room EQing

[mrouse55]

Phil Driscol came to our church in October. Showed up with a little Mackie mixer and a Driverack/PA and an effects processor in a 5-6 U rack. Ran two mic cables back to our board and and set the gain. Ran the auto-EQ and they were set for the concert. It sounded great, with no gain-before-feedback problems. They ran things about 10 dB higher than we normally do, but the general feeling was that the sound was more full, not louder. Keep in mind that our room was originally 'tuned' by a guy from a well reputed sound company with a truckload of gear (before we started going to the church). I was amazed, because it was all automatic, and made a pretty good sounding room sound even better. Now I'm waiting for the funds to become available to get one of our own.

[mrouse55]

Originally Posted by AMac

As i understand it, a parametric EQ is like the one on some boards with the sweep mids. A graphic is the one on the rack with controls for a certian frequency. I thought that the parametric was just used cause space and money and the graphic was the most controlled and best if the space and the money allow. You wouldn't put a full graphic on each channel. Now any corrections would be greatly appreciated. Thanks

Sorta. A parametric EQ will provide much better control, but is usually more expensive and is more difficult to get setup properly. The graphic, because you get an instant visual of what is being adjusted, is easier to comprehend. But because it devides each octave into the same number of segments, the Q - the number of frequencies covered by a single slider on a graphic eq - is much wider in the high frequency range giving less control in that range. On the other hand, it could take a bunch of parametric EQ's to accomplish what a single graphic can do. A fairly common configuration, which is present in digital form in most DSP units, is a graphic EQ to 'rough-in' the balance of the frequencies, followed by a parametric to fine tune the 'problem' frequencies.

So when you pull down one freq on a graphic it only changes that freq while a para will affect those around it? I think I am starting to get this now. So the parametric is to give the sound more definaition and color while the graphic is to cut out a chunk that is a problem. So graphics are used because they can effect more freqs?

[mrouse55]

Originally Posted by AMac

So when you pull down one freq on a graphic it only changes that freq while a para will affect those around it?

Actually, when you pull down a frequency on a graphic EQ, it does change those around it. How many depends on which frequency and the model of graphic EQ. For instance, on a typical 1/3 octive Graphic EQ will have 3 sliders per octave. In the low octave (20 hz to 31.5 hz) a slider may affect the selected frequency, and to a lesser extent the frequencies on either side, up to the halfway point to the next slider. The next slider will effect from the point where the first one leaves off, through the stated frequency, on to the point half-way to the stated frequency on the next slider. So lets say that you cut the 25 hz slider by 6 dB. That would mean (on most EQ's) that 23 hz would be cut by about 2 dB and 24 hz by about 4 dB. 26 would also be cut by about 4 dB and 27 by about 2 dB. Think of it like smooth, gradual adjutment of the frequencies around the stated slider frequency. This, of course is an over-simplification, because seldom is a single slider changed by itself. The interaction in the frequencies between the sliders can get quite complex.

Now, when you get to the higher frequencies, changing a slider will effect a lot more frequencies. The first octace in our typical EQ only covers a range of 39 hz. The last octave covers a range of 9,999 hz. Thus, changing the slider at 16 khz will affect a range of almost 4000 hz. This can often be way more than needed. Next example.

Our sanctuary has a resonance around 8000 hz. It is very narrow, probably less than 500 hz wide, but a tone in that range will induce feedback very quickly. In fact, a strong note on lower harmonic (4000 hz) will also cause feedback in the 8000 hz range. To kill this feedback, I have to pull down the response of the sound system at 8000 hz. Ideally, I would be able to simply pull it back between 7750 and 8250 (this is called a "notch", for obvious reasons). Unfortunately, with my Graphic EQ, pulling down the 8000 hz slider is also going to effect everything from about 7000 to 9000 hz. resulting in a void at the frequency range that brings out the shimmer of cymbals and the airiness of a flute. Bummer man......

But wait! A parametric EQ has a variable bandwidth. So I can set up my Graphic EQ without regard to the 8000 hz resonance, and then set a 500 hz wide notch at 8000 hz with a parametric. This allows the the cut to only effect the problem frequencies.

So that is my typical plan of action. Use a Graphic to get the sound in the house relatively flat, then iron out the wrinkles with a Parametric. Fine tune with the channel EQ on your mixer.

Originally Posted by AMac

I think I am starting to get this now. So the parametric is to give the sound more definaition and color while the graphic is to cut out a chunk that is a problem. So graphics are used because they can effect more freqs?

I find graphic EQ's to be primarily useful in sound reinforcement. I can't think of a single instance where I've used one on a recording. Generally, speakers do not have a flat response, and need to have certain frequencies boosted, and others pulled back to be able to produce a balanced sound. Add to this the nodes, reflections and other anomolies produced by a rooms design and furnishings and you have the primary use for a graphic EQ - to get the most balanced sound out of a set of speakers in a given space. Of course there is a lot more to system design than just EQ, and EQ can't fix problems caused by poor speaker placement etc.

But, great sounding speakers in a great sounding room is just big stereo if you can't hook up some mics and get reasonable gain before feedback. In many rooms, a parametric EQ is just what you need to get that done. Two other places that I like to use parametric's is on a 'roving' mic. A parametric with some judiciously applied notches can really tame a wireless handheld mic when it get out in the congregation.

The other place that I like to use a parametric is on a lapel mic. Since most lapel mics are worn well below the mouth, or near the collar, they are often prone to picking up breath noise, and are lacking in higher frequencies, causing them to sound 'boxy'. A parametric put into the inserts on the lapel mic's channel allows the upper frequencies to be boosted on the channel eq of the mixer, with the problems notched out.

Hope this is not too confusing.

Mark

[Simo]

I needed to do the room eq-ing a few months ago for our church. I used our graphic eq, the computer connected to the sound system, measurement mic and Audacity that we use to record the sermons. (http://audacity.sourceforge.net/).

Audacity can generate white noise and plot the spectrum of the recorded signal, even quite accuratelly. I simply looked at the spectrum and found what I was looking for. At that time, I didn't know anything about SMAART, and didn't realize I could have recorded both the mixer output and mic signal, and calculate the actual room response, as SMAART seems to do.

Anyway, the system worked. For a long time one elderly person had compained that the music was too loud, when it was not IMO. By placing the mic in front of the sofa where he usually likes to sit (yes, we have sofas in the church ) I found there was a small but measurable spike around 3 kHz which is a very irritating freq. I had never noticed that spike with my ears, and the spike was not present in most other locations of the room. Slight cut using the graphic eq, and the person has not complained anymore.

I think the spike was caused by the reflections from the low metal ceiling, because it was even more distinct when I looked at the measured response to a fast sweep from 40 to 20000 Hz. But I'm not sure, because Audacity doesn't plot the time-freq surfaces very accuratelly, and I was too lazy to use my favourite audio tool (Matlab ) for the job.

Simo

[kbob]

For sound reinforcement, the combination of the graphic and (especially fully) parametric eq's gives both the most flexibility and best "bang for the buck". For most of us in the smaller venues/churches/bands, that's as good as it gets right now. I'm with my fellow sound deadening device though-the auto eq'ing systems available now are simply amazingly cool! Droool, slooooober....

Whoops! There I go falling in lust again-and I don't even do sound much any more-they've got me hidden in the closet doing the presentation stuff. Something about me making the room too quiet...

Thanks for clearing that up for me. I am think I get it now. Ill try some of the optionslisted here and report back on em