auditorium of Sadlers Wells

Broadcasting theatre audio description #1: infrared

Theatre audio description is generally broadcast to audience members using the service, and in 2018 there are more broadcast options available than ever before. As enquiries from venues to VocalEyes about audio description broadcast technology have increased, we have decided to publish a series of articles about the options – wifi, radio, and the subject of this first article, infrared. **Spoiler alert** there is no perfect system!  Venues will have to consider what will work best in their venue for their audience.


Before we start, Emily Malen, Front of House Manager & Access Development at the Theatre Royal and Concert Hall in Nottingham makes a very important point:

‘Consultation with customers is key. Through feedback about their experiences at our venue plus finding out what users like and don’t like elsewhere – this is helping us to shape the way we move forward.’

Service user David Merkel notes that it is ‘Important to trial systems in the venue and be aware of specific issues at the venue for users.”

We are starting with infrared as it is the granddaddy of audio description broadcast systems enabling the growth of audio description in theatres over recent decades. I don’t believe current systems were designed for this purpose, they certainly used to be listed under ‘conferencing’ on suppliers’ websites, and indeed they are much criticised, but when you consider the other options infrared’s strengths become more apparent.

So how did infrared become, as Tom Hares, Technical Coordinator as Sadler’s Wells Theatre says ‘the industry standard’ for audio description delivery?  This happened because induction loop systems fell out of favour for supporting customers who had difficulty hearing shows. They were replaced by infrared systems which were considered easier to fit, more reliable and had receivers that were able to support people using a hearing aid, and people who were hard of hearing and did not use a hearing aid (induction loop signals are only received by hearing aids). Infrared systems were put into theatres so that this service for hearing aid users and hard of hearing customers was available at every performance. Because most infrared systems can broadcast two separate channels, the second channel became used to broadcast audio description.

The receivers allow customers to choose between channel 1 (usually the sound of the show relayed to support hard of hearing customers) and channel 2 (usually audio description) or channel 1 in one ear and channel 2 in the other ear – potentially enabling people to have both support hearing the show and audio description (if the levels of each are appropriate).  It seems that many theatres that VocalEyes have worked with originally had infrared fitted without any thought of using it for audio description – it was there for hard of hearing customers – but when they considered providing audio description, it was relatively easy to do – because half the equipment was already in place.

In practice

So if it is ubiquitous and works for both hard of hearing and blind/partially sighted customers – why are venues considering paying thousands of pounds on new, different systems? Wikipedia tells us that ‘Infrared radiation is electromagnetic radiation with longer wavelengths than those of visible light, and is therefore generally invisible to the human eye’.  The infrared signal broadcast in a theatre auditorium is essentially invisible light, and like a beam of light – you need a clear line of sight between source and your eye/receiver if you are to see/receive it.  The necessity of having ‘line of sight’ is probably the biggest drawback of infrared. The classic TV remote also uses infrared, if you don’t point it directly at the TV – you cannot change the channel or the volume. If a family member walks between you and the TV, you cannot change the channel or the volume. In the theatre context, infrared is broadcast from radiator panels in the auditorium; if there is not a clear line of sight between a radiator and your receiver you will not receive the signal.

As service user Edward Copisarow notes, reception is ‘specific to the seat, and whether someone tall is sat in front’.  Many venues will have infrared radiators fixed on either side of the stage, and in a traditional proscenium arch theatre this can often mean that there is not a reliable signal in the first three rows, as the angle from the radiators to the receivers is too sharp.  In these seats you will often have a ‘phasing’ effect, as you shift position in your seat you lose the signal from one radiator before then picking up signal from the radiator situated on the other side of the stage. The extreme sides of the stalls may also have the infrared signal blocked by the theatre boxes, and the rear of the stalls may be blocked by the over-hang of the circle. It is therefore essential that venues map the infrared reception in their auditorium and do not sell seats with poor reception to people using the infrared receivers. However, many partially sighted people will want to be in the first three rows to make maximum use of their sight, and then have to decide between either being close or having the audio description – frustratingly not able to have both. It is also important for venues to check reception regularly – particularly when shows change: equipment gets moved and radiators can get blocked by speaker stacks, lights – or can be temporarily unplugged or re-angled, and never put back or plugged back in. But even if you regularly check reception, as Edward notes, a tall person sat in front of you may block the signal at the last minute.

There are still some positives to broadcasting audio via invisible light, however. Unlike wifi, there is no limit to the number of people who can use the system concurrently, provided that the receivers are in line of sight of a radiator they can receive the signal. Unlike radio, you won’t have other infrared signals potentially causing interference. As Tom Hares at Sadler’s Wells says:

‘Easy to set up and with a pretty good life span, these have been a good set-up-and-forget system able to run in the background with minimal upkeep. Line of sight is essential to these though, as short patrons and infrared cameras discover to their cost. Stethosets are easy to use for the most part and the basic interaction from a staff level is simple.’

Like all of the audio description broadcast systems, infrared requires a considerable investment from venues. But it also requires considerable installation (although you won’t be taking seats up and digging up the floor as you would with an induction loop system).

The first part of the infrared system is the modulator, which converts the signal ready to be broadcast, and this is usually housed in a rack sub-stage. The radiators then need to be situated throughout the auditorium, and must be cabled to the modulator (although they can be daisy-chained). In a traditional proscenium arch theatre there is generally a minimum of two radiators per level of the auditorium (stalls, circle and gods), and this still leads to areas without reception. Some theatres have additional smaller radiators to fill in the gaps. When we discuss wifi and radio we’ll see that very often a single base station or transmitter will offer excellent coverage throughout an auditorium – making their physical set-up and upkeep much easier.


The most common design for infrared receivers is the stethoset. These have two ear-buds, below which hangs a small receiver/control unit. They have a wish-bone shape and look like the first part of a doctor’s stethoscope, but without the tube and diaphragm. This design is ubiquitous for good reason: the receiver is sited just below the chin, so relatively high, offering a better chance of being in line of site of a radiator.

Several years ago Sennheiser (the leading manufacturer of infrared equipment) re-designed their stethosets and discontinued their classic design (the (HDI 302). Service user Debi Olley offers these insights:

‘The older infrared sets are extremely reliable…with great sound quality which is rarely compromised by people walking through the signal lines…They are extremely easy to work with just 3 settings…The newer stethosets have a few issues…they can be less comfortable to wear. The volume control does not seem to have a stop point. The wheel on the front of the headset carries on rolling so you can’t tell when you get to the top volume level.’

Stethoset receiver with volume control
Sennheiser HDI830 infrared stethoset receiver

Toby Davey, service user and former Deputy Director of VocalEyes, notes that the newer Sennheiser stethosets are less comfortable because they are heavier and therefore weigh the ears down more. The older receivers certainly offer far better reception. Trialled against each other in the same theatre seat moments apart, Sennheiser’s newer stethoset offered a markedly worse sound quality. Apparently, this is because the receiver element in the newer model is now recessed within the volume control, so you are also probably cutting off your own signal when you alter the volume. While the classic stethoset had a switch to select the channel, the newer stethosets (HDI 830) have a single button that is held down for five seconds to change the channel through a sequence of channel 1 to channel 2 to both channels and then back to channel 1, with the current channel indicated visually by a light  – so it is even more important that Front of House confirm the required channel and set it before handing it to an audience member.

But despite all of this, compared to using a touch screen device to receive a WiFi signal, Edward Copisarow rates stethosets in general as ‘Best for ease of use’ and service user John Thomas says he tends ‘to favour the infrared system with its stethoset headset, though I’d like to see it modified to provide the option of a single ear device…my main reasons for this preference are familiarity, simplicity, ease of use and lack of clutter.’

Charging rack for audio description stethosets
Charging racks for Sennheiser HDI 830

The newer Sennheiser stethosets do have a couple of improvements: the balance between the two ears can be adjusted – potentially useful if you have differing levels of hearing between ears.  Some service users like to wear the stethosets with one earbud in and one earbud out – in order to better hear ambient sound in the auditorium. Being able to turn down the sound in the earbud that is not in use minimises the chance of the sound of the audio description bleeding to other patrons. But additional options are also complications and we have had instances where this setting has been fiddled with – and the stethoset has been returned as faulty. Clear improvements are that they charge much more quickly than the classic headsets (3 hours as compared to 10 hours for the old HDI 302 batteries), last longer (up to 12 hours compared to up to 5 hours on fully-charged fresh batteries) and while it was notoriously difficult to get a proper contact between battery and charger, with the new models they can be dropped either way into the charger and will generally make contact and charge.

Sennheiser EKI 830

Sennheiser now also offers a receiver (EKI 830) that is essentially a new-style stethoset with the ‘ears’ cut off into which either an induction neck loop or headphones can be plugged. It is then either clipped on to clothing or worn around the neck on a lanyard.  Sennheiser’s main rival in infrared – Williams Sound, also offer a bodypack receiver with a lanyard (WIR RX22-4N) and a stethoset (WIR RX18) but in our same seat test the stethoset came a distant third to both Sennheiser models for reception/sound quality.

Williams do have a unique further option, a set of large over-ear headphones (WIR RX15-2) which receive the infrared signal on the outside of the headphones. With their receivers sited so high and on the extreme right and left matching the positioning of the radiators to the extreme left and right in most auditoria, these offer excellent reception. Young children find them particularly comfortable, as the in-ear stethosets do not properly fit smaller ears. However, Edward Copisarow offers a perspective we have frequently heard from adult service users:

“Enclosed headphones colour the theatre experience and not in a positive way. You get hot ears and feel closed up and remote from the experience.”

It can also be difficult to hear the show, unless show sound is mixed in with the audio description to be heard in the headphones, but that can then make it difficult to hear the audio description – particularly during loud scene change music – when you really want to hear the description in order to follow the action.

What you do with it

Tom Hares’ comments about it being a ‘set-up-and-forget’ system ring very true. Infrared can work really well, but because it is such a robust workhorse (and because technicians do not always have training/knowledge or interest in access technologies), it can get forgotten about. Tom is a knowledgeable, diligent and caring technician, but you won’t necessarily find that everywhere. The radiators may not be regularly checked to make sure they are working and positioned correctly (pyros used in pantomimes can fill them full of gunk, so they may need cleaning in order to work properly).  Their life can be extended by turning them off when not in use (obviously it must be part of regular show checks to ensure they are switched back on again!) as the power of each radiator gradually diminishes with use. However, this is rarely done. How regularly do FOH clean the ear-pads on the receivers?  Are the receiver batteries charged properly? How often are the batteries replaced to ensure they can hold their charge and last through a show? You also need to feed in a strong clear signal for broadcast and be aware that you can also alter the gain on the modulator, increasing it to get rid of hiss on the system or decreasing it if the sound quality is breaking up. Gary Giles, VocalEyes’ freelance technician who has decades of experience as a theatre sound technician believes that infrared can actually offer better sound quality than radio or WiFi because of its high dynamic range – but I suspect this would surprise many people who have become used to ‘hissy’ systems that have not been well set-up, maintained or used.


  • Works for both ‘sensory’ audiences – people who are blind or partially sighted and those who are hard of hearing (adapted receivers can connect to hearing aids via the T-switch)
  • The stethoset receivers are relatively straightforward with only channel and volumes controls and these are tactile.
  • Familiar, most people who have experienced theatre AD have used infrared.
  • Ear-buds are relatively easy to clean with anti-bacterial wipes.
  • Simple self-contained and straightforward system.
  • No noticeable delay – you hear the audio describers as they speak or the show as it happens
  • If you suddenly need more receivers – currently other local venues will likely have compatible receivers they can lend.


  • A ‘line of sight’ system that requires significant set-up, not just siting the modulator, but also carefully positioning and fixing multiple radiators around the auditorium. Invariably there are still parts of the auditorium where the signal does not reach, and box offices need to be aware of this. The signal can also be blocked if a customer is relatively short and sat behind a taller customer.
  • The stethoset receivers can be uncomfortable – feeling heavy on the ears towards the end of a long show.
  • Can white out infrared cameras used in venues
  • Does not work outdoors

This is the first of three articles, written the VocalEyes Theatre Programme Manager Michael Kenyon. Add a comment below if you any questions, or contact Michael ([email protected]) directly if you have any enquiries about theatre audio description.