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[pixelated]

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?   Thanks!

[Horizons]

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?

[TheWorm]

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. (like.. a little but not much at all that's for sure)

[pixelated]

If you're curious to hear what I'm hearing , 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) .  Thanks!

[eyore]

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.

[kirky]

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.

[IllumiNations82]

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?   

[Horizons]

No, I haven't posted it.

[pixelated]

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 .

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...

[kirky]

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.

[eyore]

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

[kirky]

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.

[pixelated]

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.

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.

[eyore]

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 

[kirky]

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).