So I’ve been struggling for a while trying yo figure out the best way to accurately “delete” a mic in Helix, so I could accurately measure the eq curves the speaker cabinets in Helix, without having them “colored” by the microphone. Basically what I was looking to do was create a calibration file for a microphone, and then use it to measure speaker cabinets in helix. Once I had that calibration dialed in I could Use it to erase mics, or cabs or whatever I needed to In order to get the measurements I was after. The ticket was being able to eliminate one or other a mic or a cab.
I’ve been back and forth with this for a while. I started with speaker curves. I figured what the heck. I’ll just make an upside down version of a speaker response chart using a bunch of stacked EQ’s in helix, convert it to an IR and bingo. I’ll erase the speaker, be left with the mic curve, convert it to a house eq file, and go measure the speaker curves in helix. Here’s the problem…
This is a typical Speaker frequency response plot. I just figured even if it was smoothed out a bit, it would come out ok.. Right? Nope. The problem with trying to recreate a speaker curve and using it to reveal a Mic curve is 1 you can’t get the required resolution by using the availabe eq’s in helix, and 2 any errors you do make due to smoothing etc will probably be larger than the entire dynamic range of the microphone you are trying to measure. So needless to say then, using this erroneous curve to make determinations about other things …well lets just say it was a colossal fail and leave it at that. I gave up on it for quite a while.
My next attempt came closer. Lets try and recreate a Microphone eq curve, turn it upside down and Bingo! We can erase that sucker and reveal the speaker cabinet eq curves I’m after. So i quickly jumped onto google grabbed a SM57 microphone eq curve and dutifully recreated it using eq blocks, turned it upside down added it to a stock cab block set the mic to match and… It sucked too. Here’s the SM57 eq curve.
Problem 1.. It aint exactly Hi res. Problem 2 Its lacking critical info like how far from the speaker it is during this test. Oh hell its an SM 57 these are always pressed against the grill cloth right? Just set the helix mic distance as low as it goes and you should be good to go right? …Nope. More crappy data. I was bummed, and I gave up on this for a while. Until one day I was starting to collect EQ curve plots for my blog cause I’m planning on at least listing the mics in helix, even if I cant accurately measure them. For the sake of completeness I figured what the heck I’ll start collecting all the factory spec sheets and throw together a post. (i haven’t posted it yet btw) Thats when I found this.. It may as well be the rosetta stone as far as I’m concerned.
You really do have to admire german engineers for their thoroughness sometimes. I suddenly had the data necessary to proceed. To be honest I had seen this graph before, But I didn’t realize how it would assist me. I initially dismissed it as useless, Not knowing which bass response graph to choose. But when I took this second look at it i saw the Mic distance information, and decided to take another crack at it. I carefully reproduced the curve using the high and low shelving EQ and a couple parametric EQs from helix. and I came up with this.
Next step flip it upside down.
And lastly…. Convert it to an IR, for convenience sake.
Now to test it. But how do I do that? lets delete some cabs and look at the mic curves and see what they look like as compared to the factory spec sheets.
Ok so here in blue is a frequency sweep of a 1×12 Us deluxe followed by my IR. The data is then “calibrated” by the software and a new eq curve is created called a “house file” This file when Summed with the Input data generates a flat line shown in green. Every measurement I take now is subjected to that House file and thus anything I change is revealed in detail. Notice the deviations below 40hz and above 16khz. My test equipment has a really hard time measuring extremely quiet signals at the limits of its range.
Anyway… In my travels Looking for Mic data I also came across this beauty (once again thank you German Engineers) Unfortunately the resolution of this document doesn’t come thru in this screenshot, but Once again its a super detailed frequency response curve of a Mic in helix.
so lets switch Helix to this mic and see what we get…. Should be close I hope.
Ok well that was a bit unexpected, But it shouldn’t have been remember my test equipment is trash at measuring really low signals below 40HZ and above 15k or so. The good news is so are guitar speakers, and we generally use low and High cuts in these areas anyway. so lets try that and see what we get.
Remember its supposed to look like the U87 graph from up there 👆… and it does. First test looks ok. Lets check it against another mic I have a decent chart for. 
Ok Sennheiser 421 more detailed graph.. Lets Switch helix to Dynamic 421 and see what we get. …Fingers crossed.
Ok time for one last test. Our old friend the SM 57 remember this guy
I’ll go ahead and say thats a job well done.
Thats all for today
E
Gain Staging Geekery 
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