Basement control room

Hello everybody, I'm considering buying a house where the basement contains a room that could serve as my new studio. My goal with this post is to find out how well this room would be suited for my needs and to come up with a preliminary design. What I'm planning is a control room (no dedicated recording rooms) which will also be used for tracking vocals, guitars etc. but no live drums. Here are the room dimensions: Width: 5.41 m Length: 5.23 m Height: 2.50 m The house is in a quiet neighbourhood and the next house is about 8 m away from the studio room (corner to corner). Outer walls are 36 cm and inner walls 24 cm (stone walls). The ceiling is 20 cm concrete slab plus 12 cm floated concrete floor. Moreover, part of the considered room is underground (it's the basement). Since I won't be particularly loud (I suppose not exceeding 95 dB), I don't consider sound isolation to be a big issue in this case. Above the studio there will be the living room of my in-laws. Since they are not particularly sensitive, I consider the existing sound isolation of the ceiling to be sufficient (though I haven't done any tests yet, since the basement of the house is currently inhabited). Floor ist parquet. Here is my thought process so far: Since one of the long (5.41m) walls is almost completely covered with windows, I believe the best thing for symmetry reasons is to be facing that wall in the listening position. Since the room dimensions are almost multiple of each other, my first thought was to build a wall behind the listening position in order to decrease the 5.23 m room dimension to e.g. 4.70 m. On the other hand, I thought, that space could also be used as a giant bass absorber, which is what I went for in the attached SketchUp design. Then the front part of the room is designed to create a reflection free zone and the rear part to be diffusive in order to allow for less dead acoustics when recording instruments. Panel absorbers and super chunks are planned to be of rigid fiberglass. I am planning a budget of around 2000,- Euros, but I could go beyond that if necessary. The plan is to build everything on myself, so only material cost is included in that budget. Questions: 1. What is your general opinion on the design? What do you think will not work in that way? Any other suggestions? Completely different layouts? 2. What would you suggest for the design of the giant bass absorber in the back? Hangers? Helmholz resonators? Does this concept make sense to you? 3. With the large window in the front flush mounting the speakers seems not possible. Would you rather choose a different orientation of the speakers (putting the window to the back wall) and flush mount them? PS: I tried to upload the SketchUp file to the forum, but apparantly it is too big (15 MB). Until I find out how it works, you can download my SketchUp design here.

Neutrino, i'm not the expert... but thought i'd respond... have you looked at any ray tracing yet? I think your mix position looks a bit far back in the room and may need to be pushed toward the front wall a half meter... your aiming for 38% depth. Would it be possible to move the rear door to the center for the rear wall? I might have a few concerns about having such a large discrepancy between the left and right rear wall... you could treat the door by putting an absorptive panel on it, but it still wouldn't match the other side of the room... granted, if it's a limitation, so be it... kevin

Hi sandledfoot, thanks for the reply!

sandledfoot wrote:

have you looked at any ray tracing yet? I think your mix position looks a bit far back in the room and may need to be pushed toward the front wall a half meter... your aiming for 38% depth.

Yes, I've looked into that. However I had the impression that the 38% rule only really works with flush mounted speakers. In my case the speakers would end up really close to a large reflective surface (the window) and the spacing between the speakers would become rather small. (Remember that I'm facing the long wall of the room.) In my design above, however, the listening position is roughly 38% from the rear wall, which to my understanding should work as well.

sandledfoot wrote:

Would it be possible to move the rear door to the center for the rear wall? I might have a few concerns about having such a large discrepancy between the left and right rear wall... you could treat the door by putting an absorptive panel on it, but it still wouldn't match the other side of the room... granted, if it's a limitation, so be it...

Moving the door is not really an option. Maybe one could help the situation a bit by putting another door to flush the giant absorber surface? Or at least putting a heavy curtain there. Waldemar

Hi Waldemar, and let me add my welcome to Kevin's! :)

Yes, I've looked into that. However I had the impression that the 38% rule only really works with flush mounted speakers.

Not really. It's related to the modal response of the room. It is the best point, theoretically, where the modal issues are at their minimum in axes, and for all fundamentals and harmonics. So it applies to all rectangular rooms, or nearly-rectangular rooms. The only rooms it does not apply to, are ones that are seriously not rectangular, such as five-sided, six-sided, those with highly splayed walls, or curved walls ( :shock: ), etc.

In my case the speakers would end up really close to a large reflective surface (the window)

For practically the entire spectrum, the walls are just as reflective as the windows. There isn't a lot of difference between the coefficients of absorption for glass and drywall, for most of the audio spectrum...

and the spacing between the speakers would become rather small.

Then keep them the same distance apart, and angle them more inwards: The normal angle is 30° toe-in, but you can increase that. You can go to as much as 45° if you really need to.

In my design above, however, the listening position is roughly 38% from the rear wall, which to my understanding should work as well.

Only from the point of view of modal response, but not from the point of view of Haas times. Having your head close to the back wall in a small room means that the first reflections from the back wall arrive well within the Haas time, which is not good, and it is also hard to attenuate them sufficiently with so little space behind you. It is even harder to establish a proper low-level reverberant field in such a short distance/time. Ideally, your head should be far enough away from the rear wall that those reflections arrive later than 20ms after the direct sound, and even then at a level 20 dB down. Even in a well-designed room that is hard to do, and so you often see those numbers relaxed slightly, to 15ms and -15dB. But with your head just a few feet from the rear wall, it is not possible to attain that. There are other issues that I'd like to comment on, but I have a meeting now, so I'll have to get back to you later on those... - Stuart -

My additional comments:

Width: 5.41 m Length: 5.23 m Height: 2.50 m

That's not a good set of dimensions. The ratio is 1 : 2.09 : 2.16, which is very close to a square section, and both the width and length are very close to double the height. In fact, that ratio fails two of the three "critical tests" that the BBC uses for accepting or rejecting a room as a possible control room or listening room. I would suggest that you try to do something about improving that.

Outer walls are 36 cm and inner walls 24 cm (stone walls). The ceiling is 20 cm concrete slab plus 12 cm floated concrete floor.

Nice! You should have good isolation then. So that is not really a problem, I imagine!

On the other hand, I thought, that space could also be used as a giant bass absorber, which is what I went for in the attached SketchUp design.

Yes, but a bass trap does not change the room ratio or room dimensions. Those are measured to the hard, solid, rigid, reflective boundary of the room, which is your inner-leaf stone walls. Putting a bass trap in there does not change the dimensions.

Then the front part of the room is designed to create a reflection free zone

To do an RFZ, you need to actually angle the front part of the side walls. If you ray-trace your layout, you'll see that it is not a true reflection free zone. Putting absorption in your first reflection points helps, but not completely. Also, a true RFZ design generally needs the speakers to be soffit-mounted.

and the rear part to be diffusive in order to allow for less dead acoustics when recording instruments.

That room isn't big enough to be able to use numeric-sequence diffusers successfully. You need enough distance between the diffuser and your ears for the lobing artifacts created by the diffuser to smooth out, and there just isn't enough space for that in a small room.

1. What is your general opinion on the design? What do you think will not work in that way? Any other suggestions? Completely different layouts?

The basic plan is OK, except for the location of the mix position, and the other comments above. I'd also add that the speakers are not well located. For a room that size, they really need to be right up against the front wall, with just a small gap there for acoustic treatment between the speaker ant the wall. For the distance you show, your SBIR artifacts will be right in the low-mids, and very audible. So from that point of view, and also for the mix position, I'd suggest re-doing your speaker geometry.

2. What would you suggest for the design of the giant bass absorber in the back? Hangers? Helmholz resonators? Does this concept make sense to you?

The concept makes sense, yes, and you could do either hangers or superchunks for your rear bass traps. But definitely not Helmholtz resonators: Those are very narrow band, very hard to tune correctly, and must be placed in the exact correct location for the specific mode that it is designed to treat.

3. With the large window in the front flush mounting the speakers seems not possible. Would you rather choose a different orientation of the speakers (putting the window to the back wall) and flush mount them?

so you need that window for light or visibility? Can you cover it? - Stuart -

Hi Soundman2020, thanks for your helpful comments!

For practically the entire spectrum, the walls are just as reflective as the windows. There isn't a lot of difference between the coefficients of absorption for glass and drywall, for most of the audio spectrum...

Yes, but walls can be covered with absorbtive material, windows not :) The window is not only necessary for daylight but there is also a glass door which leads to the terrasse, that cannot be covered. Moreover, there is a large radiator below the rest of the window. When I move the listening position 38% from the front wall the left speaker ends up just in front of the glass door. Although there is also an access to the terrasse from another room, it's not the most practical solution. A different option would be to reverse the listening position and have the window in the back. Not sure whether this would be preferable...

Yes, but a bass trap does not change the room ratio or room dimensions. Those are measured to the hard, solid, rigid, reflective boundary of the room, which is your inner-leaf stone walls. Putting a bass trap in there does not change the dimensions.

I'm aware that the room dimensions are not ideal as a starting point. However, what I repeatedly read on this forum and also in the literature (Rod Gervais, Philip Newell) is that in practice they don't really matter as much as often claimed if the room is heavily treated. To my understanding the large bass absorber in the back of the room should significantly braoden the Q of the room resonance (the frequency related to the 2,50m dimension) making it much less predominant. What is your opinion on that? Changing the room dimensions by building another brick wall would mean making the room even smaller with all the problems that come with it. Any ideas of how one could change the orientation of the listening position to obtain a more satisfying solution? Mybe facing a corner of the room? Cheers, Waldemar

You can use wood or carpet for flooring as these also help in isolation of the sound.

You can use wood or carpet for flooring as these also help in isolation of the sound.

Well wood is good, yes, but carpet? For a recording studio floor? :shock: You can't be serious, surely... :?: :?: And I'd sure love to see your proof that carpet helps isolation! :) - Stuart -

I've moved the listening position to 38% from the front wall, moved the speakers accordingly and reduced their spacing a little bit. I think I could live with one of the speakers blocking the terrasse door (there is another access to the terrasse in the next room). Actually, the spacing of the speakers looks still wide enough to me (2,45m). Their distance to the back window is now about 60cm. As far as I know glass is quite transparent to low frequencies (as compared to stone wall). Therefore, it might not be as bad to place the speakers closer to the window. Am I correct? Regarding the room dimensions I don't have a solution yet... SketchUp File

Yes, but walls can be covered with absorbtive material, windows not

Actually, windows CAN get absorptive treatment! There are some interested transparent micro-perforated foils coming on the market recently that can be hung in front of windows, and provide quite significant absorption...

A different option would be to reverse the listening position and have the window in the back. Not sure whether this would be preferable...

Not really: You'd have no place to put your rear wall absorption. I wouldn't recommend the micro-perf panel for that: You need several inches of thick porous absorber back there.

However, what I repeatedly read on this forum and also in the literature (Rod Gervais, Philip Newell) is that in practice they don't really matter as much as often claimed if the room is heavily treated.

True, provided that you star out with a room that already has a reasonable ratio, where "reasonable" implies "not square"...

To my understanding the large bass absorber in the back of the room should significantly braoden the Q of the room resonance

Which resonance? A room has numerous different modes of resonance, oriented in different directions, and affected by different walls. If you only put one bass trap on the rear wall of the room, that will work fine for the modes that happen to hit that wall, but will do zilch for modes that are not associated with the wall. For example, all of the vertical room modes would be not be affected by that trap, and neither would the horizontal modes. Tangential modes that don't involve the rear wall would also not be affected.

I've moved the listening position to 38% from the front wall, moved the speakers accordingly and reduced their spacing a little bit.

Lots of problems there! The speakers are toed-in at only 18°. IT should be at least 30°. As a result, the intersection of the speaker axes is a long way behind you, somewhere over the couch, more than 4m from the front wall. The speakers are also about 10cm too high; the acoustic axis seems to be around 130 cm from the floor, but it should be about 120cm. The speakers are also no positioning symmetrically: the left speaker is about 1.4cm further into the room than the right speaker, and they are not the same distance each side of the room center-line. The speaker faces are also 95cm from the front wall, meaning that there will be SBIR issues starting at around 91 Hz. I would suggest that you fix those issues. Also, in your model you are showing the front surface of the bass trap as being made of wood: Is that just a mistake in the model, or were you really planning go make them wood?

Their distance to the back window is now about 60cm.

I think you mean the FRONT window! There's no window shown on the back wall of the room: just the large bass trap. And your speakers are positioned at about 95.9 cm from that front window, not 60 cm. All acoustic measurements on speakers are done in relation to the acoustic center of the speaker.

As far as I know glass is quite transparent to low frequencies (as compared to stone wall).

It's about the same, assuming the same surface density. Glass is quite dense: it si made from the same stuff that stone is made from! :) The density of glass is around 2500kg/m3, and granite (for example) is around 2700 kg/m3. Therefore, given the same thickness of glass or stone, they would both block sound by roughly the same amount.

Therefore, it might not be as bad to place the speakers closer to the window. Am I correct?

It seems to be the best layout to me, but you should still fix those geometry issues I mentioned above. - Stuart -

True, provided that you star out with a room that already has a reasonable ratio, where "reasonable" implies "not square"...

OK, is there something you could suggest for my situation? The only reasonable way I see to change the room dimensions here would be building an additional wall at the place where now the back-wall absorber is (maybe 60cm from the current back wall?). Changing the (already low) ceiling height is not an option and reducing the length of the long wall would be inefficient since it would make the room even more square. Would a drywall construction be sufficient for that purpose?

Lots of problems there! The speakers are toed-in at only 18°. IT should be at least 30°. As a result, the intersection of the speaker axes is a long way behind you, somewhere over the couch, more than 4m from the front wall. The speakers are also about 10cm too high; the acoustic axis seems to be around 130 cm from the floor, but it should be about 120cm. The speakers are also no positioning symmetrically: the left speaker is about 1.4cm further into the room than the right speaker, and they are not the same distance each side of the room center-line. The speaker faces are also 95cm from the front wall, meaning that there will be SBIR issues starting at around 91 Hz. I would suggest that you fix those issues.

To be honest, I haven't bothered to pay attention to these details at this stage where the room dimensions are still subject to change (I guess precise monitor placement is a subject of experimentation anyway), but I tried to correct these issues in the model now. Also I'm quite tall and the 130cm of speaker spacing from the floor was actually done on purpose :mrgreen: Concerning the SBIR: The only possibility I see is to move the speakers even closer to the window. What would you suggest?

Also, in your model you are showing the front surface of the bass trap as being made of wood: Is that just a mistake in the model, or were you really planning go make them wood?

No, not really. The wood in the model is preliminary, until I have a better idea of how the absorber should be constructed 8)

It's about the same, assuming the same surface density. Glass is quite dense: it si made from the same stuff that stone is made from! :) The density of glass is around 2500kg/m3, and granite (for example) is around 2700 kg/m3. Therefore, given the same thickness of glass or stone, they would both block sound by roughly the same amount.

OK ,but here the total thickness of the window is probably less than 1cm, so it should behave significantly different from a stone wall regarding the bass response, shouldn't it? Cheers, Waldemar

One of the keys to a Newell style room is a bass absorber at the back of the room and the ceiling that is good to below 30hz. Newell has a very specific design that he uses that takes about 1m and is a combination of membrane absorbers and hangers. This basically makes that back of the room and ceiling act like an open window, so if done correctly (the important part of this sentence is "done correctly") there will be no modes in this room making ratios irrelevant.

As I got stuck with the unfavourable room dimensions and things are moving forward, new options arise. Instead of putting the studio into the basement I could tear down one wall in the groundfloor and have a decent room with almost the same area as before, but better dimensions: Width: 4.00 m Length: 6.00 m Height: 2.70 m OK, width and length are both multiples of 2 m, but I would suppose that these room dimensions is something one could work with (also the ceileing higher). The floor plan indicates the location of the control room in the groundfloor with the superfluous wall already removed and the listening position roughly indicated. Of course this option is not without problems: 1. There is a window in the front wall, that is not positioned symmetrically. I would guess, the easiest way would be to cover it, since it is not really needed. Other ideas? 2. The rest of the windows and doors of the room is not symmetrical either. There is a wooden door on the left (as seen from the listening position), and a glass door on the right. Moreover there is a large window on the right, as can be seen on the floor plan. The good thing is that it is behind the listening position, so not in the area on the walls where absorption is absolutely essential. One way I see, how one could tackle this, is to place heavy curtains on both the window and the opposite wall to restore symmetry to a certain degree. Other suggestions? Would you prefer this room compared to the initial (square) one in the basement? Edit: I've created a SketchUp model of the room. The listening position is indicated as well as the lines along which the speakers should be placed. I think this room would even allow for flush-mounting the speakers if the front window is covered.

but better dimensions: Width: 4.00 m Length: 6.00 m Height: 2.70 m

That is a bit better, but not much, to be honest! As you mentioned, there is a direct mathematical relationship between width and length, which is not good as the modes in both directional will line up perfectly. Can you change one of those? For example, can you increase the width or length a bit?

instead of putting the studio into the basement I could tear down one wall in the groundfloor

If you did that, then what would be under the floor for that room? Would it just be earth, or would it be over the basement?

1. There is a window in the front wall, that is not positioned symmetrically. I would guess, the easiest way would be to cover it, since it is not really needed. Other ideas?

You cold leave it there, and put glass in the same position in your new inner-leaf wall, if you wanted to have natural light. Or you could just seal it up and cover it over before putting up the new walls.

The good thing is that it is behind the listening position,

Then it should not be too much of a problem. The most important part of the room for symmetry is in front of the mix position: behind isn't so important.

One way I see, how one could tackle this, is to place heavy curtains on both the window and the opposite wall to restore symmetry to a certain degree. Other suggestions?

That would work, yes.

I think this room would even allow for flush-mounting the speakers

That would be excellent! If you can do that, then it is definitely worthwhile, provided that you have speakers that can be flush-mounted. - Stuart -

That is a bit better, but not much, to be honest! As you mentioned, there is a direct mathematical relationship between width and length, which is not good as the modes in both directional will line up perfectly. Can you change one of those? For example, can you increase the width or length a bit?

No, all walls are load-bearing, so only additional drywall construction is possible.

If you did that, then what would be under the floor for that room? Would it just be earth, or would it be over the basement?

It's over the basement. In between there is a 20 cm concrete slab plus 12 cm floated concrete floor. Not sure, however how well the floating has been done, since it was not build for the purpose of a recording studio.

You cold leave it there, and put glass in the same position in your new inner-leaf wall, if you wanted to have natural light. Or you could just seal it up and cover it over before putting up the new walls.

Daylight from the front wall would be great, but I'm a bit worried since the window is not in the center of the front wall...

That would be excellent! If you can do that, then it is definitely worthwhile, provided that you have speakers that can be flush-mounted.

I currently use Event TR8 speakers that have a front bassreflex duct (therefore, I guess they would work with flush-mounting). However, I plan to upgrade my speakers anyway in the course of the move. Any suggestions about good speakers for flush-mounting are welcome, of course. Cheers, Waldemar

How about doing a REW test in your room, and posting the MDAT file here? That will reveal the acoustic secrets of the room, so we can figure out what to do about them... :) - Stuart -

How about doing a REW test in your room, and posting the MDAT file here? That will reveal the acoustic secrets of the room, so we can figure out what to do about them... :)

Yes, I will definitely do that once I have torn down the wall that is still standing in the middle of the room :-)

Since things are delayed and I'm still not able to do measurements in the room, here is a question: since the length (6m) and width (4m) are both multiples of 2m, I will most probably get a pronounced mode at 86 Hz (and multiples of it). What do you think is the best option: 1. Reduce the length of the room from 6.00 m to e.g. 5.70 m with a drywall construction in order to spread the modes a bit across the frequency spectrum. 2. Leave the room dimensions as they are and use heavy bass trapping (tuned traps if necessary). Appreciate your comments!

1. Reduce the length of the room from 6.00 m to e.g. 5.70 m with a drywall construction in order to spread the modes a bit across the frequency spectrum.

If you soffit-mount your speakers, then that would automatically reduce the room length: the new wall is the front face of the soffit. That will push into the room whatever distance is necessary to fit the speakers in, so I expect that would solve your length issue. However, don't forget that your height is 2.7m, so hopefully your length will not end up at 5.4m! I don't think that would be the case... Also, since the soffit faces are angled, that helps to break up the length axis a bit too, and just the simple fact of soffit-mounting will give you a large improvement in the overall room acoustics anyway, since it eliminates all of the artifacts associated with having speakers sitting out in the room. I would suggest taking a good look at that option, as it should solve all of your issues at once. - Stuart -

If you soffit-mount your speakers, then that would automatically reduce the room length

In case I don't opt for flush-mounting (for whatever reason). What would be your preferred option?

In case I don't opt for flush-mounting (for whatever reason). What would be your preferred option?

Put them on very heavy, massive stands just behind the desk, at roughly 28% of the room width away from the side walls, 1.2 m above the floor (acoustic axis), and angled inwards at around 30°. That's a good starting point for figuring out speaker location. - Stuart -