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Building a High End Listening Room Part 2: Fine-tuning the Acoustics

RDacoustic

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This is a second part of a post about the construction of our listening room, this time concerning acoustic adjustments. The goal: the best possible listening space.

Basic Properties of the Room
  • A quiet place, far from low frequency vibrations caused by cars, trucks, trams, etc.
  • The room is situated at the back of the building, with a garden allowing for a space for our own grounding system.
  • The building has a reinforced concrete skeleton, without any unwanted (from the point of view of acoustic) materials like plasterboard, which would be prone to resonate.
  • By erecting a new wall, getting rid of a partition which was not suitable and bricking up niches and windows, we ensured an optimal side wall ratios and symmetry of the listening space in regards to modal frequencies.
  • Conveniently, the electric mains has a transformer which is separate from the nearby industrial area.
  • The stronger-than-needed power supply is newly laid down, going directly from the main switchboard of the building.

HighEnd-Listening-room-RDacoustic-1-1-scaled.jpg



Presented are two different audio systems placed opposite of each other, the listener sits in the middle, in the ideal position given by the usual triangle. In the room, there is enough space to move the speakers and the listening spot with regards to modal frequencies.

Basic Acoustic Treatment
  • In order to disperse acoustic reflections ideally and still keep most of the acoustic energy in the room, there is a hybrid quadratic diffuser placed on the ceiling. The depth of the pattern is 200 mm.
  • The front walls by both the speaker systems are covered by a large hybrid diffuser; 5.64×2.6 m, with a pattern depth of 150 mm. It works from 128 Hz up, which is far behind the boundary of signal phase shift localisable by the human ear.
  • Direct reflections from the side walls are eliminated by a coarse fabric and small acoustic accessories.
The hybrid diffuser absorbs low frequencies more and keeps the higher ones in the space, a large advantage in comparison to acoustic treatment that only absorbs.


RDacoustic-diffuser-HighEnd-scaled1.jpg


Reverberation Time

Is, from the point of view of acoustics, one of the main measurable parameters of a listening space. Long reverberation is unpleasant to the ear: Low frequencies stay in the space for too long. Too much bass acoustic pressure fills the room and details get lost. At first hear, the bass is mighty, it can even be perceived positively by some, but when listening longer, one gets tired. In opposition, a very short reverberation in a space that is overly dampened causes perceived constraint. The ideal is somewhere in the middle between these two extremes. Our goal is a specific, concrete, hard, dynamic and precise bass. A non-specific hum of a subwoofer is not what we are looking for.

We measured the length of reverberation using a speaker placed 1 m far from the front and 1 m from the side wall. Reverberation manifests most on low frequencies and it is standardly measured up to 200 Hz. Below, you can see how the reverberation time went down from a completely empty room, through the installation of the ceiling diffuser, laying down the carpet, installing the front diffusers and finally placing the armchairs.


REW-RDacoustic-ListeningRoom-4-300x148.jpg
REW-RDacoustic-ListeningRoom-3-300x148.jpg
REW-RDacoustic-ListeningRoom-2-300x149.jpg
REW-RDacoustic-ListeningRoom-1-300x148.jpg


Simulation and Practice

Good old rules still apply. A small shift of the speakers position out of the simulated ideal point gives the sound the proper, perfect character and elocution.

The project began, as is usually the case, on paper. Once again, we found out that even the best calculations will differ from reality. Just as supercomputers fail with the prediction of weather more than five days ahead, acoustic simulation fails on the same premise. The most important thing in the equation is input data and the quality of the computational model. Just as real weather has in some places its local microclimate that is hard to simulate, so does an acoustic space have certain qualities that the computer model does not take into account. Basic acoustic parameters in spaces that are not too complex can, of course, be simulated. However, such is not the case for what the localisation of the instruments, or character and elocution of the space will be.

Measurements

Down below are measurements of the left and right speaker systems in the listening spot chosen using our ears. In a larger group of listeners, we have decidedly came to the opinion that this is the place where it sounds the best. The measurements are solely basic, for orientation. We have not made laboratory measurements of the diffusivity or absorption of the floor, walls, or the ceiling. These numbers often do not give detailed information as to the complex acoustic behaviour. Moving the microphone by 30 cm, the measured graph changes significantly.


RD-Evolution-left-300x147.jpg
RD-Evolution-right-300x146.jpg


A few pieces of general proven advice for better acoustics
  • The best speaker-listener position is always defined by an equilateral triangle (a few centimeters here and there).
  • Overly dampened or completely undampened space is wrong – too great an amount of acoustically absorptive or reflective materials.
  • Large areas of plate glass, plasterboard, wooden walls or paneling behave as resonators and are usually detrimental to good acoustics.
  • Eliminate reflections from flat opposite walls, break up large flat surfaces.
  • If the listening spot happens to be by the wall (a sofa close to the wall for example), dampen the back reflection from the wall, at least by a large pillow or a carpet.
  • Pay attention to symmetry when it comes to acoustic treatment.
  • Experiment. A blanket, a pillow… these are easily movable and you can find out by listening in which position they have a positive effect and according to the surface size and volume tested choose an acoustic canvas, tapestry or other acoustic accessories. Often, very little is needed for an acoustically “horrible” space to become “acceptable.”
  • What we frequently observe at our customers’ spaces, is flutter echo. This happens especially in large open spaces where several rooms are connected by a large passage, with not enough absorptive materials. Carpets not your thing? Place acoustically absorptive treatment on the ceiling, thick curtains and other absorptive decorations on the walls.

RDacoustic-listening-room-experiment-1-scaled.jpg


You cannot change modal frequencies of a room, but their influence can be optimised.

Elementary physics works everywhere. In most spaces where we had the possibility to take acoustic measurements, we heard with our own ears room modal frequencies. Using a five-second sweep tone where the frequency increases logarithmically, one can hear sound level fluctations. The effect is similar to changing stations quickly on an old radio; same intensity for a short time, a fluctuation, same, fluctuation. These are resonance, or modal frequencis of a room. Spaces where the distance between opposite walls forms multiples are problematic. For example, a room of 5x5x2.5 m will make one frequency and its harmonic multiples stick out. You can read more about the way sound acts in a closed space here.

In our listening space, we were able to achieve ideal ratio of the opposite wall distances, and overall symmetry.

Acoustic Interactions Inside the Listening Space

A speaker system changes electrical energy into mechanical energy. In our space of 8.7×5.45×2.6 m, plus a niche for the door, we have about 125 m3 of air. At room temperature, pressure and humidity, it weighs about 150 kg. Imagine a speaker driver, its thin membrane, as it interacts with 150 kg of air in the room. Sound is waves, its speed in air and in our conditions is approximately 345 m/s. We hear from approx. 20 Hz to 20 kHz, therefore we hear a wavelength spanning from 17 m at 20 Hz to 1.7 cm at 20 kHz. In regards to the closed space, we are interested in half waves of all frequencies; we are talking about distances between 8.5 m and 8.5 mm. As we are concerned with a closed space, in the size of audible wavelengths, resonances will occur: Certain frequencies stay in the space longer. These are modal frequencies of a room.

Moving the speakers in the space, their interaction with modal frequencies of the space changes. The weight of the speaker driver membrane, thickness of a curtain, volume of the speaker chamber – all these interact with the closed listening space, as it, on low frequencies, acts as a resonator. If you move the speakers, you can significantly influence their expression on different frequencies. The same goes for the ideal position of the listening spot.

The Ears Will Tell

If you clap your hands strongly, you will create a sound impulse that has a very wide frequency range. See the image below. The yellow intensity is background, the red the maximum value when a clap was heard. One can see that the frequency range is very wide. With your own ears, you will then easily evaluate how long does reverberation stay in a closed space and on what frequencies. You can try this simple method at home. In each room, the clap will sound differently. We can even hear modal frequencies of a room. The prerequisite is absolute silence. A strong clap in the corner of a room will sound differently than in the middle. These are the aforementioned resonance, or modal frequencies of a room.


Screenshot_20200530-101222_Spectroid.jpg


Final Acoustic Adjustments

On low frequencies, we lowered the reverberation to the required limit using hybrid diffusers, carpets and upholstered furniture. The space is still very vibrant on high frequencies, with a noticeable resonance between the side walls. We are temporarily placing a coarse fabric on the wall to get rid of the direct reflections on high frequencies. Up, down, up, down, too much or too little? The fine-tuning of acoustics is not measurable or countable; it is very subtle and subjective. The goal is for the songs to sound as best as possible, for the contour of string instruments to surface and stand out, for the piano to have exactly the colour it has in reality. We cover the door niche with a thick curtain and place fabric on the right wall. A step in the right direction. We are once again looking for the ideal listening spot… It changes with every adjustment and everybody may perceive it a bit differently.


RDacoustic-fine-tuning-room-acoustic-test-scaled.jpg


Acoustic Lens

Covering a part of the right wall with fabric has removed unwanted reflections from said wall on higher frequencies, but something else is also needed. We agree that a slight diffusivity on the side walls would definitely benefit this listening space. An acoustic lens has been created as a missing piece in the puzzle. A light accessories, easy to hang on a wall, which creates diffusivity in a wide frequency spectrum.

Acoustic Character of the Listening Space

Simply put, the sound expression is fuller, yet more detailed and with an even more powerful and precise bass undertone. Thanks to the dominant diffusers which help primarily with the spatiality of a recording, thanks to the controlled reverberation time and modal frequencies that are not overlapping, the room withstands much larger acoustic load until it, as the expression goes, falls apart. As was mentioned above, acoustic pressure of modal frequencies stays in a space longer. Once these modal frequencies add up and go over an uncomfortably loud boundary, we say that the room “falls apart.”
 

abdo123

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Incredible work!

it would have been really nice to have some 'before' and 'after' images, now I feel like the absorption is too well hidden for me to find :p

Also it would have been really nice if the roof was higher so the diffractor can be design to diffract even lower frequencies.

 

Somafunk

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Wow…….:oops:, your room looks utterly amazing and I imagine the sound reproduction in such an environment is so precise and enjoyable. Is the front wall diffusion steam bent slats?, what was the manufacturing process and did you have to do cad modelling?.
I feel kinda inadequate now with my lowly GIK treated room.

Im away to have a look at your website now.
 

amirm

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Is the front wall diffusion steam bent slats?
That would take a million years to create! :) I think it is wood veneer over shaped foam.
 

Somafunk

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Yeah i've been looking at the build process on the website, the room is quite something indeed.
 
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RDacoustic

RDacoustic

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(...) it would have been really nice to have some 'before' and 'after' images, now I feel like the absorption is too well hidden for me to find :p

Also it would have been really nice if the roof was higher so the diffractor can be design to diffract even lower frequencies. (...)

Maybe these two will give you a rough idea...
20200330_161436-scaled.jpg

RDacoustic_Listening_Room_side_view-scaled2.jpg


As for the absorption: I talked about this in the other thread. All that is responsible for absorption here is the foam parts of the diffusers, acoustic canvases on the sides, the curtain on the door and then of course the carpet/armchairs.

As for the ceiling height: Moving the ceiling was not a possibility, :) and while it looks very nice in the pic you have, what it would change is the room size, therefore its resonance frequencies. What frequencies the diffuser will work at depends on its size and depth, not on how high the ceiling is.
 
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RDacoustic

RDacoustic

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That would take a million years to create! :) I think it is wood veneer over shaped foam.

Interestingly enough, such a thing seems to exist; Berlin's Kharma Audio have the same pattern made in bent (or maybe milled?) wood.

iss-1874-5f8ebc04bf564_635x423.jpg


Full disclosure: Ours and theirs came about independently; we'd actually thought we were going to be the first to introduce the drop pattern in this size, which we built the campaign around, and had learned about this the day before launching it. :) Still, with our quadratic diffusers, I am quite sure we were first. :)

In any case, I don't think it would be such a feat either; I don't know how they did it, but we are located in a region that is famous for its bent wood furniture (https://www.ton.eu/, http://www.labernkop.cz/), it could probably be solved that way. But our idea was since the beginning diffusion+absorption in one package that can also be (to a certain degree) controlled, which obviously has its merits; acoustically, from the pow of "adjusting" to your average room, and also practically (once framed, our diffuser can just be hanged on the wall like you would a picture; full wood would be much heavier/harder to install)...
 

abdo123

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What frequencies the diffuser will work at depends on its size and depth, not on how high the ceiling is.

That's exactly the same thing i said, if the ceiling is too low and you make the diffuser thicker then the room will end up being a Super Mario level :p

IPNX2Q_kXx-DdK2HogmznOU76MU3C6TVg23kVnHWr0VDZDqU34991koiLAWUuMmIfbW9O5qZn8WVCFw_A41Tb7jb-nAU4BXp5QcZdXXu8DL2Ve2pwGZ8uAx2GxGE91RJsCEMThI_pg


you mentioned in the previous part that you used accoustic foam for the diffuser, is there a reason why you didn't use Basotect? accoustic foam does absolutely nothing under 1KHz regardless of thickness.
 
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RDacoustic

RDacoustic

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That's exactly the same thing i said, if the ceiling is too low and you make the diffuser thicker then the room will end up being a Super Mario level :p

IPNX2Q_kXx-DdK2HogmznOU76MU3C6TVg23kVnHWr0VDZDqU34991koiLAWUuMmIfbW9O5qZn8WVCFw_A41Tb7jb-nAU4BXp5QcZdXXu8DL2Ve2pwGZ8uAx2GxGE91RJsCEMThI_pg


you mentioned in the previous part that you used accoustic foam for the diffuser, is there a reason why you didn't use Basotect? accoustic foam does absolutely nothing under 1KHz regardless of thickness.

Right. :) I had smaller nuances in mind, despite your pic. The thing to consider here is how low we actually want to go. Let's look at it this way: We can discern where sound is coming from (localisation of direction given by the physiology of the human ear) on the higher part of the mids and on highs. The main purpose of a diffuser is to break up an acoustic reflection in such a way that our hearing couldn't recognise its localisation in space. (And the most important direction for this will always be a direct line towards the speakers.) So with that in mind, the diffuser does not have to be made for (on average) lower frequencies because in that part of the spectrum, we can't localise space anymore.

As for the material: We didn't know about the particular material you mention. We'll order some and test it against ours now that we do. That said, the foam we use is acoustic, UV stable, self-extinguishing, but mainly, it's an open-cell structure that will, generally speaking, work on low frequencies, even down to dozens of Hz, the thicker the lower ofc in accordance with wavelength. The effect will obviously differ depending on frequency but that acoustic foam wouldn't do anything under 1 KHz isn't right.

Then there's a third thing, though: The way low frequencies behave in a room is strongly dependent on the room size, and especially wall ratios as modal frequencies are prominent up to some 200 Hz (depending on the room). Above that, furniture and other things starts interfering. (We have an article touching on this as well https://rdacoustic.cz/en/blog/2019/03/30/room-acoustics-not-just-for-audiophiles/ ) In music itself, however, there isn't that much of the lowest of frequencies.

Now, if you buy speakers with a relatively low sensitivity whose highs won't even be heard until you pump um the volume and you stick them in a tiny square room, one of your problems will be too much bass and you'll have to treat that acoustic pressure, especially in the corners, somehow. So then you would use some sort of a bass trap, e.g. take acoustic foam and stick a bunch of it in your corners. A wave comes through it (dependent on its density) and the foam will take some of its energy. And in your particular case that may not be enough to solve your problem (because yes, low freq waves will have a lot of energy), so then you may need something more elaborate (and yet you'll never get rid of all of them), but even foam in the corners can help--we've seen this work...

There is no reason though to go the hard way if you can change the room ratio like we could, and solve your problem there. And of course, the speakers we built this for have high sensitivity. So then in our case, when we install what we have here, the diffusers are enough (there is more absorption to the sides) for the room to work well.
 
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