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[Horizons]

I have a homemade mic out there in the parks, but I'm not sure how much better or worse it is than the normal Radio Shack mic.

I have tried Kirky's mic out several times and I haven't noticed any difference in results compared to the standard Radio Shack mic, with the exception of the Illumination preshow spiels. 

[eyore]

Not much on these thing via the Internet but I did read an interesting thread regarding finding buried speaker wires on a Hi-Fi tech site. in your house and wondered if anyone had tried this in the parks for those hidden cables.
Progressive Electronics Model 701K "Tone and Probe Kit". http://technicalconnectionsinc.stores.yahoo.net/prel70cltoan.html
From what I was reading, it seems you can follow the speaker cable (hidden) and find the area with the highest signal strength (then, presumably, try the induction mic on it).
Anyone tried it?
I was going to have a look at the photos C33 posted (innards of the mic) but, alas, the pictures have been removed from the thread

Amazing how little info on their construction is available on the Internet though. They all refer back to buying one from Radio Shack!

[pixelated]

Not being as brave as Kirky, I haven't taken one to bits BUT I thought the whole point was the basis of an iron rod inside a coil - as the rod responds to the signal it becomes magnetized and creates the magnetic field so no, you won't pick up any magnet in there, it's far too weak (and only "on" when recording).

Ah...  I was thinking that the magnetic field from the speaker by itself was strong enough to induce a current in the pickup coil.  A small iron rod would explain the shape of the pickup and the sounds generated just by tapping the pickup.

Out of curiosity, did you record anything with deep bass that DID come out OK?
As we have said, it may just have been poor speakers.

This is a 20 sec. clip of a woofer playing the "Jumpin' Jellyfish" BGM.  I converted it to FLAC format straight from the original recording so it hasn't been processed at all.  The chord at the end are the deepest notes I've recorded so far (I can feel my desk vibrate when I play it ) and it sounds OK to me:
http://rapidshare.com/files/155589023/Jumpin__Jellyfish_woofer.flac.html

For this recording, the pickup was very close to the speaker but not touching it. 

[eyore]

Looks like I may be mistaken about the rod. It's certainly the movement of the rod (moved by the diaphragm ) that "makes" the sound in a standard mic so I had presumed that an induction mic would be similar minus the diaphragm  (so the air-borne sounds wouldn't get picked up) but I guess you are right that the coil itself picks up the magnetic fluctuation from the magnet in the speaker (hence the reason to get so close).
At least you have shown that the induction mic works correctly so we know we are dealing with either something to do with the speaker itself or, as you say, the low frequency causing a vibration in the mic which gets recorded if it's touching the speaker. Have you tried recording the same clip with the mic touching the housing and see if it's distorted at all?
Reproducing a problem will lead to the solution.
As a safety net, I'd buy a second mic though (not all of them are perfect so maybe you have one that's touching the plastic or something) as they are pretty cheap and, let's face it, most of us don't get to record in the parks that often so it's worth the expense .
I would think that outside vibration (within reason) shouldn't be a problem if the mic is OK (no moving parts).
At least, people pushing past me whilst I was holding one to the front of a speaker with one hand (bumping and sliding over the perforated grill) didn't alter the recording I did nor was there a problem (other than losing the sound) when the mic fell off a mushroom one and fell on the floor (no sticky dots with me)
Can others confirm that?
I know C33 took one to bits to see what was inside
Bear in mind I'm not well experienced in this form of recording (one holiday only) so take what I say with the proverbial "pinch of salt"

[pixelated]

I made a new recording of the same speaker but this time, the induction pickup was literally hanging by a thread and the pickup body was kept from touching the speaker by a springy loop of cellophane tape.  Here is the same section of "Golden Dreams" as before but this new version doesn't have the same bass distortion:
http://rapidshare.com/files/155731314/Golden_Dreams__decoupled_.flac.html

Compared to the distorted version, the bass response is much lower but I can restore most of the bass with equalization.  That's much easier than trying to clean up the distorted version.

I'll follow your suggestion and buy another pickup to see if the problem is only with this particular pickup.

If other people want to test their induction pickups, just setup your pickup and recorder as if you were checking sound levels before starting a recording session, but keep the pickup away from any magnetic fields so the recorder gets a steady input signal.  Tap the pickup with a finger or a pen while watching the recorder's VU meter to see if it jumps.  If there is no visible movement on the meter, plug a set of headphones into the recorder and tap the pickup again while listening through the headphones for any sounds caused by the tapping.

I can clearly see the VU meter spike on my recorder and I hear a nice lound thump in the headphones when I do this with my induction pickup .

[kirky]

If I understand right, what you're saying is it's possible that the bass eminating from the speaker made some vibrations that may have impacted your first recording.  For the second recording, you improvised a shock absorber of some sort and found that the same distortion was NOT evident on the recording.

This would seem to support the theory that the any movement of the pickup will produce unwanted noise in your final recording.  It may also be possible to conclude that the low frequencies are more apt to do this.  Which pretty much coincides with subwoofers effects of feeling the sounds moreso than hearing the sound.

[eyore]

As I said earlier, I don't seem to have had a problem with moving the pickup (nor even with dropping it whilst recording) but, of course, I got mine in the UK. maybe there is a slight difference inside them (i.e. coil not touching anything, shock absorber inside etc).
I'll try and experiment a little with my pickup and see if I can get it to make any unwanted sounds.
I'm sure these telephone pickups are cheaply made (i.e. not very good quality control) and thrown together in a factory somewhere (probably China) and the placing of the coil may well vary from one to another.
This is all good information we can use to get the best possible recordings!
Kirky, does your home-made one suffer from movement noise as well?
You've probably made sure the coil doesn't touch anything and is secure so that may answer my question..

[pixelated]

If I understand right, what you're saying is it's possible that the bass eminating from the speaker made some vibrations that may have impacted your first recording.  For the second recording, you improvised a shock absorber of some sort and found that the same distortion was NOT evident on the recording.

Yes kirky, that is absolutely right.  I MacGyver'd a vibration isolation mount for the induction pickup:

• Wind a short piece of sewing thread around the pickup, between the suction cup and the pickup body itself.
• Make a loop of tape, sticky side facing out, and attach it to the side of the pickup you want facing the speaker.  It keeps the pickup from touching the speaker and also anchors the pickup so it doesn't swing or twist.
  
• Tape the thread to the speaker so the pickup is hanging down in the spot you want to record.
• Place and orient the pickup the way you want and apply a little pressure so the tape loop sticks to the speaker.  There you have it!
  

The sewing thread takes up the weight of the pickup and its wiring and the tape loop provides vibration isolation.

This was a fun experiment to perform but I'd rather have a pickup that isn't sensitive to mechanical vibrations .

[kirky]

Kirky, does your home-made one suffer from movement noise as well?
You've probably made sure the coil doesn't touch anything and is secure so that may answer my question..

I used a disc about 2" in diameter and about 3/8" high.  So, it's fairly large compared the the standard one.  It is NOT encased in any plastic packaging, you can see the coil wires from the side.  It's got a little protective layer on the top and bottom to protect the soldering from contact with metal, etc.

Except for outside forces, it's probably not going to move once in position.  The cord is a heavier cord than the radio shack version so it weighs more and probably requires more permanent dots than the radio shack version.

I'm due to make another one, I've got some experimenting to do however.  I'll try to get more scientific this time.  I should post my tests, and will if I get a chance.  Everything pointed to the final prototype should have been more sensitive than the OEM one.  It's strange that it doesn't seem to work that way.

Horizons, any idea of the position of the speakers you've tried it on vs the position of the pickup?  I think the coil and the magnet should be parallel to each other.  That "might" explain why it seems to do a better job in one location than another.  Just a guess...

[Horizons]

I'm not sure Kirky.  I know the recordings that you designed it for (e.g., Space Mountain, the U.K., etc...), there was no difference at all between the two mics, trying all types of different placements.  I have only tried it on those recordings where the regular pickup wasn't doing the job.

[pixelated]

I found some induction pickups in a surplus electronics store.  They were only $2 each so I bought 4 of them.  I tried tapping each one with a pen after plugging them one at a time into my recorder; the VU meter jumped for all 4 pickups .  Tapping the pickups on the side opposite the wire seemed to have the greatest effect while tapping on the top or the bottom of the pickup seemed to have the least effect.

The next experiment with the new pickups will be to see if deep loud notes will cause them to have the same type of bass distortion as with my $8 Radio Shack pickup.

[pixelated]

Not being as brave as Kirky, I haven't taken one to bits BUT I thought the whole point was the basis of an iron rod inside a coil - as the rod responds to the signal it becomes magnetized and creates the magnetic field so no, you won't pick up any magnet in there, it's far too weak (and only "on" when recording).
There is the possibility that these particular ones don't have a core as they are so small but what's the coil wound around? I'm a little more familiar with larger stuff and induction currents - may not be quite the same.

I thought I'd revive this thread with some new information .
I took apart one of the $2 surplus store induction pickups and you are correct, there is a steel rod inside the coil.  The magnet wire is wound around a small plastic bobbin and there is a small steel rod at the center of the bobbin.  The rod seems to be very tightly held and can't move; it was probably pressed into the bobbin.  So I'm still not sure how tapping the pickup produces an electrical signal but now that I have just the coil, I can experiment further.

[kirky]

I'm not so sure it's steel, it is probably ferrite...  The coil still works without it, which makes me think it's more there to stabilize the field around the coil than anything else.  I really haven't checked into the whole physics behind an induction coil, I'm more of a trial and error type of guy.

The tapping of the device doesn't produce any signal, the tapping moves the pickup and the resulting recording has some distortion at that point int he recording.  I liken it to a doppler effect...

[pixelated]

I'm not so sure it's steel, it is probably ferrite...  The coil still works without it, which makes me think it's more there to stabilize the field around the coil than anything else.  I really haven't checked into the whole physics behind an induction coil, I'm more of a trial and error type of guy.

The tapping of the device doesn't produce any signal, the tapping moves the pickup and the resulting recording has some distortion at that point int he recording.  I liken it to a doppler effect...

The core/rod looks like steel; it's shiny with a slight blueish tinge.  I remember ferrite being a dark brown.  The core is also strongly attracted to a magnet whereas I remember ferrite being only somewhat attracted to a magnet.  Ferrite would be a better choice but this is a $2 induction pickup so steel was probably cheaper.

I'm tapping the induction pickup with a plastic pen, far away from any magnetic field (except for the Earth's own magnetic field I suppose) and I'm seeing 20 dB spikes on the VU meter on my recorder.  That's what's puzzling me.  How is the kinetic energy from that pen being transformed into electromagnetic energy?  I've disabled the phantom power on my recorder so there shouldn't be any current running through the coil to generate a magnetic field that is disturbed by my tapping.

[CAScreaminDude]

This is a 20 sec. clip of a woofer playing the "Jumpin' Jellyfish" BGM.  I converted it to FLAC format straight from the original recording so it hasn't been processed at all.  The chord at the end are the deepest notes I've recorded so far (I can feel my desk vibrate when I play it ) and it sounds OK to me:
http://rapidshare.com/files/155589023/Jumpin__Jellyfish_woofer.flac.html

For this recording, the pickup was very close to the speaker but not touching it. 

Do you still have this clip? The link has expired.