Acoustic Treatment
You may consider the use of acoustic treatment. Wall-mounted tiles made of acoustic foam placed strategically around the monitoring position is a proven way to achieve a professional monitoring environment. It is also worth considering mounting bass traps around the room and in upper room corners where bass frequencies tend to build up.
These can be made from mineral wool (as mentioned) or acoustic foam. Another effective form of acoustic treatment can be the use of ‘diffusers’ – precisely-shaped solid objects designed to break up desired frequencies, stopping them from building up. It’s likely you might require a combination of all three.
Acoustic tiles:
These absorb the reflections of high mid to high frequencies. (See image 1)
Bass traps:
These absorb the low mid and low frequencies. (See Image 2)
Diffusers:
These are shaped to fragment reflections. The one pictured is known as a ‘Quadratic-residue Diffuser’, or a ‘Schroeder Diffuser’ named after Manfred Schroeder who developed the formulae for calculating its dimensions. Their size and dimensions can be tailored to deal with certain frequencies. (See Image 3)
Before we look at where is best to place the furniture/acoustic treatment around the room, let’s first look at speaker placement with regards to the room’s surfaces.
Loudspeaker Placement
An important factor you need to be aware of is the time between the direct speaker sound and the first reflections from the walls. When this time is too short, the reflections color the direct speaker sound through a process known as ‘comb filtering’.
This is when the interaction of the direct sound waves with the slightly ‘out of phase’ reflected sound waves causes an undesirable filtering effect extending up the frequency spectrum. Increasing the initial delay of the first reflection lessens the impact of comb filtering. So it’s okay to have ‘some’ reflections; the key thing is to try our best to prevent them from reaching our ears too early, and that there isn’t an imbalance of the ‘tone’ of the reflections, such as too much low frequency compared with the rest.
The walls directly behind and beside the speakers are likely to cause issues as the reflections off these will be very short. Also, flat surfaces in front of the speakers like a table or a mixing desk can introduce reflections with very short delay times. Depending on the size of the room, there’s a good chance that the reflections off the walls either side of the listening position will be troublesome too. I will illustrate this shortly.
Putting the discussion on reflections to one side, another important aspect to consider with speaker placement is the distance between the speakers and the distance from your ears.
This is essentially what creates the ‘sound stage’. You will most likely be dealing with stereo content – some sound will come from the left, some from the right and some from the middle. The balance between these three areas is what you must get right.
If the distance between your speakers is much greater than the distance to your ears, the ‘phantom center image’ can become weak and the ‘stereo width’ too wide – as if you’re wearing headphones. Drawing the speakers closer together, shorter than the distance to your ears, the phantom center can become too strong and the stereo width too narrow. A tried and trusted way to arrive at suitable speaker locations, with respect to the listening position, is to create an ‘equilateral triangle’ between your ears and the speakers. This is known as ‘true stereo’ or ‘true 60 degrees’ (the angles within an equilateral triangle are each 60 degrees).
Recap
—Aim to neutralize the sound of the room – balance the tone of the reflections.
—Parallel walls cause standing waves – the room will resonate at certain frequencies.
—Different surface materials have different reflective properties.
—Everyday room contents can be used as acoustic treatment when placed correctly.
—Direct speaker sound is colored by the immediate reflections from the nearby walls.
—The effectiveness of the perceived soundstage is a result of the distance between the two speakers and your ears.
