11 Jul Absorption or diffusion?
This is question often asked. We know from studies that reflections that arrive after the direct signal can have a detrimental effect on the sound quality. Besides redirecting the energy away from the listener, which is done sometimes in studios with splayed walls, one can either absorb or diffuse. But what is best?
Let’s first look at the definitions of absorption and diffusion. It’s described more in detail under terminology and under the subject “Diffusion and scattering-are they the same?”, but in short one can say the following:
– Absorption: Turns the sound energy into heat and the sound doesn’t bounce back
– Diffusion: The sound is spread out evenly for the different frequencies
However, no absorbent will absorb 100% of the frequencies it’s operating in. Some sound will bounce off the absorbent but overall the sound disappears. A diffuser remains more energy, while it will also absorb some.
Diffusers create a bigger and more spacious sound field. Especially when they also perform temporal diffusion, like Modffusor and Modffractal. Absorption on the other hand gives a higher degree of direct sound, and when treating early arriving reflections; this gives the highest resolution and window into the music material.
Treating early reflections with diffusers will compared to pure absorption to some degree makes these aspects suffer and doesn’t quite give the same openness, clarity, intelligibility or localization of voices and instruments. In return though, the sound field is more spacious and more enveloping and compared to no treatment the resolution and accuracy will also certainty be improved. There are certain areas we still need to look into in order to choose the right treatment.
Area of performance:
Something we need to take into consideration is what frequency area an absorbent or diffuser operates in. A shallow absorber of 2.5 cm (1”) will not absorb effectively below approximately 2000 Hz in a small room. While a 10 cm (4”) absorber can be effective down to 300 Hz. Diffusers become very deep in order to diffuse this low in frequency. Commercial diffusers with typical 10-12 cm depth will not diffuse effectively lower than 800-1000 Hz. In addition, they don’t diffuse much above 4000 Hz. This implies that the rest of frequency area is mainly being reflected.
The length and width of an aborber or a diffuser also has a play in what frequency area it is effective in due to that size of the wavelengths. However, we have chosen to keep this out in the examples below in order to not complicate things too much.
Early arriving reflections:
Let’s look at an example with the first reflections from the nearest side wall. The reflected energy will here arrive a short time after the direct signal because the distance from the speaker to the wall is short.
If the reflection point is diffused with a diffuser that mainly operates between 800 Hz and 4000 Hz, it will not be very frequency linear. The frequency range of a reflections in a small room goes from approximately from 200-300 Hz and to 20 000 Hz. Thus we have only treated part of the reflection.
If we use a diffuser like RPG Modffractal which diffuses effectively between 450 Hz and 20 000 Hz, we have treated most of the specular reflection. However, here we encounter another challenge. Diffused energy requires distance to work. The deeper the diffuser diffuses, the longer the distance from the listener needs to be in order for the energy to be properly diffused. At 600 Hz, the distance should ideally be 1.7 m from the diffuser to the ear. And at 500 Hz minimum 2 m. Therefore, a broadband diffuser is best to use when there’s a good distance.
By placing absorbers with a depth of 10 cm in the same first reflection points, it will have an effective dampening down to approximately 300 Hz.
Thus, with the absorbers we have effetively treated almost the entire specular reflection (for most small rooms) and there is no challenge with distance like the diffusers. This implies that treatment of early reflections with thick absorbers is usually more frequency linear.
It’s also worth taking notice that with absorpstion of early reflections we do not change the recorded signal/mix. We only remove the reflected energy and this gives us the highest resolution and insight to the music material/recording. With diffusion; a more spacious and enveloping sound field is created, thus we color the music in a way. For music listening this can of course be a preference, but in a studio where the goal is to hear as accurate as possible it will not be the right choice in our opinion. Howerver, when it comes to later arriving reflections it’s different. More about that later.
A third alternative of treating side wall reflections is using a combination of diffusion and absorption. BAD Arc and BAD Panel diffuses above 800 Hz and absorbs below these frequencies.
By using BAD Arc/ BAD Panel one can treat all of the specular reflection like a thick absorbent, but with diffusion from 800 Hz and above. At 800 Hz the diffusion doesn’t need as big distance to the diffused energy, making this a treatment choice that works in most rooms. This is a way to remain more energy and ambience, while effectively treating the reflections. A very good alternative for listening rooms.
When it comes to the reflection from the opposite side wall, like the illustration below shows, the distance will be decisive factor for the choice. This reflected energy arrives later in time. However, in narrow rooms (3-5 m width) it will still be considered as an early reflection and pure diffusion will have the same challenges as previously mentioned. If the room is very wide, broadband diffusion can be a good alternative.
- We have seen that pure and thick absorption is often the most frequency linear treatment of early reflections
- Diffusion remains more energy but will also alter the music material more
- Shallow diffusers and without fractals will only treat parts of the reflection. Deep diffusers with fractals (like Modffractal) requires more distance to work optimally
- BAD ARC and BAD Panel that both diffuses and absorbs is an alternative to pure dampening and will also work well when the distance to the ears are relatively short
Treatment of late arriving reflections:
Late arrival reflections from the room also has a destructive and negative effect, but less than the reflections that come shortly after the direct sound. To achieve a great result, they should however also be treated. Since we are less sensitive to reflections arriving later in time it’s not equally important to treat these broadband, though it still will be an advantage.
Most often and unless one is sitting close the the rear wall, late arrival reflections come from the rear wall and rear side walls. If one is using dipole speakers that sends the energy behind the speakers, late arrival reflections may also come from the front wall.
First we are going to look at an example with late arrival reflections from the rear of the room.
We will say in this example that the distance from the ears to the rear wall is 2.8 m. That means that the earliest reflection from the rear wall will arrive 16 ms after the direct signal. To absorb late arriving reflections that come from the rear will easily create an overly dry and dead acoustic environment. Our brains are used to hear sound coming from the rear of a room. Therefore, we do not recommend to only use absorption there. We recommend to either use pure diffusers or a hybrid product like BAD Arc or BAD Panel. If the seating position is up close to the rear wall, some absorption should be used right behind the ears.
In this case we choose Modffractal diffusers that work very well from approximately 450 Hz and till 20 000 Hz.
To achieve more diffused energy, diffusers can be placed in a half circle and one can also place diffusers on the top of the other and turn them so they diffuse vertically and towards the ears.
In the other example we we look at an example with the front wall. Speakers with baffle will not send much of frequencies above the bass towards the front wall, as it will primarily be absorbed in the speaker enclosure. Dipoles on the other hand send as much energy backwards as they do forward.
So with dipoles we get strong reflections from the front wall. The best treatment here will depend on the distance, treatment that been chosen for the rear wall, and preferences.
When the dipole speaker is placed a good distance out from the front wall and there’s also decent distance to listening position, we believe many will prefer diffusion or combination of diffusion and absorption to music listening.
- Late arrival reflections can also have a negative effect and should be treated for the very best result
- Pure absorption of late arrival reflections can create a dead and dry environment, which we may experience as unnatural
- Diffusion or combination of diffusion and absorption will most often be the preferred choice for treating late arrival reflections
In this teaching we have focused on the frequency area above the bass. In order to receive a great result, these treatment should be combined with proper bass treatment.