The Acoustic Ramp™ Both Scattering and Reflects Energy
The Acoustic Ramp™ Both Scatters (green arrows) and Reflects (red arrows) Energy

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.

  1. The Ramp scatters acoustic energy horizontally while at the same time it reflects the energy over 4 different angles vertically.
  2. The depth of the wells of the Ramp are continuously variable leading to increased functional bandwidth.

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.

1-Dimensional Diffusers

RPG Inc's QRD 734
RPG Inc's QRD 734

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:

  1. RPG Inc’s QRD Diffusor (http://www.rpginc.com/products/qrd734/index.htm) This is the diffuser based most closely on Manfred Schroeder’s academic work on diffusers.
  2. RealTraps Diffusor (http://www.realtraps.com/diffusor.htm) This diffuser combines the QRD (Quadratic Residue Diffusion) math from Schroeder’s work with low frequency absorption.
  3. Primacoustic Razorblade Quadratic Diffuser (http://www.primacoustic.com/razorblade.htm)  The diffuser uses a sequence of depths that doesn’t appear to be strictly a quadratic residue sequence, in that it is aperiodic and the width of the zero-depth reflectors isn’t consistent. The diffuser does offer a tremendous amount of phase-complexity and probably works very well indeed.

2-Dimensional Diffusers

RPG's Skyline Diffusor
RPG Inc's Skyline Diffuser

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:

  1. RPG Inc’s Skyline Diffuser (http://www.rpginc.com/products/skyline/index.htm) This diffuser is based on a primitive root number sequence instead of the more commonly used quadratic residue number sequence.
  2. Art Diffusor Model W from Acoustics First (http://www.acousticsfirst.com/diffuser-art-diffusor-model-w.htm) This diffuser uses both the more linear 1-D architecture with various block heights of the Skyline diffuser in a unique way.
  3. Auralex Acoustics’ Wave Prism (http://www.auralex.com/sustain/waveprism.asp) The Wave Prism uses a grid of dividers to separate the blocks of different heights which, according to Schroeder, offers better diffusion properties.
Schroeder Diffuser
A standardized Schroeder diffuser based on the prime number = 7 quadratic residue sequence

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).

\lambda=\frac{c}{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).

\bot f= \frac{c}{(4)(depth)}

The high frequency limit of effectiveness is defined by the frequency whose wavelength is 2 times with width of the wells (or blocks).

\top f= \frac{c}{(2)(width)}

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:

\bot f is 375 Hz \approx \frac{1130}{3}

\top f is 1700 Hz \approx \frac{1130}{.67}

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.

Acoustic Ramp vs. QRD
A comparison of a standard QRD in the foreground with the Acoustic Ramp in the background

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.

Horizontal Array of Acoustic Ramps
Horizontal Array of Acoustic Ramps Reflects Diffused Energy to the Side Walls

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