Digital EQ with Ultracurve DEQ2496

Introduction

One of the keys to creating the sound system that works best for you is actually to use that one thing we've been told is evil. Tone controls are for wimps and graphic equilizers - forget it! No, no, we're serious audiophiles here, and please don't mention that word digital.

I use digital EQ in my system and it seems everyone who hears it agrees with me - it's a clear improvement. At times I hear much more expensive systems, and yet my typical response is that I find my own system more enjoyable. The main reason is that I've been able to create a target response curve that sounds best to my ears. If I had to choose, I'd rather have that than a megabuck system that doesn't allow enough shape the response. If it sounds extreme then you realise how serious I am about making this point - EQ makes a huge difference.

Benefits of EQ

It's a very rare system that won't benefit from EQ. In a nutshell, EQ makes a system sound balanced. Without realising it, you may have adjusted to the strengths and weaknesses of your own system. Let's say you have exaggerated bass in your room - other systems will typically sound weak in the bass. Or let's say the treble is tilted up. You may feel that flatter systems are lacking in detail and sparkle. Experimenting with EQ is a learning process that helps you to learn more about your own preferences. Chances are, they will become more refined and you'll gain much more control of the sound of your system.

The benefits are quite compelling:

• room modes that destroy bass accuracy can be tamed
• active speakers can have the levels better matched
• sub volume can be better chosen
• detailed control of the tonal balance is possible

Beyond basic EQ, some advanced options are available:

• dynamic loudness control compensates for the ear's insensitivity to bass at low levels and can supply the right amount of boost/cut at all levels
• dynamic settings can reduce treble at higher volumes, subjectively creating a more relaxed sound at high output
• limiters can protect the system from being over driven by others
• level dependent limiters can reduce bass at high output to protect drivers from damage (an alternative to rumble filters)
  

In a nutshell - it's all about gaining control and learning how to create the system you want. In that respect, EQ is critical. In reality, any system has some amount of EQ, even if it's simple part of the passive crossover. The difference here is that the contro is in your hands. If you are prepared to put in some effort, it can be very rewarding.

Getting a flat response

In my system I use Behringer Ultracurve DEQ2496. It's a powerful unit and great value for money.

The first step is getting a flat response. You need a measurement mic which has a flat response - definitely not a dynamic mic. A DIY mic using the Panasonic capsule or Behringer ECM8000 is suitable. Place the mic in the listening position and run "Auto EQ." Ultracurve will play pink noise and automatically apply eq until a flat response is achieved. This takes about 5 minutes.

If you listen to music once you've set it up as completely flat at the listening position, you'll be surprised to find how bad it sounds.

Trying to do two things at once

We are actually aiming to do two things at once. We want to EQ the bass at the listening position, but at the same time we actually want to get the speakers flat at 1m on axis. In my particular room, I can effectively do these two things at once. I place the mic in the listening position and select the "room eq" option which applies a 1db/octave slope boosting the bass end. When I place the mic at 1m while pink noise is playing, I can see in the RTA window that the response is flat. When I place the mic in the listening position, the response tilts down towards the top end. This tilt is about the same as the 1db/octave applied by the room eq option. The result is that the eq actually makes the speaker flat at 1m for the midrange and above while correcting bass response.

In typical rooms this should work well. In a very live or dull sounding room a different approach may be needed.

Improving on flat

Once a slightly modified version of flat has been achieved, we can now shape the response a little more. This is where experimentation and personal preferences come into play. In my system I use two filters as a minimum. Firstly, a gentle roll off of the treble, starting at around 1kHz and -2.5 dB @ 20 kHz. Then, for music I add bass boost - 6dB boost at 40 Hz which rolls off either side. The boost often becomes overpowering on movies, so no boost for movies is often better. The original mix usually has exaggerated bass already.

Another reason you need subwoofers

Obviously, 5" vented woofers aren't well suited to this kind of EQ. The power and excursion demands are just too much in many cases. We are now into subwoofer territory, and if you haven't considered this option already, then it's time to think seriously about crossing to subs below 80 Hz. If you have not heard a serious sub system set up for music as well as home theatre, then you owe it to yourself to experience one.

My settings

After running AutoEQ with my DIY omni speakers and Rythmik subs, these are the settings used to achieve a flat response:



You can see that most of the eq is correcting for room modes. In virtually any arrangement I get a peak around 40 Hz and a series of peaks and dips around 80 - 200 Hz.

I use parametric EQ filters to shape the response:



You can see 6db boost centred around 40 Hz, which subjectively makes the bass twice as loud as a completely flat system. For music this generally sounds "just right." For movies I often remove this boost, as I find the result is often overpowering.

There is a "psychoacoustic dip" around 3.5k as well as a 2 db reduction of the treble. I find this gives a more relaxed sound that I prefer.

Some prefer to use PEQ to deal with modes, but I find it more intuitive to use these filters to shape a flat response. I like to see what I'm getting.

But wait, there's more!

There are a few more benefits and features I haven't mentioned. Let's say you want to try open baffle and need to compensate for the resultant roll off. No problem - simply run through the EQ process again and store a new preset. 5 minutes and it's all done. Let's say you want to make your vented box sealed. Simple again. If you like you can even recreate the same response and apply the necessary EQ. Or what if you want to re-arrange your room for a party that means you have to sit in a less than ideal spot, or push the speakers back towards the wall so the sound gets boomy. Again, not a problem - you can easily compensate and take the boom out of the bass.

A few more features that are nice to have:

• stereo width control - expand the stereo effect
  
• RTA - some say it's like watching a fire, I like to see the shape of what I'm hearing and how bass the bass goes
• SPL meter
• digital delay to subs

More information

It started here for me:
Ultracurve Review by Thorsten Loesch
Note: the unit reviewed has been replaced by DEQ2496

Ultracurve on the Behringer website

An article on the "house curve" concept at the Home Theatre Forum

DIY Rythmik Servo subwoofers

Introduction

Experience has taught me that the best bass doesn't always come from a subwoofer. There are still many subs that don't match the accuracy of a good woofer. My Rythmik subs restored my faith in subwoofers.

Admitting to bias

I should admit up front that I can't be completely unbiased when talking about Rythmik audio. Back in 2006 I suspected that they had a great product backed by very sound theory, and proposed a website redesign. I had to hear them for myself, and I also wanted to know if they really were as good at the website asserts. It didn't take me long to verify that they really are exceptional.

First impressions

It didn't take long to realise I was listening to accurate subs. The absence of distortion was instantly recognizable and the sound was dry from the start. The sound reminded me of the sound of some Focal woofers I'd heard at Aranmah acoustics. The cost of those woofers is about the same as buying the Rythmik driver and amp. Not bad considering the Rythmik has greater extension and output.

Set up and calibration

Set up correctly, the Rythmik subs are magic. But like any subwoofer, it's possible to set them up so that they sound terrible. In my system I use Behringer Ultracurve with a Behringer ECM mic for EQ. I use it to shape the response and this includes the bass response. It also helps determining suitable sub locations. In my room placement isn't critical - the room has a lot of bass damping, which means most practical locations for subs work well.

I use an 80 Hz crossover point and a 4th order electrical high pass supplied by the plate amp.

Measurements

An earlier version of a Rythmik sub has been independently measured at the Home Theatre Shack:

HT shack Rythmik measurements

One aspect clearly shown in the measurements is that the distortion is low at all drive levels within the range required for music. The distortion below 40 Hz climbs fairly high, but this is mostly related to more artificial home theatre LFE use, for which the ear often has no real life reference. Subjectively the Rythmik subs are very clean and dry.

Dipole experiments

My previous subs sounded better in a H frame dipole arrangement, but I was surprised to find that the Rythmik subs sound just as accurate in a sealed box. I believe the reason is related to the higher level of damping control provided by the servo which eliminates box coloration in a similar way to infinite baffle and dipole subs. I've written more about this in my post about my open baffle speakers.

Final thoughts

I have no hesitation in recommending these subs to anyone for serious home theatre and music listening. They don't use the highest excursion drivers or the most powerful amps, but I believe they operate in a sweet spot. Where more output is desired, I suggest multiple subwoofers, which is actually better anyway in terms of room acoustics.

Rythmik Audio Website