Effects of magnetic fields

There are very accurate meters to measure magnetic field. We used them to investigate the effect of fields on our inertial missile guidance system, and it is a fact that the earth's magnetic field affected instrument performance (Gyro drift, Accelerometer Scale Factor) and that the system performace varied depending on which way it was pointed (geographically). This was in spite of the fact that the system itself included very strong magnets in the gimbal torque motors. An earlier generation of guidance system had a vidicon (tube) camera for the star sensor, and this was also affected by which way it was pointed when being tested.

A CRT (TV picture tube) is very sensitive to DC magnetic field because the electrons "fly" a long way between the gun and the tube face, and the whole design is aimed at allowing the electron beam to be easily deflected so as to make the picture. An amplification tube, on the other hand, has closely spaced emitter and plate, and a little deflection of the electrons doesn't matter as long as they all end up on the plate.

Loudspeakers have a DC field. AC magnetic fields are another matter and can cause hum by interacting with any element of the amplifier circuitry, most often interconnects.

Thanks for the response. Yes I have heard of the Texas Instruments shield and of course mu-metal. Since my listening room is small and I have many toys including Legacy Sig III's , 2 subs , and a zillion power cords I may be in a magnetic jungle.

Wish I could answer your question as to how to measure the magnetic field, but I don't know.

Jim Aud of Purist brought a device to my home some years ago and could read all the fields in my room, including the location of wire in the walls.

He built a isolation platform that contained a shield, I assume similar to TI shield and it could be placed in between my sub and the Soundlab and get identical improvement as moving the sub. I think they were built for Japan and never marketed here.

The TI shield was at one time sold by Mike Percy. I have not tried it but assume it performs similar function.

Darrylhifi,

The effect I heard was from moving the sub woofer SIDEWAYS, away from the Soundlab Ultimates.

Since the Soundlab is a dipole, the sub at it's side has very little effect acoustically. Anyway, I'm discussing the sound with the sub woofer (un- shielded box) in an inoperative state, merely it's presence both magnetically and electrically.

The magnetic field in the sub is strong due to the 15" woofer and even worse, the built in plate amp (200 watt) effected the power supply of the Soundlab. I assume the step up transformer or bias circuit was being effected but cannot say for certain.

Herman is correct that moving speakers have great effect sonically but in this instance the sub was definitely effecting the Soundlab, other than acoustically.

Since magnetic fields are so easily seen on televisions when speaker fields are near I don't see much stretch in imagining it could effect some circuits during music playback.

Changing the position of your speakers will change how they sound mostly because of how they are interacting with the room and their relationship to your listening position. Im most situations these factors will far outweigh any effects due to magnetic fields. All of your equipment is also affected by the vibrations in the room and the strength of these vibrations is different at different points in the room.

If you move equipment other than the speakers and the hum level goes up or down then that is probably due to magnetic fields, but if some other aspect changes with no difference in hum levels then that is probably due to differences in pressure (vibration) levels.

You are correct, there are many variables and it is difficult to isolate them.

Albert is their any way to easily measure it ? Do not know too much about it , except I believe the positive magnetic field generally moves forward, the negative , backwards , as in the back of the speakers, is that correct ? I wonder, in the context of how my equipment always seems to sound different depending upon where it is placed in my room . Trying to do Audio and Video, their are so many variables . Thanks !

I'm still trying to figure out what the magnet in a Verdier TT must do to the field around a phono cartridge. Must not be a problem but you would think there would be some effect.

Yes I thought so. I was told today by Chris at Legacy Audio that when they ship speakers on an Airplane by law they must be 18 inches from certain parts of the aircraft because of the magnetic field. Also I notice the performance of my Audio Magic Eclipse PLC seems to waiver if its in too close in proximity to large woofers. Also my speakers tend to sound better if they are spead apart further than conventional suggestions. Could it be because when they are spead out , they are further from my electronics ? Interesting.

Yes it's possible to hear the effect.

The proximity of my HT sub woofers effect the sonics of my Soundlab Ultimates by disrupting the back-plate (power supply). The sound is improved proportionately by increasing distance between the large woofer and the Soundlab as well as removing the power cord of the sub from the wall outlet.

Guess the Soundlab doesn't like the field from the amp either.

That's an interesting question. Any flow of electrons has an associated magnetic field. Any magnetic field will have an effect on the flow of electrons in it's vicinity. That's how transformers work and how magnets can affect a picture tube.

So the short answer is yes, all of these fields and flowing electrons interact. Is it audible? It depends on the strength of the signal being affected compared to the orientation and the strength of the magnetic fields trying to affect it.

Put a phono stage dealing with very small signals next to a big power amplifier with large transformers and it is very easy to hear the humming that results because the large fields from the amp transformers induce currents in the phono stage, but the phono stage will have a much smaller effect on the amp.

It is directly proportional to the strength of the field, the distance betwen the devices, and how they are oriented.