XIX AcousticsEffective Diffusion Bandwidth: 300 Hz – 4 kHz (see actual data)
Effective Scattering Reflection Bandwidth: 300 Hz – 20 kHz (see actual data)
Quadratic Residue Sequence: Prime = 7
Height: 44 7/8″
Width: 24″
Depth: 12 1/4″
Weight: 40 lbs.
Reflective Material: Furniture Grade 1/2″ Pre-Finished Maple Plywood
Divider Material: Powder-coated 1/8″ Aluminum Stock
Most Effective Installation Location: Rear Wall of Rectilinear Rooms
Acoustic Diffusion is not an easy thing to test or describe. In fact in the academic acoustics world there is a big controversy about the best way to describe and measure diffusion: the Scattering Coefficient or the Diffusion Coefficient.
“You will know it works when you hear it,” is true but probably not a good explanation. I am going to avoid this whole dispute by publishing the actual data of all frequencies and amplitudes in my tests.
I did several rounds of acoustic testing on the Ramps and here is what I found:
The Ramp does both of these things at the same time. So if you install the diffusers on their sides, it only changes which orientation scatters and which one reflects and scatters.
When sound strikes a hard flat surface most of the sound bounces off the wall as an echo. That echo can interfere with the sound that is headed towards the wall by cancelling out certain frequencies and emphasizing other frequencies. This effect is called comb-filtering. You just cannot hear sound accurately in a room that has comb-filtering problems. Scattering the sound breaks up echoes into many little tiny echoes diminishing their effect. The Ramp also reflects sound away from the sound source, further minimizing the effect of comb-filtering.
Reflections can also be controlled with absorption of course, but absorption-only treatments tend to yield rooms that sound dead and lifeless.
Those diagrams are called “sonograms” and they were generated from data gathered using the software package ARTA. There will be a later post about the testing procedure, but for right now let’s focus on the diagrams.
The Sonogram shows Amplitude (color), Frequency (x-axis), and Directivity (y-axis). The above diagram shows what happens when you place a loudspeaker directly in front of a flat panel. The bright yellows and reds show loud levels and the blues and dark blues show quiet levels. The stripe of brightness that goes horizontally across the center of the diagram shows that sound is bouncing right back towards the speaker. At about 500 Hz the sound is bouncing at about 30 degrees and 50 degrees off-center. And from about 300 Hz and below the reflection pattern is pretty wide.
If there is pretty good diffusion at a specific frequency the diagram will show a vertical band of similar colors. Horizontal bands of similar color means that there is an intense specular reflection at a specific angle. For instance, in the above flat reflector model, there is a very strong reflection at 0 degrees, meaning that sound hitting the reflector is bouncing straight back to the loudspeaker. If the reflector had been angled at 45°, we would expect there to be an intense band at around 45° as well.
In the horizontal test of the Acoustic Ramp™ you can see that nearly all of the energy is directed away from bounding back towards the sound source because the center of the diagram shows darker blue colors.
In the vertical test of the Ramp you can see the scattering of frequencies from about 300 Hz to around 4 kHz.
There are several reasons why the Acoustic Ramp™ works better than traditional diffusion treatments, but we’d like to focus on two major improvements in this post.
In order to understand why these factors are important we need to discuss a few things about diffusion in general. There are two different types of diffusers that have been thoroughly tested and evaluated in the academic and in the ‘real-life’ community: the 1-Dimensional (1D) Diffuser and the 2-Dimensional (2D) Diffuser.
1D Diffusers are probably the most commonly used diffusers in studios and critical listening rooms and offer very predictable results. These diffusers scatter energy in a semi-circular pattern horizontally. Some examples of this type of diffuser are the following:
2D Diffusers are most commonly referred to as Skyline diffusers after RPG Inc’s model with that name. They scatter acoustic energy in a hemispherical pattern, both horizontally and vertically. Here are some examples of 2D diffusers:
According to Schroeder’s research, both types of diffusers work on the same basic principles. The width of the wells (or the blocks) determines the upper frequency limit. The depth of the wells (or the blocks) determines the low frequency limit. The math behind this is as follows:
The wavelength (λ) of a frequency is equal to the speed of sound (c) divided by the frequency (f).
The low frequency limit of effectiveness is defined by frequency whose wavelength is four times the depth of the deepest well (or tallest block).
The high frequency limit of effectiveness is defined by the frequency whose wavelength is 2 times with width of the wells (or blocks).
Let’s use RPG Inc’s QRD 734 as an example. The wells of the 734 are roughly 9 inches deep and roughly 4 inches wide providing the following bandwidth:
Or roughly from 375 Hz to 1700 Hz.
In independent tests performed on RPG’s QRD, the actual bandwidth of diffusion extended to approximately double what Schroeder’s theory predicted. We also found in our testing that the upper limit was significantly higher than the formulas predicted.
The depth of the wells is another area where the Acoustic Ramp™ has a major improvement over previous diffusive treatments. The wedge-shape of the diffuser means that the depth of the wells are continuously variable, which means that the frequency response is much wider than a typical 1-D diffuser.
Additionally, the Acoustic Ramp™ varies between a depth of 12 inches to a deep of less than a half inch. The deepest part of the diffuser is usually installed in the corner where the ceiling meets the wall, taking advantage of typically unused space in the room. The diffuser tapers as it descends the wall allowing racks, furniture and other equipment to be placed against the wall without trapping space.
The angles formed by the wedge shape of the wells allow the installer to direct reflections away from the sound source. In the most common installation, the wedge shape would direct reflections down toward the floor. In alternative installations, and array of Ramps could be used to direct reflections towards the side walls. In both of these cases, directing sound energy away from the sound sources helps to significantly reduce the effects of comb filtering.
The following slideshow is from ECHO Boston (http://echoecho.us), a studio in Brighton, MA, that was designed by XIX Acoustic’s Hendrik Gideonse. The photos were shot by Townsend Colon and Art Directed by Farida Amar. Makeup was done by Leila Rivers.
