Assessment of Stray Magnetic Fields
To get an idea on what is involved, we started by assessing the stray magnetic fields of objects and equipment typically used in a Sound Studio or Archive. The measured values are tabulated in the Appendix. All test objects were tested for AC and DC fields. Where there is no entry for one of these it was non-existent, or negligible, that is, below 0.5 Oe.
In all DC measurements, the magnetic field of the earth was not compensated for (this would have complicated the measurements unnecessarily), so there is an uncertainty of ± 0.4 Oe which we found acceptable for our purpose.
From the table it can be seen that all pieces of equipment incorporating permanent magnets exhi-bit strong DC stray fields, i.e., loudspeakers (especially small ones), dynamic microphones, dynamic headphones (Beyer DT 48), moving-coil instruments, etc. AC stray fields originate of course from power transformers in mains-operated equipment (including tape recorders).
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Stray Magnetic Field Test Results |
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Test Objects:
Measure values: Test Equipment: Bell Inc. Model 640 Incremental Gaussmeter |
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| Test Object | Measurement Location | H (Oe) | D (mm) |
| Loudspeakers | |||
| TANNOY Arden | front | 19.0 | 150 |
| back | 17.0 | 130 | |
| side | 3.0 | ||
| top | 5.0 | ||
| TANNOY Berkeley | front | 20.0 | 150 |
| back | 36.0 | 150 | |
| side | 4.0 | ||
| top | 1.0 | ||
| ACRON 100c | front | 150.0 | 120 |
| back | 60.0 | 120 | |
| side | 50.0 | 100 | |
| top | 160.0 | 80 | |
| B & O Type M | front | 30.0 | 20 |
| back | 1.0 | ||
| side | 1.0 | ||
| top | 1.0 | ||
| Microphones | |||
| AKG D19 | front | 90.0 | 30 |
| side | 30.0 | 12 | |
| AKG D900 | side | 30.0 | 2 |
| AKG D190 | front | 90.0 | 30 |
| side | 30.0 | 12 | |
| AKG D58 | front | 90.0 | 30 |
| side | 30.0 | 12 | |
| AKG D160 | front | 90.0 | 30 |
| side | 30.0 | 12 | |
| AKG D24 | front | 90.0 | 30 |
| side | 30.0 | 12 | |
| Headphones | |||
| KOSS Pro-5 LG | inside | 100.0 | 50 |
| outside | 20.0 | 20 | |
| KOSS ESP-9 (electrostatic) | power supply | 10.0 AC | |
| AKG K240 | inside | 100.0 | 25 |
| outside | 20.0 | 5 | |
| AKG K140 | inside | 45.0 | 20 |
| outside | 25.0 | 10 | |
| BEYER DT48 | inside | 230.0 | 70 |
| outside | 150.0 | 70 | |
| side | 80.0 | 50 | |
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Tape recorders |
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| STUDER B67 | maximum AC | 0.5 AC | |
| loudspeaker top surface | 30.0 DC | ||
| in tape path | 7.0 DC | ||
| pinch roller sol.* top surface | 15.0 DC | ||
| in tape path | 7.0 DC | ||
| STUDER B62 | pinch roller sol.* top surface | 10.0 DC | |
| in tape path | 6.0 DC | ||
| STUDER A67 | pinch roller sol.* top surface | 11.0 DC | |
| in tape path | 5.0 DC | ||
| VU-meters | 20.0 | 6 | |
| REVOX A77 | pinch roller sol.* top surface | 20.0 DC | |
| in tape path | 10.0 DC | ||
| NAGRA 4.2 | loudspeaker top surface | 8.0 | |
| in tape path | 5.0 | ||
| side | 15.0 | 20 | |
| Stellavox SP7 | loudspeaker top surface | 8.0 | |
| in tape path | 5.0 | ||
| power supply | 1.0 AC | ||
| Tandberg Type 11 | loudspeaker top surface | 20.0 | 20 |
| in tape path | 5.0 | ||
| Uher 40001C | loudspeaker front | 20.0 | 30 |
| in tape path | 12.0 | ||
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Miscellaneous Equipment |
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| LEEVERS-RICH Bulk Eraser | in tape path (10.5" reel) | 600 to 800 AC | 150 |
| bottom | 10.0 AC | 100 | |
| sides | 10.0 AC | 100 | |
| Hand-held De-Magnetizer Type 1 | tip | 800.0 AC | |
| shaft | 400.0 AC | ||
| handle | 500.0 AC | ||
| back | 1500.0 AC | 150 | |
| Hand-held De-Magnetizer Type 2 | tip | 600.0 AC | |
| back | 1000.0 AC | 120 | |
| Welding Transformer 1.8 kVA Model PROMETHEUS | idling | 40.0 AC | 100 |
| short circuit | 400.0 AC | 500 | |
| Desk-Top Telephone | back of receiver | 22.0 DC | 30 |
| left side of case | 4.0 DC | ||
| PPM Instruments Ernest Turner | front | 100.0 DC | 50 |
| Mixing Console (FELS) | 0.8 AC | ||
| Mixing Console (Custom) | 0.5 AC | ||
| Tone Generator (Heathkit) | 3.0 AC | ||
| Tone Generator (Custom) | 0.2 AC | ||
| Universal AF Filter | 5.0 AC | ||
| Pro-Amplifier (QUAD) | 0.6 AC | ||
| Pro-Amplifier (Kenwood) | 0.5 AC | ||
| Pro-Amplifier (Custom) | 0.5 AC | ||
| Multimeter UNIGOR 6e | top | 70.0 DC | 60 |
| side | 15.0 DC | 50 | |
| bottom | 10.0 DC | 10 | |
| 60 W Fluoresent Lamp | near internal choke | 2.0 AC | |
| Typewriter IBM | 0.8 AC | ||
| Typewriter OLIVETTI Editor 3 | top | 1.5 AC | |
| bottom | 3.0 DC | ||
*sol.= solenoid
