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  #16  
Old 02-12-2010, 03:00 PM
DupleMeter DupleMeter is offline
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Originally Posted by rick-slo View Post
This is true Steve, the body of the acoustic guitar gives off some lower frequencies, which is part of the reason why when I do cut off low frequencies I do not begin evaluating that close to the lowest string frequency. Also there is a bell curve, not really a point, around the cutoff frequency selected, plus some filter artifacts. However as always use your ears to decide.
Yes - but low freq. energy is exponential. So you start the roll off in the frequencies you still want to hear. The lowest frequencies you produce on the guitar are way too powerful. You need to cut them back a bit, and @ -6dB/Oct you begin to return the response to a more linear curve. Especially if you used a directional mic closer than 4 feet or so. There is low-end buildup that you need to worry about. Plus, a good lowcut filter (band pass) is going to have a slight bump right above the cutoff point (a baxandall curve) - use that to your advantage.

So - yes, start very close to the target frequency...it's likely already affected by things like mic type & proximity, room modes, et al. You want to return it to a more natural gain structure.

I could math this out - and bore the crap out of everyone...but just use your ears, you will hear it working.

As to filter artifacts - ummm....get a better EQ. Don't know a better way to say it. You should not be hearing any artifacts from your EQ filters if they are any good. And if you are hearing artifacts...they're no good, go get something else. Life is too short to put up with tools that make more work.
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Old 02-12-2010, 04:24 PM
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Steve, I am not following the comment about low frequency energy being exponential and especially in the digital realm but in any case regarding decibel levels in my recording environment low frequencies are at fairly low levels and most of what there is originates from the guitar.

For example a snapshot of background frequencies down to -60db (no filters being used)




and a chord played on the guitar




Regarding artifacts there is always going to be something with whatever use although it may be minimal. I usually use Waves REQ with the Baxandall curve and it works well. BTW the Baxandall curve is a wide one (low Q) which is another reason I feel you don’t need to set it as high a 70 hertz in most cases.
I do agree 100% to use your ears as the final determinant.

My apologies to Sean for going off topic somewhat.
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  #18  
Old 02-12-2010, 05:49 PM
DupleMeter DupleMeter is offline
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Yeah - you need something like Spectra-Foo to see what I'm talking about.

Here's the deal with power vs. frequency. Low frequency waves are exponentially more powerful than higher frequency wave.

If I were to take your recording and play it through my big system here (DynAudio M4s) you would be amazed at how much low end noise was in the recording. By beginning your lowcut within the audible (and desireable) frequencies you tame that power and avoid internal buss clipping (which happens quite easily when you start stacking those plugins). That, in turn, limits headroom and collapses the stereo field.

I've been known to start a rolloff as high as 100Hz for solo acoustic guitar recordings (even when tuned to an A415!). The odd thing is that you now appear to have more bass in the recording because you have removed the subharmonic distortions and have tamed the overload of power that results from the low end. What's left is a very punchy and big low end.

Take a look at these FFT Coefficient graphs showing power vs. frequency:




I hope that explains it better. It's a very counter-intuitive idea. I didn't believe it when I was first shown, either. But I had to take it seriously & really give it some credibility considering the guy who showed this to me was one of the best engineers/producers of his time (possibly of any time).
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  #19  
Old 02-12-2010, 08:20 PM
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Not sure how the graphs are supposed to relate. The Fourier series used to represent a periodic function by decomposing it into sum of much simpler (smaller) components but a spectrograph based on it may not accurately portray energies in the live sound via phase issues. But more to the point say the graphs more closely mimic what you would see in a fundamental tone and its harmonic series. It is anologous to what we would all agree on, that is if you have a fundamental tone on a guitar string the energy contributed by each ascending harmonic number (higher and higher overtone if you wish) is less and less (energy in proportion to inverse of the harmonic number in an ideal situation of which the guitar isn't).
You could say the graphs represent something along those lines. However that says nothing about starting with a lower fundamental tone, adding up its series of harmonic numbers and getting more total energy (amplitude), let alone exponentially. That low frequency waves are exponentially more powerful than higher frequency waves taken to it end points should be prima facie obviously not the case.

As far as rolling off at 100hz you will lose a lot of body and warmth. The only case in solo guitar I can see where it might help is if you recorded in a really bad location with lots low frequency noises. Hopefully that is not the case.

Sorry to be way off topic of the OP and boring to boot.
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Last edited by rick-slo; 02-13-2010 at 07:38 AM.
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  #20  
Old 02-12-2010, 10:25 PM
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DupleMeter, what kinds of guitar tracks are you recording? I see a lot of confusion in discussions about recording acoustic guitar because of different musical styles. Recording a rhythm guitar for use with bands, you'll probably want to kill lots of lows, maybe even higher than 100Hz depending on what's going on and the sound you want. For solo guitar, like Rick and Sdelsorey, I'd think less so. I personally kill the extreme lows with a sharp filter, say below 20-30 Hz, but often boost the low end in general, if anything, because I want that bottom end and warmth on a solo fingerstyle piece, at least for the style I play.
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  #21  
Old 02-13-2010, 06:25 PM
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I think you may need a refresher course in engineering & the physics of audio - and this is not the place for it.

As you were...
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  #22  
Old 02-13-2010, 06:39 PM
DupleMeter DupleMeter is offline
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oops - and I realize I may caused some confusion.

The X Axis on those graphs should be labeled "Freq." - my bad...working too quickly.

So - with that in mind - you can clearly see the power relation, as this was an equal loudness sweep.
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  #23  
Old 02-13-2010, 08:33 PM
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Actually the graph was labeled correctly with Harmonic number, n as it is interger multiples of the fundamental used in the Fourier equations to get this plot. The graph looks like the one that represents the harmonics (partials) you would need to add to a fundamental pure tone to get a sawtooth wave form.
In regard to your last comment I concur
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Last edited by rick-slo; 02-13-2010 at 09:05 PM.
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  #24  
Old 02-14-2010, 08:13 AM
DupleMeter DupleMeter is offline
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Originally Posted by rick-slo View Post
Actually the graph was labeled correctly with Harmonic number, n as it is interger multiples of the fundamental used in the Fourier equations to get this plot. The graph looks like the one that represents the harmonics (partials) you would need to add to a fundamental pure tone to get a sawtooth wave form.
In regard to your last comment I concur
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
-- Mark Twain
No - I swapped labels from another project I was working on at the same time. this was a 0-30Khz sweep vs power (in pascals). I'll repost the correct graph when I can rerun the test (I didn't save it).
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  #25  
Old 02-14-2010, 08:26 AM
DupleMeter DupleMeter is offline
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Originally Posted by Doug Young View Post
DupleMeter, what kinds of guitar tracks are you recording? I see a lot of confusion in discussions about recording acoustic guitar because of different musical styles. Recording a rhythm guitar for use with bands, you'll probably want to kill lots of lows, maybe even higher than 100Hz depending on what's going on and the sound you want. For solo guitar, like Rick and Sdelsorey, I'd think less so. I personally kill the extreme lows with a sharp filter, say below 20-30 Hz, but often boost the low end in general, if anything, because I want that bottom end and warmth on a solo fingerstyle piece, at least for the style I play.
You're taking my statement out of context - I said I *have* started a roll-off that high...not that that would be standard. And I mentioned that that was a solo acoustic recording (of a rather boomy gibson jumbo).

And all of this just seems like running in circles, because for any of what you say to be true would mean you are playing a guitar that is greater than 13 feet in width at some point along the soundboard - because there is no other method for the instrument to produce it's own low note without the use of room modes (a 82.4Hz wave is about 13.7 feet long, a large guitar may have a 17 inch lower bout - leaving a discrepancy of about 12.2 feet of vibrating material to produce that low E). The guitar is producing a very complex wave of overtones to make you believe you're getting that (much like an upright bass has no chance of every producing its lowest note of 41.2Hz, only its overtones).

So you see - while what you believe may seem perfectly logical. In the real world of physics & sound, it's implausible. Which is why it is perfectly acceptable (and desirable) to begin your roll-off within the range of frequencies you want - you're not adversely affecting the overtones and will end up with a stronger low end because of it.

All that you have down there are a buildup of room modes and subharmonic resonances. When you clean that up you can boost around 100-150Hz or so and add warmth and not mud because you won't be adding power to the low end mud that exists below 80Hz. Remember all frequencies below 80Hz are omnidirectional. They permeate a recording and mess with the soundstage and sense of depth.
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  #26  
Old 02-14-2010, 10:59 AM
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And all of this just seems like running in circles, because for any of what you say to be true would mean you are playing a guitar that is greater than 13 feet in width at some point along the soundboard - because there is no other method for the instrument to produce it's own low note without the use of room modes (a 82.4Hz wave is about 13.7 feet long, a large guitar may have a 17 inch lower bout - leaving a discrepancy of about 12.2 feet of vibrating material to produce that low E). The guitar is producing a very complex wave of overtones to make you believe you're getting that (much like an upright bass has no chance of every producing its lowest note of 41.2Hz, only its overtones).
Interesting.
You are saying that an acoustic guitar can not produce a frequency below its maximum body dimension in relation to wavelength. So given a guitar's maximum body dimension which is in the neighborhood of 19 to 20" vertically the lowest note it can produce is about 700 hz. Any perceived tones below that is just a blend of yet higher frequencies which are deceiving the ears. Phantom fundamentals do exist but it is not the explanation here. One has only to look at the graphs posted here to see it is not just some psychoacoustic trick of our ears but that these lower fundamentals are present and measureable.
My headphones will produce sounds down to 18 hz and my 10" speaker drivers down to 28.5 hz. It is much less about size than what the driving forces are. Also with a guitar it is not just the length of flat pieces of wood. We are talking about an enclosed resonant cavity and most of the sound volume being produced by the guitar acting as an air pump.
That said in a decent recording environment and where people are not walking around I do not think there is a lot of very low frequency mucking about although there is some and it does not hurt to filter that out.
On another note it was mentioned that the spectrum graphs I posted earlier were not showing the low frequency energy properly and I should look at a Fourier graph.
Here is a snapshot of the same tune and the preponderence of the energy is right where one would expect it to be.
.



Also at same recording just a moment before I started to play the guitar.
This shows that the lower frequency energy that was present was due to the guitar itself and room reverberations of its sound.
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Last edited by rick-slo; 02-15-2010 at 08:19 AM.
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  #27  
Old 02-14-2010, 01:07 PM
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And all of this just seems like running in circles, because for any of what you say to be true would mean you are playing a guitar that is greater than 13 feet in width at some point along the soundboard - because there is no other method for the instrument to produce it's own low note without the use of room modes (a 82.4Hz wave is about 13.7 feet long, a large guitar may have a 17 inch lower bout - leaving a discrepancy of about 12.2 feet of vibrating material to produce that low E). The guitar is producing a very complex wave of overtones to make you believe you're getting that (much like an upright bass has no chance of every producing its lowest note of 41.2Hz, only its overtones).
Wrong.......
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Old 02-14-2010, 02:07 PM
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...So you see - while what you believe may seem perfectly logical. In the real world of physics & sound, it's implausible.
Hi Duple…
I don't mean to be rude or take this topic off track - you really need to listen to some of Doug's recordings. He's been recording very successfully for a while now...


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Old 02-19-2010, 09:15 AM
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Amazing how a really practical thread can digress so quickly into big science - wow. You guys rock. Makes me wonder what some of your day jobs are....

I'm just hoping this brain trust can bring the thread back to some practical tips for the lay engineer. I know there's room for endless debate and ultimately the ears are the test, but...it would sure be nice to have the high theory boiled down to some practical things one could try out by turning the actual knobs. If so, this is an incredibly useful discussion!
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Old 02-22-2010, 10:41 PM
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Amazing how a really practical thread can digress so quickly into big science - wow. You guys rock. Makes me wonder what some of your day jobs are....

I'm just hoping this brain trust can bring the thread back to some practical tips for the lay engineer. I know there's room for endless debate and ultimately the ears are the test, but...it would sure be nice to have the high theory boiled down to some practical things one could try out by turning the actual knobs. If so, this is an incredibly useful discussion!
Too right, I thought the learning curve was steep enough.... I might as well give up on recording and go climb Everest

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