Room Acoustics - Part I
ROOM ACOUSTICS - THE LAST BARRIER, OR THE MOST IMPORTANT FACTOR?
hifi-advice.com - info and reviews by Christiaan Punter - April 2011
It's no coincidence that it took so long for me to publish an article about room acoustics. Despite all I've learned about audio,
I haven't yet been able to fully tackle this last barrier. In this article I'd like to share what I have learned so far.
Room acoustics are difficult to get a good grip on. Even for experienced audiophiles it is often still a mystery why one room sounds great while the other sounds awful. Sadly I have to admit that I am also still somewhat in the dark here. I know something about room modes, resonances, standing waves, flutter echo, first reflections, absorption, diffusion, lively room/dead room, bass reinforcement/bass cancelling, aligning speakers both vertically as well as horizontally and equidistant from the rear- and side walls etc ect. But for few of these properties I can say that I have fully mastered everything there is to know about. When you live in one room for an extended period, have found the sweet spot for the speakers and you don't change the speakers during that time, you may start to feel that you know your speakers very well and that you know exactly how they sound. You may even feel that you now know how your room works and that setting up a speaker isn't all that difficult. Until you change to a different kind of speakers, or move house. Say for instance that you change your closed cabinet speakers to bass reflex ported ones. All of a sudden the speakers can sound bass heavy, boomy and coloured. But it can also be that you finally get full, deep bass where before it was tight but perhaps lacking body. Or if you move from a small to a large room and keep the same speakers, all of a sudden the sound can literally drown in the room, making for thin, sloppy, unclear bass, phasey mids and confused highs. Alternatively it can be that you finally get clean bass without that annoying boomy emphasis at certain frequencies. Or it could go the other way around when moving from a big to a small room and all of a sudden it seems like you have a monster subwoofer in the system because the bass is now finally fully extended where before it was merely thin and correct. But when going from a big room to a small one you can also lose a lot of precision and articulation in the bass and the mids may become very forward and even harsh. At this stage I can continue making examples but that's just boring and I suppose you got the picture two examples ago. Suffice to say that I have learned that results can often go both ways. Rooms just aren't that predictable at all. Or am I just ignorant?
Of course I'm not entirely oblivient in this matter. I do know a thing or two and hope to give you, dear reader, some insights into why room acoustics are such a delicate matter.
I've learned the hard way how to position my speakers best: by painstakingly moving them around the room, inch by inch, centimeter by centimer and eventually even on the milimeter accurately. At some point I learned how to position the toe in just so that the soundstage was nice and large but still focused. Likewise I figured out how to optimise bass performance just so it has good depth while maintaining articulation. The thing is: every listening room I tried needed different placement from the rear wall and each other. There were even differences from room to room, so large that they validated choosing an entirely different amplifier in order to maintain the soundbalance the way I like it. This is another reason why I haven't written about this before: no matter what the experts say, there seem to be no simple universal rules that guarantee good sound.
Cardas Golden Ratio
But there are other ways to decide on speaker placement. The mathematical ways. Regular readers know that I am not all that much a mathematical guy, I more trust my instinct and have a good feel for when speakers are optimally placed, I just cannot explain to anyone how I do this. And this is where maths come in. Even though in the electronical world measurements often don't tell us much about how any component sounds, in the world of acoustics, it is possible to calculate the optimum place for your speakers. There are various guides for deciding/calculating the optimal place for your speakers. George Cardas for example uses the so called Golden Ratio for room calculations as well as cable design. The formula, according to Wikipedia, is as follows: "In mathematics and the arts, two quantities are in the golden ratio if the ratio of the sum of the quantities to the larger quantity is equal to the ratio of the larger quantity to the smaller one. The golden ratio is an irrational mathematical constant, approximately 1.6180339887." While the formula itself is beyond me, I understand that it is all about having dimensions not being dividable by each other in an effort to cancel, not amplify resonances.
Wilson's method
There's also an interesting guide from David Wilson which is really very simple and all you need is your own voice. I simplified the method here as it is just for illustration purposes, not a proper guide. First determine the zone relative to the back wall, so start speaking at the back wall, in front of the listening position, walking toward the listening position while speaking. As you approach the listening position, the voice will appear to 'free up' as it is relieved of the low frequency energy imparted by the closeness to the rear wall. In other words, you are listening for the point in the room where the rear wall stops to reinforce bass. When this point is reached, mark the floor with a piece of tape. Using the same procedures, but starting speaking at the side walls and walking across the room in front of the listening position, you can now determine the left and right boundaries of the Zone. Again, as the person speaking walks away from the side walls, you should hear the voice freed of the bass reinforcement provided by the side walls, eventually reaching a point where the opposite wall begins to interact with the voice. Repeat this for the left and right side walls, again marking the borders with tape. This marks the rear border Zone of neutrality. While continuing to walk toward the listening position while talking at the same level, listen for the next change in the voice's tonal quality. This will be the first interaction of the opposite wall and should sound like a loss of focus that seems to resonate because of the side and rear wall reflections. Again, mark the position on the floor when the artifact first appeared with tape. You now have the front and back borders of the Zone of Neutrality. Now you may find that the borders are marked fairly different for the various walls. Simply using these borders would result in an asymmetrical arrangement of the Speakers. So take some measurements and average all the different Borders you came up with. Then re-arrange the tape to reflect the symmetrical average of your various positions. After performing these procedures you will have two zones, in which your speakers should sound the least coloured. But of course, while having the least coloration may be nice, it may not be the holy grail. Perhaps a very wide soundstage or a liquid presentation it is more important to you or perhaps you even want some coloration or bass reinforcement. I'm just saying. There are many variables to take into account. But if you like to dive in, there are plenty more methods and guides, as evidenced by a Google search.
There are also various computerprograms that can do some calculations for you. All you have to do is to enter the room dimensions and some other parameters and they will come up with what is expected to be a good place for your speakers. I have no doubt that calculations like these can yield good results but they are neccesarily very coarse and can really only be used as a starting point after which you have to do the finetuning by ear.
The bass problem
Ever since I moved house, I wish that I knew a little more about the principles at work here. Because once more I am at loss to explain why my Magnepan MG3.6R's sound marvellous in my large new listening room while the B&W N804's (that are normally in the much smaller cinema room) sound awful there. No matter where I place them, they sound coloured, boomy, lack bass and basically just don't sound anything like Nautili. I guess it must have something to do with standing waves, room modes and nulling. The thing is that my room isn't square. It is very wide where the speakers are (2 meters to the left speaker and 6 meters to the right speaker), and narrower at the listening position (5 meters). The large space at the right speaker is the kitchen. But where such a gap can normally result in phasey bass, in my case the bass is very precise and articulate and absolutely not phasey. As long as the Magnepans play. When I use regular box speakers (N804's and Synthese Floating 2 (respectively ported/transmission line products), it becomes almost impossible to get satisfactory results. The best sound I achieved yet was with the speakers almost in the middle of the room. And still they wouldn't sound like anything I heard them do before in other rooms. What is always easily achieved by means of carefully positioning the speakers and adding some damping materials at key places is smooth treble, good focus and a good soundstage, and I have no problems there. Early reflections and too much liveliness can be easily dealt with. It is the bass however that is always causing me problems. It's not difficult to find the best spot simply by moving the speakers around the room until you like what you hear. But it takes an incredible amount of time and perseverance. I can really be doing this for hours and hours at end, traveling from sofa to the speakers, do some situps by moving the speakers and inch to or fro and travel back to the sofa, only to disaprove what I hear and repeat the cycle dozens of times until I find a good working point, go to bed happily, and have to repeat the same process the next day because it turns out that I was focusing too much on one aspect of the sound. This continues until I like what I hear and eventually I always find the right spot. But it is a tedious process. And all muscles in my legs always hurt afterwards:-)