Reflections

Reflections



  1. Tenson
    Lets talk about reflections in a little more detail because they are very important in the mid and high frequencies. Yes, low frequencies reflect too, but because of their large wavelength they don't tend to reflect at angles, they just bounce between opposing surfaces. In the specular range, we can consider reflections to seem like a source of sound in their own right, coming from behind the boundary. Just as when looking at yourself in a mirror, it seems like a person is behind it, not on the surface of it.

    Reflecting-Image.jpg

    The angle at which the sound hits the reflecting surface changes the angle it bounces back at, and as I have said earlier, the angle of incidence equals the angle of reflection. This is hugely important in a listening room because on every room boundary there is a point at which the sound from the speaker needs only reflect once to arrive at the listener position. It's called a first reflection point, because only needing to reflect one time, these are always the first reflections to arrive after the direct sound from the source.

    A few other shapes of surface, other than the flat room boundary are important to know about. These are convex, concave and right angle corners.

    A convex surface is basically a outwardly curved area. When the convex shape is smaller than half a wavelength of the sound meeting it, the sound ignores the shape and reflects much like it had hit a flat surface. When the shape is larger than half a wavelength of the sound, it is roughly dispersed in the shape of the curve. This is a simple but limited way to encourage fewer strong reflections and gain a diffuse sound field. However, the even distribution of the reflected sound is not as ideal as it looks in the drawn diagram below, so purpose made diffusors exist, and I'll talk more about those later.

    Convex-1.jpg


    A concave surface is of course the opposite. We find these in our listening rooms most commonly in the form of a bay window area or lofted ceiling. The effect of this shape of surface is to focus sound reflecting from it to a single point in front. This tends not to be a good thing in listening rooms since, for reasons I'll discuss later, we should avoid reflections arriving together at exactly the same time. To put your speaker in front of a convex boundary, and then sit between them would focus all rear-radiated sound from the speakers directly at you.

    Concave.jpg

    The right angle corner is interesting. It doesn't matter what angle the sound hits this surface (or two surfaces) it always returns heading in the direction it came from.

    Corner-Reflection.jpg