Room Correction Dsp

[Allen Yerke]

Ihave experimented with REW (Room EQ Wizard) and room correction, and how to apply it in Roon, and decided to write a guide about it. This is what produced best result for me (I sit in a small room with concrete walls), but the way I did it should work well in bigger rooms and for different walls as well. This guide assumes you have a 2.0 or 2.1 system, and works best if you have a specific listening position and you sit in the middle of the sound, with an equal distance to left and right speaker. By the way, 2.1 systems is measured just like a 2.0 system, with the sub-woofer active during the left/right speaker measurement.

Some of you might wonder why not use the normal way of placing the microphone where your head normally is, and move away while doing the measurement. For me, that way produced a worse result, probably because I sit in a small room and the effect my body and head has on the sound waves becomes relative big. Using sweeps will also give false indications of higher frequencies needed adjustment, due to how short waves the higher frequencies has, and generally speaking a less reliable result. Having said that, if you prefer to use sweeps that do so, both way works.

Updated the guide to generate stereo wav files instead of mono (step 14), if generating mono files I think you also need to write .cfg convolution files, which should not be needed when using stereo wav files (maybe some Roon tech guy an confirm this?).

It must be my lack of tech knowledge, but I get the impression that (this way of) room correction resembles adjusting the EQ (a bit less/more bass, treble etcetera), thus changing the balance the producers and artists have intended. I prefer to hear my music as pure and as close to the way it was recorded. Will room correction improve or deteriorate the original recording balance?

Take the below pics, first is my left speaker and right speaker in my room without any room correction, the last picture is after all adjustments are done (as described in this guide). The sound wave should be close to the blue target line, and I am pretty sure no artists intended it to sound like it does for me without any room corrections (i.e. the first 2 pictures)

Yes, its basically a glorified equalizer, but with very high resolution and ways to decide the steepness of a modification ( the Q value). But unlike old analog equalizers, it works on a digital stream which is handled and modified by Roon in various ways (for example volume leveling), and then converted to an analog signal by your DAC.

Its the data for this equalizer that REW can auto-generate (doing it manually is very hard due to the Q value), and then you can either manually input the data into Roon PEQ or generate wav files for the Roon convolution engine.

Think of the most advanced EQ you can imagine - not only adjusting frequencies throughout the entire audible spectrum (or the parts you specify), but also dealing with reflections (think echos and reverberation from your room).

The easiest mistake to do when doing room correction is if you do one measurement sweep where your head is, and then make REW generate filters from 20 Hz to 20k Hz. Chances are, voices, electric guitars etc will sound very strange after that. The reason for this is that at higher frequency, the sound waves are very short so a few cm difference in measure position might make a big difference, which makes it very easy to overcompensate.

My personal experience (admittedly only using Dirac) is that full range DRC from 20 hz to 20khz can give fantastic results. I understand the reasoning behind limiting to the lower registers, but I think the right software can still deal with the high frequency effectively.

I think that if you modify the higher frequencies smart, which means no sharp modifications (high Q value), it will work. But from a single measurement, its easy to think that a specific small range needs adjustment, for example 1400 - 1700 Hz is 6 dB to high. Modifying that is probably not a good idea, since it probably comes from a faulty reading.

I did measurements and correction in Dirac, which was incredible easy and intuitive. And the result was pretty impressive. I compared to my REW room correction by adding Dirac as its own zone in Roon and playing same tune in both Roon - Dirac and Roon - REW.

Dirac has worked miracles in my setup (difficult room, lots of reflection, bass suckout at listening space). Bass has cleaned up significantly and a bit over overexcitement in the 3000-4000 range has been effectively tempered (my main filter is setup correct up to 4200).

I absolutely believe these filter settings are bringing some improvement and I am far from giving a verdict without having listened to music in this room. But to me the combination of pictures plus measurements is strongly hinting to the conclusion that the first things to do is improving speaker positioning and room acoustics, not finding EQ settings to counter what the room is doing to the sound.

The main things I read from the measurements: It seems you are facing severe problems with resonances and standing waves all over the bass region from 25 to 150Hz. Several peaks of such significance plus some dips are usually indicating problems in the time domain plus cancellation effects you both simply cannot EQ. You can milden the problems but not solve them digitally.

The other thing I noticed is that the increasing level in the treble region >5k has been corrected. That might milden problems with sharpness and metallic-sounding reverb but again it is not solving the problem. The problem is you have too much energy in the room in this region and too less in the area 2k-5k. You need absorbtion to solve this, not an EQ. Are the B&W all D3 generation?

And yes room acoustics would lift the SQ even higher. BUT - I has to look good at the same time - and that is not easy to find. Placing absorber in the ceiling could an idea - but if this bring the impact is hard for me to say.

You nailed the acoustic problem @Kuryan but I would not go so far to declare a compromise impossible. It is just in my opinion the combination of obviously significant reverb in the room, speaker placement close to the wall and fairly large distance to the listener plus choice of loudspeakers being known for substantial low bass and wide directivity pattern in the treble region - that is a recipe for acoustical problems being absolutely un-EQ-able.

Whenever I have heard convincing sound quality from the B&W 80x D3 (and they can offer world-class treble resolution!) it always turned out that the room had been damped heavily such as in a home theater environment. That is certainly not the case here.

Yes we could all move speakers and listening position to the best place in the room but it would not make it practical at all for 99% of people or families. We adjust as good as we can to make things work. DSP does the rest. It solves my bass issues none of which I could without DSP and I have tried with many different speakers always the same. There is too much naysaying on DSP in audiophile circles and they really are missing out on what their system could actually sound like.

We all agree that DSPs are useful and people who dismiss using one in most of cases have to live with acoustical problems which could be solved easily. In my eyes the opposite belief that a DSP can solve everything is just as unhealthy as it in many cases leads to carelessness when choosing and positioning speakers.

The point about DSP is understanding what they cannot do. In other words: You have to create a proper base by avoiding problems in the time domain (such as bass booming and too much of reflexions at the listening position) which cannot be solved electronically. In most cases this means improving speaker positioning, choice of speakers and overall level of reflections/reverb in the room.

It is fully understood that no-one wants to live in a recording studio environment. But in many cases room optimization as a base for EQable reproduction means measures such as moving your speakers 50cm towards the listener, away from the corner, angling them in, choosing a slightly bigger carpet and maybe exchanging curtains in front of the windows. These 5 things together with choosing suitable speakers in terms of directivity after knowing your RT60 and acting accordingly in my experience solves more than 90% of all problems.

As a rule of thumb I would recommend to narrow down the listening distance to a level you get a center channel grade localization in stereo. Plus optimizing positioning and choice of speakers until dips do not exceed -5dB and peaks are not audible in the time domain (i.e. booming bass). Yes that sometimes means pushing a sofa towards the speakers or getting one out of the corner. But in most cases this is a pretty good base for starting the digital optimization.

I do not miss the point but have actually experienced many times that people do not put their effort into what is really solution-oriented in terms of choosing equipment. Many also do not evaluate the sound of speakers they are going to buy in their own room. In my opinion the single most crucial mistake leading to a lot of dissatisfaction and everlasting quest for new gear.

Yes hi-fi people tend to alter the position of their speakers a lot and angle them in and out. And it might change things to the better or the worse depending on the music played, no-one knows. But that is not changing the actual problems caused by the room and not what I would call a strategy.

For me a strategy is to first take an RT60 measurement of the room and then theoretically plus practically figure out what is the ideal (maximum) distance between speakers and listener by doing a quick monaural/localization test. If this already leads to an unsolvable contradiction (and it does in many cases), speakers have to be exchanged for an acoustically suiting model.

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