Low end rubbish

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Low end rubbish

Post by Stig Ø »

I posted this in the Mac forum,but thought maybe it was more appropriate here. Mods, please move or tell me off if this isn’t how I should be going about it. Anyway:

I have just recorded a couple of acoustic ditties - solo steel string arrangements. I’m working with eq and compression to give it some polish. In the multimeter in Logic I see some low end rubbish going all the way down to 16Hz! I can’t figure out how to get rid of it - I’ve high-pass filtered the track, both with the channel eq and with a one band on the main buss. What am I missing? I’ve made sure that the meter is placed after the eq.

Edit to add that it’s low in volume, below -60dBFS. I’m still curious as to how to handle it. Let’s say I were to have it pressed on vinyl (wishful thinking), I would prefer to hand over files that were free from stray low end noise, as an example.

- Stig
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Re: Low end rubbish

Post by The Elf »

Meters don't always show the truth - especially at LF. Can you *hear* any of this low end?
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Re: Low end rubbish

Post by RichardT »

Yes, at the low end, meters can be misleading sometimes. What filter cutoff frequency and slope are you using? With that instrumentation you should be able to start quite high up with a gentle filter, which reduces artefacts.
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Re: Low end rubbish

Post by The Elf »

A technique I use for assessing low end is to place a finger *gently* against the edge of a speaker cone. You can learn a lot about the low end using this technique.
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Re: Low end rubbish

Post by resistorman »

The Elf wrote: Mon Jul 04, 2022 5:42 pm A technique I use for assessing low end is to place a finger *gently* against the edge of a speaker cone. You can learn a lot about the low end using this technique.

:clap: Nice, hadn't thought of that!
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Re: Low end rubbish

Post by Hugh Robjohns »

The guys are right in that FFT displays attached to EQs can be very misleading at the low end because they often have very poor resolution to keep the processing overhead down. Some FFTs allow you to increase the block length and select alternate windowing options which can radically improve LF detail.

You haven't said if these acoustic guitars are being recorded via mics or bugs/pickups. The latter are notorious for generating subsonic mush. Its unusual to suffer lots of VLF from mics — especially directional mics — unless its mechanical vibrstions through the stand (foot tapping?) Or breezes from air con or open windows. Good shockmounts and wind gags are the antidote there.

But removal will need a steep high pass filter ( or two at staggered frequencies) . Ideally 18dB/oct from as high as you can without losing wanted tone and as the very first step before any compression. You might need more after compression, depending on what you're doing/using.
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Re: Low end rubbish

Post by The Elf »

resistorman wrote: Mon Jul 04, 2022 6:14 pm
The Elf wrote: Mon Jul 04, 2022 5:42 pm A technique I use for assessing low end is to place a finger *gently* against the edge of a speaker cone. You can learn a lot about the low end using this technique.

:clap: Nice, hadn't thought of that!

I started doing it way back in the late 70s, but as time has gone on I've had other engineers tell me they do it too! Just shows ya...

I believe the last time we discussed this is was dubbed the 'Elven finger'! :lol:
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Re: Low end rubbish

Post by Stig Ø »

Thanks, everyone - wasn't aware of the properties of FFT at low frequencies. I cannot hear it on my small KRK monitors - it's below -60dBFS, and goes from around 30 down to 16Hz. As to how the guitar was recorded, it was miked with a Røde (can't remember the model name, but it's 20-odd years old) LDC, pointed at the 12th fret from 20-30cm distance. The guitar in question is a Dreadnought, so the neck meets the body at the 14th fret. I'm positive that neither foot-tapping nor wind noise caused the offending sub content. The filters had center frequencies at approx. 100Hz, and the one-band HP on the master buss has a 24bB/octave slope. The lowest frequencies rear their ugly heads when I make a somewhat forceful strum - kind of a rasguedo - at certain points in the tune. I'll try the fingertip-on-surround technique mentioned. The funny thing is, I just purchased Mike Senior's book, and suddenly remembered reading the exact same thing there, yesterday or the day before. :blush:
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Re: Low end rubbish

Post by RichardT »

Stig Ø wrote: Mon Jul 04, 2022 8:55 pm Thanks, everyone - wasn't aware of the properties of FFT at low frequencies. I cannot hear it on my small KRK monitors - it's below -60dBFS, and goes from around 30 down to 16Hz. As to how the guitar was recorded, it was miked with a Røde (can't remember the model name, but it's 20-odd years old) LDC, pointed at the 12th fret from 20-30cm distance. The guitar in question is a Dreadnought, so the neck meets the body at the 14th fret. I'm positive that neither foot-tapping nor wind noise caused the offending sub content. The filters had center frequencies at approx. 100Hz, and the one-band HP on the master buss has a 24bB/octave slope. The lowest frequencies rear their ugly heads when I make a somewhat forceful strum - kind of a rasguedo - at certain points in the tune. I'll try the fingertip-on-surround technique mentioned. The funny thing is, I just purchased Mike Senior's book, and suddenly remembered reading the exact same thing there, yesterday or the day before. :blush:

24dB is pretty steep for an HP filter - but with little or no wanted material <100Hz, it probably doesn’t matter much! At 30Hz that filter is already about 40dB down, almost two octaves, so your noise is about 100dB down. I would say you don’t need your second filter!
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Re: Low end rubbish

Post by Tim Gillett »

The low E note on a guitar is in the 80 Hz region but that doesnt mean other lower freq. energy cant be produced playing the guitar in certain ways.

Maybe you're actually making that sound when you the "rasguedo" thing.

As so often, an audio file example would probably speed up diagnosis.
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Re: Low end rubbish

Post by Hugh Robjohns »

Stig Ø wrote: Mon Jul 04, 2022 8:55 pmThe filters had center frequencies at approx. 100Hz, and the one-band HP on the master buss has a 24bB/octave slope.

The master buss is waaaayyyy too late for filtering out subsonics. You've got to get rid of them at the earliest point in the chain. Otherwise all your other processing -- especially compression -- is likely to make the situation far worse than it need be.

So if you have a subsonic filter on the mic, use it. If that's not doing enough and you have a high-pass filter on the preamp, use that too... or instead.

And once the signal is in the digital domain, the very first plugin should be a third-order (18dB/oct) high-pass filter. Some filters are steeper -- analogue synth filters are commonly 24dB/octave and digital filter algorithms can often provide some seriously steep filter slopes -- but 18dB/oct is about the best compromise for effective subsonic removeable with minimal audible side-effects.

If your plugins have a linear-phase option, use that too because steep filters do inherently mess up the low-end phase response and you might not like the audible effect (it can start to sound slightly unnatural). Experiment with the high-pass filter's corner frequency (centre frequency applies only to band-pass filters) to find a setting that minimises unwanted subsonics without affecting tone.
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Re: Low end rubbish

Post by RichardT »

Tim Gillett wrote: Tue Jul 05, 2022 4:20 am The low E note on a guitar is in the 80 Hz region but that doesnt mean other lower freq. energy cant be produced playing the guitar in certain ways.

Maybe you're actually making that sound when you the "rasguedo" thing.

As so often, an audio file example would probably speed up diagnosis.

That sounds highly likely to me based on what the OP is saying. Rasgueado does create a lot of noise as well as tonal sounds.
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