Bass roll-off question

The principle which applies is that the earlier in the chain you get the signal corrected, the better off you are. As Harvey pointed out, the gentle slope of a mic's rolloff is more pleasing than what you could expect from say a Mackie's low shelving eq. Plus, you've sent a more manageable signal to your preamp, and subsequent points in the chain.

As for bass and bit-conservation, perhaps it's useful to think in terms of the amount of wattage required to push bass and sub bass frequencies versus higher stuff. Think of a 50 watt guitar amp vs. a 50 watt bass amp. The bass requires a much more powerful amp to produce a comparable level. Highpassing subsonic frequencies enables engineers to squeeze more dbs to get the hottest levels, when the client demands more.
-kent
 
Whoa, we're actually getting into two different subjects here. First of all, the bass rolloff switch in most mics is primarily for one single purpose; to eliminate the effects of bass boost due to the proximity effect. It's usually a gentle roll off that starts around 250 to 400 Hz, and just keeps going, rolling off more bass the lower it goes.

Board EQ usually doesn't work that way. You have shelving EQ which drops the signal to a certain level and then keeps everything below that at the same level. Or you have peaking EQ, which is usually some form of sweeping so that it achieves maximum cut at one frequency and then returns to normal below that. Here's a picture that might help:

micfiltr.gif


The red line would be the way a shelving EQ works, in this case, the corner frequency (80Hz) is down 3dB, then the cut drops to -12dB and stays there.

In the case of peaking EQ (the green line) , the 12dB down point is centered at 80Hz, and comes back up to normal on both sides of the center frequency.

The blue line represents a typical proximity filter built into the bass rolloff switch. It starts way higher and just gradually rolls off everything more and more as it goes lower.

Now let's talk about bass power and why it's a good idea to filter some of it out, and the various methods available.

It takes more power from the amp to drive a speaker as the notes get lower in frequency to maintain the same signal level in the room, assuming a perfectly flat speaker. How much more power does it take? This is gonna surprise some of you. Each octave lower takes DOUBLE the power to maintain the same signal level as the preceding octave.

Let's see how that works out:

Let's say you're putting out 1/2 watt at 20,000 Hz.
You'll need 1 watt to get the same sound level at 10,000 Hz,
2 watts at 5,000 Hz,
4 watts at 2,500 Hz,
8 watts at 1,250
16 watts at 625 Hz,
32 watts at 312 Hz,
64 watts at 156 Hz,
128 watts at 78 Hz,
256 watts at 39 Hz,
and 512 watts at 20 Hz.

Most CDs start rolling off the extreme bass below 30 or 40 Hz, simply because most speakers can't handle those kinds of power levels, and at those very low frequencies, the cones would just kinda flap around in the wind till the woofer's voice coil heats up and they automatically self-destruct.

The range that most people associate with "Wow, listen to that huge bottom end" is usually around 50 to 80Hz. Since most speakers don't really go much below 50Hz, you're just taxing the system and wasting power by trying to boost stuff below that point. That's the purpose of a "subsonic filter".

The other thing to remember is that just because it says you're boosting 80 Hz, the EQ is also boosting frequencies on either side of 80 Hz as well. Start cranking the bass on some channels and you may really be building up "mud" an octave or more above the points you think you're boosting.
 
Harvey, one question.
What about the preamp stage? Is it also better to cut bass right in the mic, or is it the same to do it in the board or in the preamp? I mean, disregarding the difference in the eq curves that you showed in the previous chart.
 
Remember that on most mixers, the EQ section is after the mic preamp, so you're reducing your available dynamic range when you let the full range signal come thru the preamp to be cut later on with the board EQ. If you're gonna cut bottom, it's always better to cut as early as possible to increase headroom in later stages.
 
thanks, I understand it.
Are there external cut-off switches that you know, or something that you can build easily? Something that can operate at mic level to add that feature to say, the Mars V67.
Cheers, Andrés
 
Thanks, Harvey! I always wondered why some manufacturers offer "1000 watt" amplifiers with their active subwoofers.

Mark H.
 
Harvey, what about the Bass cut-off switch on the mixer? It's before the preamp, isn't it? Which of the lines does it resemble, if any?
 
Most aren't - I'd hafta look at the block diagram.

Simple test: Plug the output from the channel insert jack directly into the recording unit and flick the rolloff switch switch while recording. If you can hear a difference on playback, it's probably before the EQ section.

Second test: If the channel has a clip light see if the light will go out when you hit the channel with a big low end signal (like a low end synth signal plugged into the mic pre), and you hit the bass rolloff switch. If the clip light stays on, the bass rolloff switch is after the preamp.
 
Great thread:)

Fantastic explanations everyone.

I never thought about the wasted head room



Thanks.

F.S.
 
Last edited:
cordura21 said:
There's a page in Ethan Winer's site that shows how to do rolloff circuits and also mic pads:

Mike Pads and other Small Gadgets

Is there a commercial standalone rolloff for mics?

Cheers, ANdrés
Click to expand...

I'd like to know if there is something like this as well or any other solution for the matter (is there any software that can cut eq before it gets recorded in say cubase ?), I bought 2 of the older C1s that dont have the switches.
 
Last edited:
you can use a band pass (or band reject) filter. There is one free for Pro Tools, and I'm pretty sure there must be a free one for vst. You can also use a shelving eq with very deep envelope.

The idea of an analog one (be in the microphone itself) or stand alone, is just better cause you reduce the bandwith use before the signal hits the convertors. Right?
 
This thread on bass rolloff is very interesting to me right now. I have a pair of MXL 603s mics, and used with a Tascam DA-P1 DAT recorder to record a live performance at Lukenbach Dance Hall a couple months back. Unfortunately I was way in the back and the bass was really boomy in there. I kept noticing that I had this sporadic harsh sounding distortion that did not seem to go away when lowering the recording levels, none of the peaks appeared even close to 0dB. It was very frustrating. That place had a wooden floor and I finally concluded that it must be acoustic coupling into the mic stand that caused this. I did not have shock mounts installed for the mics. I'm pretty new at this stuff, so could someone confirm that this was a classic mistake I made? Or if not, perhaps a bass rolloff as suggested above might have helped out?

Thanks a bunch!

Drewboy
 
You must log in or register to reply here.
Back
Top