One of the big advantages of magnetic tape over any other recording medium is that the stored information can be erased and the tape used over and over again. If, however, tape is to be used as a permanent store of information, as in a Sound Archive, this property is a very big disadvantage indeed. It would be fine if one could - by some still unknown process similar to that of fixing a photographic picture - freeze the once recorded sound for good.

Aside from complete erasure, which can be considered the extreme case, the magnetic inscription on the tape can be influenced by a number of factors including:

• Humidity
• Fungus
• Chemicals
• Age
• Print-through

In addition, and perhaps most dangerously, the magnetic inscription can be influenced by external magnetic fields.

G.A. Knight and others have thoroughly investigated the influence of AC and DC magnetic fields on stored tape. In particular, they studied print-through, partial erasure, increase in distortion and noise, etc., caused by external fields originating from electric machines, lightning strikes and the like.

In his paper G.A. Knight suggests that a value of 25 Oe (not distinguishing between AC and DC) may be accepted as a level “below which very little degradation is likely to occur”. No information is given, however, on the degree of degradation that is considered to be acceptable. Also, there appears to be no literature on what happens to tape being moved through a stray magnetic field.

At the Phonogrammarchiv, we came across this question when we discovered that many technical devices used in a studio exhibit quite strong stray magnetic fields - in fact, even tape recorders of prominent manufacture show field strengths which, at first, shocked us and then prompted a closer study. The question is, what happens to a pre-recorded tape travelling through a magnetic field, AC or DC, of, say, 5 Oe, or 50 Oe, or more.

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).

Stray Magnetic Field Test Results
Test Objects: various pieces of equipment usualy found in a Sound Archive were tested for magnetic stray fields. Where necessary, explanatory remarks are given. Measure values: H- Magnetic Field Strength (AC or DC) in Oersteds, measured on the surface of the test object (unless mentioned otherwise). Note: 1 Oe = 80 A/m. D - Distance in mm from test object surface where measured field strength is 5 Oersteds. Test Equipment: Bell Inc. Model 640 Incremental Gaussmeter
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
Tape recorders
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
Miscellaneous Equipment
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

Our next concern then was to assess the effect of stray fields on tape moving on a tape recorder or tape winder, and especially, to find a maximum permissible value of field strength below which no deterioration occurs.

What we were looking for in our tests were partial erasure or loss in playback level, increase in distortion, increase in noise and, ultimately, recording of the stray field onto the tape.

Depending on its source, a stray magnetic field can take many forms from a very homogenous one (from a distant source) to a rather sharply focused one (from a source near or in the tape machine). As the latter case was thought to be most detrimental and as we deliberately wanted to establish the worst case conditions, we decided on a test setup where the stray field in the tape path was generated by an iron-core inductor with a large air gap of some 7 mm. Also, again looking for the worst case, we found that stray fields perpendicular to the tape surface had the largest effect.

Our tests were carried out using a Nagra 4.2 L tape machine and Agfa PE-46 tape, since it was our aim to test a tape right in the middle of the coercitivity range to obtain typical results which would also be of significance for other tapes. Pure sinewave signals of different frequencies and magnetization levels were recorded and the tape subjected to a calibrated stray field during playback. Playback level and waveform were monitored by an RMS voltmeter (weighted or unweighted), an oscilloscope and a distortion analyzer.

It soon became clear, however, that it was not possible to obtain conclusive results with that method. After getting some very puzzling test results we decided to analyze the playback waveform on a spectrum analyzer and there we found the reasons for the misleading test results using conventional equipment. Also, the spectral analysis proved extremely instructive, and in some cases surprising.

It would exceed the scope of this report to give details on all the many tests we made. They can be summarized as follows:

• DC stray field: increase in low-frequency noise starting at 50 Oe, loss in playback level starting at 100 Oe, no appreciable increase in distortion up to 200 Oe;
• 50 Hz AC stray field: partial erasure of original signal, and recording of 50 Hz stray field starting at 10-50 Oe.

It is interesting to note that the stray field is recorded not only in the form of the 50 Hz fundamental and its harmonics but also in the form of sidebands around the original recorded signal and its harmonics. The number and amplitude of these sidebands increase with increasing stray field strength and their effect is clearly audible even for stray fields as low as 20 Oe.

Much to our surprise, we also found that small stray fields between 10 and 50 Oe add 50 Hz sidebands to the even-numbered harmonics of the original signal only (Fig. l. a - d). The mechanism causing this peculiarity is not yet fully understood and there is some evidence that it may in part be due to the form of the magnetic field used in the test setup. Further work will be necessary to find a satisfactory explanation. It should be noted that all tests were made using 50 Hz power line frequency fields as these are most commonly encountered in practice (60 Hz in the USA will give similar results). Stray magnetic fields of higher frequency are most unlikely in a studio.

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Figure 1 Spectrograms of pre-recorded tape subjected to AC stray field during playback

Parameters:

Recorded test fequency 1 kHz with harmonics
Record level 0 dB
Tape machine Nagra 4.2L
Tape Agfa PE46
Stray field 50 Hz AC, sinusoidal

Figure 1a: Stray Field 0 Oe

 

Figure 1b: Stray Field 10 Oe

 

Figure 1c: Stray Field 20 Oe

 

Figure 1d: Stray Field 50 Oe

As our study has shown that already rather weak stray fields can harm archival tapes and that these values are within the range exhibited by equipment used in a studio, it would seem wise to have some safeguard against any “magnetic accidents”.

We propose, therefore, to establish a new warning label to be attached to any piece of equipment liable to emit stray magnetic fields, similar to the well-known radioactivity warning label. It should show a universally understood symbol for a magnet and should indicate the nature of the field, AC or DC, and give the worst-case 5 Oe-distance or enveloping radius in mm.

It has been shown that stray magnetic fields as low as 10 Oe AC or 50 Oe DC can cause permanent damage to recorded magnetic tape. Including a safety factor of 2, it is recommended to keep tapes away from fields exceeding 5 Oe AC or 25 Oe DC.

In practice, special care must be taken with carrying cases housing tape recorder, recorded tapes, dynamic microphones and dynamic headphones adjacent to each other. On the other hand, loudspeakers (except those in miniature enclosures) are not nearly as dangerous as has been thought up to now. Also, the AC and DC stray fields found in the tape path of tape recorders can be considered harmless. They could, however, easily be avoided completely by proper design of the machine.

This paper was presented at the IASA Technical Committee Session in Salzburg and some very interesting suggestions were made in the discussion period. These focused on magnetic hazards to tape during transport. These problems will be dealt with in a subsequent report which we hope will also include data on metal detectors, magnetic gates (in airports, etc.), electric trolleys, fork lift trucks, electric railways, air freight, etc.

Acknowledgements

Dipl.Ing. Franz Lechleitner (Phonogrammarchiv der Österreichischen Akademie der Wissenschaften) for invaluable help with measurements;

Dr. Werner A. Deutsch (Kommission für Schallforschung der Osterr. Akademie der Wissenschaften) for doing the spectral analyses;

Institute für Allgemeine Elektrotechnik der Technischen Universität Wien, for assistance in instrumentation.

Bibliography

G.A. Knight: “Factors Relating to Long Term Storage of Magnetic Tape,” Phonographic Bulletin, no. 18, (July 1977), pp. 15-35.