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UNPLANNED DOWNTIME: 12th Oct 23:45
Mix Room Acoustic Treatment - Let the fun (again) begin.. Update #11
Base theme by DesignModo & ported to Powered by Vanilla by Chris Ireland, modified by the "theFB" team.
Comments
Be good to see what you do with the space you finally end up with!
Thought I'd take a moment to take a breath and recap a little.
We've effectively got to a point now where we've significantly improved early reflections, decay time, resonances and as a consequence of removing energy (to achieve the above) have improved the frequency response. Note I said as a consequence here. The unfortunate truth is that whilst it's relatively easy to improve the first 3 it's much, much harder to improve the 4th especially in the low/sub region. Physics is just against us and, as I mentioned before, low/sub energy is like a rampaging elephant and the tools we have are just nowhere near effective enough to stop it; we can slow it a little but beyond that. We will eventually use some tools beyond acoustic treatment to add the final gloss to the FR but we have to understand what is impacting this as some things can be improved using other tools and some things can't. We'll use some further analysis of the data to identify these and treat accordingly.
In terms of outcomes from analysis of the data the following areas are where we want to focus attention for phase 2 in order of priority;
- Reduce reflections around 12.5 and 17ms
- Reduce modal resonances between 50 & 100hz
- Reduce window resonances
- Bring down decay times above 1k to circa 175-200ms
- Flatten decay time from 200hz - 1k.
- Smooth overall frequency response
- Match speaker response in both FR and time domain
We'll start to work through each of these over the next few weeks and see where we end up.Finally I thought it might be worth talking about how the room actually sounds subjectively now as after all that's the reason I'm doing it. In short, the change is so profound it's almost a laughable! It's like I'm listening in a completely different room (which I am acoustically speaking!). Imaging is sharp and the the sound literally jumps out of the speakers. 3D sound field presentation is amazing with left/right/front/back/up/down easily portrayed on decent mixes. ER/verb/delay treatments are now exposed in all their glory as indeed is everything else including technical issues, horrible synth patch choices etc
In summary it's already sounding great
Si
Before we go any further on the project I wanted to talk a little about the biggest challenge I face in my room; the sub 100hz region. My current speakers roll off at 12db/oct from 100hz and as such in the absence of any impacts from my room/boundaries (a theoretical construct!) I should see this reflected in the recorded frequency response. I don't as obviously I'm in a room! So what's the deal here? The deal is that my room is built from hard stuff and pretty much all of the energy is contained within the space. Why is this such a problem?
Imagine my room was reconstructed at exactly the same dimensions but, instead of the brick/plaster, the walls were actually a wooden frame covered in fabric that allowed energy to flow through it. This hypothetical room is standing in a field with nothing around it and hence energy can freely dissipate (I'm conveniently ignoring the floor here). if we were to pump energy into the room and measure the frequency response at my listening spot it would be very close to the signal coming from the speakers themselves.
The problem with rooms is that the energy creates pressure at the boundaries and, at frequencies that match the physical dimensions of the room (and its multiples), standing waves are created. These establish themselves and literally form waves that vary in amplitude if you move through them at the relevant frequencies. Think of these waves as ripples in amplitude that overlay each other and, as we go higher up in frequency, more and more overlay to create a complex pattern. We can calculate these standing waves (modes) using a tool like amroc online but bear in mind these are theoretical models and as such reality often diverges. To hear this in action simply calculate a mode (low axial modes are the best), play a sine wave and move around the room. You will hear the response vary widely and at the lowest modes the tone you hear will literally disappear in the middle of the room along the relevant axis and sound way louder at the boundaries.
So how do we fix it? What absorbers are we gong to install to remove all this energy and solve the FR problem? None; we can't! The problem here is that there is no passive or active absorber that has the horsepower to flatten a frequency response by any significant margin below 100hz. Sure if we are lucky and we spend a load of time, money and literally fill the room with deep treatment or high Q pressure traps we might shave a db or two off a peak or a null - given I have swings of circa 10db I have literally no chance at these frequencies. Treatment on all surfaces is good as we go higher in the spectrum as the tools become more effective (and we get some additional benefits), but at the bass/sub..forget it. Remember I am talking frequency response here not time domain.
How do 'proper' rooms minimise this problem then? They have special boundaries (within boundaries) that are designed to soak up as much of this energy as possible - remember our acoustically invisible room example in a field above.
There is actually another problem. Part of our strategy is at some point to use digital room correction to smooth the frequency response. Easy then, let's just EQ out the problem. Without wanting to get too heavy the very low frequency response isn't minimum phase and as such EQ just won't cut it to any significant degree. We'll look at minimum/excess phase at some point soon.
Is it actually a problem? We know that our sensitivity to changes in amplitude and at low frequencies it is very poor but the problem is we are actually quite sensitive to peaks. We tend to hear those peaks more easily and that's problematic.
So what do we do. Option 1 is to live with it. Don't like the sound of that. Option 2 is to use the only practical tool we can actually implement and we'll be doing that at some point. Welcome to the world of multi-sub woofer room mode management.
My room will broadly have the same issues irrespective of the speakers I use and their respective FR. It's built into the DNA of the room itself.
Si
Back to the drawing board...
R.
Eqd Speaker Cranker clone
Monte Allums TR-2 Plus mod kit
Trading feedback: http://www.thefretboard.co.uk/discussion/60602/
So come on Si, what's the plan?
We know that we're going to deal with the sub 100hz region using subs (and we may also get some help from other things we do) so what are we going to focus on prior to this.
I highlighted that time domain specifically between 50hz and 100hz is something I need to control tightly and if you look at the FR between 100hz and 300hz it could definately benefit from some smoothing out if possible. I face a couple of constraints though; I don't want to suck out the high end further and I also face physical space constraints. The problem is that velocity absorbers fail to address both of the constraints adequately. The problem with the lower frequencies of this range is that the power of even my 400mm deep panels is just limited and they will literally eat my room physically. At 100hz these panels have an absorption coefficient of about 0.6 so effective but not as effective as I would like.
What we need here is some treatment that is broadband between enough to cover the region 50-300hz, most effective around 100-150hz and has a small physical depth footprint such that we can implement it without taking up too much space. It needs to be simple, effective and something that we can build at home with available materials. It's a big ask but, luckily for us, some clever folks in Germany have a possible answer..
We've talked about two types of treatment so far; velocity and pressure. Most pressure traps are amazingly effective but they have a couple of significant problems; they have a high Q so they can only really tackle very specific problems and they are hard to build at home and be effective. Not that we can't build them, I've had great success with MLV traps but it can be tough and for every success in the past I've had failures and wasted time/money. The odds get even worse with things like Helmholtz devices.
A little while ago some folks came up with a rather novel idea. Why not take a thin metal plate, attach it to a substrate that had specific properties and damp the vibration of the plate to remove energy as it is excited by sound. This solution is somewhat elegant and in some respects is the answer to many a home acoustics prayer. To be able to build something effective from circa 50hz, has lower Q charateristics and that is only 4" deep seems just too good to be true. The data seems to bear it out however.
I'm going to build 4 off these and we'll find out..
Back soon.
Si
Thought I'd have some fun today as progress is a little slow on the passive treatment side.
I mentioned before that I basically cannot do anything about the lumpiness in the low end of my setup with any of the treatment options. Basically there's nothing I can put in the room to improve things. There's only one option here.. multiple subs..
So I rigged up a miniDSP 2x4HD with 2 Eve Audio TS107 subs.. both subs are in positioned in my front corners and I managed get a couple of hours setting up a very basic, quick and dirty setup with the sub integration. The sub positioning isn't technically optimal however, I'm not just interested in a flatter FR; I want to ensure my imaging is as good as I can get it. I'm running the XO a bit higher than I would like as my main speakers roll off from 100hz. When I change them I'll run it down lower.
This also allowed me to move my main speakers to a much better near field position without the 50hz range falling into the abyss.
Using subs like this allows us to excite the room modes more evenly and smooth out the low end in a way that no treatment can. We also get the added benefit of unloading the sub/low from the mains and improving the mid-range reproduction a little..
So.. here's an FR from REW that shows my untreated room (left and right individually) along with a combined L+R+subs from today all at 1/48.. you can ignore the high end as this always rolls off when measuring two speakers at the same time
Starting to look (and sound) pretty sweet!
Deadmau5 playing at the mo so that I can enjoy all that low end..I do love a filter sweep or two
Si
Despite a distinct and frustrating lack of progress from a treatment perspective I thought I'd jump ahead a little and at least implement another piece of the final puzzle. I'll have to do this again but it's fun so..
Basic treatment is in place, ER's are covered, decay time/balance is sorted (to a degree) and subs are in place and well integrated. It's as good as it's gonna get with the current treatment but it just isn't good enough!
What's a chap to do eh?
Just imagine if we could turn the chart in Update 10 into this..
1/48 oct smoothing:
1/3 oct smoothing:
Impressive eh? The above is only half of the story as it represents frequency only. Step response and group delay are pretty much perfect so we have the time domain covered as well.
As you may be able to imagine the difference is pretty stunning; very balanced with amazing imaging and clarity/detail.
What is this magic? Welcome to the world of 'proper' DSP.. and by proper I don't mean Sonarworks marketing bull.. I mean proper room correction.
Will expand soon..
Si
I used REW and a MiniDSP to set up room correction in my listening room and the results were quite spectacular
Did you use Rephase as part of the process to tackle time domain after working in REW? Are you deploying FIR filters or just exporting EQ from REW into MiniDSP?
If not and you want to take another step up in quality I can guide you to something that is well worth the effort! It's not as good as the tool I use for a number of reasons but you can do something similarish for free..
Si
https://www.youtube.com/watch?v=O65Y_7pUkSg&list=PLChqFga45IcGGojIToCucGnoJQiq1Yxi4&index=1
Si
Eqd Speaker Cranker clone
Monte Allums TR-2 Plus mod kit
Trading feedback: http://www.thefretboard.co.uk/discussion/60602/
You can talk us through the state of the art then and I can talk about the state of the art now
Thought I'd take a couple of minutes to talk about the room correction stuff I referred to in my last update.
So, just in case anyone is a) actually reading this and b) actually interested, here's a very high level overview of what this means and some thoughts.
When we sit in the listening position we're hearing the speakers and the room. As we know the room is effecting what we hear. Modes mess with the lower frequencies and as we move up the frequency range modes influence less and reflections more. When we use acoustic treatment we try to reduce the influence of these things. Some stuff is easier to influence; decay times (harder as we go low in frequency) and early reflections are the easiest. We can influence the frequency response down to a certain point by reducing modal energy and reducing reflection energy. Worth bearing in mind that it's these two things that change a nice flat frequency response from the ideal to what we experience.
So what does DSP do to help us. Once we get to a certain point it's almost impossible to remove modal energy and the influence of reflections. We either run out of space, money, will to live etc. and in the days of old that was all we could do; we had to live with it. Today however we have tools that can literally perform audio magic.
In it's most simple 'room correction' form DSP allows us to invert the things we don't want to happen and remove them from the audio stream that enters the speakers. By the time the sound has traveled to the ears (influenced still by the room) we get what we want. Imagine if some modal energy is boosting 60hz such that in the listening position it is 6db higher than the original signal. If we remove 6db at source then the modal boost will bring 60hz to 0db when it gets to our ears. We get a desired flat response at the listening position.
Simple EQ has been used to achieve some of these objectives in the past but it is a very blunt tool. It also has the disadvantage of impacting on signal phase. If we introduce a traditional filter we are effectively changing phase and that is problematic. The other issue with traditional EQ is that it is a very blunt tool. We are limited to the number of bands etc.
Let's say however that we could build a digital filter that had thousands of (or way more) EQ bands. Let's say this filter was also able to do this without phase changes. Let's say we could also do the opposite; change phase without changing frequency. The power would be enormous. Not only is this a reality today, but we can do it with very modest computing power. We can also do way, way more and actually create systems that are incredibly accurate.
So what's the catch? As per all basic economics textbooks - there's no such thing as a free lunch. One price we pay is in time. It takes time to do this processing. We can however work around this by creating filters that are uber-high quality for when we are critically listening and slightly lower quality for mixing (as an example) where instant feedback is required. We can switch between them in real-time depending on the task at hand and get all of the benefits and none of the disadvantages. The other issue is that DSP can't do anything to reduce decay time in our rooms (not strictly true, but too advanced for this post). We simply have to get these under control using acoustic treatment in one form or another.
I'll leave today with a last point. With power comes control. Obviously not everyone wants to learn some of this stuff. There are solutions for folks who want a 'point and shoot' experience but they're just not as powerful and the results less optimal.
Is all this effort worth it? Completely.. go and look at Update 10 again - this took me 30 minutes and is still nowhere near what will be achieved when I come to do it properly at the end. At that point I'll also define linear phase digital crossovers, time align everything perfectly and linearize the subs before doing the final correction piece.
There are folks who think that this sort of thing is cheating/less valid. I'll leave them to live in the past. Few of us have the hundreds of thousands of pounds it can take to build 'perfect' listening conditions but today we can get frighteningly close to this for a fraction of the cost. It's not lost on anyone who follows this sort of stuff that many speaker manufacturers are including some of this capability directly into their products.
Si