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I've read that anything below 300 Hz should be cut in mono on vinyl; others say 100 Hz should be the threshold. I always thought it mattered most how much music was cramped onto each side, because the bass sounds required horizontal space. Metallica, e.g., reissued one of their early albums as a double issue, because the single disc issue “sounded like shit” (sorry, can't remember which one, only that it was one of the records from Ride the Lightning to … And Justice for All); bands with complex audioscapes, such as Dream Theater, prefer no more than around 15′ per side, further strengthening my belief.

But belief is not enough, of course. What is the hard reality? Does it matter whether one has more horizontal space per groove for the overall low frequency sound experience? And are there any ways to overcome the 100/300 MHz limit (Which is it?) on stereo sound?

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Let's start by saying, even if you could, it doesn't make a great deal of sense to have a heavy stereo split in the bass frequencies.

Let's try to analyse why that would be...

First, your directional senses are very poor at low frequencies, so assuming you were sitting slap bang in the middle of your huge hi-fi speakers you'd still barely be able to tell where it was coming from.
It would be slightly stranger on headphones, but they're an 'artificial' stereo anyway & can mimic sounds coming from 'inside your head' which can't happen on any other reproduction system.

Next, take a club, playing current dance music. The bass is panned heavily to one side... over there everybody's enjoying it, over here where you are, you can barely hear it at all.

So, even on CD or wav/flac/mp3 it doesn't function as you'd think.

...and that's before we even get to vinyl.

Vinyl is a physical medium where a fairly literal analogue representation of the sound wave has to be cut into the surface; deep enough for a stylus to be able to track it & read it.
Vinyl already suffers from being capable of throwing the needle out of the groove, even with current cutting techniques. Records are cut using a special frequency EQ curve known as an RIAA curve. This massively reduces the amount of bass that needs to be cut into the vinyl & to compensate, an opposite curve is used in a phono pre-amp to put it all back again as it's played.

The groove, as a representation of the analogue sound wave uses a V-shaped groove, one side is the left channel, the other is the right. Each side must independently move from side to side depending on the current frequency it it reproducing. The higher the frequency, the quicker & smaller this movement is.
By the time you get to the bass frequencies, this is producing quite a considerable amount of movement in such a small space.

If the bass is mono & centred, then the two sides of the groove move in sync with each other. This is the best you can achieve & all you then need to beware of is how much volume you're cutting.
This is why if you want a lot of depth &/or volume, you need to make each side of your album shorter, to get the grooves deep enough & separated enough that you don't let the needle simply cross from one groove to the next if they get too close.

If the bass is hard-panned [ie all in one speaker] or even worse, out of phase* , then the two sides are no longer in sync. This results in the groove getting by turn deeper/wider & shallower/narrower. You don't need to be too far out of phase before it will simply toss the needle out of the groove.

The 'fix' employed by cutting engineers & smarter mix engineers is two-fold. First, you don't pan the bass away from the centre to start with.
Secondly, for stray frequencies from reverbs or echoes etc which could throw some complex phase discrepancies into the bottom end of the completed mix without being truly audible, you can employ a device [or these days plugin] that will sum the stereo to mono, only below a chosen frequency. It does this gradually, so you will never notice that suddenly at one frequency you're in mono.
The frequency [& sometimes curve] are chosen by the mix engineer - or cutting engineer if the mixer didn't know it had to be done.
I normally set mine at around 70 - 120 Hz, depending on the source material. Sometimes the only way to judge is to use your ears.

But them we come to another controversial issue...

Once you get below about 45Hz it becomes even harder to make the needle stay on the groove.
So, to combat this, another EQ is strapped across the entire mix & frequencies below that are dialled out, again in a slope so you don't hear it abruptly stop.

Modern [80's] lathes can cut louder & to lower frequencies than earlier machines, because they can read the groove ahead & adjust the track gap automatically - but they still have a mechanical limit.

I realise, reading this back , that it may be a bit rambling. If anything isn't clear, drop in a comment & I'll attempt to clarify.

Phase* - phase would take a whole page to explain, but the very short version is that it would make the two sides of the groove move at different times, causing the overall groove to narrow & widen unpredictably, which can throw the needle out of the groove

Re: genre - for pretty much all modern music types it makes sense to pan the bass centre. Checking for 'true' mono at lower frequencies is less important in modern digital delivery/listening methods [anything from CD to FLAC, MP3 etc] as they don't suffer the physical limits of vinyl, but it can still affect the overall 'punch' of the sound, so I would still expect the mix or mastering engineer to at least pay it some attention.

Things are slightly different in classical music. For one, the basses are traditionally at the rear right from the conductor/audience's perspective. There is also a massive amount of room sound [reverb, reflection from the room itself] contributing to the overall sound of an orchestra & this will induce phase issues.
However, the relative volumes when compared to modern 'pop' make it far less of an issue at the cut & [I'm guessing, I've only ever mastered one classical track & that was choral, so no bass to speak of] that these could be mainly ignored.

  • This is an excellent and enlightening answer, which gives the necessary background to understand the basics of the problem. Could you add some parenthetical explanations of hard-panned and phase (left and right, i.e. + and −?) in “If the bass is hard-panned or even worse, out of phase …” You explain well why bass has to be mono (there really is no gain in having it stereo, listening-wise), and this necessarily has to apply to any low-frequency sound, but where (our in which area) is the threshold? And does the genre matter (e.g. metal vs. funk vs. baroque vs. full-blown romantic Wagner)? – Canned Man Mar 2 '17 at 20:51
  • sure - end of day for me right now, but I'll add more info in the morning :) – Tetsujin Mar 2 '17 at 20:53
  • Excellent, I'm looking forward to it! Take care and good night. – Canned Man Mar 2 '17 at 20:58
  • Made some tweaks - sorry, was a busy week :) – Tetsujin Mar 8 '17 at 9:38

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