Graham's Blog Archive
Why Rumble Filters Are Bad
The beginner to records might think a rumble filter to be a good idea because it attenuates warps and bearing noise. However, it isn’t all that clever because it also attenuates the low bass frequencies — and worse.
You could argue that your speakers only go as low as 75 cycles (which isn’t low bass), so what does it matter? And if so, why are you concerned your cones flap with such low energy?
To attenuate “rumble” the filter must have sharp cut because the frequencies where warps and bearing noise happen is only a couple of octaves below the lowest piano note, if that.
With a first order filter you’re only going to get 12dB attenuation (6dB per octave) so the “rumble” will only sound just less than half as loud.
The phase shift however is 45 degrees at the filter turnover so if it’s at say 27 cycles (lowest piano note), the low frequency phase response is going to be out up to around 270 cycles (where it’s 6 degrees out).Phase Response
The effect it has is on perceived timing: your hearing is so sensitive that it can detect such phase differences and tell you where the sound is relative to other sounds.Phase Shift
Taking phase differences to extremes you get the phasing or “flanging” effects such as you’ll hear on records such as Skywriter (Jackson Five). This demonstrates how changes in time alignment can be heard. As a musical effect it’s OK, but distorted otherwise.
To get more “rumble” attenuation means using more filter sections than a simple first order filter. Each additional section displaces the phase response another 6 degrees at frequencies 10 times that of the filter turnover frequency. So in-between we will hear the effect of phase shift – but not musically like the Jackson’s hit – it will just sound out of step – so wrong.
But also consider this: the record companies started to record with bass cut also. This was around the beginning of the microgroove record, so virtually all your records have some bass cut which simply isn’t equalised in the RIAA stage, because it’s in addition to the recording curve. Why did they do it? To squeeze more playing time per side.
With the introduction of CD such bass cut became evident because CD didn’t have the groove restraints of vinyl.
A rumble filter simply adds to the bass cut making your already “cut” records sound bass light. Plus, it “time-steps” the bass in relation to the higher frequencies (where our hearing is most sensitive) so much so it can confuse the ear into believing the bass is playing out of tempo.
So rumble filters are a bad idea IMO. But if you’re getting cone flap and you’re worried about damage to your woofers (which you shouldn’t be unless they’re audibly hitting the stops), then it isn’t bearing rumble. What it is, is arm/cartridge resonance, where warps make the arm/cartridge exaggerate the warp. This is where you need to select your arm and cartridge carefully. It’s OK chasing after review hype, but if it leads to a mismatch then you’re simply not going to get good results.
Here I’ll hand you over to the experts who will explain that:
In most cases a rumble filter is a sticking plaster get-by. Better to do without.