Behringer DEQ2496

**sg2** · 16 October 2006, 17:08 Monday

Hi,

All religious considerations with respect to equalizers left aside, there is no scientific ground to any low frequency (let's say < 100 Hz) room correction with reasonably sized "absorbers". The reason is very simple: for an absorber to be efficient, its thickness has to be at least 1/4 the wavelength of the target frequency. This is a physic law, one that you cannot workaround.

Sound in air propagates at speed = 340 m/s, and the wave length calculates as wave length (m) = speed (m/s) / frequency (Hz). A 6 m room dimension yields a 28 Hz room resonance mode (wall to wall round trip), which is a 12 m wave length. Thus, there is nothing you can do to efficiently dampen it short of putting more than 12 / 4 = 3 meters (!) of saw tooth shaped open cell foam absorber on one side. Obviously this is not what WAF will allow you to do.

Note, those 3 meters (quarter wavelength) can also more easily calculate as 1/4rd of 2x (resonance needs a round trip) the wall to wall distance

Conversely, for the very same reasons, it is very easy to tune down the "bright" character of a room, by covering a fraction (no more than 1/3rd usually) of the surface of the walls or roof or ground, with any medium to treble absorbent material, and that will be thin, because the wave length etc

Now, compensating this low frequency problem *before* the sound is amplified, by inserting a filter which will have a transfer function inverse that of the room, is an "easy" way to do away with room modes.

What you need in that case, is a filter for which you can define a set of "taps" (one per room mode you want to linearize), each tap with 3 parameters:
- a central frequency in Hz (one of the direct or indirect room mode frequencies as calculated from the room dimensions)
- a gain in dB, alias attenuation or amplification coefficient (yes, some modes deafen some frequencies as much as some others are boomy)
- a band width in Hz (to make the filter sharp or wide)

This is the definition of a "parametric equalizer", as opposed to a "graphic equalizer” with settable gains for fixed frequency bands.

Ideally, to cover the ordinary needs of a parallelepipedic room, you need to be able to specify each tap's frequency with 0.1 Hz accuracy, the band width with same accuracy, and gain between -16 and +16 dB.

Now, reality kicks in, and says that no such filter is available without "side effects". Especially, any such filter modifies the phase of the fraction of the signal on which it has a non-unitary gain. This can be noticed when you have sharp filters with important gain, at one point there starts to be audible distortion (but you'll hear <100 Hz distortion way less than medium or treble distortion; typical mid to high end subwoofers yield more than 25% distortion at the bottom of their response curve – which is not true for the servo-controlled ones like the Digital Drive series from VELODYNE for example).

Unfortunately, some rooms would require such sharp and strong filters. Luckily for you, this is probably not the case with your room, as you have taken some dampening actions already (even if poorly efficient, due to law stated above).

Okay, I've typed all that to assess the fact that a parametric equalizer is the only (scientifically grounded) solution to low frequency room mode problems.

That said, if we look on the market for such products, we have 2 main product families:
- Analog equalizers
- Digital equalizers

Analog parametric equalizers (Cello did one) have their virtues, but also so many drawbacks that I won't comment. You can safely consider that they are things of the past, plus for them to sound good, there is no way short of putting up a ton of $$$.

Digital parametric equalizers are ubiquitous in room correction applications (in this case, Digital Room Correction or DRC).

To start with audiophile products down to cheaper alternatives, you find them for instance (non exhaustive list of course):
- In the very efficient multi channel Lexicon MC12V5+EQ. It has tons more features, but I'm seriously considering one myself, just because I'd like to apply DRC also to linearize the response of my rear speakers. Problem, it costs almost the price of a pair of 800D's: $15.000. Subjective listening (including mine) suggest that this box does a really good job (I won’t expand because I hate subjective terminology à la “open, transparent, etc.”).

- In the more or less equivalent TACT TCS (multi channel digital processor with room correction) which sells for $10.000… I haven’t had the chance to listen to a TCS equipped system, but I’ve heard it does a good job too.

- In the TACT RCS2.x: this is a 2 channel only device but at approx. $4.000 it is still overly expensive if you are only into room mode equalization. Note, the TACT only has digital I/O as standard; they offer analog out as an optional board. Subjective listening (including mine) suggest that this box does a very good job too.

- In most of the higher end AV receivers, YAMAHA, DENON etc. now almost all have a room correction feature. I consider this built-in feature gives you little fine control, and the "auto calibration" approach yields poor (coarse) results, suitable for most users of those devices, but not for the one who pursuits a real speaker/room linearization. Note, DENON and other manufacturers rely on 3rd party technology for their room correction feature, which often comes from AUDISSEY (http://www.audyssey.com/).

- AUDISSEY recently started selling their equalizer technology as a standalone multi channel box, but it is only available through their registered installers, and as such, it does not seem possible to buy it and set it up yourself. Plus, it only works on unbalanced analog input and output signals, which I consider bad, because it takes another round of A/D and D/A conversion… and you have to figure out a way to insert it in the analog chain, after source selection but before volume control. I would tend to think that most people working with separate preamplifier and amplifier use balanced interconnects between the two (where it would be appropriate to insert the AUDISSEY box)… I haven’t had the chance to listen to an AUDISSEY equalizer equipped system (apart from integrated DENON’s which fall into the previous category).

- In the form of freeware PC software (Google for "DRC"). If you want to go this way, prepare for a long journey into math, convolution etc., because you are basically left on your own to assemble all the building blocks (you need “BRUTEFIR” to run the filters that you have calculated using DRC, and it takes a lot of work and thinking to complete such a setup). For me it is too much work, and I don’t like the idea of a PC fan anywhere near my audio setup, nor the hassle of fine-tuning the DRC/BRUTEFIR couple, nor the idea of having to start a PC, load all the apps etc. just to listen to a CD. But, without ever having heard such a system, for me there is no doubt it does a fine job, given it applies the same kind of calculation as all the other room correction oriented solutions described here.

- In many cheap pro studio devices, like the BEHRINGER’s digital audio processors, which range from $200 to $500. In fact, most of their digital sound processing racks have a "feedback destroyer" feature, which is exactly what we want: a multi tap parametric equalizer, with .1 Hz frequency accuracy, a +/- 12 dB gain per tap etc. Their feedback destroyers are usually utilized by scene or studio people who want a dynamic approach to Larsen feedback: the DSP in the rack will constantly auto track resonant frequencies and kill them. The DSP auto sets all the notch filter parameters! But that's not how we will use them, we will set them manually.

The DEQ2496 is in fact a graphic equalizer (you'll probably not use that feature) which has a set of 10 parametric filter taps that you can freely define.

You could as well use any other BEHRINGER rack with a feedback destroyer feature.

I personally use the DSP8024 (the ancestor to the DEQ2496) with only 6 taps, and it does the job of calming my room modes very well. I use it only for this purpose.

If you want to bypass a rather tedious learning curve with those boxes, here are a couple of hints on how to set them right:

- As the filters operate in the digital domain, it only makes sense to feed them with a digital signal all the way from a digital source. Otherwise you'll end up multiplying A/D and D/A conversions, which is bad.

- for the same reason (digital), they will yield poor results if not fed with a full range dynamic signal (they do their math on 24 bit signals, so if you only use the least significant 3 bits, don't expect miracles

. So they have to be inserted BEFORE the volume control. The ideal position is thus, between your CD source and your preamplifier/amplifier. For an audiophile (!) setup, that would be: CD transport -> DEQ -> DAC -> PRE -> AMP.

But in fact, the BEHRINGER’s are equipped with very good DAC's on the output channels (AKM chips of the same family found in the TACT's; I measured their noise floor, dynamics, linearity, diaphonic interference etc). In my setup, this is CD transport -> DEQ -> integrated amp and it works very well.

In terms of analog input/output, the BEHRINGER’s usually come with balanced inputs and outputs. But you can use those balanced plugs as single ended, the rack manual tells you how to wire the XLR plugs to do so.

- The DEQ has an "auto set" feature dedicated to room correction. My advice: don't use it, it yields poor results (typically with too sharp high gain filters -> audible distortion), and you'll be much better off calculating the room modes yourself and inputting your filter's frequencies and gain manually. BUT, the nice thing is, for this auto set feature, the DEQ comes with both a high resolution signal generator (sine, pink, white noise) and a measurement microphone input, to which you can attach the very cheap ($30 + an XLR/XLR 10 m pro cable for another $10) BUT nonetheless very linear BEHRINGER ECM8000 microphone. This means, you can use both the signal generator and the mike, to plot a real-time room response, for either channel or both, and see in real-time the effect of your filters. This is smart and this is the way to use the DEQ.

- The filters you define should "globally have a gain which will not exceed 1". This means, if you are to amplify some frequencies, compensate that by lowering the global equalizer gain. Otherwise you'll hear the saturation distortion (you cannot miss it).

- The DEQ has a ton more interesting features. They have a real time RMS level display, a real time frequency spectrum analyzer, they are controllable from a PC through a MIDI interface (the PC software shows you the response curve of your filters and the associated phase shift curve) and tons more things. Personally, I only use it (a DSP8024 in my case, but it is the same device) for 6-tap room correction on my 2 main channels.

Finally:

- As you may have guessed, my personal intimate conviction is that any decently programmed DSP with adequate resolution and I/O, fed from full dynamic signal, will do the same job at room mode processing. So my advice is to go the cheap way, and BEHRINGER’s are very good candidates.

- A word on BEHRINGER: you'll hear their stuff is low end non reliable non audiophile crap. This might have been partly true in the past, but I’ve used various signal processing racks from them, and 1) they are well built, simple, no nonsense, and just as much reliable as anything else, and 2) I’ve ran them through the lab at work, and they don't generate any more noise nor distortion nor digital artifacts than you would expect from 24 bit devices.

- A word about me: I'm a physics PhD (all the babble above, over simplified, stands on a solid math ground, and I'm open for discussion on my statements

. My system is based on a pair of 802D's. I would not want to hurt anyone, so I will refrain from telling you what my CD player is, what my cables are etc

- Last, I do not work for BEHRINGER, nor have any vested interest in their company

Regards,
--
Stéphane - Forgive my non-native broken English please

**ThomasW** · 16 October 2006, 17:29 Monday

Underconstruction but should get you started...

http://home.comcast.net/~thomasw-2/SubwooferSetupandEQpageDEQ2496.html

The AutoEQ works with only with the GEQ function. One can then go in and fine tune with PEQ where necessary. There's really no such thing as fully automatic anything since moving the mic as little as 12" can completely change the curve. So it's usually best to take measurements from several places in the room and create an 'average' response curve.

There's nothing else out there for the modest price of the DEQ2496. It's surprisingly transparent.

What's fun for sealed box sub owners is the ability of the DEQ2496 to create a Linkwitz-Transform circuit. This boosts low bass while maintaining a low Qtc.

Understand that the lowest hinge point is 20Hz. One can create boost/cut lower than 20Hz depending on the choice of the passband.

People boosting ported boxes need to know the port tuning frequency so they don't boost too low causing the woofer to 'unload'

The ECM8000 mic used with the DEQ2496 is sold separately. Also you'll need to buy a mic cable.

Depending on how clever you are the great little freeware program 'Room EQ Wizard' (google to find it) runs on a Mac.

**Kal Rubinson** · 16 October 2006, 18:33 Monday

Originally posted by sg2

Digital parametric equalizers are ubiquitous in room correction applications (in this case, Digital Room Correction or DRC).

To start with audiophile products down to cheaper alternatives, you find them for instance:
- in the very efficient Lexicon MC12V5+EQ. To my knowledge, this is the only (non pro-studio-only) such product which is multi channel. It has tons more features, but I'm seriously considering one myself, just because I'd like to apply DRC also to linearize the response of my rear speakers. Problem, it costs almost the price of a pair of 800D's.

- in the TACT, but this is a 2 channel only device, and it is overly expensive imho.

Insert comments of the Meridian MRC, the TacT TCX (multichannel), the DEQX and the new Audyssey Pro.

Kal

**dknightd** · 16 October 2006, 21:09 Monday

Thanks for the information so far.
I use Room EQ wizard on my Mac to measure the room (using a RadioShack SPL meter as a microphone). It does run on a Mac, but, it has some quirks, and frequently crashes. Still VERY useful, and the price is right!

My plan would be to use the measurements from it, and/or the measurement function of the DEQ, to determin room modes (which I've already done). I'd use the PEQ (parametric equilization) to squash the peaks (and maybe if I'm brave reduce the valleys a little bit). I'm not sure if I'd use the automatic correction feature - I might give it a try, then mimic its response using the parametric equalizer.

One nice feature of the DEQ is the ability to store 64 different processing settings. So I could try different things, save them, and easily compare the results. I could also store one setting optimised for my normal seating postion, and another that was a comprimize that worked for most seats in the room. I could set one up with a fletcher curve to correct for frequency effects at low volume listening.

It really does seem like a very powerful tool. . .

I could wait for somebody to write a nice software PEQ program for the Mac, but I don't see that happening any time soon (too bad, since my computer is my transport it makes much more sense to process the signal there before sending out to the DAC1 rather than sticking another box in the chain.)

**Glenee** · 16 October 2006, 21:23 Monday

sg2, That was absolutely outstanding. In my humble opinon. I would like to add a small tidbit to anyone using EQ. The demands on your Amplifier needs will increase due to people using just a SPL meter up till this point. The amp only has to drive 1 octave for it to register top db on the SPL Meter. When you hook up with a spectrum analyzer and go to achieve a near balanced line across the spectrum your amp could work as much as 3X's the load driving all octaves the same SPL. Be carefull when doing this can smoke a amp in a hurry. This is why it is advisable to only EQ down on high spikes never up on low octave. Besides messing with too many octaves at once will only make you chase your tail and pull your hair out. Once again very good job sg2.
Excellent.
Glenee

**warnerwh** · 01 November 2006, 23:48 Wednesday

My room is LEDE with four large bass traps, one in each corner. Adding the Behringer DEQ 2496 is worth every penny and then some. It's the best deal in audio tweaks I know of after 30+ years in this hobby.

If you get the Behringer unit be sure to read the manual a couple of times before you get it. Also you'll want to get the Behringer ECM 8000 microphone for about 50 bucks. The mic is really a necessity. This device can take over where the room treatments leave off in a very positive way.

Even with all the treatments and traps in my room I still have a significant amplitude variation which of course everyone with walls does. Best of luck. If you have any problems getting things going email me and I'll gladly help you.

**Kal Rubinson** · 02 November 2006, 08:45 Thursday

sq2-

Very interesting analysis but you seem to deal only with amplitude issues and not with time/decay issues. How do you deal with these? Do you assume that even the amplitude problems are the same throughout the room?

Kal

**Martyn** · 02 November 2006, 16:23 Thursday

"I second that emotion"...

Originally posted by Glenee

sg2, That was absolutely outstanding.

Stephane, I freely admit to having difficulty understanding the meaning of expressions like "parametric equalisation", and even of understanding exactly what many of the Behringer type of products do as a result of the abundant jargon. Your dissertation was an education for me; it's very useful to have such a clear and well-written piece of work. Thank you.

**warnerwh** · 02 November 2006, 18:36 Thursday

Alot of people seem to underestimate what the amplitude variations are in their rooms. The average room varies by plus or minus 15db. Imagine letting a child randomly adjust a graphic equalizer for you. That's what the room is doing to the sound.

If you've ever played with bass/treble tone controls you can imagine what making major adjustments across the audible frequency spectrum is doing to the sound.

The Behringer or better yet, the Tact, can minimize the profound effects of these amplitude variations making the sound far more realistic and natural. The Tact can adjust time and phase on top of that.

Everything from the bass through the treble sounds so much better I think it should almost be a law that any system worth over 2 grand should have at least the Behringer installed if room treatments are not an option.

Time and phase issues can be minimized a few different ways. One is to use a speaker with drivers wired in phase with first order networks. Next make sure the first reflection points from sidewalls, ceiling and floor are damped. Another one I like is to angle the speakers in so they crossfire in front of you,this assumes sound will be good at that angle of course.

dknightd: If you get the Behringer and use the autoeq mode you need to understand how to set an equalization curve. You do NOT want a flat in room response. This will sound thin in the bass and bright on top.

You need to bump up the region below 125 about 3db dropping to maybe 1db at 125 or so. I personally like a bump of a db or so in the region under 400.

At about 2khz you want to make a curve downard toward 20khz. You should end up being down at least 3-4db at 20khz. Also many people like to add a dip of 2-3db in the 4-6khz region. I think this is known as the British sound. I like that also.

I don't know what your amp or speakers are capable of. When you're done be sure to bring down the the boosts that the Behringer's graphic equalizer did for you. You can leave a boost of 2-3 db if you're comfortable your amp or speakers won't have a problem. I also won't let the Behring er push down more than 6-7db with the graphic equalizer because the bands are too wide imo.

When you're done with that listen to alot of various music. As you know recordings vary alot the way they were processed. You can then use the parametric equalizer to adjust the sound to your taste more closely. At this point I only make .5db adjustments. Be patient! Only make small adjustments of 1db in one place when you start out. Take your time lest you mess things all up like someone I know did

ops:

One other thing I use the parametric equalizer for. Your room has at least three dimensions. Get the modecalc program for Windows off of the realtraps.com site. Then put in to the nearest inch the dimensions of your room, hit enter and see what the first mode is of each dimension.

These frequencies should be attenuated with the parametric equalizer. You can set the parametric equalizer to 1/10 octave. Give these three a good 3db cut. Being as you have good room treatment the Behringer will provide the icing on the cake.

I love the DEQ 2496, if/when it dies I'll be at a pro audio shop within the hour paying full price for a new one.

Martyn: The only difference between a parametric equalizer and a graphic equalizer is that you can adjust the frequency that you want to control with the parametric equalizer. A graphic equalizer has fixed bands. With the parametric equalizer you can adjust exactly the bands that are causing problems in your room.

Behringer DEQ2496

Behringer DEQ2496

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