Tuesday, 26 November 2013

Upon Reflection

Positioning your loudspeakers in your listening room for optimum performance is an arcane art.  Three factors must be taken into account.  First, you want to avoid exciting  your listening room’s natural bass resonances; Second, you want to throw a good and accurate stereo image; and Third, there will be any number of purely practical considerations that you cannot avoid and have to work around - for example, it's best if you don’t block the doorway.

The first of these factors is well understood, although not to the extent that the correct answer can be exactly derived a priori.  The final solution will depend on the acoustic properties of each of the listening rooms walls, floor, and ceiling, as well as the speaker’s dispersion pattern, and not forgetting all of the furnishings.  There is a commonly adopted tool called the Rule of Thirds, where the speakers are each placed one third of the way into the room from the side wall, and one third of the way in from the back wall.  The listener then sits one third of the way in from the wall behind him.  This is usually a good place to start.  A variant of this rule is the Rule of Fifths, which is pretty much the same with all the “Thirds” replaced by “Fifths”.  But this post is not about this aspect, so lets move on.

The third factor is also something that I cannot help you with in a FaceBook post.  You had best check out Good Housekeeping, or something.  So this post is going to focus on the second factor, obtaining a good stereo image - indeed this post is about one very specific aspect of it.

It turns out that, of all the factors via which speaker positioning affects the creation of a solid stereo image, the most important is usually the so-called First Reflection.  Sound leaving the loudspeaker cones makes its way to your ear either directly, or by reflection off the walls, floor, and ceiling.  The sound can take many paths, bouncing all round the room, but the most important path is the one that travels from speaker to ear via a bounce off one surface only.  In most listening rooms, the ceiling is generally flat and unobstructed and is the same across the whole room.  Therefore the sound from either speaker will bounce off the ceiling in a similar manner on its journey to your ear.  As a consequence it does not generally impact the stereo image, at least to a first approximation.  The same can be said for the floor, although in most situations furniture and carpeting can interrupt the path.  However, the walls affect the speakers asymmetrically.  The right speaker is close to the right wall but far from the left wall, so its First Reflection will be dominated by reflections off the right wall.  The opposite will be the case for the left speaker.  This asymmetry is partly responsible for the perceived stereo imaging of the system.

There are two things the user can use to control these First Reflections from the side walls.  The first is to adjust the proximity of the speaker to the side wall.  The closer it is, the less the time delay between the arrival at the ear of the direct signal and the reflected signal.  The second is to adjust the speaker’s toe-in (the extent to which the speaker points towards the centre of the room rather than straight along the room’s axis).  Unless you have a true omnidirectional loudspeaker, the speaker’s horizontal dispersion pattern will peak at its straight-ahead position, gradually falling off as you move to one side or the other.  Therefore, the amount of toe-in controls the proportion of the reflected signal to the direct signal.  If your listening room has sonically reflective side walls (plain painted walls, for example), you will probably require a greater degree of toe-in than if you have heavily textured wallpapered side walls, or furniture that will scatter the sound as opposed to reflecting it (such as bookshelves).

I have attached two photographs of my own listening room.  The side walls are flat and painted, and are quite reflective, therefore my loudspeakers have quite a large degree of toe-in.  Also, along the wall beside one of them I have a glass door close to the speaker.  With the door closed, the First Reflection comes off a combination of the wall and the glass door.  However, with the door open, the First Reflection comes off a combination of the glass door and a big hole (which clearly does not reflect at all).  Therefore, on my system, the imaging is severely impacted if I listen with the door open.

The other thing you need to bear in mind is that your best strategy is to control these First Reflections rather than work to merely eliminate them.  As a rule, placing highly absorbent panels right where the Reflection is going to strike is not going to help the sound too much.  The fact that reflections are so important generally means that you don’t want your room to be too acoustically dead.  An empty room with painted flat walls can have a horrible echoing acoustic, but it only takes a small amount of furnishing to break it up.  The echoing or “liveliness” of a room is usually measured by a property called RT60.  This is the time it takes for a reverberation in the room (caused, for example, by clapping your hands) to fall to 60dB below its initial value.  A good number for a listening room would be 0.3 - 0.5 seconds.  If your room has a larger RT60 value, then you will probably need to deaden it with a judiciously placed acoustic panel.  But how big of a panel, and where to place it, is a very complicated subject in itself.  My room has a big absorbing panel, about 6’ x 4’, affixed to the ceiling between and behind the speakers.  I also prefer to listen with the heavy floor-to-ceiling curtains on the wall behind my listening chair drawn.

Of course, every time you make a significant change to the acoustics of your listening room, the chances are good that you are going to need to reposition your speakers.  Changes that affect the RT60 may well impact the optimum positioning, so you may have to go through the whole procedure again.  Reposition, then fine-tune the toe-in and the tilt.  My B&W 802 Diamonds weigh 160lb each, and are the most cussedly awkward things to grasp if you ever want to move them, so that is something I don’t like to get involved with on a whim.  Because of the First Reflection factor, if your listening room is such that the First Reflection surface has a high acoustic reflectivity, then be aware that the distance of the speaker from the side wall will probably have to be set to an accuracy of half an inch.  Likewise, the toe-in and tilt can require great precision for optimal results.

If your loudspeakers are not set up to image as well as they can, then you are going to find it that much harder to optimize other aspects of your system setup.