Inductive Loop in vehicle

[TiM]

Hello,

I'm currently in the process of designing an induction loop amplifier
(as used to transmit audio to hearing aid users)
to work within a Taxi. The Loop is intended to be fitted within the
headlining of the passenger compartment (rear of the Taxi) as there is
a glass divide between the Driver and passengers.

I'm like to know if anyone here has had any experience with Inductive
Loops in
Cars and if there are any issues that other people have had to deal
with.

I realize that the metal-work of the vehicle may influence the
operation of the loop, but as yet i'm not sure to what degree. Does
adding more loops help with this?

Thanks in advance for any info anyone may have,

TiM

 

[mike]

Hello,

I'm currently in the process of designing an induction loop amplifier
(as used to transmit audio to hearing aid users)
to work within a Taxi. The Loop is intended to be fitted within the
headlining of the passenger compartment (rear of the Taxi) as there is
a glass divide between the Driver and passengers.

I'm like to know if anyone here has had any experience with Inductive
Loops in
Cars and if there are any issues that other people have had to deal
with.

I realize that the metal-work of the vehicle may influence the
operation of the loop, but as yet i'm not sure to what degree. Does
adding more loops help with this?

Thanks in advance for any info anyone may have,

TiM

I expect you'd get such a wide range of coupling to different hearing
aid models that the downside for those who got blasted would be greater
than the benefit to those who liked it. Head alignment would also be
an issue for magnetic coupling. But you've already solved those
problems in the lab and just need auto installation info???
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[John Woodgate]

I read in sci.electronics.design that TiM <[email protected]>

Hello,

I'm currently in the process of designing an induction loop amplifier
(as used to transmit audio to hearing aid users)
to work within a Taxi. The Loop is intended to be fitted within the
headlining of the passenger compartment (rear of the Taxi) as there is
a glass divide between the Driver and passengers.

I'm like to know if anyone here has had any experience with Inductive
Loops in
Cars and if there are any issues that other people have had to deal
with.

I realize that the metal-work of the vehicle may influence the
operation of the loop, but as yet i'm not sure to what degree. Does
adding more loops help with this?

Ampetronic have done some work on this, but you may be sure that they
won't disclose any proprietary information.

First, you need to check the magnetic noise level in your vehicle. If
it's too high, there is no point going any further until you've fixed
that, if it can be fixed. Don't forget, however, that there is no point,
for most hearing-aid users, in trying to make the magnetic noise level
far lower than the acoustic noise level in the vehicle, because the
listeners can hear that pretty well; it's the higher frequencies that
they can't hear. As long as the magnetic noise level is around 10 dB
below the acoustic noise level, that should be OK unless the magnetic
noise has a distinct tonal characteristic.

Next, you need to experiment. Thin sheet metal a few centimetres from
the loop may have only a small effect. There are ways of calculating the
effects, but they are very complicated and not freely available. Just go
by experiment. A larger loop is likely to give you better coverage than
a small one. You do need to current-drive it, because the optimum number
of turns is likely to be greater than one (depending on your amplifier
design and the loop dimensions).

 

[John Woodgate]

I expect you'd get such a wide range of coupling to different hearing
aid models

Why do you expect that?

that the downside for those who got blasted would be greater
than the benefit to those who liked it.

'Blasting' is liable to occur if the loop is small and thus has to
produce a high field strength close to in order to get enough further
away.

Head alignment would also be
an issue for magnetic coupling.

That's another reason for using a large loop rather than a small one.

 

[John Popelish]

Hello,

I'm currently in the process of designing an induction loop amplifier
(as used to transmit audio to hearing aid users)
to work within a Taxi. The Loop is intended to be fitted within the
headlining of the passenger compartment (rear of the Taxi) as there is
a glass divide between the Driver and passengers.

I'm like to know if anyone here has had any experience with Inductive
Loops in
Cars and if there are any issues that other people have had to deal
with.

I realize that the metal-work of the vehicle may influence the
operation of the loop, but as yet i'm not sure to what degree. Does
adding more loops help with this?

Thanks in advance for any info anyone may have,

TiM

I think your biggest problems may revolve around the frequency
response of the coil/sheet metal combination. The metal will
circulate eddy currents that will vary with frequency. But the
coupling to various kinds of hearing aids will also have various
frequency responses. But then, the biggest variation in frequency
response is probably the ears of hearing impaired riders.

 

[John Woodgate]

I read in sci.electronics.design that John Popelish <[email protected]>

I think your biggest problems may revolve around the frequency response
of the coil/sheet metal combination. The metal will circulate eddy
currents that will vary with frequency.

The effect CAN be surprisingly small. OTOH, Murphy says that sometimes
it isn't.

But the coupling to various
kinds of hearing aids will also have various frequency responses.

By convention, this is not taken into account. The field strength should
be substantially constant at **400**mA/m during the loud parts of the
signal (not 100 mA/m) from 100 Hz to 5 kHz. (IEC 601184; it's poorly-
written but we are revising it).

But
then, the biggest variation in frequency response is probably the ears
of hearing impaired riders.

If you take the whole hearing-aid using population, yes. But the
majority have a greater or lesser amount of high-frequency loss. (By
that I mean between 30 and 70 dB at 3 kHz: you can lose a lot of hearing
and still cope).