In the last post, we discussed the fundamentals of sound diffusers, where they can be used, and their limitations. We broadly categorized them on the basis of their impact on the respective components of a soundwave - Phase/Amplitude. In this post, we will discuss the various types of diffusers based on their calculations/design languages.
General Knowledge - Dr. Manfred R. Schroeder is regarded as the father of phase-grating diffusion designs by many acousticians throughout the world. Along with many other achievements and discoveries in the field of acoustics, Schroeder came up with the concept of number-theoretic diffuser designs, such as MLS and QRD sequences, which marked the birth of phase grating diffusers.
1. Quadratic Residue Diffusers (QRD):
Construction Basics:
QRD diffusers consist of evenly spaced wells of varying depths, based on quadratic residues. Different well dimensions are designed to work on the different sets of frequencies in a specified range. The width determines the higher frequency limit, and the maximum depth determines the lower frequency limit.
Working Principles:
QRD sequence is originally a one-dimensional phase grating design. The uneven structure not only scatters the sound in multiple directions but this carefully designed device ensures even scattering throughout the design range of the spectrum spatially in a fan-like shape perpendicular to the diffuser front. Added to that, temporal diffusion is rather the more desirable trait of such diffusers, hence - phase grating.
Sound Interaction:
As sound waves interact with the QRD diffuser, the wells cause constructive and destructive interference, dispersing the waves temporally, and multiple internal reflections break down and redirect the sound in multiple directions. The resulting sound is less intense, spread out, and rather amorphous creating an illusion of a large diffused field. This illusion is one of the most sought-out reasons for these designs.
Unwanted Side-Effects:
Lobing effects, Flat reflection of lower frequencies, and unwanted absorption due to object edge diffraction are common side effects that cannot be negated, and hence wise placement is important. Errors in design may cause re-emission and flat reflections off the diffuser for the crossover range which cannot diffract through the diffuser as a whole but can through the individual wells. Based on choice of material and edge contours, unexpected absorption can be observed from frictional losses via edge diffraction effects. Uncalculated designs and placement can result in poor intelligibility, unexpected results and rather uneven sound field. Some DIYers attempt with some random online calculators, without actually knowing what they are going for, what kind of design should be suitable for their purpose, or if they need any diffuser at all. This results in a very obvious but poor sound field - uneven. Stereo image might suffer along with the tonal response, lack of intelligibility, poor low-end response, and lack of high frequencies can be a few from the long list!
Considerations for Implementation:
Accurate calculations and diffuser design are essential to achieve the desired diffusion pattern. Proper positioning and integration into the overall acoustic treatment plan are crucial for avoiding unwanted side effects.
Example:
A popular commercial product in this category is the RPG
Quadratic Residue Diffuser. It has been widely used in recording studios and concert halls to achieve controlled diffusion of sound, providing a balanced and immersive listening experience. Soon, we are coming up with our proprietary improvement design based on a variation of QRD - GrayFuse - SPAN. 2 broadband and precision-tuned diffusers are currently under testing and shall be available in our inventory soon. Contact us for more info! GrayFuse - MINI SPAN is already available in our catalog as a fractal hybrid QRD diffuser for live spaces. Contact us to know more!
2. Binary Amplitude Diffusers (BAD):
Construction Basics:
BAD diffusers are constructed with wells all of the same size, alternated by nothing in place of a well with theoretically 0 depth, hence a binary sequence - 1, 0. Modulating the amplitude of sound waves, unlike QRD and PRD, BAD diffusers focus on grating amplitude, not phase. RPG is considered to be the pioneer of these designs.
Working Principles:
When sound waves interact with the alternating-depth wells of a BAD diffuser, amplitude modulation causes scattering and diffusion in different directions. Or, so they say!
In our observations, BAD design majorly works on object edge diffraction and reflection principles. When paired on top of a velocity absorber, it essentially acts as a range limiter, preserving some of the higher frequencies.
Sound Interaction:
When sound strikes the surface, it has 3 probabilities: 1) It would strike the hard surface, and a limited range of high frequencies (Design based) will simply reflect off. The frequencies which have wavelengths larger than that of the well size, i.e. comparatively lower frequencies, would wrap around the well, and go through it like it's not even there. This is part 1 of Diffraction. 2) It would interact with the 0 well, and simply go in the absorber and get absorbed. 3) It interacts with the edge of the well, and as a part of the diffraction effect - gets attenuated with friction, and is altered temporally as well, hence if anything reflects back, it would be on a different temporal plane.
Hence resultant sound is an illusion of preserved high range.
Unwanted Side-Effects:
Improperly designed BAD diffusers may cause irregular scattering patterns. Apart from that, based on edge designs, there could be an unforeseen and unwanted high-frequency absorption, creating a psycho-acoustic illusion of a dead room, while it's not actually dead.
Considerations for Implementation:
Precise calculations and optimization are necessary to ensure consistent and balanced diffusion. Careful positioning and integration with other diffuser types can mitigate unwanted side effects. For beginners, we recommend tapering/sloping/contouring/rounding their well-edges to stay safe from over-absorption of high frequencies.
Example:
A renowned example of a commercially available Binary Amplitude Diffuser is the RPG BAD Arc. It has been effectively utilized in home theaters and recording studios to control sound reflections and achieve an immersive audio experience.
GrayVee Amplitude Grating Plates is a custom option for our buyers, which is more inclined towards the aesthetics of the room. We are not following the binary sequence here, but the other aspects of the diffusion plate make it very flexible to not produce any unwanted side-effects, yet provide a liveliness in the room with any demanded design by our clients.
3. Primitive Root Diffusers:
Construction Basics:
PRDs are constructed with wells of varying depths based on primitive roots and prime number sequences. Also widely known by the RPG trademark name of skyline diffusers, this design is so far one of the best and most reliant diffuser designs. Although there's a debate on credits, many like to believe PRD is a sequel derived from QRD.
Working Principles:
This 2-dimensional diffuser is based on block-shaped wells of different depths. Like QRD well depth, and well width respectively determine the lower and higher limits of the diffusion range in the tonal spectrum. The resultant sound is a spherically diffused sound field.
Sound Interaction:
When sound waves encounter the wells of a primitive root diffuser, based on the well width, the different ranges of higher frequencies either get reflected, diffracted and absorbed, or just get trapped inside only to reflect multiple times and get scattered in the room as a temporally and spatially diffused and ultimately attenuated and morphed sound field. The relatively lower frequencies on the other end, following diffraction principles keep enveloping the front shield of wells, ultimately getting trapped in, and redirected back in the room as a diffused sound.
Unwanted Side-Effects: Same as QRD
Considerations for Implementation:
Accurate predictions of diffuser diffusion characteristics require careful analysis of the room's dimensions and desired frequency response. Adjusting the diffuser design can optimize performance and mitigate unwanted side effects.
Example:
An illustrative example of a primitive root diffuser in action is the RPG Skyline, one of the pioneers in design.
Northward use their variation of PRDs - Omni Diffuser in their FTB Rooms. Although there are no published data on the company's designs, this particular statement is picked up from an informal statement made by the admin on a public forum. The authenticity of the data is not verified and might be wrong.
GrayFuse Quasi-Projection diffusers are our lineup of optimally tuned and sequenced variation of Primitive root sequence, often found in high-end music studios and critical listening rooms. Its diffraction properties help create a balanced and natural sound environment. We have limited designs that only are listed when fully developed with critical testing. Currently available in a 156 matrix, 2 ranges - 900-3250Hz, 450-3250 Hz. Meant for respectively larger rooms.
We are developing a MINI Projection diffuser model which would be aimed at small non-critical spaces. It's gonna be exciting, stay tuned!
4. Polycylindrical Diffusers:
Construction Basics:
Polycylindrical diffusers are constructed with curved surfaces and resemble cylinders. They operate on diffraction and reflection principles.
Working Principles:
When sound waves interact with the curved surface of a poly cylindrical diffuser, while the higher frequencies directly bounce off, it forms a confusing illusive surface for the mid/lower-midrange (Depending on design), and diffracts, and reflects certain frequencies in certain different directions.
Sound Interaction:
Sound waves striking the curved surface of a poly cylindrical diffuser experience diffraction, scattering them in multiple directions for diffusion.
Since the surface is curved, it appears to be small initially for the wave, through which it tries to diffract, only to reflect back after reaching the point where the surface is significantly larger than the wavelength. Hence, the sound reflects but not all frequencies in the same direction.
Unwanted Side-Effects:
Polycylindrical diffusers may effective in high-frequency diffusion, in certain room geometries, they can amplify specific frequencies, leading to an uneven sound response.
The design calculations are much more complex than it seems. Arc Radius, extensions, and placements are needed to be very careful. Redirection of frequencies depends largely on the size of the poly, but also on the angles and placements since the surface would be perceived wildly differently by a sound coming from one direction and the same sound from another! If miscalculated, scattering can be highly unpredictable messing up the entire stereo image.
The reflected sound loses a lot of energy, in the midrange especially. While this "lot" doesn't feel substantial in practice, if calculations are not precise to decimals ( a little exaggeration here ;)), some frequencies can get actually be affected by "a lot".
Considerations for Implementation:
To optimize the performance of polycylindrical diffusers, proper planning, construction, positioning, and orientation in the room are essential. Combining them with other diffuser types can help achieve a more balanced diffusion spectrum.
Example:
The Vicoustic Cinema Round series is a well-known example of polycylindrical diffusers used in auditoriums and concert halls. Its curved surface design ensures controlled sound diffusion while adding an aesthetic touch to the room.
Acoustical Surfaces Curved Diffuser claim to be a hybrid Design with phase coherent diffusion paired with a bass trap:
Source: Acoustical Surfaces Curved DIffuser 5. Fractal Diffusers:
Construction Basics:
Fractal diffusers are constructed based on fractal geometry principles and have a self-replicating pattern. They are typically made of materials with repeating geometric shapes.
Working Principles:
The intricate fractal pattern of diffusers scatters sound waves across a wide frequency range, achieving balanced diffusion.
Sound Interaction:
Sound waves interact with the complex fractal pattern, undergoing multiple scatterings, and diffusing in various directions.
Unwanted Side-Effects:
Inadequately designed fractal diffusers may introduce unwanted resonances or uneven diffusion. Over-diffusion can also result in a loss of sound energy and reduced clarity.
Considerations for Implementation:
Precise calculations and careful design considerations are vital for creating effective fractal diffusers. Integrating them strategically with other diffusion techniques can yield superior diffusion performance.
Example:
Real-life facilities such as renowned concert halls have incorporated the RPG FlutterFree Fractal Diffuser to achieve remarkable diffusion results. Its self-replicating pattern ensures a balanced soundscape for both performers and audience members.
6. Micro-Perforated Panel Diffusers (MPP):
Construction Basics:
MPP diffusers consist of panels with small perforations, allowing sound to pass through and scatter. The perforations are typically micro-sized, creating a large surface area for sound diffusion.
Working Principles:
As sound waves pass through the micro-perforations in the panel, they are scattered in different directions due to diffraction and interference effects. Some sound energy is also absorbed by the panel.
Sound Interaction:
Sound waves interact with the micro-perforations, undergoing diffraction and scattering in multiple directions, while some energy is absorbed by the panel.
Unwanted Side-Effects:
MPP diffusers may exhibit limited low-frequency reflection. Excessive high-frequency absorption can also lead to a loss of live energy and create an illusion of acoustically "dead" space.
Considerations for Implementation:
Precise calculations are necessary to strike the right balance between diffusion and absorption. Proper positioning and combination with other diffuser types can yield optimum diffusion results.
Example:
The Artnovion Micro-Perforated Panels have found applications in home theaters and recording studios to manage early reflections and achieve a balanced acoustic environment. Their high-quality micro-perforations ensure effective diffusion while preserving sound clarity.
Conclusion:
Understanding the various types of sound diffusers and their working principles is crucial for creating an acoustically pleasing environment in music, audio, and film spaces. Careful planning, precise calculations, and thoughtful integration of diffusers into the acoustic treatment plan are essential for achieving transparent sound delivery and immersive sound experiences. By utilizing the information provided in this comprehensive guide, professionals in the music, audio, and film sectors can enhance their workflow and deliver exceptional sound quality in their projects. Implementing diffusers strategically and selecting appropriate examples will result in optimum diffusion performance and an enriched sonic environment for all listening and recording endeavors.
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