May 1, 2014

RVAAC Communications Sub-committee Conference Call

1:00 – 3:00 PM

888.396.7314 or 773.756.0935

8020595

Real time captioning

Streamtext.net/text.aspx2event=050114USAB100pm

Today’s meeting will begin with a presentation from Mr. Tom Lane of Ampetronic with a 

brief Q&A to follow on the topic of induction loop systems on rail vehicles. 

Additional committee information available at http://www.access-board.gov/guidelines-andstandards/transportation/vehicles/rail-vehicles-access-advisory-committee/

Excerpts from an email from Tom Lane, today’s presenter, that may be useful to 

committee members.

We have received a number of enquiries regarding Assistive Listening Induction Loops (Hearing 

Loops) for projects in the USA over the last year (especially in the Autumn), so in some ways it 

makes sense to learn that the US RVAAC is reviewing the ADA regulations.

Previous Experience

Ampetronic has some experience of working with rail vehicle manufacturers to achieve 

successful Hearing Loop installations on rail vehicles.

With Alstom, we have systems:

· delivered on Dublin LUAS (Citadis 301 and 401),

· currently being delivered by Alstom to the Nottingham NET project (also Citadis).

We have also participated in a number of pending rail vehicle bids with Alstom in the recent 

past.

With other manufacturers / installers, we have achieved successful projects on heavy rail trains 

in

· New Zealand and

· Australia.

In all these cases, the system provides effective assistance to hearing impaired passengers (in 

some cases known through user feedback), and has been shown not to affect other on-board 

systems (including carefully monitored out-of-hours testing with Alstom on the first LUAS 

project).

We have also demonstrated success with similar systems aboard public buses (including 

articulated vehicles).

It is worth noting that (unlike most of the other suppliers who have attempted to do this), 

Ampetronic have worked to create systems that meet the requirements of the relevant 

international standards and had considerable success in doing so.

However, I can quite understand Alstom’s concerns about hearing loop provision on rail 

vehicles. We also have had to address most of these issues.
Basic Explanation of Hearing Loops

Perhaps it would be a good to summarise a Hearing Loop system’s main features, as relevant to 

rail vehicle applications. You may know most of this but it is still useful to create a summary for 

reference.

· An Audio Frequency Induction Loop System (or AFILS), more commonly known 

as a ‘Hearing Loop’, is an assistive listening system which couples an audio signal 

directly to a hearing aid user’s earpiece. This is beneficial because the signal then does 

not include the unwanted sounds in the environment – in a rail vehicle this might be the 

noise of the vehicle’s motion, the voices of fellow travellers, etc.

o Note that the hearing aid user carries their own receiver for the system. The 

transport operator does not have to supply anything extra to any hearing aid user 

whose aid is fitted with the ‘T’ coil, and is not expected to do so. This is reported 

to be true for the majority of hearing aids – over 99% in the UK, over 65% and 

rising in the USA. In a public transport environment, this makes hearing Loops 

the only practical option for users who are ‘transient’ (i.e. do not stay in the 

transport environment for long enough to make issue of a receiving device 

viable).

· The Hearing Loop does this by generating a magnetic field at audio frequency in 

the desired listening space. This is done by passing the audio signal as a controlled 

current through a wire which generally surrounds the listening space, forming a ‘loop’.

o The Hearing Loop system can be thought of as a really badly coupled, mostly

air-cored, transformer, with almost zero power transfer!

· The loop current (and hence the magnetic field generated) is a direct analogue of 

the audio signal, and occupies the audio frequency band of nominally 100Hz – 5kHz 

(although hearing aids often do not reproduce much signal below 300hz or so).

· A hearing loop system generally consists of a loop of wire, surrounding the 

listening space (usually a little above or below the listener); this is connected to a ‘loop 

driver’ (the amplifier) which outputs a controlled audio current (i.e. the rated output 

quantity is current, not voltage, although obviously voltage is also present!). This is 

connected to a power supply (of some kind) and one or more audio input sources. These 

might be a PA system, an intercom, or another source. The signal format might be 

speaker line or small-signal, and a PA system might in turn be connected to various 

sources –e.g. recorded announcement, live microphone (e.g. driver or conductor/guard), 

intercom system, or remote live audio (e.g. a control centre via radio communication), or 

a combination of one or more.

· The performance of the overall system (i.e. the audio magnetic field that is output 

for the hearing aid user to receive) is specified by the international standard IEC 60118-

4:2006. A summary of the requirements is:

o Maximum Field strength at least 400 mAm-1 @ 1kHz (0.125mS rms 

measurment) (measured as the vertical component of the field, at the listening 

height);

o Variation in field strength +/- 3 dB across the area (can be 9dB range in some 

cases);

o frequency response +/- 3dB re: 1kHz over the range 100 Hz to 5 kHz

o magnetic background noise preferred < -32 dB re: 400mAm-1, recommended 

< -22 dB re: 400 mAm-1.
That is about it for a basic explanation!

Alstom’s Questions

To answer your specific questions:

· Power Transfer: Does the loop current “power” the receiving device by the 

mutual inductance?

o No, it is only a transfer of the audio signal: the hearing aid is battery powered 

and amplifies a very small received signal just as it would the signal received 

from the hearing aid’s built-in microphone.

· Installation of loop wire – practical considerations, interaction with train systems, 

etc.

o Wire Position: You are correct that, to achieve even an approximately even 

magnetic field strength in the area of coverage, the loop has to enclose the area 

where coverage is wanted. I am aware that the cabling design of many rail 

vehicles does not have a standard route in this location, but such a route has been 

achieved in each case so far.

On the Citadis trams, the route is perhaps not as easy to install as would be 

desirable (by virtue of the vehicle design), but in the heavy rail applications the 

loop wire was integrated into the body build quite successfully. There are 

apertures in most body structural members (to reduce weight) and when run inside 

flexible conduit and mounted on stand-offs from the bodyside, this has been 

reported a viable and repeatable install.

The loop path can vary up and down to some degree; the overall path should be 

moderately constant in most cases. This would be part of an installation-specific 

design and test (we can simulate the field generated by the loop for any given 

path).

o Train cabling interaction: Again, this is a valid consideration. The loop wire 

does carry significant peak current over a significant frequency range. However, 

as per the item above, the loop needs to go around the car sides, so the loop wire 

is – by design – then separated from the other train cabling, and is likely to cross 

most other cabling at near right angles. If other potentially susceptible on-train 

systems are designed to be immune to external magnetic fields, then they should 

have small flow-return loop areas as a matter of good EMC design, and would 

therefore not suffer interference. No interaction has ever been reported or 

suspected in tests (I’m not saying something could never happen of course, it is 

just unlikely and avoidable by basic good design).

· EMI considerations – immunity and emissions compatibility.

o Immunity: The loop equipment itself is certified to EN 50121-3 and is not 

likely to be affected by systems on the train. However, the reception of the 

intended audio magnetic signal can potentially be affected by magnetic 

interference, as the loop system and the interference are ‘independent sources’. In 

tests on UK mainline rail, and tram and urban rail in a number of countries, it has 

been found that although the overall amplitude of interference can on occasions 

exceed the recommended levels (sometimes up to -18 dB re: 400 mAm-1), this did 

not affect user reception of the audio if the loop system generated a signal that 

was of standards-compliant field strength. Obviously if the field strength is 
inadequate, and/or the audio unduly distorted, the reception was not as clear: it is 

a case of maintaining good signal-noise ratio. This does mean that a good ‘clean’ 

audio input is desirable so a line-level feed before the PA amplifier is desirable.

o One reason why the traction and similar interference has been less of an impact 

than expected may be that the interfering signals are very different from spoken 

audio. The spectral content of the interference is usually lower frequencies and 

constant frequency. It is also intermittent depending on the stage of traction / 

braking involved.

o Interfereing magnetic EMC experienced on-board can be minimised by good 

vehicle design: arranging common routes for traction current flow/return (e.g. 

between power inverters and traction motors, and ensuring all ground paths are on 

the same route as power and explicit (i.e. use braided links across moving joins)), 

positioning filter inductors a little away from main passenger spaces, designing 

current flow through braking resistors to be low-interference, etc, all help. These 

methods are also good for reducing overall vehicle emissions

o Whilst EMC immunity of the hearing loop system is an important 

consideration, I will raise one point: In the USA, almost all longer distance stock 

is diesel-hauled, so traction related interference is really not a consideration in 

that case. On urban transport, it is usually electric traction, so then the issues do 

apply.

o Emissions: The loop system generates a magnetic field. This could, in theory, 

interfere with other critical circuits on the train and lineside. However, this has 

been shown to be very unlikely due to the relative magnetic field strengths and 

signal types involved. Railway Signalling control loops & track circuits use spot 

frequencies if AC, and would not be susceptible to broad-spectrum audio at the 

amplitudes used. DC circuits would of course be no problem as audio is AC. It 

has been calculated by LUL technical teams that even for London Underground’s 

most sensitive (delta) track circuits, interference is not a risk. Generally, the 

magnetic fields generated by traction motors which are very close to the track are 

a far greater risk due to the relative position.

o Also, the magnetic fields are such that there is no risk to passengers or workers 

from magnetic exposure according to EU limits (calculated by Alstom for bid 

documentation in some recent projects).

· Other considerations:Metal in the car construction

o The loop system is a magnetic system. The car is usually a metal shell and 

structure. This a metal structure would be expected to affect the magnetic field 

produced by the loop – and it does. The metal effectively absorbs some of the 

loop’s magnetic field, creating a reduction in field strength and changing the 

frequency response.

o The type of metal involved affects how serious the loss of energy and change 

of frequency response may be. Stainless steel is not a great problem; mild steel is 

rather more of concern, and Aluminium causes the biggest loss of energy of 

commonly used metals.

o If designed properly, the loop system cable route and the loop driver 

specification & design can overcome the effect of the loss of energy (by a higher 
current in the loop), and a properly designed loop driver will provide correction of 

the frequency response to achieve standards compliance. 

I hope this is a useful set of answers for you. I’m sure that additional questions and thoughts will 

arise, and of course we’d be happy to answer and/or discuss any matters that might arise.

Please contact me if there is any way we can help you further.