May 1, 2014
RVAAC Communications Sub-committee Conference Call
1:00 – 3:00 PM
888.396.7314 or 773.756.0935
Real time captioning:

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

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.
    • 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’.
    • 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:
    • 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);
    • Variation in field strength +/- 3 dB across the area (can be 9dB range in some cases);
    • frequency response +/- 3dB re: 1kHz over the range 100 Hz to 5 kHz
    • 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?
    • 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.
    • 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).

    • 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.
    • 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.
    • 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.
    • 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
    • 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.
    • 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.
    • 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
    • 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.
    • 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.
    • 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.