the court, to the events that unfold in the play. Laughs, gasps of astonishment or the general chatter that accompanies the entrance of new witnesses into the courtroom. The speakers are also used for specific spot effects, within the witness testimonies. These include sounds of breaking glass, dripping taps, clocks, and fragments of conversation and laughter as events are recalled from the witness box. 

Photo of Portcullis speaker

Using these systems integrates the sound design into the historical fabric of the building in which it is being performed, enhancing the site-specific nature of the production.

Create Diegetic audio content for the play for the following purposes:

• To Establish location

• To provide the sounds required by the script.

Diegetic Sound is sound that would be heard by people present in the location of the scene being performed. In the court scenes this would include the three loud knocks on the door prior to the Judge entering and the vocal reactions of those present in the courtroom. In the scenes not set in the court, in particular the docks at Limehouse, diegetic sound sets the location. The noise of water lapping and mournful ships horns.  In the scenes set in Sir Wilfred’s office, birdsong, church clock chimes and the sounds of the newspaper sellers from Fleet Street and the Strand, which are very close to the Inns of Court, where many lawyers have their chambers.

Diegetic sound is usually a fairly close rendering of the actual sounds in real life. In our production they are enhanced, in way that makes them slightly hyper real.  The water lapping may be gloopier and darker, the door knocks louder and slightly more threatening and drum like, but they are still accurate and recognisable recordings.


Create Non-Diegetic audio content for the play utilising appropriate sound design techniques for the following purposes.

• To heighten the underlying tension in the play by underscoring key sequences of action and dialogue with a series of very specific soundscapes.

• Reflect the grandeur and epic scale of County Hall and propel the narrative arc from scene to scene by choosing appropriate music

Non-diegetic sound is more abstract. It includes the soundscapes that underscore key passages of the text, in particular the testimony of witnesses recollecting events they are being questioned about, and also the music which occurs between scenes.

Sometimes, in productions where less thought has gone into the sound design, very generalised sounds are used to create dramatic underscores to heighten tension. Low frequency drones, heartbeats, bowed metal, and other tension building sonic staples, which often become repetitive and tedious.  To avoid this trap, the soundscapes in WFTP are very specific to the action and dialogue they underscore and are based on specific information that can be found by looking carefully at the text of the play.

A good example is the soundscapes that underscore recollections of events that happen in the flat that Leonard and Romaine live in. In the first scene Leonard tells Sir Wilfred he has a “a tiny maisonette over a shop behind Euston Station” When the events in this flat are recalled in court they are underscored by elements that might be heard if you lived close to a railway, combined with musical elements distorted through ring modulation, an electronic music technique which adds sum and difference frequencies which are not naturally harmonically related  to the original notes of the music. These sections also make use of Shephard Tones, the audio equivalent of the barbers pole, which give the illusion of continuous falling pitch. The classic Shepard Tone is built using sine waves, but in WFTP the sine waves are replaced by ring modulated low orchestral strings.

These elements are distorted and combined using the techniques of musique concrete, a manipulation and arrangement of sound recordings of real-world things and natural ambiences to produce a composition.

Here is an example of a sequence, being played back from the sound control software:

Leonard and Romaine’s Flat in Euston Soundscape.

Events recalled in Miss French’s house, are underscored by wind through leaves, percussive stings of glass breaking and muffled speech fragments, again layered and precisely placed in the text, as a deliberate accompaniment to the actors’ words.

As a contrast to these highly abstracted compositions, the scene changes are propelled dynamically and urgently through the use of more conventional instrumental music, played either by a large string orchestra or a string quartet. Sometimes productions use specially composed music, but WFTP uses an existing composition, Company composed by Philip Glass.

The orchestral version of this work complements the grandeur of the court room, the power of the court in determining matters of life and death and provides a compelling score for the highly choreographed scene changes which set and dismantle the scenes not set in the court.

For scene changes to quieter locations, excerpts from the same piece of music are used but played by a string quartet.

Make the actors clearly audible to the audience.

The debating chamber at County Hall is a fifty-foot high, marble walled, piece of architectural magnificence. Unfortunately, while visually splendid, it is widely regarded as a problematic space for speech.  When it was opened in 1922 there were constant letters and articles in the press bemoaning poor audibility of debates, The Daily Express dubbed it ‘The L.C.C.’s Hall of Murmurs’, councillors, press and public were unanimous in finding the acoustics dreadful. and this continued to be an ongoing issue throughout its lifetime as a council chamber.

The municipal journal in 1922,

“When the London County Council planned its new County Hall it made an unfortunate blunder in designing a Council Chamber which is four times as large as the old. Consequently, the members find that they cannot be heard when they join in the debates.”

The surveyor in 1930

“It was said at the last meeting of the London County Council that the council chamber at the County Hall is entirely unfitted for the purpose for which it was intended. … Members cannot hear one another’s speeches, and the debates are consequently dull and lifeless”.

And in Hansard in 1943

“To address the London County Council is to make a speech in a tomb”

After the LCC was disbanded and people started using the chamber for other purposes things were no better.  In 2010 a theatre piece staged in the chamber received this review in The Guardian.

“It must have seemed a bright idea …….to stage this verbatim piece about democracy in the historic debating chamber of County Hall. But, although the material is fascinating, the acoustics are terrible.”

The fact the venue was so hostile to the spoken word was quite a challenge to address particularly as, in our play, scenes not set in the courtroom, are played in the central area of the chamber on a raised platform, in the round. Although in the round productions have an immediacy and inclusivity, one of the sound problems is that by its very nature a speaking actor is going to be facing away from a large portion of the audience at any time. This also applies to our Lawyers in the court scenes , as they are allowed a bit of dramatic license to wander around, which wouldn’t actually happen in an actual English court.

When we first looked at the acoustics of the chamber, it became obvious that voices speaking 6 metres away from a listener had very low intelligibility as the speech was cloaked in a dense reverberation (like in a cathedral or church) where indirect sound was reaching the listener after bouncing around the marble walls for a few seconds, masking the direct sound from the actor. Speaking up didn’t really help either, as this just excited the reverberation further.  However, unsurprisingly, conversation between people standing a metre apart was perfectly clear. It was obvious that what we needed to do was to provide the same ratio of direct to indirect sound a listener 1 m away from the person speaking would experience, to an actor speaking 6-20 metres away from an audience member.

This required the use of microphones, wired microphones in the case of the fixed court positions, judge, clerk and witness box, and microphones hidden in the hairlines of actors who were performing in the central space, connected to wireless transmitters (radio microphones) concealed in their costumes.

The speech picked up by these microphones is then sent to loudspeakers within 1m of each audience member. (Slightly further away for audiences in the gallery). These speakers had to be visually unobtrusive, so very small speakers were attached to the desks in front of the audience members, 1 speaker for every 3 people (over 100 speakers in total). 

Photo of Small speaker in position

This solved the clarity and intelligibility issues, but now it was very apparent that the actors’ voices seemed to be mainly coming out of these small speakers, and it was difficult to determine the direction of the sound, and therefore which actor was talking, particularly if you couldn’t see their mouths.

To restore the impression that the sound was coming directly from the actor we use a simple bit of physics, known as the Haas effect (discovered in 1949 by Dr Helmut Haas). This states that the apparent source of a sound for a listener will be the sound that arrives at their ears first. Sound travels relatively slowly (343 metres per second) so if an actor is 20m away from a member of the audienc, who has a speaker 1m away from them, the sound from the speaker will arrive at their ears around 50 milliseconds before the actual sound of the actor speaking. If we electronically delay the sound going to the loudspeaker by 60ms, the sound from the speaker now arrives at the listeners ears 10ms later than the actors actual voice, so the audience member perceives the sound as coming from the actor’s mouth and the speaker is no longer acoustically prominent.

To further aid the clarity we ensure that only microphones that are being spoken into are live, so the sound operator performs line by line mixing, switching on each performers microphone just before they speak and off again just after. A job requiring great dexterity and concentration.

In the round, one audience member may be 20m away from a performer, but an audience member on the other side of the theatre may only be 2 m away, and therefore requires a much shorter delay time to make the sound appear to come from the performer. To make sure that all audience members have the correct delay times we must divide the auditorium up into much smaller sections, and each microphone in use has to have individual delay times set for each section. In our sound system we have 19 different audience zones and 6 different microphone positions. We therefore have 19×6=114 different delay times set, so every microphone is perfectly delayed for each audience member.

Screen shot of Matrix settings for multiple delay times
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Design and engineer a sound system, to enable the delivery, control, and timing of all the above elements, to deliver an excellent audience experience.

The music and sound effects are cued by the operator at precisely the time they are required, following the lines spoken by the actors of the actions they perform (visual cues).

All the recorded elements are kept as separate events so their precise timing can be varied from performance to performance. The actors never have to follow the sound. We always respond to what they do. This necessitates the sound operator pushing the GO button for the sound playback system around 300 times a show. Within these 300 cues, there are a total of over 1000 individual sound elements and each of these have to be sent to the correct levels to the 16 loudspeaker systems that are used for the replay of recorded sound in the show.

This complexity (remember the sound operator is also mixing the microphones ‘line by line’) can only be achieved by loading all the sound into a computer. Once in the computer it is programmed using an industry standard solution for show control and sound playback in theatres, called QLab. Here’s a section of Inspector Hearne’s Testimony programmed in QLab.

Screen Shot of QLab programming for a complex sound sequence
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The numbered cues are where the operator presses the button, everything else is recorded and remembered so it can be reproduced accurately, potentially thousands of times during shows run. At the bottom of the screen you can see the level setting screen, which shows one of the component sounds in the sequence being sent to 7 loudspeaker systems.

The sound design for a play as complex as WFTP requires thousands of individual items of sound equipment. These include:

The microphones both wired and wireless

The sound mixing console that allows the operator to control all these microphones ‘line by line’

The complex electronics that distribute all these signals to the loudspeaker systems and apply the correct time delay for each signal to all the audience zones

The show control computers that allow complicated sequences of sound to be played back and cued to follow the actors’ performances.

The 150 loudspeakers and the amplifiers that drive them.

The miles of cable that connect all these items of equipment together to from a complete sound system.

Here’s a very simplified version of how all the equipment is connected together.

Excel Spreadsheet showing Sound Equipment Interconnections
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And here’s a drawing of the chamber which shows  the speaker locations and main audience zones:

Drawing of County Hall showing loudspeaker locations
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