08 Mar WHERE TO PLACE ACOUSTIC TREATMENT
Placing acoustic treatment based on measurements is always the best way. Where this is not possible, one can follow general guidelines and practices in acoustical treatments. Below is some information concerning this.
The room modes will most often terminate in trihedral corners. Other intersections, for instance where the floor meets the wall and wall meets the ceiling are also considered to be corners and places the low frequencies will build up. In the trihedral corners we recommend Modex Edge bass traps (come in two sizes) and stack them from floor to ceiling if possible. Measurements indicate they are highly effective from as low as 20/30 Hz till 350 Hz, quite a unique performance for a bass trap. The fact that they reflect higher frequencies will in most situations be an advantage as it avoids deadening the room.
Another place, which in almost all small rooms will benefit from low frequency absorption, is the front wall. The front wall is the wall seen from the listening position, i.e. the wall behind the speakers. For instance can SBIR (Speaker Boundary Interference Response) be an issue at the front wall, caused by to two sound waves colliding. The same happens at the sidewall by the speaker. The reason for this is that low frequencies in small rooms become omni-directional.
Breaking up these reflections with bass trapping will minimize both SBIR and standing waves in the room. Modex Plate and Modex Broadband are particularly well suited in these scenarios due to their shallow profile and high efficiency. While Modex Plate works from either 30 to 100 Hz or 50 to 500 Hz, the Modex Broadband works higher up in frequency and may be a better solution behind dipoles or when higher frequency absorption is needed.
The rear wall is also almost always in need of low frequency absorption, especially at the mentioned intersections. RPG Modex Plate and Broadband would be the preferred choice here as well and Broadsorbor the budget option and when absorption higher in frequency is required.
Midrange and treble
The choice of treatment above the bass region will depend on what acoustic concept one chooses to follow. We can be of assistance in giving recommendations to customers in choosing what products to purchase. In order to give you the best advice, please advice us what intended usage of the room and if possible attach a drawing and pictures of the room. Here are some foundational principles that can be used as a starting point.
Absorbents dampen the sound. In a very small room, absorption would be the primary type of treatment. The most important surfaces to treat are those where the reflections arrive the earliest after the direct sound.
Usually this will be the floor, ceiling and sidewalls. To locate these reflection points, one can use the mirror method. Position one person in the listening position while another person holds a mirror at the mentioned surfaces. Where the mirror is in a place where the person in the listening position can see the speaker through it is the reflection point in which to place the absorbent.
We recommend treating a somewhat bigger area than what’s seen in the mirror and place the absorbent mainly according to the image of midrange and tweeter drivers. The closest sidewall to each speaker is important area unless the room is very wide.
The floor is unpractical to treat in a proper way, but usually one thick or two wool carpets on top of each other is an acceptable solution. If the listening position is close the back wall, you will also benefit from placing absorbents here in order to attenuate the reflections.
Whether one wishes to dampen the sidewall reflections on the opposite wall to each speaker depends on the width of the room and when these reflections arrive. In a small and narrow room it’s recommended to absorb these. If the room is wider, diffusion or a combination product (BAD Arc/BAD panel) can be a good alternative. The drawing below shows reflections arriving from the closest sidewall and from the opposite.
As mentioned, diffusion is an alternative on opposite sidewalls in a wider room. Diffusion requires a certain distance in order to function optimally. A good rule of thumb is that the listening distance should be three times the wavelengths of the diffused frequency. When a diffuser diffuses well down to 500 Hz, it means that the minimum distance from the ears to the diffuser should be 2 m. Some would be of the opinion that this rule isn’t absolute and that sitting closer also works fine, but we recommend staying close to this principle for the best result.
Due to the distance needed to diffusers, the placement of diffusion will often be on the rear wall and also on the rear side walls. With dipole speakers, diffusion behind the speakers (front wall) is also an option. Some do prefer diffusion in the ceiling which gives a feeling of the ceiling having infinite height. However, thick absorbents in a room with low ceiling height (common height in homes) is more neutral frequency-wise.
Considering that both the front and rear wall would benefit from bass traps, the diffusers often need to be placed on the outside of these. Optionally one can use BAD panels/BAD Arc that performs combined absorption and diffusion. BAD will not diffuse as low in frequency as a diffuser with deeper wells and doesn’t absorb as low as bass traps, but it’s a good alternative when there’s a need to control specular reflections while preserving some ambiance. The BAD doesn’t require as much distance to the listener as they primarily diffuse from 800 Hz upwards, where the wavelengths are smaller.