Science Buddies: "Ask an Expert"

I'm trying a slight variation on the Wire Recorder (https://www.sciencebuddies.org/science- ... shtml#help) for an art project. My objective is to record audio onto thick steel (a couple millimeters or more). I've been successful recording onto thin steel shims with a pre-made tape head taken out of a tape recorder. Adding a high frequency signal (to act as the bias) into a mixer along with the microphone audio helped improve the sound a lot. But the thicker the metal the worse/inaudible the audio becomes.

I'm thinking I need to compensate for the loss by either substantially increasing the output voltage (when recording, playing, or both?) or increasing the magnetism in the tape head. I tried making a large recording/playback head similar to the head listed on the sciencebuddies page. The magnet was about 1" diameter with a .3" hole in the center. I made a small gap with my diamond blade and dremel. I used 40 gauge magnet wire and wrapped about 300 times, giving the wire a resistance of about 500 ohms when checked with my multi-meter. It would not even work on the thin steel. I've read that modern tape heads have a piece of gold in the gap for better transfer. Filling the gap with a 14k gold wire did not help.

What is the key to to record onto thicker steel? Am I correct in thinking increasing the output voltage and/or increasing the magnetism in the tape head will work? If so, how do I make a very strong head? Was I going in the right direction with the big ferrite core and large magnet wire bundle? Or, is there any equipment that will increase the output more? Currently I'm using a powered mixing board (connected to large-ish powered speakers for playback). This video claims (around 1:10) recording can be done on scissors, knives, etc, and I don't image this $40 wire recorder kit contains anything so hi-tech that I can't reproduce those results: http://www.youtube.com/watch?feature=pl ... QiVFEkVzy0

Any suggestions you can offer would be greatly appreciated. Thanks in advance!

sps5582 wrote:What is the key to to record onto thicker steel?

What makes you think that one can magnetically record a signal onto thick steel? All of the commercally used magnetic recording media that I'm familiar with are all thin magnetic layers on top of a thicker non-magnetic substrate.

Hi Craig,
Assuming this is accurate, the wire recorder kit here shows recording onto thicker items such as scissor and knife blades (at about 1 min and 10 sec in):
http://www.youtube.com/watch?feature=pl ... QiVFEkVzy0. So I imagine it can be done, as I have been somewhat successful on different gauge steel shims, but the loss is great as I increase the gauge.

sps5582 wrote:the wire recorder kit here shows recording onto thicker items such as scissor and knife blades

Scissors and knife blades are often alloys and may even be plated with a thin layer of a different material. If the plating material is one that is easily magnetized compared to the base alloy, then you are in fact recording on a thin layer of magnetic material. Alloys that contain a combination of magnetic and non-magnetic materials. The non-magnetic material won't drain energy from the recording magnetic field and will make the material behave more like a thin magnetic film material for recording and playback purposes.

sps5582 wrote:I have been somewhat successful on different gauge steel shims, but the loss is great as I increase the gauge.

This results matches my expectations. It takes more energy to create a local magnetic domain (north or south pole) and the domain is less defined when more material is involved. To record a signal, you have to have alternating domains to represent the sound waveform. To represent higher frequency sounds, you have to either be able to create more domains per inch or use more inches per second of recording media. The faster you move the media, the more concentrated the recording magnetic field needs to be in order to create permanent domains.