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What Sort of Acoustics Should I Expect?

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Posted on August 12, 2009 at 07:59:10
cawson@onetel.com
Audiophile

Posts: 380
Joined: September 27, 2004
 In 3 months time I'll be moving home into a new-build apartment with an oddly shaped living room. What sort of acoustics should I expect?

The room is approximately 800 sq ft with 8 ft suspended plastered ceiling (6-8" void above) and wooden flooring with rugs covering 25 or 30%. It has one continuous curved wall (roughly parabola-shaped), that is glazed floor-to-ceiling and one effectively straight wall of 45 ft joining the ends of the curved wall. There’s one column about 4 ft x 1 ft in the middle of the room along the centre of the room (not across it).

Put another way, the building is shaped like an elongated egg and my living room is the pointed end – approx 25% of the total egg!

The glass wall will be curtained in lightweight, cotton fabric (voile?) - usually left open in summer.

I have big floor-standing horn speakers and I would be interested to hear inmates’ ideas on the acoustics I might expect and possible speaker positioning. Being horns (Avantgarde Unos) the effect of walls/windows behind them should be minimal, I hope.

Thanks

Peter

PS Apologies - this is a double posting (originally in General) - I hadn't realised there was a specialist forum here!
I'd second Ethan's placement recommendations but I'm more positive about the curved surface…, posted on August 12, 2009 at 13:41:18
David Aiken
Audiophile

Posts: 5108
Location: Brisbane
Joined: September 25, 1999
 because of one thing—your speakers.

I once heard the Unos in a space I would have thought horrendous for any speaker, much less a speaker the size of the Unos: way too small, glass walls on both sides (one wall was the store window, the other a dividing wall) and a highly reflective plate metal covering on the wall behind them. They actually sounded very good, I suspect largely because of the directivity provided by the horns which greatly weakened the front and side wall reflections at mid frequencies and above.

That directivity will help minimise a lot of the early reflections so I wouldn't think you would have much problem with them. I assume that you're going to want the view so that means that you're going to be sitting against the flat wall and the speakers are going to be somewhere in front of you with glass behind them.

As far as placement goes, I'd opt for a symmetrical placement both because of Ethan's comments about sound and also for the view but I think you're going to have to experiment a bit with both where the speakers go in the "pointed end" of the shape and how far forward of the wall you sit. I'd try to sit a fair way from it and as close to the speakers as you can go without interfering with good integration of the sound from the different drivers—in other words at a point where you have the shortest direct sound path that delivers good sound while maximising the length of the reflection paths. The early reflections from the glass are going to be low frequency because of the directivity of the horns, and glass is "lossy" at low frequencies so the frequency balance of reflections from the glass is going to be nothing like the frequency balance of the direct sound. That's not good so you want to make the direct sound as strong as you can relative to the strength of the reflected sound so the tonal balance of the direct sound dominates.

You're going to have standing waves or, more precisely, axial room modes related to the length and breadth of the room, despite the curves of the wall. The frequencies of those modes aren't going to be simple as they are in a rectangular room, the modes are going to be spread over a wide frequency range rather than simply occurring at specific frequencies. That may be a plus since you probably won't get much in the way of strong modes apart from the floor to ceiling modes. The problem is that you aren't going to have much in the way of corners which are the obvious points to place bass traps, really just 2 corners where the curved wall joins your back wall. I'd definitely consider bass traps at those 2 points.

I think you're going to need some other treatment but I also think it's going to be hard to predict just what you'll need. Normally I'd recommend absorption at the rear wall reflection points but with the way the glass is going to respond to reflections and the fact that it's also going to be reflecting sound reflected from the flat wall, it may actually help to have a fair bit of diffusion on the back wall in order to counteract the focussing effect of the curved wall. I wouldn't make any decisions until I moved in and then I'd experiment with absorption and diffusion to get a feel for how they work in the room.

As for the light curtains, I think that's a good idea. I'd break the curtaining up into small sections which can be "opened/closed" separately and experiment with what areas of the window I left uncovered and where I bunched up the curtains. That way you can have your view but locate the bunched curtains where they can absorb the brightest high frequency reflections from the glass if you get that sort of problem from sound being re-reflected from the back wall. If you need more in the way of absorption at different points along the glass, I'd go for traps like the RealTraps Mondo Trap on floorstands so that you can easily locate them at the right spots. They're going to look very functional but I'd make a point of that by decorating the room in a very functional style but I'd also choose furniture with a strong visual presence so that the acoustic panels don't dominate visually. Make sure the seating is as sound absorbent as possible so go for wool or similar natural fabric coverings rather than leather which is reflective at high frequencies since you're going to have enough high frequency reflective surfaces with the large glass area.

It's going to be an interesting room and, as some have said, potentially there are some big problems with such a shape. I think your speakers are the ideal choice for this sort of room and I think that's a big plus in your favour right from the outset and I think you should be able to get some good results in the room, and I can understand your choice of a home with such a room. I really like modernist architecture with lots of floor to ceiling glass walls though that certainly doesn't describe my home, and I've often daydreamed about how I would work with such a space. Horns such as the Avantgardes have always figured in the solutions I've considered. I'd actually love to be in your position. I think you may have a bit of work to do with experimenting in order to get the best results but I think you can get good results and I think good sound in a room as visually interesting as yours sounds could be a wonderful combination.



David Aiken
RE: I'd second Ethan's placement recommendations but I'm more positive about the curved surface…, posted on August 13, 2009 at 10:48:04
cawson@onetel.com
Audiophile

Posts: 380
Joined: September 27, 2004
 Hi David. Where do I start? Firstly thanks for the comprehensive reply and intelligent ideas - often lacking if one asks a slightly obscure question!

As you can imagine, I’m eagerly awaiting completion of the apartment and decided to pose the question for ideas from someone like yourself. It was unexpected to get such a thoughtful response.

I bought the Unos some years ago after a short but unexpectedly disappointing ownership of ATC SCM50 Active speakers. This was in my previous apartment in London. It was an “easy” room being roughly 5 sided with no parallel walls, only one right-angle between walls and 10 ft high ceilings. The ATCs were far too “in your face” and oppressive – I wanted to push them back another 20 ft – impossible in a room of 300 sq ft! I bought the Unos after reading the Stereophile review and learning that they were chosen as their “Speaker of the Year” jointly with $80K Dynaudio speakers. They were a huge improvement in my view and sounded just as I hoped they would. Driven by SET monoblock amplifiers, imaging in particular is amazing and the degree of musical detail is astonishing.

I’m glad you share by confidence that these speakers will sound good in the new apartment. They also have the advantages of adjustable bass level, bass crossover frequency and extreme bottom end cut-off frequency if necessary.

Ideally I would agree with your suggestion to install the speakers symmetrically in the pointed end, placed 1 m or so away from the windows, but the problem is the central column. This is probably right at the best listening position, although I could move my listening chair back a bit but it would be facing the end of the column – possibly not ideal aesthetically even if it is acoustically.

The straight wall should be quite good acoustically as there’s an open-plan kitchen one side of the entrance door and built-in bookcase and storage the other, so a pretty non-reflective surface.

I will certainly take up your suggestion of breaking up the curtains into shorter runs. In any event there will be at least 4 runs as there are columns in the curved wall about 2/3 the distance from the front.

It’s all very well and quite fun discussing the possibilities and the likely effects, but I’ll have to wait till I get in before being able to properly experiment. I’m pleased that no one has suggested it will be disastrous!

Best wishes

Peter
RE: I'd second Ethan's placement recommendations but I'm more positive about the curved surface…, posted on August 13, 2009 at 15:37:26
David Aiken
Audiophile

Posts: 5108
Location: Brisbane
Joined: September 25, 1999
 "I’m pleased that no one has suggested it will be disastrous!"

That's often a prediction to take with a grain of salt when it's given.

When my wife and I moved house a bit over 7 years ago, we had an agreement that I could have a room for the audio system which I could set up or treat as I liked. I had the choice of 2 rooms/areas in the new house. One was an L-shaped room carpeted wall to wall, windows on 2 opposite sides and 2 wide archway entrances on the dog-leg side of the L, one leading to the hallway adjacent to the front door and the other to the kitchen and a large open plan area. The other choice was the open plan area with ceramic tiled floor, a lot of glass window and door area down one side, a very wide archway on the opposite side to the windows, and the kitchen space at one end. It was also the larger area. Both appeared problematic but I went with the L-shaped area because the other area sounded more reverberant in comparison (those floors) and the archway led to a hall leading to the bedrooms and toilets so there was going to be heavy traffic through the space which would place a lot of limitations on setup options. The L-shape was a space with no traffic through it, which gave me a lot more scope on placement. Using a nearfield listening type setup and a bit of acoustic treatment I get what I regard as superb sound, and I think better results than I could have got in the other space. I've also got a fair idea of what sort of results I could get in the other space since setting up a small HT system out there.

Odd and irregular shaped rooms certainly have their problems and it's easy to focus on them and become overly negative, but there are no perfect spaces and I've found it can be quite surprising what can be achieved when you start to work with a space. One thing I've found is that the nearfield approach, with the direct sound path significantly shorter than the first reflection paths, is a really good starting point since it reduces the impact of the room on the sound significantly just on its own. That makes any problems you have left to deal with a lot simpler to solve though the odd room shape can also make it harder to track down the location of the problem. While I've always had very good sound in my room, I have made a couple of changes in the past year of so which have yielded what I think are significant improvements and I've been surprised each time. I often find it amusing that I can still discover new things about the acoustics in my room and the effect of different things within the room and even just outside it on the other side of the archways even after 7 years. I doubt that would be the case if I had that nice rectangular room with doors I've craved for years and never had.

You definitely are going to have to experiment and I suspect that will continue for longer than you expect. I think you can probably set things up to get quite good sound in a reasonably short period but getting the results from quite good to really good will take more time and experimenting. In many ways I think there's a hidden benefit in that: you can learn an awful lot along the way and that knowledge is very handy if you ever come to set things up in a new space again, or help others set things up.

As I said, I'd love to have a room like yours to play in and that's both with the audio setup and the furnishing and decorating. It sounds like the sort of space I'd really enjoy.



David Aiken
RE: What Sort of Acoustics Should I Expect?, posted on August 12, 2009 at 12:13:22
Ethan Winer
Manufacturer

Posts: 1554
Location: New Milford, CT USA
Joined: December 3, 2003
 Curved surfaces are generally not good for acoustics because they tend to focus sound toward a single point. Focusing is the opposite of diffusion which of course is desirable. But how bad the problem is depends on the amount of curving. The good news is absorber panels at key places can solve any problems.

As for positioning, left-right symmetry is very important for good imaging. It's also best to have the speakers fire the longer way down to room, to put the rear wall behind you farther away. More here:

How to set up a room

--Ethan

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