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Kvalsvoll

Bulding the Room2 listening room

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Kvalsvoll    71

I have posted about the new Room2 in the bass punch threshold thread, and announced that I would start a thread on the building of this room.

 

I plan to present a complete article describing what the acoustic improvements do for the sound, from start to finish, and I also have measurements form all stages of the build process.

 

Here is how the room looked initially, after removing subwoofers and audiophile mains, and some furniture, including the table, which will be replaced: 

 

post-181-0-67178900-1462029080_thumb.jpg

 

 

Preview of the process:

 

post-181-0-48507800-1462029125_thumb.jpg

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Kvalsvoll    71

The table was partly due to design considerations, the old one did not look good in the room now.

 

But it works fine for acoustics - it does no harm to sound, compared to a table with a large reflective surface.

It is not large enough to have any significant effect on the floor reflection alone.

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SME    201

This table is a great idea!  I'm thinking of doing something similar, except maybe an ottoman instead.  I was thinking of just doing fabric-covered OC-705 for the top, but I'm not sure how durable that would be.  OTOH, I'm not seeing nearly as much floor bounce with my new larger baffle speakers, and I expect it to diminish more once the speakers are permanently installed at a higher elevation than they are now.  I may be able to substantially improve things by having the subwoofers (on the floor) help out by way of some clever DSP tricks.

 

In another thread, Kvalsvoll wrote the following:

 

 

Absorption in the right places will improve things, how much remains to be seen.

The radiation pattern of the speaker I am using in this room now is difficult, this speaker is a down-sized version of something that works a lot better in the lower midrange.

 

I have already installed side wall absorption, and this has some effect on low frequency decay, but it does not improve the frequency response across the midrange.

A possible solution is to reduce the thickness of this absorption, some absorption is necessary because the direct sound from the opposing speaker illuminates the side walls.

 

The room must perform well from 100hz an up, the low bass already has good decay, and the bass response will be fixed by the bass system.

 

Why are you thinking of reducing the absorption thickness?  This will tend to absorb low frequencies less, not more.  If your problem is early reflections, then you want as much absorption there as you can get, even if your steady-state frequency response looks worse.  Have you compared the responses, before and after, with a relatively short window?  Try a 10 ms window and turn off all frequency response smoothing.  That might give you a very different picture.

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Kvalsvoll    71
..
Why are you thinking of reducing the absorption thickness?  This will tend to absorb low frequencies less, not more.  If your problem is early reflections, then you want as much absorption there as you can get, even if your steady-state frequency response looks worse.  Have you compared the responses, before and after, with a relatively short window?  Try a 10 ms window and turn off all frequency response smoothing.  That might give you a very different picture.

 

A surface will give boundary gain when the frequency gets low enough, below the first cancellation.

It will no longer act as a reflection, it has become part of the original sound source.

This is why the front wall absorption is 20cm deep, and not 60cm - it absorbs from around 100hz and up, and then the wall supports the low frequency response below that.

 

Further up in frequency, the many reflections from boundaries around the speaker and close to the listening position will sum up and tend to flatten the response.

When most - but not all - of those are removed, there is a good possibility that there will be large deviations in the frequency response.

 

Measuring with and without side wall absorption in this room reveals that the side wall absorption does more good than bad in here, so it stays.

 

The room is nearly completed, there are a couple of issues that remains to be looked into.

In general, the decay is very good at lower frequencies, but there is potential for improvement in the midrange 200hz-2khz.

Fortunately, I have a selection of speakers I can measure and try, so that I can avoid compensating for problems caused by faults in one speaker design.

 

I use decay analysis a lot when working on this, it is a very good tool to see what is going on, and it correlates well with listening impressions.

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SME    201

I see what you are saying.  I thought you were talking about reducing the absorption thickness on the side-walls.  Though I get it if you are space constrained there.

 

On the front-wall, I'm not sure I'd opt for much absorption at all unless you really need it for decay control.  Even then, it might be good to not cover the entire surface unless you are trying to kill modes in the lowest bass.  You'll never be able to kill the bass part of the reflection completely, and if the coverage is uniform, then the reflection will be qualitatively similar.  OTOH, if you use multiple panels and if you vary the separation distance somewhat, the left-over reflection will be a bit more diffuse, which might help with the response.

 

Depending on how close the woofers are to the wall, maybe throw up some 10-15 cm thick panels just to the inside of the woofers.  With toe-in, my woofers are centered about 300 mm from the wall and I am doing fine with 10 cm OC703.  Actually, the response isn't a whole lot worse without the panels, presumably because the 425 mm wide baffles already limit the amount of sound hitting the front wall quite a bit.  With my older mains with 200 mm baffles, they made a bigger difference.  For my center channel, I'm planning on installing plywood at an angle between each edge of the speaker and the wall to maximize beneficial boundary gain.  The speakers are only 225 mm or so deep, for this express purpose.

 

Decay analysis is definitively helpful, but I still use the spectrogram more because it is so flexible and (with some fiddling of the window settings) can be made to reveal both early reflections and decay times as a function of frequency.  The IR/ETC being visually biased toward high frequencies is helpful for finding early reflection points, but any high frequency absorption at those points will tend to "clean up" the ETC even though the bass reflection is still there.

 

Before I started work on treating my room, I did a lot of research on Gearslutz and other forums.  There I learned that a common mistake is to over-treat a room, making it too dry.  In small rooms, this primarily applies to mid and high frequencies, given that bass is so troublesome to deal with.  Unfortunately, even thick absorbers tend to absorb treble better than bass, so unless some kind of membrane or facing material is used to reflect high frequencies, treatments will tend to suck way too much high frequency energy out of the room.  One nice thing about horn speakers compared to domes is they limit the number of surfaces that require high frequency absorption, making it easier to retain that energy that is so easily lost.  I don't recall where I read it, but my understanding is that human hearing system perceives treble best when the direct sound is supported by a diffuse field.  You don't want the reflected sound to overwhelm the direct sound, but lower level diffuse reflections actually help with intelligibility.

 

Anyway, I'm curious as to what you'll find out when you try other speakers.  I'm willing to bet the differences between designs with different baffle widths will not be subtle.

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Kvalsvoll    71

I have tried several other speakers, such as these:

 

post-181-0-83370600-1462828458_thumb.jpg

 

They measure like this, decay plot 20ms lines 20ms rise time:

 

post-181-0-60074600-1462828384_thumb.png

 

The differences between horn and small domes are interesting, they radiate very different.

With horns it is easy to achieve a low early reflection level, and since the sound is more focused on the reflective surface at the back of a room, it is also easier to preserve and control the ambient sound from the room.

 

Testing subwoofers in the room today, so far it does not sound better with subs, which means I did not get it right.

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Kvalsvoll    71

Now I got it right, the bass sounds good and also measures quite well.

 

Decent bass was possible to achieve with 2 subwoofers placed in front.

DSP is necessary, and the delay setting on the mains is what makes it work.

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dgage    67

Testing subwoofers in the room today, so far it does not sound better with subs, which means I did not get it right.

Blasphemous! Go wash your mouth out and stand in the corner until you remember everything sounds better with subs. :D

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TTS56A    3

Now I got it right, the bass sounds good and also measures quite well.

 

Decent bass was possible to achieve with 2 subwoofers placed in front.

DSP is necessary, and the delay setting on the mains is what makes it work.

 

Interesting, what about the step response?  :)

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Kvalsvoll    71

Blasphemous! Go wash your mouth out and stand in the corner until you remember everything sounds better with subs. :D

 

Of course it does, real bass makes the big difference.

 

Unfortunately the myth that subwoofers are not good for music still lives on, and that is due to bad set-up.

Properly configured it will sound better because the overall response has less peaks and nulls, part form the obvious extended bass response.

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Kvalsvoll    71

Interesting, what about the step response?  :)

 

The step response can give some usable information for setting the timing, but the shape of it rarely looks nice, so it may be difficult to see what the best setting is.

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3ll3d00d    71

  For my center channel, I'm planning on installing plywood at an angle between each edge of the speaker and the wall to maximize beneficial boundary gain.  The speakers are only 225 mm or so deep, for this express purpose.

 

do you mean so that the shape presented is like this or something else? i.e. the square is your cabinet and the slashes are the angled plywood

 

     ______

\ |     | /

 \|_____|/

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SME    201

do you mean so that the shape presented is like this or something else? i.e. the square is your cabinet and the slashes are the angled plywood

 

     ______

\ |     | /

 \|_____|/

 

Correct.  While this will neither eliminate the baffle diffraction nor the reflection off the wall behind it, I believe it will diminish the negative effects of both by an amount roughly inversely proportional to the angle at the front face.  A box without the angled plywood has a 90 degree angle, so plywood at 45 degrees should reduce the diffraction wavefront by half or -6 dB.  At 22.5 degrees, it's one quarter or -12 dB, and so on.  Note also that the diminishing of the wall reflection does not harm the response for frequencies well below the quarter wavelength because of the reduction of the baffle step loss in the first place.

 

Now if I'm feeling real ambitious, I could make the plywood extensions multi-segmented into kind of an S-shape to spread out the area across which both the diffraction and reflection take place, but there are diminishing returns as far as smoothing the transition is concerned.  That is because the wavelengths of concern are all relatively long.  I see many examples online of people who go through the trouble to put big round-overs on their cabinets in the hope of reducing diffraction, but in many cases, these are of little benefit, particularly when using horns for high frequencies.  (The situation is a bit different for dome tweeters that radiate a significant amount of sound across the front of the baffle.)  The trouble is that those large round-overs aren't actually that large as far as low frequencies are concerned.  For example, consider a 90 degree corner rounded out with a 2" radius, which I imagine adds a lot of complexity to the construction of a typical enclosure.  With the round-over, the diffraction is spread out across a surface of (pi/2*r) = 3.14".  When analyzing acoustics, it's beneficial to always keep in mind the speed of sound, which indicates that the diffraction wave front will be spread out in a region of time of about 0.23 ms.  That's about 1/4 of a cycle at 1 kHz, which means that the benefit of that "big" round-over has already substantially diminished by that frequency.  Again, if you're using a dome tweeter or small woofers, that might help a little bit, but probably not as much as you'd think.  Why not?  Because the wavefront travelling across the front baffle doesn't arrive at the edges all at the same time.  The wavefront from a circular woofer will remain circular.  The larger the baffle, the greater the region in time over which the diffraction wave-fronts are spread out, even without round-overs.

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3ll3d00d    71

I made my boxes a bit like that. I wasn't sure whether it would have a significant effect on diffraction or not as I couldn't find any tool to model that shape. Once I get the boxes finished then I could compare against my test box (which is just a box) to see how it looks.

 

post-1440-0-44102400-1462992214_thumb.png

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SME    201

I made my boxes a bit like that. I wasn't sure whether it would have a significant effect on diffraction or not as I couldn't find any tool to model that shape. Once I get the boxes finished then I could compare against my test box (which is just a box) to see how it looks.

 

attachicon.gifpic.png

 

Nice!  I bet those are going to kick some butt.

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Kvalsvoll    71

do you mean so that the shape presented is like this or something else? i.e. the square is your cabinet and the slashes are the angled plywood

 

     ______

\ |     | /

 \|_____|/

 

Nice effort, and it may improve, but be prepared that it also might get worse.

A rectangular box actually has smoother frequency response than a sphere.

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Kvalsvoll    71

Bass installed.

2 S6-14 subwoofers, how can anyone need more..

 

post-181-0-14537800-1463021513_thumb.jpg

 

Frequency response improves, it is now kind of full range, at least to the unaware:

 

post-181-0-67757600-1463021645_thumb.png

 

 

Decay response, compare it to the previous I posted, with the trad hifi-speakers:

 

post-181-0-88043300-1463021589_thumb.png

 

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Kvalsvoll    71

The room got a new player, I installed a small laptop, in matching white color: 

 

post-181-0-62392200-1463021835_thumb.jpg

 

Runs Kodi, just like the other player in the Moderate Cinema demo room:

 

post-181-0-25651900-1463021895_thumb.jpg

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SME    201

Nice effort, and it may improve, but be prepared that it also might get worse.

A rectangular box actually has smoother frequency response than a sphere.

 

If you think about what I said above about how the part of the wave-front that travels across the baffle face is essentially circular, it makes intuitive sense how and why a sphere might have inferior response vs. a rectangular box.  I believe a baffle shaped like a very thin disk will be worse, perhaps even the worst shape possible, at least in free space because all of the wave-front travelling across the front of the baffle will reach the hard edge and diffract at the exact same time.

 

Also realize that the slanted shape (at least in my design) is specifically intended for use with the speaker placed directly against the wall.  It eases the transition between the baffle and wall.  I believe this is very beneficial, even though speakers are rarely designed to do this.

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Kvalsvoll    71

If you think about what I said above about how the part of the wave-front that travels across the baffle face is essentially circular, it makes intuitive sense how and why a sphere might have inferior response vs. a rectangular box.  I believe a baffle shaped like a very thin disk will be worse, perhaps even the worst shape possible, at least in free space because all of the wave-front travelling across the front of the baffle will reach the hard edge and diffract at the exact same time.

 

Also realize that the slanted shape (at least in my design) is specifically intended for use with the speaker placed directly against the wall.  It eases the transition between the baffle and wall.  I believe this is very beneficial, even though speakers are rarely designed to do this.

 

It may be better.

But the acoustic properties of the wall behind will have greater impact.

 

This is a problem mainly for surround and center speakers, because they are meant for placement against a wall.

The problem is worse for wall placement because the distance between the front baffle and the wall is so small that the cancellations and peaks will end up far into the midrange frequency range.

 

For the S1.2 surround speakers the shape of the cabinet gives a smoother transition to the wall boundary, for the C1 center I designed the baffle a little wider.

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