Techies: a question about frequency curves

[Chris Fitzgerald]

My new Focus 2R has the following specs for the EQ section:

Three Band EQ:
Bass Control: Shelving type, ±15 dB @ 60 Hz
Mid Control: ±15 dB @ 650 Hz
Treble Control: Shelving type, ±15 dB @ 10 kHz
Switchable Notch/High Pass Filter
Notch Filter: -12 dB sweepable from 30 to 700 Hz
Notch Control lets you reduce the volume of a single frequency to help fight a feedback or resonance problem
High Pass Filter: -12 dB/octave sweepable from 30 to 700 Hz
The High Pass filter lets you reduce the response below a frequency of your choice, to fight the mud on boomy stages and in similar situations

I was getting too much bass flab out of one of the channels last night even with the hi pass filter engaged, but had not experimented with really lowering the bass control way down on that channel for fear of losing too many "sweet" bass frequencies. Tonight, I'm going to simply twiddle the bass knob on that channels to my ear's content and go with what sounds best.

That said, my question is this: if I drastically cut the bass shelf filter on that channel, what kind of EQ curve is likely to be produced between the cut centered at 60 hz and the center of the next frequency (at 650 hz), which I plan to leave flat at first? 60hz, in my recording experience, is a frequency that I almost always cut when mixing, but how much will those other frequencies between 60 and 650 be cut if I really crank that control way down? Clearly, I'm just going to screw around with the knob and go with what sounds best, but inquiring minds and all....

[larry]

Chris,

I played around with an EQ plugin I have that draws a line over the frequency spectrum to show what is happening. I set it for a low shelf, -15db @ 60hz. Interesting enough, at 60hz it only cuts half of that, 7.5db. Maybe someone with more experience here can explain that. Regardless, you can see the proportion of the rest of the cuts below, bearing in mind that every EQ will have its own slope. Yours could be steeper or more shallow, but probably not by much. If yours really does cut 15db at 60hz, you could probably just double these numbers and be in the ballpark. Hopefully this helps:

800hz Flat
500hz -.25db
400hz -.5db
300hz -1db
200hz -2db
100hz -5db
80hz -6db
60hz -7.5db

FYI (and off-topic), I know you are into recording as well, and I know you have expressed some displeasure with what homerecording.com has turned into. Have you checked out Gearslutz.com? There are some heavy recording engineers on that site. Check out some of the recording DB threads and you'll read posts from guys who actually did sessions with Ron Carter, etc.. I'll post some links in a little bit.

Edit - Here's a good one: http://www.gearslutz.com/board/showthread.php?t=64710

[robgrow]

Chris,

There's a tone control simulator available on the Duncan Amps web site: http://www.duncanamps.com/tsc/tsc_setup.exe

Run the setup file and go to the "James" tab. You can then move the sliders and see what shelving EQ "looks" like.

BTW the James circuit is very similar to the bass and treble controls on the old Ampeg B-15N amps. Other equalizer designs will vary in where the turnover points are, but the Duncan tone control simulator gives you a general idea how shelving EQ works.

Finally, many years ago, a very well known recording engineer told me to listen and not look, meaning don't look at the where the knobs are on the EQ. Just listen to what the sound is. I've always found that to be very good advice.

[Chris Fitzgerald]

Larry - thanks for the specs and the link: I bookmarked it and will look into that later when school's out. HR.com is a wasteland these days..."BeSt CoNdEnSuR fOr RaP fOr LeSs ThAn $150???".

Oy.

Rob - I wholly agree about the ears/brain thing. I'm just curious about what the term "shelving EQ" really means, and what kind of curve that is. In Digital Performer, I have a bunch or parametric EQ plugins which allow you to select the type of curve you use for EQ cuts. I've gotten pretty good at using those - I think there are five curve types for each band - but I don't know what any of the ones that aren't hi or lo pass are called. The Duncan site thing doesn't work on a Mac.

So is a "shelving" EQ the one with a dip in the center frequency that gradually slopes upward on both sides toward the end of the curve? (Yes, I'm a techno dummy). Thanks, guys!

[larry]

Originally Posted by Chris Fitzgerald

So is a "shelving" EQ the one with a dip in the center frequency that gradually slopes upward on both sides toward the end of the curve? (Yes, I'm a techno dummy). Thanks, guys!

Not really; A shelf begins a cut or boost at some point and continues to the end of the spectrum. Some EQs let you define the slope, some do not. A bell-shaped filter that can cut or boost in between like you describe is a bandpass or parametric.

Read more here: http://www.prorec.com/prorec/article...2565F4005D82B3

[Chris Fitzgerald]

Larry - great link! So if I understand that terminology correctly, a low shelf filter could be used to cut all frequencies below the set frequency equally below the cutoff point, right? (I never use EQ to boost frequencies if I can help it)

If that is correct, then I'm still a little confused because the specs that I quoted above say, "Bass Control: Shelving type, ±15 dB @ 60 Hz", while in the Focus manual, it says, "Tone: The bass control is a shelving-type that affects frequencies below 250 hz and with a maximum boost/cut of 15 dB". I assumed there was a kind of curve thing going on to connect the "60hz" number in the specs to the "250hz" number in the manual, but apparently not. Can anyone help make any sense of these numbers? Thanks.

[robgrow]

Originally Posted by Chris Fitzgerald

...I'm still a little confused because the specs that I quoted above say, "Bass Control: Shelving type, ±15 dB @ 60 Hz", while in the Focus manual, it says, "Tone: The bass control is a shelving-type that affects frequencies below 250 hz and with a maximum boost/cut of 15 dB". I assumed there was a kind of curve thing going on to connect the "60hz" number in the specs to the "250hz" number in the manual, but apparently not. Can anyone help make any sense of these numbers? Thanks.

Chris,

Sounds like you've got the general idea already. With shelving EQ, there's a slope that begins at a certain frequency -- 250Hz in this case, and then things level off to a shelf beyond another frequency, which is 60Hz here. In this particular instance, the slope is about 2 octaves wide which is fairly typical of many shelving EQ designs. (BTW the frequencies specified are sometimes called the turnover or hinge points.)

[larry]

To me, that would mean that when the "bass" knob is rotated fully to the left, you would have a cut of -15db @ 60hz, and the cut would gradually slope up to 0db (flat) at 250hz. So you probably have about -7db at around 150, about -10db @ 100hz, etc.

I'm still not sure why the EQ program on my computer cuts half the amount I plug in.

I had a gig last week where my sound was awful. Sometimes, you just get in a bad part of the room and no EQ will fix it. Try moving the speaker around. The next night in a different place, everything was back to normal. (Normal being that I really dig my sound right now )

[fdeck]

Download my analyzer program and measure the response of your preamp. Start by verifying that the curve is flat when the tone knobs are centered. Then you can map out the effect of the different controls. No need to measure the speaker output -- just take the pre out.