Bass distortion in induction recording

pixelated

Active member
I noticed that I'm getting some bass distortion in one of my induction recordings.  I'm recording what looks like a 6 in. woofer in a small 2-way speaker, using a good ol' Radio Shack telephone pickup.  The midrange and highs sound fine and the live audio did not sound "hot" at all so I'm curious as to the how and why the distortion is happening.  Do low-frequency magnetic waves need a different pickup coil shape or configuration?  Do I need to wind my own pickups? ;D  Thanks!
 
What does it sound like?  Some of my recordings, like the Pixar Place BGM, has a slight distortion sound, even though the levels never peaked.  Post processing made it sound a lot better though.  Any luck with that?
 
I guess I've never heard an induction recording of a woofer but I'm not sure I've ever heard any bass at all in anything recordings I've ever listened to. :p (like.. a little but not much at all that's for sure)
 
If you're curious to hear what I'm hearing :D, I've uploaded to RapidShare two 20-second snippets in FLAC format from the recording:
http://rapidshare.com/files/155127653/Soarin___distorted_.flac.html
http://rapidshare.com/files/155127654/Golden_Dreams__distorted_.flac.html

For purposes of comparison, the last 7 seconds of the "Golden Dreams" snippet don't have whatever accompaniment is distorting; that section of audio sounds OK.
If there is a way to clean up or minimize the distortion, I would be very grateful (and happy) ;D.  Thanks!
 
Obviously my playback on the PC can't be that good as although I could hear a bit of "crackle" on the first one, the second just sounded like percussion but I don't have decent recordings of either to compare with so it may sound bad on a decent Hi-Fi.
Did the actual audio (what you heard with your ears) sound cleaner? The percussion does sound a bit "clicky" on the recording - should it be there at all?
You only get what comes out of the speaker signal and if there's a problem with the actual signal (ie a bit of static or the like) it'll come out on the recording. It's recording the magnetic fluctuations from the magnet in the speaker so, if that's on the way out............................
Some of the speakers have seen better days. :(
Only a few thoughts as I'm new to induction recordings. May be Way out.
 
You absolutely can wind your own pickups...

Not sure by listening to these as to what the problem is.  In my experience, what you hear is actually what the pickups record as well.  A crappy speaker yields a crappy recording.  You can filter out the bass if you want.  I know many poeple use battery poxes with bass roll off when recording with normal microphones.  While it won't work with an induction pickup, it's the same principle.  Personally, I would rather have the original recording and then alter it in post to suit my needs.  Once you eliminate that bass, it's impossible to get it back.

You'll definitely have to do some post processing with any induction recording, or at least that's been my experience.  Some sound pretty darn food, but all of them can at the very least be enhanced with a little noise reduction and some other minor tweaks.
 
Horizons on a slightly off topic that you brought up above, have you happened to post your Pixar Place BGM loop yet and I just missed it (and am a horrible searcher!), or are ya holding it for now?  ;)
 
eyore said:
Did the actual audio (what you heard with your ears) sound cleaner? The percussion does sound a bit "clicky" on the recording - should it be there at all?
You only get what comes out of the speaker signal and if there's a problem with the actual signal (ie a bit of static or the like) it'll come out on the recording. It's recording the magnetic fluctuations from the magnet in the speaker so, if that's on the way out............................

Yes, the actual live audio I was listening to sounded fine.  I normally check several speakers (when possible) for signal strength and audio quality before choosing one to record.  This time I noticed that one of the speaker enclosures sounded like it was physically vibrating or resonating on some of the lower notes so I definitely didn't pick that speaker :D.

And that brings up a hypothesis: could it be caused by mechanical vibration of the pickup?  I've noticed that if the induction pickup falls over because of a twist in the wire when I'm recording a ground speaker, that is recorded as a noticeable <thump> sound.  That's the pickup just rolling over onto its side from its upright position, not falling off the speaker and hitting the ground.  As a test, I tapped the pickup with my finger and I could hear it in my headphones when I did that.

I'm physically attaching the pickup to the speaker enclosure with some putty when I record.  Maybe the vibration of some of the more booming bass notes is being mechanically transmitted through the enclosure and putty into the pickup, causing a more musical "thump" (like distorted bass) to be recorded.  Next time, I'm going to try to mechanically isolate the pickup by hanging it from a thread instead of attaching it directly.

The things we do to get a good recording... ;D
 
Yes, any physical movement WILL alter the recording.  I liken it to a "doppler" effect.  You're moving the received within a field, so if your location within that field moves, even the slightest, it will be apparent in the recording.

I don't think you're recoridng the sound of the movement more so than the stretch or squash of the magnetic field.
 
I think you may have answered your question there.
If the bass notes were making a speaker vibrate, I suggest that the actual notes were so low they were beyond the capability of the speakers to reproduce and maybe so low that they were inaudible to the human ear. They may, however, be within the range of the induction mic but would be "translated" as best it could. The putty should have absorbed any physical vibration but it's possible that the low notes could be causing the mic to vibrate as well.
Like any mic (or speaker), the induction mic has a frequency limitation and maybe those low bass notes were just outside it.
Yes, a thump will be recorded because it will cause a movement of the magnet inside so it's not the sound of the fall that's recorded, it's the movement of the magnet which it translates into a sound (usually a low one because it's a slow movement) - if you follow.
There's some interesting possibilities there.
I wonder if one could wind a mic with a greater frequency response?
After all telephones don't have that good a sound, do they?
Wish I knew more about electronics!
Maybe get a spare mic and compare how they each work (bad mics do exist) with some very low bass notes on your Hi-Fi.
Now there's a possibility for a University paper ;D
 
I have been doing some experiments, with some success, and some failures.

For the casual fan, the winding wire is the prohibitive factor.  It's quite expensive unless you find a "deal".  I picked up some 40 AWG and 44 AWG (maybe it's 42) wire on eBay.  It's thinner than a human hair it seems, which my guy tells me is the key to frequency response.  I haven't quite figured out what the key is to sensitivity.  Is it more or less wire, or possibly a larger or smaller coil?

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.
 
eyore said:
If the bass notes were making a speaker vibrate, I suggest that the actual notes were so low they were beyond the capability of the speakers to reproduce and maybe so low that they were inaudible to the human ear. They may, however, be within the range of the induction mic but would be "translated" as best it could. The putty should have absorbed any physical vibration but it's possible that the low notes could be causing the mic to vibrate as well.
Like any mic (or speaker), the induction mic has a frequency limitation and maybe those low bass notes were just outside it.
Yes, a thump will be recorded because it will cause a movement of the magnet inside so it's not the sound of the fall that's recorded, it's the movement of the magnet which it translates into a sound (usually a low one because it's a slow movement) - if you follow.

I don't think there is a magnet in an induction pickup; I just checked mine with a magnetic compass and the compass needle did not deflect at all.  I think the induction pickup is just a small coil of copper wire.

I once did a short test recording of a big 12 in. woofer by holding the pickup next to the speaker enclosure but without actually touching it.  The recording has some good deep bass tones like you would expect but no noticeable distortion.  I'll try a longer recording to see if that gives me any insight into the low-end frequency response of the pickup.

kirky said:
I haven't quite figured out what the key is to sensitivity.  Is it more or less wire, or possibly a larger or smaller coil?

Hmm, for greater sensitivity, I would guess you want to capture more of the magnetic field, so that means a bigger coil.  More wire on the same sized coil might just mean more electrical resistance though, especially with fine gauge wire, and that would lead to decreased signal strength.

Yes, this does indeed make for some interesting home-brew research.
 
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.
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.
There may be some compromise to get the high and low frequencies satisfactory or there may even be a theoretical limit to the possible frequencies that CAN be captured by this method.
Not being a physicist I can't answer that (but I know a man who can)
see
http://www.freepatentsonline.com/3851117.html Some clues there.

maybe we'll end up inventing the perfect Disney induction mic  ;D
 
My homemade coil DOES NOT have any iron core...  My suspicion is that the iron core stabilizes the field somewhat, so it's not necessary, but wold probably help create a cleaner signal.  In my tests, I've heard a very rapid high pitched beeping (it's easy to filter out in post) that the OEM pickups don't produce.  My pickup is also significantly larger in diameter (the indie diameter is close to the outside diameter of the OEM pickup.

One thing I have confirmed is the direction of the coil (axis) DOES matter in relation to the speaker magnet.  You record the same content, but the sensitivity seems to be better when it's aligned parallel to the speaker magnet.  At least in my home tests (with both the OEM and homebrew mic).
 
kirky said:
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. 
 
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!
 
eyore said:
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.

eyore said:
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. 
 
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) ;D
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" ;)
 
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 :(.
 
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.
 
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..
 
kirky said:
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 ;D.
 
eyore said:
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...
 
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.
 
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.
 
eyore said:
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 ;D.
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.
 
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...
 
kirky said:
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.
 
pixelated said:
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.
 
CAScreaminDude said:
pixelated said:
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.

I've re-uploaded it to MediaFire.  This link won't expire anytime soon:
http://www.mediafire.com/file/ynzzjnezykn/Jumpin' Jellyfish woofer.flac
 
Thanks. The bass is definitely present and not distorted, from what I can tell. Obviously, the higher pitches are somewhat muted, similar to the Pier (Boardwalk) induction of yours. Whatever method you used for that recording sure seemed to work. But, which speaker did you find near Jelly that allows for that close of a recording?
 
CAScreaminDude said:
Thanks. The bass is definitely present and not distorted, from what I can tell. Obviously, the higher pitches are somewhat muted, similar to the Pier (Boardwalk) induction of yours. Whatever method you used for that recording sure seemed to work. But, which speaker did you find near Jelly that allows for that close of a recording?

I used a five-foot extension to reach the woofer of one of the pole-mounted 3-way speakers near Jumpin'.  As for the recorded audio, you shouldn't hear a significant amount of treble tones coming from a woofer since it wasn't designed to play those tones.  The crossover circuitry in the speaker will route most of the treble tones to the midrange driver and the tweeter.
 
How did you get it up there!!!!
Or was that a telescopic doo-dah that was 5 feet?
I know some here do carry things like radio aerials around with them to reach the speakers.
That's dedication ;D
 
eyore said:
How did you get it up there!!!!
Or was that a telescopic doo-dah that was 5 feet?
I know some here do carry things like radio aerials around with them to reach the speakers.
That's dedication ;D

You are correct, it's a 5 ft. telescoping rod.  The audio extension cable was 6 ft. long.

CAScreaminDude said:
How long did you stand there with the five foot cable ? ;D Sounds like we should fashion a suction cup mic to throw up at the box.

I think I was there for only around 5-7 mins. because:
1) it's hard to keep the end of the rod steady and since that's where the induction pickup is, that makes the signal fade in and out and change tone
2) your arms get tired from trying to keep a 5 ft. long object steady and from just holding the thing in the air
3) it's not very stealthy

The bottom of the speaker is just over 11 ft. from the ground so the chances of a thrown/tossed sticky mic actually sticking to the speaker enclosure are are not good and perhaps more importantly, how do you make the mic un-stick itself on command once you're done recording?
 
After 2 years and a simple Wikipedia search, I think I've answered my own question about why induction pickups seem to be sensitive to vibration.  I've highlighted the important sentence in red:

http://en.wikipedia.org/wiki/Inductor
Air core inductor
The term air core coil describes an inductor that does not use a magnetic core made of a ferromagnetic material. The term refers to coils wound on plastic, ceramic, or other nonmagnetic forms, as well as those that actually have air inside the windings. Air core coils have lower inductance than ferromagnetic core coils, but are often used at high frequencies because they are free from energy losses called core losses that occur in ferromagnetic cores, which increase with frequency. A side effect that can occur in air core coils in which the winding is not rigidly supported on a form is 'microphony': mechanical vibration of the windings can cause variations in the inductance.
The telephone pickups that I've opened do have a mild steel core so they are not air core inductors but the actual wire coil itself is wound around a plastic bobbin, sometimes loosely.  The wire is not coated with something like lacquer or varnish to make the coil a single solid mass, so the windings can vibrate.  That causes a change in the inductance which in turn changes the induced current going into the recorder.

The same Wikipedia article says:
Core materials with a higher permeability than air increase the magnetic field and confine it closely to the inductor, thereby increasing the inductance.
Plain steel has a relative permeability of 100 and air has a relative permeability of 1 (http://en.wikipedia.org/wiki/Permeability_(electromagnetism)) so if I remove the mild steel core from a pickup, the pickup's sensitivity should drop.  And if I replace the steel core with a Permalloy core (relative permeability of about 8,000 to 100,000 depending on the alloy), the sensitivity should increase.

Something to try on a rainy day... ;D
 
Interesting.
I wonder if coating the winding with something like nail varnish would fix the basic vibration problem?
Or is that too simple?
I've been fairly lucky with the few induction recordings I have made (plus nothing to compare them to) but better results can't be bad regardless.
Do let us know if you have success replacing the core.  At least the things are cheap enough to have a few failures and any improvement on an already fairly successful method of recording would be great.
 
eyore said:
Interesting.
I wonder if coating the winding with something like nail varnish would fix the basic vibration problem?
Or is that too simple?
That would probably only stabilize the outer windings.  If the entire coil could be thoroughly soaked, that might work.

I've been fairly lucky with the few induction recordings I have made (plus nothing to compare them to) but better results can't be bad regardless.
I've only really noticed the distortion on speakers that were being driven at high volumes, where you could easily feel the entire housing buzzing.

Do let us know if you have success replacing the core.  At least the things are cheap enough to have a few failures and any improvement on an already fairly successful method of recording would be great.
I haven't gotten an actual price for Permalloy or other similar high-mu alloys but what I've read say they are pretty expensive.  I've been told ferrite alloy cores are almost as good and significantly cheaper so I'm going to try those first.

If you're keen on designing your own induction pickup, give this a read:
"Magnetic Field Measurement" by Steven A. Macintyre
http://www.radio-astronomy.org/library/Magnetic%20Field%20Measurement.pdf

An induction pickup is just a specialized magnetometer after all ;).
 
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