Explain how a stereo or amplifier EQ works for me

Factual Questions
1

Millions,

I was fidgeting with the EQ on my stereo a few nights ago when it dawned on me: I didn’t really know what I was doing. We’re talking one of those set-ups with the vertical sliders; about seven of 'em. So there I was raising the ones of the left, since I think those are bass, and certainly, I notice the low end getting louder. Same with the high end. So, in this truly vague sense, I understand how they work. But, when we start talking about specific frequencies and cutting certain mid-ranges out, I get lost, lost, lost.

Which, I should mention, troubles me. I play bass and my amp is veritable rack of EQ knobs, buttons, and sliders. I get a sound I enjoy, but I just don’t know what I’m doing. So, in practice, I’m effective. But I was hoping someone could explain, well, the intricacies of EQ to me: what each specific number is, what happens when you max them all out and cut everything, exactly WHAT is going on, sonically, mathmatically, and otherwise.

I might not be explaining this as well as I could be—I blame a festive 1st. So I’ll probably chime in with requests for clarification and other worthwhile queries. In the meantime, I’m in your capable hands.

2

All an EQ does is adjust the volume of a specific band of audio frequencies relative to the others. The way you’re supposed to adjust it is to use a white noise generator and an audio spectrum analyzer. You plug the white noise generator into your stereo system, and use the spectrum analyzer through a microphone to “see” what the frequency response looks like at the spot you would be listening. Then, you adjust the EQ sliders until the spectrum analyzer shows a flat response. At that point, if you listen to music from that same location, it will sound exactly as the sound engineers who recorded the track intended. The reason for this is because materials in the listening area absorb or reflect sound at different frequencies depending on construction, composition and other factors. The EQ is meant to “balance out” these factors.

The way MOST people use an EQ is to adjust the sliders until they get the sound quality they like best, whether the emphasis should be on the bass, treble or midrange. There’s nothing wrong with this approach, since the idea is to enjoy the music the way that you want to.

3

Mmm. Thanks Q.E.D.
I’m familiar only with the way most folks deal with EQ, which is the only way I’ve ever known how. That’s how I approach my bass amp when actually (shock! horror!) playing music. It just dawned on me that I really have very little idea what I’m actually doing, just that I know the sound I’m aiming for. I feel a little ignorant, but it’s blissful, so I’ve been going with it.

4

A little more information then. Each slider is labeled with the frequency band it controls. They are almost always arranged from the lowest frequencies on the left to the highest on the right. Remember, the adjustments are relative to each other. Therefore, if you max them all out, the frequency response will be identical to having them all set to midrange, only the overall sound will be a bit louder. I neglected also to mention that the other purpose of an EQ is to compensate for shortcomings in the frequency response of the equipment, like the speakers and amplifier, as well as factors which exist in the listening environment. So, if your speakers don’t reproduce bass very well, you may want to compensate by raising the level of the leftmost few sliders, for example. You might also want to enhance a particular range to bring out, for instance, the vocals or perhaps some other instrument. If, in this example, you have trouble making out the vocal portion of a piece, you can try adjusting the midrange slider(s) which correspond to the human vocal range, which is roughly 300-3500 Hz or so.

5

Double Mmm. Nay, triple Mmm.

I have this terrible old beater bass, purchased new for $300 that’s never done me wrong. I suppose, judging by your more recent response, that I can now assume: 1) my bass has no high end; 2) my bass has no low end; 3) my bass is terribly awesome.

The range of the human voice (or, your approximation of the frequencies) will be very helpful, since, as a whole, the band and I are fairly ignorant of the technical aspects of sound. Not a source of pride for us, just of, as I mentioned before, blissful ignorance. The PA we use is…shoddy (to put it oh so kindly), so this information will come in plenty useful. I’ve been hesitant to raise the mids on the PA, if only because, in my lack of understanding, I thought that maxed-out mids would give the vocals a totally different tone, as opposed to simply bringing out the natural range instead.

Learning, learning, learning…

6

A little more about what I believe happens when you move one of the slides.

Imagine a “white noise” signal (approximately equal sound across the spectrum) displayed as a graph with frequency along the X-axis, db on the Y. If you slide one of the EQ bands up, a hump will appear along the otherwise straight line. the center of the hump should appear at the frequency listed below the slide.

Parametric equalizers (which your PA may have) generally have two dials associated with each slide. One will let you vary the frequency of the centerpoint around the listed frequency, and the other will let you vary how wide the hump is. Having one helped me understand what I was doing to the sound.

As for your bass, I assume you’ve tried it on other amps and PAs. If you find that it has little high or low end (compared to more expensive basses on the same equipment), consider upgrading the pickups and tone knobs on it. If you haven’t replaced the strings recently, a new set will give it a nice shiny high end (for about a month).

7

Just to see how far we can push this before your brain hurts, Birdmonster, let me chime in here.

Each slider on your Graphic Equalizer controls what is known as a bandpass filter. The sound signal is split between all these filters, each of which is tuned to a different frequency, and the results of equalization are mixed back together. A bandpass filter allows only sound at the frequency it’s tuned to, and to a lesser extent, the frequencies above and below that out to a certain interval, known as the rolloff of the filter.

Given that a graphic equalizer is not only meant to filter specific frequencies, but the overall sound in general, the rolloff on the filters is fairly moderate. If a particular frequency needs to be emphasized or deemphasized and one of the filters isn’t tuned to it, you have to play with the sliders on either side of that frequency and hope for a good result. If you really want to geek out on equalization, get a hold of a parametric equalizer, where for each filter, you can adjust the tuned frequency as well as the width of the passed band. This gives you ultimate control.

8

Indeed I have tried the bass on other amps & PAs and change my strings bi-weekly (we play several times a week and it’s a necessity to keep the high-end bright with our songs). I’m a little nerved about changing the pick-ups, because they’re the reason I still use this terrible little beater-bass. They have this honest, hard-working snarl to them I’ve been able to recreate only on tube-amp heads, which are about $2K and far far out of my meager pricerange. Hell, my Straight Dope membership dropped me below the poverty line.

Glilly: Thanks for the graph explanation. Makes complete and total sense.

Re: Parametric Equalizers: They aren’t those little boxes with, say, 20 different sliders on them by any chance are they? I guess googling’s just as easy, but I’ve heard what I described above (the box with 20 sliders—I’ve used one that was set up as a pedal and occupied the signal space between the instrument & the amp) would be a worthy purchase for someone like me: nitpicky & cheap.

(I’m heading into the studio to record a full length in a few weeks, so you’ll all be seeing more questions about sound in the following weeks)

9

The human hearing range is often referred to as 20 Hz to 20 kHz (20,000 Hz), but this is just some nice round numbers that happen to be reasonably close to average. In reality, most people don’t hear this full range, and some rare folks can hear beyond it.

Every octave is double the frequency. So, if you play the A string on your bass, it’s going to be 55 Hz. Go up an octave (2nd fret on the highest string) and it’s 110 Hz. Go up another octave, and it’s 220 Hz. Go up one more octave and you are at the A that is above middle C on the piano, which is 440 Hz, which, coincidentally, is the standard we use for music these days. You can google “440 Hz” for more info on how this came about.

If you pluck the A string, it is going to vibrate at 55 Hz. But, if you were to look at it through a slow motion camera, you’d find that it’s not a perfect vibration, especially since you pluck it from near one end of the string. Instead, you’ll find that there are all kinds of little harmonic oscillations going on up and down the string. These higher frequency components are what makes each instrument sound unique. Woodwinds have very few higher frequency components to the sound waves they make. The sound out of a woodwind is almost a perfect sine wave. A brass instrument is at the other end of the spectrum. Brass instruments have huge amounts of higher frequency components to their sound. A bass guitar isn’t going to have a lot of high frequency components to its sound, but it will have some.

If you use your EQ to filter off all the high frequency sounds, what you’ll find is that you mostly hear only the main frequency that each string is vibrating at (called the fundamental frequency). This is often desired when the type of music has a lot of other higher frequency instruments, and the bass is there just to fill out the low end of the sound. You’ll also note that most of the “plucking” sound gets filtered off too. The bass is left as more of a background instrument. If you want the bass to be featured more, or you want it to fill in some of the sound, then you will want to include more higher frequencies on the EQ. The most extreme example I can think of for this would be the rock band Rush, who uses the bass guitar not only for the bass notes of the songs, but also as a countermelody to the guitar, so in their case it is important that the bass stand out, and not blend into the background. No matter what style you use, there’s not much on the really high end of the sound spectrum from a bass guitar, so you can often kill off the really high frequency bands on the EQ to help get rid of noise.

An EQ allows you to have a lot more control than just low and high frequencies. For example, you can filter off the lows and highs both, and just keep the mid range frequencies. This will give your bass a sound that isn’t booming loud on the low end, and blends in fairly well with other mid range instruments, and yet isn’t so punchy that you have to worry about it intefering with the sounds of guitars and other higher frequency instruments. Think of the warm blended sounds of the 80’s. These days, most bands prefer a more open and aggressive sound, which you get by keeping more low end and high end frequncies, but cutting back a bit on the midrange frequencies.

If your bass has two pickups, then you’ll want to adjust the volume controls on each one in addition to adjusting the EQ. The pickup closer to the middle of the string tends to get more of the fundamental tones of the strings. The pickup near the bridge gets more of the higher frequency tones. You wouldn’t want to use the rear pickup (near the bridge) and then filter off all of the high frequencies in the EQ, since all that would remain are the lower frequencies which don’t get picked up so well by that pickup, for example.

10

ECG: Highly informative, thanks. Strange that, after playing an instrument for more than a decade, I’m really rather ignorant of this aspect.

Question: Does cutting the midrange actually make the notes in that range quieter? Obviously, if I cut the 85Hz (seems like a good midrange example or I am off here also?) that note doesn’t dissapear, but it does become a bit quieter, correct?

Also, with some serious ignorance: Does cutting a certain frequency alter ALL notes played? As in, are there frequencies that resonate in all notes? My guess is no, from what I’ve learned, but sometimes I need to double…and perhaps triple…check.

As always, thanks a million…millions.

11

I’m no expert, but my experience is that when you alter a frequency, it has much more of a tendency to change the sound of a note than the volume of the note. If you find a home stereo with an equalizer, and you slide the 1kHz slide (the middle one that human voice tends to be close to), it doesn’t so much make the voice quiter than just make the sound of the song sound different. It doesn’t have nearly the effect as the individual input volumes on a PA system do as far as changing the relative volumes of instruments.

Part of that goes back to the “hump” an individual slide will make. Most instruments will have a soundform larger than that hump, so you can’t decrease the volume of the instrument, just part of it. But you will also decrease that part of other instruments (if the equalizer is equalizing the entire sonund, not just on the individual instrument line).

Technically, altering a equalizer level will not alter all sounds and instruments, but may hit several, especially the higer frequency slides. You can get a lot of treble out of a bass, and the treble slides will affect that. The bass slides will not affect a symbol much.

As far as your bass, what ecg said is good. You may also find that if there are two sets of pickups, there may be a toggle switch that controls the front, back, or both. This will also change the sound. The pickup closer to the bridge will give a twangier sound, the other, a smoother, lower sound.

You said your bass has a snarl to it. In general, I’ve found that inherent or added “personality” to a bass tends to drop off its low end and (to a lesser extent) the high end. To fight it, layers of equalization may help. Bump the bass and treble on the amp and the PA. If you can’t, or it’s not good enough, check if there’s a cheap pedal with equalization and do the same.

Understanding how it all works will help you find the optimum sound faster, but it will come down to being more of an art than a science. But I think you already know that.

12

My bass’s pickups work depending on where you strike the string. No toggle on there. In fact, it was almost too cheap to include knobs.

As for your last paragraph: Exactly. I’m going into the studio with a well-honed tone, but I’m fully aware that sound quality is a lot different in an iso-room so I wanted to be more educated in the almost certain event that I will have to do some fiddling. Plus, music, while art, also has some serious science behind it. Right now, I’m incredibly top-heavy.

Still learning…

13

This is true, because a note is not just one frequency. A note is a fundamental frequency plus a series of overtone frequencies. The settings on an eq will make even a single note take on different tone qualities but not wipe out the note altogether. Unless you try it with a pure 1KHz sine wave.

BTW 1KHz is pretty high for the fundamental of a human voice note. 440 Hz is A above middle C, so 1KHz is more than an octave above that, which is pretty high.