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..
This is the only way to get a fairly uniform field over a large volume. 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. 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.
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.
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
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.
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.
I read in sci.electronics.design that Jim Denton wrote (in ) about 'Audio EMF coil?', on Sun, 9 Jan 2005:
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.
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.
No. Much too small, and remember, a magnetic field can have only ONE direction at any point in space.
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?
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Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
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I read in sci.electronics.design that Ben Bradley wrote (in ) about 'Audio EMF coil?', on Mon, 10 Jan 2005:
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.
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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
I read in sci.electronics.design that Tim Shoppa wrote (in ) about 'Audio EMF coil?', on Mon, 10 Jan 2005:
First find you spherical room.
I have somewhere a paper about a circular room with a hollow pillar in the middle which allowed a toroidal coil to be installed! But the very uniform field is horizontal, of course, whereas hearing aid pick-up coils are almost always vertical.
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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
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 ?
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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.
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.
I read in sci.electronics.design that Jim Denton wrote (in ) about 'Audio EMF coil?', on Tue, 11 Jan 2005:
You may well need to reduce the bandwidth at the high frequency end. The low end can go down to DC if you want!
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.
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
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.
Yes but audio frequency radio waves are something I expect them to simply ignore.
Experiments are good. We should all do experiments. BUT! Over the decades, I've seen many experiments die under their own weight. Experimenters tend toward asking for the moon on the first experiment. They don't want anything obfuscating their experimental results. They waste their energy on perfecting the experiment and lose sight of the goal.
It makes no sense to try to develop an extremely uniform field when you have no idea that ANY field will cause an effect. Biological experiments take a LONG time. Non-uniformity of the field is an asset. You get multiple results from the same test run on different parts of the plant. You'd be better off with multiple simultaneous small non-uniform fields operating with different stimulus waveforms on different plants. Once you determine that something, anything is happening, move on to narrow the testing and removing unknowns. If field strength matters, it will be obvious in the growth patterns. It's hard to get anywhere in experimental biology without massive parallelism.
If you find a method to make your "plants" grow faster/better, publish it here. Automated procedures would beat the hell out of "singing to it" all the time...although it can make you feel like singing...or so I'm told. ;-) mike
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