Audio EMF coil?

[Jim Denton]

I would like to generate a relatively uniform magnetic field from
complex audio frequencies within a room-size area. The amplifier I
have does 100W into 4 ohms.

In terms of coil configuration what is the most straightforward and/or
effective approach? I am assuming air core here.

Perhaps a series of 100W cross-over coils adding up to 4 ohms. One
each could be aligned to x, y and z axis.

Or a wide area coil, one on the floor and one on the ceiling,
Helmholtz style.

Hope I haven't frightened anyone. Just fishing for suggestions..

Thanks,

Jim

 

[Ken Smith]

In article <41E1F6E8.A08A771@rica.net>,
John Popelish <jpopelish@rica.net> wrote:
[...]

Or a wide area coil, one on the floor and one on the ceiling,
Helmholtz style.

This is the only way to get a fairly uniform field over a large
volume.

Not quite, you could have a 3rd coil half way between the floor and the
ceiling too.

If the distance from the floor to the ceiling is greater than 1/2 the
width of the room, the extra winding will need to be in the aiding
direction.

You may need to put a big ballast resistor in series with a one or two
turn coil to keep the low frequency impedance above the minimum for
the amplifier.

This is almost certain to be needed unless thin wire is used.

Also, you need to calculate (or measure) the inductance of the coil. If
you want uniform amplitude over a large frequency range, you may need to
add a "peaking capacitor" to boost the high end.


I think I would try a coil made of 2 conductor lamp

cord and connect the strands in series for a two turn coil. I like
the idea of putting one around the floor and one around the ceiling.
I would also try these in series, first.

--
John Popelish

--
--
kensmith@rahul.net forging knowledge

 

[Ben Bradley]

On Sun, 09 Jan 2005 21:29:18 +1100, Jim Denton
<jimdenton@rtonline.com> wrote:

I would like to generate a relatively uniform magnetic field from
complex audio frequencies within a room-size area. The amplifier I
have does 100W into 4 ohms.

There are largish coils to do this for magnetic pickups used for/in
hearing aids - a foot-diameter coil may transmit two or three feet. To
do a whole room uniformly you just need a coil that's as large as the
room.

In terms of coil configuration what is the most straightforward and/or
effective approach? I am assuming air core here.

Perhaps a series of 100W cross-over coils adding up to 4 ohms. One
each could be aligned to x, y and z axis.

These coils would all add up as vectors to a single field going at
an angle between two opposite corners, and if your receiving coil is
at 90 degrees to that angle it will be in a null and get practically
nothing. You might as well just have two coils, one in the floor and
one in the ceiling, as others have discussed, and orient your
receiving coil so it stays in the horizontal plane.
Unlike the other suggestions, I would use more turns, maybe 10 to
100 turns (though this gets to be a LOT of wire for a coil diameter of
8 or more feet, to give full room coverage in even a small room),
depending on where the treble response starts to fall off
unacceptably. For a practical example, five turns of telephone cable
for in-house wiring with the four conductors wired in series gives you
20 turns, with (for a 10 foot diameter) 160 feet of cable.

Or a wide area coil, one on the floor and one on the ceiling,
Helmholtz style.

Hope I haven't frightened anyone. Just fishing for suggestions..

Thanks,

Jim

-----
http://mindspring.com/~benbradley

 

[John Woodgate]

I read in sci.electronics.design that Jim Denton
<jimdenton@rtonline.com> wrote (in <i412u0psi72c8791bqhirud0pdbnvtsf67@4
ax.com&gt about 'Audio EMF coil?', on Sun, 9 Jan 2005:

I would like to generate a relatively uniform magnetic field from
complex audio frequencies within a room-size area.

There is no practicable way of making a *uniform field*. Ye canna'
change the laws o' physics, Cap'n. You will almost certainly need to
accept a variation of at least +/- 3dB (aka -30/+40%).

What is this for? What field strength do you want to get (A/m or
microtesla)? How big is the room? What frequency range do you want to
cover? Hint: the two octaves from 5 kHz to 20 kHz are usually far more
trouble than they are worth.

There is a large amount of information around on these 'audio-frequency
induction loop systems' (AFILS), some of it correct. I have the correct
stuff and I can let you have it if I know more about exactly what you
want to do. But please come back quickly because I'll be off-line from
Wednesday to Friday.

The amplifier I have
does 100W into 4 ohms.

It's not really suitable; you will have to jump through one or two hoops
to get good results with it. AFILS normally use current-source
amplifiers.

In terms of coil configuration what is the most straightforward and/or
effective approach? I am assuming air core here.

Perhaps a series of 100W cross-over coils adding up to 4 ohms. One each
could be aligned to x, y and z axis.

No. Much too small, and remember, a magnetic field can have only ONE
direction at any point in space.

Or a wide area coil, one on the floor and one on the ceiling, Helmholtz
style.

Well, possibly, but you may only need one. Are there any adjacent rooms
(up and down as well as side-to-side), into which the field should not
spread?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Woodgate]

I read in sci.electronics.design that Ben Bradley <ben_nospam_bradley@mi
ndspring.com> wrote (in <ur74u0p1791df2kd09aqddb0f90s89ema9@4ax.com&gt
about 'Audio EMF coil?', on Mon, 10 Jan 2005:

Unlike the other suggestions, I would use more turns, maybe 10 to 100
turns (though this gets to be a LOT of wire for a coil diameter of 8 or
more feet, to give full room coverage in even a small room), depending
on where the treble response starts to fall off unacceptably. For a
practical example, five turns of telephone cable for in-house wiring
with the four conductors wired in series gives you 20 turns, with (for a
10 foot diameter) 160 feet of cable.

The other guys gave misleading information, but that is so bad it's not
even wrong. Try calculating the inductive reactance of such a coil at
say 5 kHz, remembering that you get 1.6 to 2 microhenrys per metre of
perimeter, multiplied by the square of the number of turns (assuming
closely-bunched conductors). Now compare that with the resistance to
find the high-frequency -3dB frequency.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Tim Shoppa]

There is no practicable way of making a *uniform field*.

The other extreme of terminology is "partial vacuum". There's no
perfect vacuum, so of course all vacuums are partial vacuums...

Interestingly, it is possible to do significantly better than the
Helmholz coil for uniformity. See, for example,

http://www.iop.org/EJ/abstract/0950-7671/43/7/311

I think that actually winding the sperhical coils is left as an
exercise for the reader!

Tim.

 

[Tim Shoppa]

First find you spherical room.

I'm not sure my spherical cows will appreciate the uniformity of the
magnetic field .

Tim.

 

[Ken Smith]

In article <1105361603.082010.196320@c13g2000cwb.googlegroups.com>,
Tim Shoppa <shoppa@trailing-edge.com> wrote:

There is no practicable way of making a *uniform field*.

The other extreme of terminology is "partial vacuum". There's no
perfect vacuum, so of course all vacuums are partial vacuums...

Interestingly, it is possible to do significantly better than the
Helmholz coil for uniformity. See, for example,

http://www.iop.org/EJ/abstract/0950-7671/43/7/311

I think that actually winding the sperhical coils is left as an
exercise for the reader!

Its easy. Just wind two hemispherical coils and hook them together.

But seriously:

Depending on how uniform "uniform" means, a 4 coil set can have a sweet
spot with about 1/2 the radius of the coils used.

--
--
kensmith@rahul.net forging knowledge

 

[Adrian Jansen]

Jim Denton wrote:

The coil is intended for an experimental setup pertaining to my
research on plant response to EMF's, and it seems Helmholtz is the way
to go. Signals are complex audio, not single frequency.

The exact bandwidth has not yet been established. So I suppose I am
initially looking at compromised performance over the entire 100-20KHz
range. I can progressivley narrow this down according to my
obervations.

Since I have a 100W audio amp, and don't need that much power, a major
degree of inefficiency can be tolerated, provided the waveforms are
not distorted. Preferred coils diameter would be 1-2 metres.

Not to add much fruitful to the discussion, but how do your plants know
the difference between "complex audio" and "distorted" ?
And maybe you should investigate the effects of the field direction -
vertical, horizontal,etc ?

--
Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

 

[Tim Shoppa]

The coil is intended for an experimental setup pertaining to my
research on plant response to EMF's,

I suspect that iron-bearing minerals in the soil will mess up all your
efforts at making uniform magnetic fields.

Oh well, there's no effect to measure anyway, so why bother with
ultra-uniform magnetic fields? "If it isn't worth doing, it isn't
worth doing well."

Tim.

 

[John Woodgate]

I read in sci.electronics.design that Jim Denton
<jimdenton@rtonline.com> wrote (in <9rt5u09ibdj1aho7dd1fg4g5u15e6456gp@4
ax.com&gt about 'Audio EMF coil?', on Tue, 11 Jan 2005:

Many thanks for your kind offer John.

The coil is intended for an experimental setup pertaining to my
research on plant response to EMF's, and it seems Helmholtz is the way
to go. Signals are complex audio, not single frequency.

The exact bandwidth has not yet been established. So I suppose I am
initially looking at compromised performance over the entire 100-20KHz
range. I can progressivley narrow this down according to my
obervations.

You may well need to reduce the bandwidth at the high frequency end. The
low end can go down to DC if you want!

Since I have a 100W audio amp, and don't need that much power,

Because the coils are almost always significantly inductive, power is
not really relevant. It's best to consider the voltage and current
capacities of your amplifier separately. In addition, the inductive load
causes very significant extra heating of the output devices. Since you
don't say what field strength you are aiming for, I can't tell whether
your 20 V/5 A amplifier is enough or not.

Depending on the design of the short-circuit protection in the
amplifier, you may be able to get more than 5 A out of it provided the
voltage is lower than 20 V, i.e. the load impedance is less than 4 ohms.
Audio amplifiers are often designed to do that, to allow for high
transient currents demanded by loudspeakers on some types of signal.

a major
degree of inefficiency can be tolerated, provided the waveforms are
not distorted. Preferred coils diameter would be 1-2 metres.

If you can just offer a good starting point, and the key design
considerations, I can hopefully take it from there, ie. number of
turns, wire size, any passives, etc.

The starting points have to be the field strength you require in the

'uniform' volume, and the formula (for your particular implementation of
the Helmholtz construction) relating that to the 'sheet current' in the
coils. The sheet current is the actual current multiplied by the number
of turns.

Because the resistance of the coil is proportional to the number of
turns, but the inductance is proportional to the square of the number of
turns, you use as few turns as possible if you are trying to use a
conventional amplifier. The point is that the frequency response is -3
dB at the (high) frequency at which the inductive reactance is equal to
the resistance. A single turn 1 m diameter coil has an inductance of
about 5 uH, which is about 0.63 ohms at 20 kHz. A 10 turn coil has an
inductance of about 90 times as much (90, not 100 because the coupling
between turns isn't perfect.)

I can't go any further without the field strength and 'current sheet'
data. In any case, I'm going off-line until Saturday, but maybe the
above will help a bit.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Ken Smith]

In article <1105450500.177012.81950@f14g2000cwb.googlegroups.com>,
Tim Shoppa <shoppa@trailing-edge.com> wrote:
[...]

I suspect that iron-bearing minerals in the soil will mess up all your
efforts at making uniform magnetic fields.

Unless you set out to do so, you won't end up with soil that has a mu more
than about 20% more than free space.

Oh well, there's no effect to measure anyway, so why bother with
ultra-uniform magnetic fields? "If it isn't worth doing, it isn't
worth doing well."

I can't see that a uniform field would matter much if there is an effect
to be detected either.

--
--
kensmith@rahul.net forging knowledge

 

[Rich Grise]

On Tue, 11 Jan 2005 14:44:24 +0000, Ken Smith wrote:

In article <1105450500.177012.81950@f14g2000cwb.googlegroups.com>,
Tim Shoppa <shoppa@trailing-edge.com> wrote:
[...]
I suspect that iron-bearing minerals in the soil will mess up all your
efforts at making uniform magnetic fields.

Unless you set out to do so, you won't end up with soil that has a mu more
than about 20% more than free space.

Oh well, there's no effect to measure anyway, so why bother with
ultra-uniform magnetic fields? "If it isn't worth doing, it isn't
worth doing well."

I can't see that a uniform field would matter much if there is an effect
to be detected either.

I don't know how either of you can know what will happen, since he hasn't
even made his tests yet.

Sheesh! Talk about blind faith!

I'd like to see what happens if he can make a toroidal field, like what
you'd see in a toroidal core, but I have no idea how you'd do that in
practice, air-core, without running a conductor right through the plant.

But I'm looking forward to hearing if he has any results. Plants _do_
respond to stimuli, you know.

Thanks,
Rich

 

[Ken Smith]

In article <pan.2005.01.11.19.29.49.962090@example.net>,
Rich Grise <richgrise@example.net> wrote:

On Tue, 11 Jan 2005 14:44:24 +0000, Ken Smith wrote:
[...]
I can't see that a uniform field would matter much if there is an effect
to be detected either.


I don't know how either of you can know what will happen, since he hasn't
even made his tests yet.

Sheesh! Talk about blind faith!

Can you think of any way that the uniformity of the field would matter to
his detecting an effect? Even though I doubt there is an effect, I didn't
say there wouldn't be.

But I'm looking forward to hearing if he has any results. Plants _do_
respond to stimuli, you know.

Yes but audio frequency radio waves are something I expect them to simply
ignore.
--
--
kensmith@rahul.net forging knowledge

 

[Dirk Bruere at Neopax]

Jim Denton wrote:

I would like to generate a relatively uniform magnetic field from
complex audio frequencies within a room-size area. The amplifier I
have does 100W into 4 ohms.

In terms of coil configuration what is the most straightforward and/or
effective approach? I am assuming air core here.

Perhaps a series of 100W cross-over coils adding up to 4 ohms. One
each could be aligned to x, y and z axis.

Or a wide area coil, one on the floor and one on the ceiling,
Helmholtz style.

Hope I haven't frightened anyone. Just fishing for suggestions..

Any relation to my putative TMS project for a whole roomfull of people?

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org