|Above: this is a typical room treatment setup using Real Traps products. Bass traps are used in the corners and portable absorbers are placed on side wall first reflection points.|
In a room there are three regions that need to be considered:
1. Modal region (light red)
This region is dominated by room modes. You can see the peaks and dips although in many rooms this will be worse. Some EQ has been used to reduce a very large peak around 45 Hz and some care has been taken to tame this room. Typical rooms will often see +/- 20 db.
2. Geometric region
Above the transition point, called the Shroeder frequency, room modes become spaced together so closely that they smooth out. Thus we tend to overlook the fact that modes still exist and instead we pay attention to the fact that sound travels in a straight line and is reflected as geometry would tend to dictate.
3. Pressure region
As wavelengths begin to become large compared to room dimensions, we no longer see peaks and dips but instead the room becomes uniformly pressurised. In this region, the bass response is smooth and if the room enclosure is adequately constructed, room gain will extend low frequency response. The lowest mode in the room measured above is around 45 Hz so the pressure region starts a little below that. This region needs no treatment.
It's interesting to note that in a car, due to the small cabin size the entire bass range is in the pressure region. This is why cars have so much bass even with small speakers.
In the modal range, the goal is to add bass damping without adding any unwanted full range absorption. This will flatten out the frequency response and help the bass to "stop faster."
Have you ever been inside a completely empty room with no furniture, curtains or pictures? Because your ear can tell the difference between the sound of your voice and the room reverberation, you realise instantly that the room needs taming. In the bass range, the ear is not able to make that distinction, so the added reverberation is simply heard as bass that is lacking control and tightness. The subwoofer itself will often be blamed. Those who have not heard a properly treated room will also not realise the level of realism that can be achieved.
The answer is to use a bass trap, which is a purpose-built bass absorber. These are normally placed in room corners where they will have the most effect. For more information about bass traps, you may like to read:
Bass trap primer
Absroption and diffusion are used to improve imaging and create the right level of reverberation. The amount and arrangement of treatment depends on the type of speakers used and a level of personal preference. Speaker such as open baffle and waveguide based designs which control directivity tend to favour less treatment, while more common speakers with wider dispersion tend to require more. It's important to achieve a balance. While you probably can't have too much bass absorbers (traps), you can certainly kill a room with too much full range absorption.
Diffusors break up specular reflections, and instead cause the reflection to radiate equally in all directions. The result is that sound waves travel a longer path before reaching your ears. Ambience is retained, but the detrimental impact of early reflections is reduced. A room will tend to sound as if it were bigger and this is generally an advantage in domestic rooms.
|A popular skyline diffusor|
Diffusors should not be placed too close to your listening position - that would place you in their nearfield region. They require a certain distance to work correctly. If you listen in the nearfield, you will have certain artifacts that will interfere with imaging. As a general rule they should not be placed within 3m of the listening position by direct line of sight.
As with most things, use in moderation. Consider them on the front wall, ceiling and side walls near the speakers, although they should not cover the entire surface.
The first place these are used is on first reflection points, as shown in the first image at the top. This is the convention, although with speakers that control directivity more, this may not be necessary.
Early reflections are the ones that tend to confuse imaging. More delayed reflections are more readily picked up by the brain as being part of the room sound and so imaging is not affected.
Another less common use for absorbers is where non-ideal placement of speakers is required. Placing surround speakers on walls causes problems with the response, even if they are flush mounted. Treatment should be used. The same is true of main speakers which are placed close to a wall.
It's important to realise that the thickness of the material and it's distance from the wall impact the bandwidth. Very thin material placed on the wall will only affect high frequencies. To work down lower into the midrange, the panel needs to be thicker.
Next: room treatment suggestions