A new type of bass trap/diffuser room treatment?(long)

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darkmoebius

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Joined 2003
An externally hosted image should be here but it was not working when we last tested it.



My question is - is there anything we DIY'ers can learn form this new device to enhance the popular DIY fiberglass insulation based tube trap?
Can we somehow figure out what they've done and clone it?
Or, is all a bunch of hype?

Italian company Acoustica Applicata has created what it claims to be a new form of bass trap/diffuser, called the DAAD(see closeup pics here), which greatly outperforms the industry standard ASC Tube Trap (which AA manufactured/distributed for years) and
RPG Diffusers.

According to this long description(MS Word doc) of the DAAD's conception/evolution, there are 3 important factors to DAAD's superior performance: 1) the external metal screen/grate(closeup pic) serves the diffusing purpose, 2) the internal resistive/absorptive material sandwich, and 3) the oval/lobe shape of the traps as the link above shows.

To save the impatient a little time, I've culled some of the most important concepts/claims from their literature.

Regarding the external screen/diffusor:

The truth is that we had to realise rather quickly that the ‘density’ of the reticule{re:screen} is actually really influential. If there were too many and narrow holes, the ‘s’ became excessively hissing, and if they were too large, vocals sounded darker.

But these changes did not only affect high frequencies, because low frequencies also behaved differently. If the cover grille allowed more air to go through and reach the inside of the trap, it worked by absorbing larger quantities of frequencies if they were higher than 100 Hz, but it became poorly effective below that number. Whenever we used a more dense cover grille, the amount of absorption diminished drastically but the trap could also work at lower frequencies. In other words, the different types of cover grilles determined the quantity and quality of absorption at low frequencies. This has its logic in a trap that works through differences in pressure. For example, in the case of a dense cover grille, the amount of air reaching the resistive material is small if compared to a grille that has fewer but bigger holes. The quantity of resistive material inside is fixed. Therefore, air that enters the trap through denser and smaller holes ‘sees’ a greater quantity of sound absorbent material and has a higher pressure. In this way the trap absorbs a smaller quantity of sound because it has less air to deal with, but it activates itself at lower frequencies. On the other hand, if it has bigger holes, the trap receives more air and absorbs a greater quantity of sound, but, because of the fact that the pressure is lower, it activates itself at higher frequencies.

The pressed and micro-perforated metal sheet turned out to be a very ‘powerful’ and flexible material. Whenever it was put in place instead of the original cloth covering the Tube Traps results were better. We were still not satisfied however....

Tube Traps covered with a pressed metal sheet offered both good control over resounding frequencies and an acceptable amount of sound diffusion at high frequencies, but...didn’t allow music to breathe the way we wanted.
Click to expand...

Regarding the absorptive insulation:

The resistive material inside of Tube Traps is glass wool that has excellent properties as a sound absorbent. Its thickness is calculated on the basis of the volume of air inside the trap. If one introduces compressed air inside a Tube Trap (thus creating a practically reversed situation compared to normal utilization), the air that comes out of the trap is almost non-existent. In other words, the added air is, for the most part, converted by the glass wool into heat through a powerful friction. But, if you think this through, this also means that it will take longer for the trap to return to its original pressure state. If one considers a long sequence of sound transients, it is quite likely that a device with a considerable quantity of sound absorbent material will succeed in handling the first difference in pressure, but then won’t have time enough to get ready for the second and some of the following ones. Therefore, a slow trap works only in intermittence.

.... In order to get what we wanted, we had to experiment with other materials and thicknesses that allowed air to penetrate the trap quickly and get out again after a given time. These new materials shouldn’t create excessive friction to the air passing through them, in order not to slow down the functioning of the entire acoustic device with regard to the timing of music transients that follow each other. What we wanted was a ‘fast’ trap! After some substantial additional research we finally found a satisfactory combination of two materials.
Click to expand...

Regardig the shape of the trap:

So all we had to do now was to define the final shape of our new acoustic device.
The shape of a lobe seemed the most suitable one, for the following reasons:
1. its internal volume being equal to a cylinder, a lobe-shaped device ‘penetrates’ the corners of a room more deeply, thus capturing the resounding frequencies more easily;
2. its shape facilitates the simultaneous use of different materials for the resistive layer of the device;
3. like a cylinder, but unlike a flat panel, a lobe device allows one to have an inner volume with air and a thickness able to create ‘acoustic shade’. In other words, it provides a very good ratio between the space used and the results that are reached;
4. like a cylinder, but unlike a flat panel, a lobe device can be rotated on itself. Having lobes with different diffusion characteristics allows one to position them in several ways and to change room acoustics according to one’s personal needs or tastes.

But the most interesting and lucky discovery actually came when we realised how the chosen lobe shape tended to ‘remix’ energy: once it receives a sound wave, the DAAD works on it in a way that not only delays its re-release but also diffuses it homogeneously all around it.

We were about to reach the finish line. All we had to do now was to find the right ratio between the thickness of the resistive material and the degree of ‘permeability’ to air of the pressed metal sheet

We only thought to have reached our goal after we decided to reduce the thickness of the resistive material, when the device was put in a condition to work faster. The presence of the pressed metal sheet and the shape of the trap obviously allowed us to use sound absorbing material with more moderation. It is the combination of these three things – the shape, metal sheet, and quality and thickness of the resistive material - that allows DAADs to behave both as a fast acoustic trap for resounding low frequencies and as a diffusion–diffraction device that turns early reflections into more delayed ones.
Click to expand...

So, people of jury, if you've made it this far - what say you?

Any opinons on whether this method actually works better than the standard bass trap with a reflective skin on one side?

Any thoughts on what they might use as absorptive material?
 
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ThomasW

Member
Joined 2001
I popped this thread back up since I had an opportunity to hear a room treated with these at the RMAF-2005.

Now there was $230K worth of gear in the room so that probably had something to do with it. But it was the finest sounding room I've ever heard.

Here's a pic of that set-up

An externally hosted image should be here but it was not working when we last tested it.


If anyone is interested in reverse engineering these traps count me in.

re: materials, despite the text a fiberglass or acoustic foam core should be fine, might vary with the frequencies being targeted. There's a plastic film on one side of the trap so it can be rotated between diffusion and absorption as is the case with most of these devices.

Finding something similar to their coated screen outer, might be a bit trickey

DAAD2
absorbents of sound for reflections
posterior laterals and p/resonances above of 120Hz
diameter max. 22cm - 110cm

DAAD3
absorbents of sound for all
the primary reflections above of 70Hz
diameter max. 28cm - 110 cm

DAAD4
absorbents of sound for all
the reflections anteriorers above of 50Hz
diameter max. 39cm - 110 cm
 
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