Taming a Square Room With Two Subs, REW and the Mini-DSP HD

This was my room when we began...


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Nasty isn't it? 

Review equipment:

Sony ZD9 75"
(Sony TV review)
Denon AVC X8500H

(Denon AVC X8500H review)

Dynaudio Confidence 1 Platinum
( Dynaudio C1 Platinum Review )

Sony UDP X800

(Sony UBP X800 Review )
Sony UDP X700

Dynaudio Confidence C1 and Centre Platinum front and centre

Usher 520 side surround back

Monitor Audio RXFX in dipole mode for rear surrounds

JL Audio E112 subwoofer

REL 1205 subwoofer

Anthony Gallo A'Diva for ceiling Atmos placement

QED XT Revelations for front and centre speakers

QED Micro speaker cable for the surrounds

Monoprice Premium Cables from 4k players to amp and onwards to the TV

Audioquest Snake subwoofer cable 

Blue Jean subwoofer cable

Wireworld Oasis 6 & 7 Interconnects

Wireworld Oasis 6 power cables

Assorted Xindak, PS Audio and other power cables

MK wall power sockets

PS Audio Juice Bar

Rhodium Right Angle Plug adapters for USA power cables

Auralex subdude platform and Mopads under centre speaker

http://peteswrite.blogspot.com/2019/04/my-setup-42019.html

So I finally managed to move into a new den, complete with a nice sized TV, posters, my kind of wallpaper (not the stuff you see on monitors) but a real WALL – paper. I position my kit, taking care to do the bass crawl, set the right levels etc and then I sit back and turn on my setup… and to my dismay, the results was not what I expected!

Stereo performance was fine, surround sound immersion was great, but the bass…. nasty

So what was wrong?

The bass had no rumble and no chest thump. It was muddy, and unclear. More importantly, there was a certain compressive feel in my ears, especially the right one.

Now firstly, I have to describe my room. I have a dedicated power line coming into it from the mains, which is not corrupted by power going to the other high current items like the aircon, kitchen appliances and the pumps.

I also have a wooden beam ceiling with almost 80kg of Rockwool on top of it. I also have diffusers to spread the sound and strategically placed absorbers.

But none of it could fix the room dimensions.. you see, I have a square room. It’s 4.02m wide, 3.98m long and 3.1m high. If the ceiling was any higher, I would have a perfect cube. The kind of room that would give most hifi enthusiasts the shudders..

This kind of room is exciting in the wrong way.. there are exciting room nodes which can really give a very uneven bass response. The bad word here is 40.. 40 hz… that’s the usual number that crops up and gives the pressurised feel.

So with the aid of a few buddies, we set out to fix this. At our disposal were four subs:

JL Audio E112

REL HT1205

Two Starke Sub24 8”

All of them are sealed subs and the first two are 12” cones.

Using the two Starke Sub24s gave a tight bass. The 12” had impact but the bass was muddy and without impact. If you tried to increase the SPL to get some tactile feel, it made the pressurised feel worse without real improvements in sound quality.

So we have to bring in more sophisticated kit.

Drum roll… introducing the miniDSP 2×4 HD DAC+DSP, UMIK-1 USB calibrated microphone, and 2×4 HD1 software plug-in. This little box is quite dainty and rather inexpensive, but in the right hands, it can do some serious magic.

(https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd)

Now that’s the main caveat.. you need to be familiar with the software and the tweaking process. I’m not guru, but I’ll try and explain the process here, but you won’t simple be able to do it on your own without some help. It’s not that easy.

What do you need for fixing this room:

REW software – this is a free download, but do considering making a donation, the creators are superb and without it we will not be able to do anything..

(https://www.roomeqwizard.com/)

A microphone – I use the UMIK : miniDSP UMIK-1 calibrated USB measurement microphone

(https://www.amazon.com/gp/product/B00N4Q25R8/ref=as_li_tl?imprToken=gms9sAZEk5mpkv9TRYlwGg&slotNum=0&ie=UTF8&camp=1789&creative=9325&creativeASIN=B00N4Q25R8&linkCode=w61&tag=roomeqwizar07-20&linkId=d3063b35f3c13b526a79c93a5b3c5024)

A microphone stand

And the Mini DSP

Of course a few able bodies friends who are willing to help you move the subs around does help very much.

You need a low noise floor and anything that causes noise should be turned off. Without my aircon and hard discs running, my room measures around 30-35 Db.

You also reset the AVR to the default settings, for distance and turn off Audyssey. Point the Umik upwards and use the 90 degree calibration file.

So the process:

Use the REW sim function to find some potential spots. These may not be the ones your sub crawl suggests.

Well the first hour was a bit of a snafu... we didn't know why the Mini DSP had no output.. we switched laptops, cables etc until we realised we were using the wrong outputs!

Anyway, use output 1 and 3 if you have two subs.

Then with the lowest noise floor, measure your room’s inherent room node. In my case, it was 43Hz. That was the evil number.. This was the exciting node along with any multiples of it eg 86Hz.

There was also a secondary node at 60Hz.

Figure 1: Spectrograph showing the exciting nodes

We then set out to find the places with the lowest 43Hz excitement. We did this by playing a Pink PN note at 43Hz and going around with the SPL meter. We did the same with 60Hz.

At this point, you need to set the SPL levels to get about 75db – on my Denon, we had to dial it to about -20 (but this will vary in your room).

We found that a Near Field placement for the JL E112 and a secondary position at the Right rear back proved to have the lowest SPL measurements for these two frequencies.

Now for the mid bass punch, my friend suggested placing the REL such that the woofer faced the rear wall and was 6 inches from the wall. The use of reflected energy from my solid concrete wall was inspirational.

It also helped that these positions were practical and had good WAF (wife acceptance factor) with less cables running around. It helped that I had pre-placed some subwoofer cables in that corner along with power points too – I had almost 20 power sockets in my HT den.

So then we placed the E 112 on a Subdude platform, and measured the two subs so that they had the same volume and SPL output. Ideally, one could employ two subs of the same make and model, but we make do with what we own..

Then came the phase and time alignment parts.

I’ll try and do a more detailed report on what are the settings within REW and the Mini DSP program when I post the review on the MiniDSP itself.

But basically you measure the sub outputs individually, check the frequency responses and then the levels, then the time alignment and you then smoothen out the response.

Figure 2: The measurements of the subs before time alignment

You do this manually within the MiniDSP program, getting both subs to output the same SPL, making sure they do not excite the room nodes, and are working in synchrony together.

Once you get the two subs in phase and alignment you then measure and assess what you need to do to cut the peaks, then once you know how high they are, you then decide how much to cut this to allow the troughs to be evened out.

Then the Mini DSP attempts to smooth out the peaks, and bring up the troughs to get a more even response and also do some time alignment.

Figure 3: the original sum of the two subs

Figure 4 : after manual time alignment

Figure 5 : What are the areas that will be cut off to eliminate the peaks and fill in the troughs.

Figure 6: Before and after calibration SPL measurements.

Now after getting a more flat bass response curve, that’s not all! You then want to get a good crossover to the main speakers.

We then did a measurement of the two subs alone, then along with the mains, and each individual speaker. Then we found the ideal crossover frequency.

Figure 7: Getting the right crossover

The crossover frequency can be different and do not feel that if you have a large speaker, the crossover must be lower down. The room will interact with your speakers and you need to measure to know what’s best.

It will help if you have individual crossovers for each channel.

After that, you go onto to use Audyssey to get the entire system time aligned. Do the full eight points and be as quiet as possible.

The microphone should be at the nose level, facing up. Make sure it clears the headrest of your chair though.

We found that the initial setup have a different time alignment between the main speakers and the subs, so we had to manually measure the time difference and correct for that. You can do that within the MiniDSP program or in the amp. We choose the MiniDSP so that if we re-run the Audyssey it won’t be affected.

Bear in mind: Each time you change a component, add a bass trap or even change the position of the gear, you need to repeat this process.

We then measured and we found a secondary exciting node at 44-46 Hz and we used the PEQ function to lower that.

It’s no mean feat considering it took us about 5-6 hours to get this far.

(thank you buddies!)

Figure 8: The final curve – yeah….

Figure 9: Final Spectrograph

This wasn’t what we settled for… we made the lower frequencies a little hotter, by adding a Parametric EQ with a low shelf from the 30Hz mark downwards with a Q of 0.7 and a gain of 2. This gave more low rumble. We also increased the gain on the subs from the MiniDSP program to boost the low end when we listen at lower volumes.

We did not use Dynamic EQ, as it muddied the bass and made the surround channels too loud. This “bass boost” function should be turned off. It’s an easy cop out way to increase the low end, but it made the bass loose and whilst it’s a lot more work to work on the time alignment and PEQ, the rewards are there.

We use Dyn Volume at “Light”.

Last words
Just some points for the enthusiast who wants to do the same thing..
This isn't something you undertake without doing some homework... actually, a lot of homework.. 
Understanding how the device works, how to do the calibration is just part of the work needed. Knowing how to use REW and the various parameters is the foundation. 

It also helps if you have a computer in the same room, be it a laptop or in my case, my Mac is within HDMI cable reach of my amp and also near to the positions of the sub, so I can use a long USB cable to access the Mini DSP. 


I don't mean to dampen anyone's enthusiasm, but this kind of work is really rocket science. I'm no pro at calibration or the use of the computer, but this is a very systematic means to get the right curve and sound you want, and it's effective, even in such a difficult room :) 

A very useful tutorial: