[SML] infrared hearing systems

[e-mail frank.wood95]

In 1991 I was responsible for installing an IR system in the Questors
Theatre. This is a Tyrone Guthrie arena stage, but adaptable, so that
it can be used as anything from a straight proscenium theatre, with
the arena rostra stepping down to form a sunk pit, to full
in-the-round, with seating rostra onthe proscenium stage. (Or any
other seating arrangement that a zany designer can think of!) We
selected IR because it is, quite simply, the best.

Due to the odd shape of the auditorium, we felt that four microphones
were needed. These are sited among the stage lights, so they have head
amplifiers adjacent to the microphone itself, which feed back to the
control room at high level through a transformer balance and star quad
cable, to give the greatest possible immunity to interference from the
lighting system. Inthe control room there is a wholly passive
transformer-balanced mixer which feeds the IR control. It also
provides a feed to the general stage sound system.

This is the Sennheiser system. It contains a volume control, a
compressor or a limiter, selectable, and the RF modulator. This sends
an RF signal at either 95KHz or 250KHz to the IR radiators. Several
radiators may be connected in series with a 50ohm co-axial cable. We
found that two radiators covered the whole auditorium, but bigger
theatres will need more. Four satisfiy the Roual Opera House, Covent
Garden.

The link between the microphones and the control box is designed with
a very high overload capacity, to cater for maroons and firearms.
Nomially at 2V RMS Peak, it will send up to 40V peak without any
significant clipping in the electronics. The microphones we use are
AKG short rifles, and there is a power supply for them and the head
amplifiers in the control room.

The headset recievers present some administrative problems. They are
issued and recovered by FOH staff, who also have the duty of charging
the batteries. These do not have an infinite life, and need charging
overnight. Periodical replacement will be needed. Receivers which will
talk to hearing aids are available.

The system has worked well since it was installed, although we have
recently found it neccessary to replace our two radiators.

As for costs, the system basically cost £4'000 in 1991. This does not
include the head amplifiers or mixer and power supply which, I
confess, came from my then employer. Nor the time and skill I put into
the design of the system. Our then manager took the view that if
nobody had broken anything he had a lot of good engineers wanting to
employ theyr skills in advancing themselves, and looked kindly on this
sort of thing. That was how we got our first computer: we designed and
built it ourselves. Those were the days!

On 24 November 2014 at 16:06, Stephen Litterst via Stagecraft
<stagecraft at theatrical.net> wrote:
> On 11/24/14 10:44 AM, Garry Brown via Stagecraft wrote:
>>
>>    I've been asked to gather information on infrared hearing systems for
>> our two theatres. So far, I've done a couple of searches on said systems
>> to determine what exactly we might need and a ballpark cost estimate,
>> but I really don't know anything about them beyond the basics of a feed
>> of some sort, an amp/router, and headset receivers.
>
>
> First off, is there a reason you're going the infrared route?  There are
> variety of assisted listening system technology, ranging from IR, to RF to
> induction loop.  Depending on your needs, one of the other technologies may
> better suit your application.
>
> In brief:
>
> Infrared:  requires every seat to be in view of the IR emitter, and for the
> patrons' devices to have an unobstructed line-of-sight to the emitter.  For
> large theatres, this may require installing multiple emitters.
>
> Radio Frequence (RF):  Essentially a low-power FM radio station.  An antenna
> from the transmitter broadcasts the signal to the patrons' devices.
> Line-of-sight is not an issue, but building consruction or poor antenna
> placement can create dead zones.
>
> Induction Loop:  A copper conductor is looped around the audience, allowing
> patrons with the proper hearing aids to tune directly into the feed
> (stand-alone receivers are available as well.)  Can be cost-prohibitive to
> retrofit into an older building.
>
> I think the most important part of any system is the quality of the feed
> into it.  I know of a theatre that spent in excess of $20k to replace their
> RF system with an induction loop, only to find the problem wasn't the AL
> system, but the feed to it.  They then spent further $$ to upgrade the
> microphones which then made both systems sound much better.  It doesn't
> matter how you deliver the sound, if you're not capturing clean sound to
> begin with.
>
> Steve L.
>
>
> --
> Stephen Litterst            Technical Operations Supervisor
> litterst at udel.edu           Mitchell Hall
> 302/831-0601                University of Delaware
>
>
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-- 
Frank Wood