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.... in home systems for Room correction and general system EQ

Folks,

This is from my own Yahoo group, but may be of interest here....

Since writing this I have in fact changed my speakers to dipoles and do not currently use a PADEQ (it will return though).

Suggested target curves and setup techniques for Pro Audio Digital Equalisers in home systems for Room correction and general system EQ

Scope and Objective 
 
The following are some notes on how to use Pro Audio Digital Equalisers (PADEQ from here on) and especially the Behringer Ultracurve (UC from here on) and how to combine it with other measures to achieve the best results. It will only cover the EQ settings, room acoustics, suggested target curves etc. It is assumed that correct level setting; connections, modifications etc. have been sorted. Whoever needs more info on this may be directed to my review at enjoythemusic.com.

http://enjoythemusic.com/magazine/equipment/0101/behringer8024.htm

Now, the Behringer, just like the Derringer it is some pun-wise compared to, is a specific tool for a specific job. Correctly applied the effect of either is mind-blowing. Used wrongly it can make a bad situation worse.
 
It is ESSENTIAL to understand a little about speakers in general and room acoustics, as well as understanding the specifics of your speakers. As a result this whole "little" article has to become quite long.

Lets start at the beginning - the room.....

The room of doom - why listening to a stereo indoors is bad idea  
 
For a room in which a speaker plays we find three predominant frequency ranges through which the room behaves dramatically different.

For frequencies in the midrange and treble, where the room dimensions are large, compared to the wavelength of the frequency reproduced the room behaves in general diffuse but consistent. We shall call this range the reverberant range, as this describes its behavior well. In this range we can think of sound to be similar to a ray of light and being reflected, diffused or absorbed by room surfaces.

Depending upon the directionality of the speaker and the reflectivity of surfaces in the room that sound is radiated towards the room will reflect (or not) and generate a reverberant field that merges with the direct sound of the speaker. If two or more speaker are used (stereo, multichannel) it is essential for correct reproduction that the acoustic environment around each speaker is as symmetrical and identical as possible.

Gross asymmetry (like heavy drapes on the left wall and a naked wall on the right side) MUST be avoided at all cost. Please also note that few absorbing materials are consistently absorbing over a wide frequency range. I personally prefer the use of diffusion as much as possible with some absorption behind and around the listening position to excessive damping, as the latter often leads to a "dead" and unnatural, lifeless and amusical sound.

Contrary to many claims made by their manufacturers, there is NO WAY to effectively deal with the reflections in the reverberant range using digital correction and the only real way to achieve a resolution of problems there is physical, be in using controlled directivity speakers or absorptive/diffusive room treatment.

In most modern living quarters the reverberant range starts somewhere in the 200 - 300Hz range. The larger the room the lower extends the reverberant range, though usually the ceiling/floor distance is the limiting factor. The common 2.4m high ceilings will lead to a main mode at 143Hz and a strong second mode at 286Hz, demarking basically the reverberant frequency range.

Below the reverberant range the room transit into the modal range. Here the behavior of sound in the room is dominated by resonances related to its dimensions, in effect the room becomes a resonant system where both the position of sound source and sound receiver (ear) with respect to room boundaries becomes a determining factor in the resulting frequency response. In the modal range deviations from a flat response by more than 20db are common.

Very few effective ways exist to deal with the problem in a mechanical fashion (room treatment) and even the various "Bass Trap" products are contrary to their marketing NOT effective in combating these problems. In studios very large passive (bending plate) wall sized absorbers are common and some active absorbers exist, neither is common in domestic settings.

In the modal range we will encounter both pressure maxima (resonant reinforcement) and pressure minima (resonant cancellation). Certain effective speaker setup systems like Wilson Audio's WASP rely on optimising the speaker and listener position such that the number of frequencies at which the listener receives either a boosted or cancelled signal are minimised. However, again effectiveness is limited.

As a rule, pressure maxima exist near all room boundaries and pressure minima are scattered like "black holes" across the room. In a pressure minima there is simply no sound pressure, thus no sound and deep notches occur. In a maxima there is a boost and thus at the room mode a strong peak.

While there is no way "to fill in" a "black hole" a number of solutions exist to allow peaks to be dealt with. As you are reading this in the context of using a PADEQ it is assumed that you will chose to equalise these peaks out. This usually requires parametric equalisers or dedicated room correction systems (like TACT), simple graphic equalisers (like Accuphase) are not able to help much, as they offer to coarse a view of the frequency spectrum.

Below the modal range our room behaves as more or less ideal pressure chamber. Room modes stop to exist at a frequency where 1/2 of Wavelength of the tone is longer than the longest room dimension. In my listening room this dimension is depending how you look at it 5.5m or very long (through the open door into the hall). For a 5.5m dimension the room becomes a pressure chamber (in my case a leaky one) below around 30Hz and here a notable bass boost is present helping to extend the bass of large speakers down as far as one wishes, assuming there is still SOME usable output.

In this region the positioning of listener and speaker becomes completely irrelevant. It should be noted that the demarcation between all three of these ranges is somewhat fluid so it is a question more of abstract judgment where one starts and the other ends than of concrete fact. However once we are in the modal region all sorts of Fit hits the Shan and live becomes VERY interesting.

In the reverberant region things are interesting too, but primarily due to the stupidity of modern speaker designers. Only in the pressure region at very low frequencies is life easy and predictable. But usually not only do we have a lot of problems with the room, we often have as bad or worse problems with the speaker....

Friend or foe - the Loudspeaker designer and his products  

Considering the extensive body of knowledge that exists on room acoustics, the appearance of the "average room" and so on it comes again and again as a surprise to me how ill considered and conceived most so-called "High Fidelity" loudspeakers are, especially if one compares the situation to the sound reinforcement sector and serious studio monitors (meaning NOT the Yamaha NS-10 or BBC LS 3/5 or similar "Scherzo's").

Speakers interact with the room in various ways, both due their own behavior and due to the different regions of the room discussed above. The key to much of perceived tonality is the behavior of room and speaker though the reverberant range, because in this range we find the fundamentals and formants of most instruments and the human voice as well as all the harmonics. Above I commented that in reverberant range the speaker behaves a lot like a light source.

For a long time two specific types of speakers have been sold to the unsuspecting public that have by design severe behavioral problems in this range. One type is the Full Range Dipole (Electrostatic or Planar) and the full range omnipolar speaker. Neither type is found in serious Studio or sound reinforcement applications for exactly this reason.

Dipoles and Omnipolar Speakers create way more problems than they solve, by radiating energy over a very WIDE frequency range and also a very wide area. Think simply of a naked lightbulb in a room. This is your omnipolar speaker and to a lesser degree your dipole. This makes the frequency response in room within the reverberant range maximally dependant on the room symmetry, absorption etc. and thus will require major efforts on room treatment to correct for the problems caused. Of course, a notable minority of listeners actually likes the presentation of omni's and full range dipoles and they by all means are welcome to their preference.

Another major problem source are modern "conventional" HiFi Speakers. They misuse a semi omni directional HF unit (the dome tweeter was designed by Stu Hegeman for applications radiating upwards and giving semi omni directional hemispherical response) in combination with a cone unit that at higher frequencies coming up to the crossover frequency is actually quite directional and as a result the directionality changes strongly with frequency. The use of non coincident sound sources also adds to the problem as now in at least one plane (lateral or vertical depending upon orientation) the radiation is turned into several "beams" going off on sides of the main (forward) one.

All in one unholy mess and one that has done much to cause the rather unnatural, upper midrange emphasised sound of modern HiFi systems, by placing a depression in the upper fundamentals and a strong emphasis onto the lower harmonics for the response off axis, assuming a flat on axis response. Here you cannot really do anything with equalisation, as "fixing" the problem in the overall response will severely unbalance the on axis response and thus the first arrival of sound.

THAT SAID, if no other measures can be taken to address the problem, equalising the in room reverberant response flat will sound subjectively better. However, a better solution is to either switch to controlled dispersion loudspeakers or if a change of speakers is not possible, at least all reasonable means should be employed to correct especially the excessively wide dispersion in the upper midrange of common dome tweeters. One of best solution are the type of diffraction control felt/foam rings embodied by the AIG "Imagers" and found also on BBC Monitors and Dunlavy/Spica Speakers.

http://www.audio-ideas.com/tweaks.html

Room treatment of various sorts can be used effectively in this range to eliminate first reflection points (primary ceiling and floor). Also an arrangement that places the speakers along the long room wall wide apart and strongly toed in can further help to reduce problems from non too ideal directivity of the speakers by directing less sound to walls and making the way for these unwanted sounds "long", thus attenuating them notably. A prime example of this approach is the Hawkesford/Audio Physics speaker setup.

I generally advise against removing sidewall reflections with damping material, as the sidewall reflections somewhat match the behavior of a concert hall and thus are more or less similar to the natural behavior of music and can in modest amounts enhance the naturalness of presentation, diffusion is MUCH preferable to absorption.

If fullrange dipole speakers are used (Maneplanars, ESL's etc...) it is essential that the rear wave is in some way diffused and damped, to avoid strong reflections. My preferred suggestion would be to place diffusers fairly close behind the Speaker, like for example variations on the Argent Room-Lense theme. Unlike with dynamic speakers, where a location close to rear walls is entirely possible and indeed even desirable (more later), full range dipoles require "room to breath" behind them to both delay and attenuate the rear wave. The area behind a dipole speaker should in addition be quite absorptive, after all we need to kill as much of the rear output as we can manage.

As an aside, Dipole Speakers using dynamic cone drivers tend to remain dipole only to around 300-5000Hz, with the rear output attenuated at higher frequencies by the lowpass filter formed by basket and trapped air plus the acoustic "shadow" of the magnet system. So, unless extra rear midrange and tweeters are used to re-establish the dipolar pattern at higher frequencies cone driver dipoles tend to minimise the rear output through much of the reverberant range, one more advantage to recommend them.

Similar principles of course applies also to Omni directional Speakers, but things are much worse there, because while dipoles have at least a "null" (an area with no sound radiation) on their sides omni's radiate to everywhere, while really radiation is only desirable into the direction of the listener. All the extra radiation needs to be diffused and damped.

From the above it should be clear that before you attempt to equalise your speaker flat throughout the reverberant range (or before you use other digital room correction products) you need to address as much of the directionality problems of your speakers as possible and to address any problems with asymmetric room acoustics etc.

Of course, some of us do not have such problems, this includes people with 12" - 15" Coaxial speakers, larger fullrange speakers (especially if additionally loaded into a front horn), well designed Dynamic Driver dipoles and so on. I'll come back to this in the section on the modal region - but the old Hartley Concert Master Speaker (22" Dipole Woofer and 10" sealed box fullrange speaker) is surprisingly close to an ideal domestic speaker and the usual common 2-Way floor standing of stand mounting 2-Way "High Fidelity" speaker is surprisingly far from being such.

Forever onwards Equalisers
 
Now in to the meatier chunks. Above I said that in the modal range the room becomes resonant. One option is of course to locate speakers and listeners in pressure maxima, which are near walls. So if you move your listening position as close to one of the long walls, centered and place the speakers along the other long wall, strongly toed in and (if necessary) Imagered, you will have sorted many of the problems for the reverberant range and you will have coupled yourself (listener) and the speakers maximally to room modes. Of course, the bass is going to sound hugely boomy, but we have an equaliser for that.

The one problem we will have is the floor/ceiling mode where we, due to a seated height of around 90cm - 1m are precariously close to at least one pressure minima. This usually falls around the 100 - 150Hz range and not much can be done about this, it will fall whichever way it will. in the context of the other main modes however both speakers and listener are well placed.

As a further freebie placing speakers and listener close to the walls will both extend and increase their bass output, which after equalising the resulting system will have less actual power input at low frequencies and thus less distortion, so in the bass the system will player lower and louder without strain using such placement and equalisation. Of course, such an approach only makes sense if a suitable equaliser or room compensator is in the system, it effectively becomes an absolute requirement.  Tact is quite astute in recommending exactly such setups with their Room correction gear.

Finally, below the modal range our best choice (should we wish to extend our response that low) is a simple monopole subwoofer, either sealed, passive radiator or vented with a suitably low cutoff.

A Dipolar sidetrack... 

There is of course another trick here that we have so far missed. While all normal enclosed dynamic speakers operate in the modal region effectively as omni directional radiator (regardless of being a domestic LF Horn, Reflex, sealed), using a DIPOLE to cover the modal range will actually result in a rather well behaved Bass reproduction mostly free of extreme resonance’s in room modes. The way the dipole interacts with room modes will result in a much more even LF response by exciting room modes less.

A dipole will maximally energies room modes if placed in the areas that with conventional speakers turn into "black holes" and the least if placed where normal speakers energies room modes most. I am sure you have heard remarks that "speakers sound best where they do the maximum visual damage" and "speakers sound worst where my wife wants them...". Well, dipoles, for low frequencies at least are the opposite. They sound best where your wife wants them and worst where they do the greatest visual damage to room decor.... 

What follows from this is of course that 90% of ALL HiFi speaker designers got it all ass forward. They make hybrid Electrostatic or ribbon Dipole speakers that are dipoles where they should directional (midrange/treble) and that omni directional where they SHOULD be dipolar (bass) or they make speakers that have a totally uncontrolled and widely varying directivity with frequency. Normal Box speakers really can only take refuge to special room setups or equalisation, they DO NOT WORK without them, if High Fidelity is desired.

It seems in the last 70 or so years of sound reproduction only three or four companies ever "GOT IT" (or at least got it to some degree), namely the earlier mentioned Hartley, Celestion with their SL-600 & dipole Subwoofer combo, Gradient in Finland and the now defunked Audio Artistry headed up by Sigfried Linkwitz. Surprisingly, the pre-stereo area music center and/or large Valve radio tended to be equally dipolar at low frequencies and even many early stereo music centers where equally dipolar in nature.

Well, as we know, that was when designers trusted their ears more than their measurements (as accurate measurements where difficult and measurements showed poor correlation with what was heard), before people started to believe that because it could be measured it was meaningful and that a good performance in the few and one-dimensional areas that where measured actually somehow equated to audible performance. But that as they say is another story, back to room acoustics.

How flat are you - equalising with (un)common and musical sense
 
I hope this somewhat exhaustive coverage of room and speaker acoustics has helped a little to understand what can be done and what not. Now onwards, onwards Equalisers - we are going to put some of the above to use.

Hopefully we have re-arranged the speakers and listener for maximal coupling to room boundaries or have changed our speakers to a pair of Oris 200 horns with a major bad dude style dipole woofer system, or if we insist on using badly designed speakers (strictly in the sense of working well within an acoustically small room) we have treated the room suitably and so on.

Our first task is to understand the limitations of our speakers. Let's for arguments sake take an excellent candidate for equalisation, namely a Lowther Fidelio or Acousta. While offering fairly decent directivity, the response of the actual speaker is extremely uneven. If we do not employ an added active subwoofer (which can be integrated much better if a PADEQ is employed BTW) the LF response is realistically no lower than around 80Hz. Below this there is not much happening and while we could use the full range of the Behringers various EQ sections to make the System flat down to 20Hz, power handling would suffer dramatically.

Thus first be aware of the limitations of the speaker. Then consider the classic "rule of the 400000". This states that if the speaker is 3db down at a given lower frequency it should be 3db down at an HF point that will multiplied with the LF limit give 400000. So if your LF cutoff is 80Hz and we will equalise the system to be flat at 80Hz (or 3db down) then we should retain a flat response up to 5KHz and then roll off at an equal rate as the LF roll off.

Thus before running ANYTHING like Auto Q please set your target curve in our case to flat from 80Hz - 5KHz and then with a gentle roll off (say 6db) towards 40Hz and 10KHz with a steeper roll off below 40Hz and above 10KHz. This will ensure that the resulting speaker will play musically coherent and "sounds right" plus, it will not attempt to make speaker do something it is not capable of, with potentially fatal consequences for the driver.

Being lucky enough to own a system that is capable of a flat response from 20Hz - 20KHz (equalised) I chose to make my system for starters flat from 20Hz - 20KHz. Due to the sophisticated memory management of the Behringer EQ it is possible to overlay any "psycho acoustic" and personal curves later, so may as well start with clean sheet.

With some other systems (especially TACT but also the Accuphase DG- 28) it is necessary to overlay any such psycho acoustic curves on top of the target curve, as a later addition of this is a little tricky. I come back to psycho acoustic and "generic" recording compensation is a bit, for now we are dealing with room modes on the Behringer and similar PADEQ's. The TACT does that job for you and well, the Accuphase does it not at all, so the section now is "Behringer only" while all else said above holds a more generic applicability.

While discussing the room modes I suggested that only a parametric EQ can actually effectively deal with them. Luckily the Behringer has three (ten on the 2496) per channel on board. They have to be set manually and this takes time, plus a modified radio shack SPL Meter (and a tone generator for the DEQ 2496 - download a free one for your PC from www.marchandelec.com), the 8024 has it on board), but the results are very convincing.

To access the parametric EQ's choose the Feedback destroyer section, and set all filter to manual. Then enter the analyser mode, choosing the front panel bypass allows you to do the setup without all noise blasting you away. Go into the Analyser setup and select the analyser output to be a sine wave, suitably low in level. (Added later, applies only to UC 8024 - other methods must be employed with the DEQ 2496 and other Manufacturers EQ's)

As the Rives Audio Parc (www.rivesaudio.com) works very similar to the use of the parametric section on a PADEQ you may also employ their method and spreadsheets etc., check their site and the PARC manual as well the measurements section.

You can then use that sine wave (or one from your generator) together with your SPL meter to sine wave sweep the 20Hz - 300Hz range, one channel at a time.  If you use speakers with limited LF power handling PLEASE go easy on the volume.

You need to make a first quick run to establish all major peaks. In my case these where around 45Hz (related to the mode of room and hall) and 60Hz (related to the 5.5m room length). Just note how much these peaks are above the rest of the spectrum in this frequency range and the "peak" frequency.

Program the attenuation and frequency into the parametric EQ, normally the three filters should suffice (the 10 do so definitely), if you have more major peaks, go for the highest level ones. The bandwidth is a bit tricky, you could work that out from the measurements, but for most folks a simple interactive method of cut/try will work better. It takes time, but you don't do that kind of job often. Start with a setting of 20/60 Octaves for the bandwidth, that is 1/3rd octave.

Repeat the sweep and vary bandwidth and attenuation (and if necessary center frequency) until you get a response around the former peak that is well integrated with adjacent frequencies. Basically we don't want any drops or lifts, if the peak is not entirely symmetric it is better to allow a slightly depressed area than a peak.

This process will take a while - once done write down the settings and save them to a program slot. Now you can pull the Measurement Mike out and connect it up to your PADEQ. Place it on mike-stand (real or improvised) so that the mike is as close to the point where your ears will be when listening. Make sure your analyser is set up correctly (pink noise output) at a sensible level (mine is at -28db), that whatever target curve you require is selected and that you have selected a program containing the previously established settings for the parametric EQ's to kill the major room modes.

Now use the Auto EQ function and let her rip. The result should about match your target curve, though some speakers may require more boost at certain frequencies than the PADEQ/UC is happy to apply. If you feel this should be corrected, you can boost the sliders left out a little so that PADEQ/UC picks them up on the next run and repeat.

I would recommend to look out for individual sliders that are either extremely boosted or attenuated compared to the rest of the sliders on either side of them. Take those back into normal regions.

I would suggest to apply no more than around 6db equalisation, if you need much more than this it's either an extreme resonance mode somewhere or the Target curve was still unrealistic of the speakers actual abilities, so correct whatever is wrong. In the end, both channels will be as flat as they can sensibly get. You can listen now but you may find the resultant too bright, as most recordings are mixed and mastered in non-flat frequency response environments.

This brings us to some "default" psycho acoustic corrections that are expedient to apply. I do so by keeping a truly flat 20-20 Programme, one with my "psycho acoustic" curve and a third that combines both. This way I can easily go back and forth. I would recommend the following EQ Applications for "pleasant" sound with most modern (post mid 1960's) recordings or re-masters of older recordings:

1) Boost the range below 125Hz uniformly by 2 - 4db (depending upon taste - I use 2db) and take the sliders above 125Hz so the form a falling slope back to 0db, with 0.5db (1 step) per slider or 1.5db per octave. I personally also have 20Hz at -1db compared to 125Hz and 31.5Hz at -0.5db. This is simply to slightly limit the LF Boost.

2) Apply a similar slope (0.5db per slider / 1.5db per Octave) from 2KHz upwards, meaning -0.5db @ 2.5KHz and then on to -5db @ 20KHz. You may experiment with increasing the point where the roll off begins somewhat.

3) Put a 4db notch into the response around 2.5 - 3KHz, returning to flat at 1Khz and 6.3KHz. This is the classic "BBC Dip".

The resulting curve is what I use on a daily basis and was arrived at based on the study of the various literature and a noting down of the most often applied EQ settings. It offers a good compromise between neutrality, sweetness and pleasant sound. But feel free to vary this basic recipe to taste and experiment.

Having fun with an Equaliser and "Blauert Bands"....  
 
In the 1970's a Major Dude in Germany called Jens Blauert wrote about the effect of boosting/cutting narrow on the spatial localisation of sound sources. This is a piece of work on psycho Acoustics that is still not well known even in Germany, never mind anywhere else (Jens Blauert "Raeumliches Hoeren" S. Hirzel Verlag, Stuttgart, 1974). Anyway, the upshot of Mr. Blauerts work is that five critical bands exists that effect the localisation as front/back and high. In the context of Stereo controlling these narrow bands by a few db will give more "presence” or a more diffuse, distant sound perception.

As most if not all modern and even many early recordings are clearly miked way too close and give a way to "present" sound for my liking I choose to investigate my "psycho acoustic" target curve for my digital EQ.

A more present perception can be attained by boosting the 270...550Hz range and the 2.7...5.5KHz range (note the difference of a whole decade and identical bandwidth of these two bands), or indeed, reducing the level in those bands can reduce the "presence".

I choose originally to boost the range below 125Hz uniformly by around 2.5db with a gentle roll off towards flat that reaches 0db at 315Hz, thus effectively reducing the output in the 300...500Hz, so already one layer of de-presencing has been completed. In the upper midrange I choose to put a fairly deep (-3db) notch around 2.5KHz, which I felt left a good vocal intelligibility while making the sound much less "in yer face". Whenever I went higher with the center frequency I found that the sound lost too much "clarity", very much in line with Mr. Blauerts ascertains. Both these changes where arrived at purely by listening, without knowledge of Mr. Blauerts work. They have been longstanding in my system in addition to a gentle HF roll off (1db/Octave) above 2.5KHz.

Based on Mr. Blauerts points about the "height" band 7...9KHz I boosted this by about 3db and indeed got a bigger impression of Soundscape height by further boosting the range up to 20KHz in accordance I got more depth. Equally, a corresponding band is a decade lower between 700Hz and 1700Hz, which I also boosted by around 3db.

The effect of those additions to my "standard" psycho acoustic correction curve dramatically increased the sense of spaciousness and the backwards reach of the soundscape. I now recommend a target curve for the presetting of room equalisers and room corrections as follows:

Look at the capabilities of your speakers in terms of low and high frequency reproduction as detailed above.

Add to the above curve a 1db/octave (on the Behringer UC EQ this means each slider is moved by the smallest possible step) above 2.5KHz (meaning 2.5KHz is FLAT).

Now OVERLAY a secondary curve that boosts the range below 125Hz by 2 - 3db (more boost - warmer sound) with a gentle roll off towards higher frequencies, reaching back to flat at 315 Hz.

Boost the range between 800Hz and 1.6KHz uniformly by around 2db with a 1db boost in the two adjacent bands (630Hz and 2KHz).

Place a -3db dip at 2.5KHz with a 3db/octave slope.

Boost the range between 8KHz and 16KHz uniformly by 2db with 1db in the adjacent bands (6.3KHz and 20KHz).

The addition of the above fairly complex described here frequency function will give a much more "pleasant" sound while it fits within the TRUE capabilities of the speaker into the tolerance field recommended by the IRT (German Industry body lead by the federal radio stations, similar to the BBC's now defunked research center) for domestic/studio monitoring.

If your speaker system genuinely is capable of covering the 20Hz - 20KHz range your speaker system will still be 100% compliant with the IRT recommendations for Monitors, while having a subjective tonality and spatiality that is much improved over a fully linear frequency response.

Well, that's all folks....

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