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Posted in Absorption, Art Galleries, Articles, Auditorium, Broadcast Facilities, Diffusion, Home Entertainment, Home Theater, HOW TO, Industrial Facilities, Media Room, Multipurpose Rooms, Music Rehearsal Spaces, Offices, Product Applications, Recording Facilities, Studio Control Room, Teleconferencing, Theater on April 29, 2022
For the May 2022 edition of “The Construction Specifier,” Acoustics First was asked to illustrate the use of absorption and diffusion in creating optimal acoustic spaces. The article is a great reference for understanding the types of acoustic absorbers and diffusers, as well as some use scenarios like offices, critical listening spaces, and larger communal spaces.
Note: This version has been edited and the advertisements are removed. The full published version of the May 2022 digital edition can be found on The Construction Specifier’s website here.
The philosophical thought experiment of “does something need to be perceived to exist,” has been around since the beginning of time. This allows for human extrapolation into concepts such as quantum mechanics (Schrödinger’s cat) and advanced Artificial Intelligence principals. Albert Einstein was effectively “unfriended” for asking the question of his colleague, Abraham Pais,
“Do you really believe that the moon only exists if you look at it?”Albert Einstein to Abraham Pais
Pais prescribing to “the majority view of the quantum mechanics community then (and arguably to this day) that existence in the absence of an observer is at best a conjecture, a conclusion that can neither be proven nor disproven.”
But the question still exists. “If a tree falls in a forest and there is no one to hear it, does it make a sound?”
The eminently interesting, Dr. Irving Lirpa asked the burning question…
“If observation is proof, can we calculate the Amplification Coefficient of Human Perception upon something that is ‘likely’ into ‘truth’ versus the amplification of ‘hogwash’ – which will always remain ‘hogwash?’ Because if something is able to be perceived, it must exist in some degree, as amplifying the perception of the non-existent is akin to multiplying by zero.”
Dr. I. Lirpa further posited that: “while the observation of sound proves its existence, the lack of observation does not disprove it… it merely has not been amplified by the scrutiny of human perception.”
He further affirmed that the tree would indeed create sound, but with a much lower intensity due to the Human Perception Amplification Coefficient… henceforth, there would still be sound because it exists – but it would fail to be amplified by human perception.
This seminal work calculated a Maximum Reverb Time of only 0.04 @ 1000 Hz “Without Audience” in a full-leafed, deciduous biome common to the vernal mid-temperate zone. Further calculations found that the Human Perception Amplification Coefficient is equivalent to the reverb time being amplified by 42 TIMES per frequency band upon being observed – which coincides exactly with the calculation made by the supercomputer DEEP THOUGHT on the “Ultimate Question.”
Coincidence? We think not.
“If a tree was to fall in the forest and there was no one there to hear it, would there be any sound?”
“Yes, but there would be 42 Times more sound if there was someone there to hear it.”
Glass is a universal building material that is attractive to architects and clients, while posing a variety of challenges to acousticians.
Due to its transparent nature, glass creates an open and pleasing atmosphere. Curtain walls, skylights and windows allow for a view both outward and inward; connecting occupants to the building’s natural or urban setting. The use of natural light can lower electricity bills, brighten the rooms of a building, boosting the mood of the occupants. Glass is also a renewable building material, with 30% of new glass comprised of recycled materials. For all these reasons and more, glass will continue to play a major role in architecture in the future.
However, glass has a number of acoustical properties that can contribute to a poor occupant experience. To illustrate this, let’s take a closer look at what happens when sound interacts with glass.
When sound encounters a window, the glass converts some energy into thermal and kinetic energy (resonate vibrations), allows some sound to pass through, and reflects the rest back.
Glass only “absorbs” sound near its resonant frequency (and subsequent harmonics). The resonant frequency of glass is dependent on many factors, including density, thickness and panel size. As is the case with many “hard” building materials, the absorbed sound accounts for only a small fraction of sound energy’s interaction; most sound is either reflected or transmitted through the glass. Sound reflection and sound transmission are two separate acoustic issues with separate solutions.
Sound Reflection – Reflected acoustic energy from an internal sound source can cause a number of issues for occupants. Large, uninterrupted spans of hard materials like glass and gypsum cause specular reflections (echoes) and contribute to excessive reverberation and noise levels. These conditions can contribute to a poor acoustic environment in which speech is difficult to understand and music clarity suffers.
Specular reflections are compounded when there are other hard surfaces in the room. Flutter echo, heard as “ringing”, happens when sound bounces back-and-forth between parallel reflective surfaces (between walls or floor-to-ceiling). Flutter echoes greatly degrade speech intelligibility and music definition. This is a big problem in studios, offices, conference rooms and theater/media rooms. If there is an abundance of reflective surfaces, background “noise” from latent energy will cover up or distort the direct sound.
Typically, these issues are corrected with sound absorbing materials. However, we cannot simply “resurface” the glass with sound absorption, like we would with concrete or gypsum, without impacting transparency. Until someone invents invisible acoustic foam or fiberglass, sound reflections off glass will continue to be a challenge that needs accounted for.
Sound absorptive materials like thick curtains or acoustic shades provide adequate sound absorption and coverage flexibility. Other creative solutions include “stand alone” furnishings like tall, leafy plants or translucent perforated plastic sheets mounted over top the window. Essentially, any irregular surface you can introduce in front of the glass will help diffuse sound and break up harmful wall-to-wall reflections.
Sound Transmission – More than 90% of all exterior noise comes in through doors and windows. This can be partially attributed to poor weather stripping. “Leaky” windows will not only cause uncomfortable drafts, but allow sound to more easily work its way into our homes and businesses. Sound is a little like water; it will “pour out” through any gaps in the building assembly. Improving sound-loss across glass often starts with replacing the weather stripping and properly sealing any joints with non-hardening acoustic caulk.
Air-tight, limp, massive materials are the best at blocking sound. Glass is rigid, and its heft is limited by transparency requirements that keep it thin. Glass transmits a lot of sound energy, particularly at low frequencies. Laminated glass and insulated glazing assemblies both reduce sound transmission through glass by reducing resonance and adding air-space.
Including an acoustic consultant early in the design process will allow architects and owners to make well-informed decisions. An acoustical consultant will best identify potential pitfalls of using glass and recommend glazing systems and construction techniques to minimize future headaches. This measured approach will result in more beautiful looking (and sounding) spaces!
Sometimes you just don’t have the wall space for acoustic treatment. When this is the case, you will often see treatments move to the ceiling… but what do you do if you have lower ceilings, or many ceiling fans and fixtures?
This ceiling was sloped toward the massive windows on the outside wall, and it had lights and fans running right up the center. Complicating things further, the opposite wall had sconce lighting, doorways, HVAC, and even more windows. Finally, the floor was not carpeted to facilitate cleaning – as is the norm in many dining spaces.
Direct-mount acoustic panels are a great solution in these scenarios. Here we see an array of 4’x4’x2″ Sonora® panels attached to the ceiling in rows running down the length of the space. While our Tone Tiles® are often selected for their ability to blend in aesthetically, this particular installation proves that Sonora® panels wrapped in fabric are also a solid choice.
Cathedrals, mosques, synagogues and temples are often decorated with an abundance of architectural details (deep coffers, arches, columns, sculptures, intricate engravings etc.). These features are not only beautiful to look at, but also serve the vital acoustic purpose of sound diffusion. Large, uninterrupted spans of hard, flat surfaces reflect sound in a singular, specular wave, which creates discrete echoes and comb filtering (In acoustics, comb filtering is when a delayed reflection interferes with, and distorts the original sound wave). These conditions can contribute to an acoustically uncomfortable environment, in which speech is difficult to understand, and music can be hard to perform and enjoy. Irregular surfaces, on the other hand, scatter these reflections, minimizing comb filtering and distracting echoes. In a “diffused” worship environment, speech is intelligible, music is clear and warm, and there is a sense of envelopment which greatly enhances the congregants’ worship experience.
Absorptive treatment can also be used to control echoes and harmful reflections. Instead of redistributing reflections, these “fluffy” materials (drapery, padded pews, acoustic panels etc.) reduce the overall sound energy from the reflections in the room. However, using to too much sound absorption in a room can often make a space sound ‘dry’ or ‘dead’. Determining whether your space needs absorption, diffusion, or a combination of both is dependent upon the acoustic properties of the space, as well as the type of worship service being conducted.
Acoustic Properties of the Worship Space: Reverberation, a principle acoustic factor, is the sound energy that remains in a listening environment as a result of lingering reflections. The dimensions, construction materials, and furnishings of a given worship space determine its reverberation time (RT or RT60). Large halls with reflective materials (glass, wood, concrete) have longer reverb times, while small rooms with absorptive materials (drop acoustic ceiling, carpet, curtains etc.) will have shorter reverb times. Incorporating sound absorptive materials, Such as fabric wrapped acoustic wall panels, is often the best way to reduce the overall reverberation in a room to a suitable level. However, the target reverb time also depends on the nature of the worship service being conducted in a particular space.
Type of Worship Service: Ideal reverb times for worship environments vary widely. Non-musical, spoken-word worship requires a very short reverb time (.5-.8s range), ensuring that speech is intelligible. At the other end of the spectrum, cathedrals can tolerate an extremely long reverb time (2s and above) due to the traditional nature of their liturgy. Choir, organ and plainchant worship will actually benefit from longer reverb times that create a sense of ambience and spaciousness by sustaining musical notes. These spaces will often lack a sound system, and instead utilize the hard surfaces to propagate sound throughout the room.
Traditional worship may be enhanced by long reverb times, but contemporary worship requires a significantly shorter reverb time. In these environments, drums, guitars, bass and other amplified instruments are critical to the high intensity worship experience, but have far different acoustical needs compared to the choir and organ in a more traditional service. Contemporary “high impact” churches require a reverb time in the .8-1.3s range to ensure that the music won’t become too “muddy” and indistinct. Contemporary churches must also be more cognizant of late specular reflections (slap echoes) which can inhibit the timing of musicians and contribute to poor music clarity.
Let’s take a look a few common scenarios when it comes to treating traditional and contemporary worship spaces.
Scenario 1: Conversion from Traditional to Contemporary worship
A growing contemporary church moves into a larger, traditional sanctuary and is confronted by a raucous acoustic environment during their first rehearsal… This “live” space was perfect for traditional music, but is not conducive to a “high impact” contemporary worship service. To reduce the excessive reverberation and distracting echoes, sound absorption should be added to the rear wall (opposite stage) and side walls. Also, if using on-stage monitors, the stage walls should be treated to manage stage volume. Spot diffusive treatment that provides low-frequency absorption would also be beneficial. A good choice for this would be traditional ‘barrel’ diffusers. These are one of the oldest tried and true solutions for controlling bass issues in a performance space. Also, since these units function as both absorbers and diffusers, you get the benefits of both.
Scenario 2: Mix of Traditional and Contemporary worship services.
A worship facility decides to offer a contemporary service in addition to their traditional services… As more absorption is introduced to cut down on distracting reflections, we want to retain the envelopment and spaciousness which benefits congregational singing and traditional worship music. Sound diffusive treatment would be a great way to control echoes and specular reflections, while keeping the energy in the space. A mix of absorption and diffusion is usually best. Multipurpose spaces can also benefit from variable acoustic treatment which allows the room to “adapt” to each service, but that is a subject for another article.
Scenario 3: Poor Music Clarity in Traditional Worship space
A traditional worship space renovates their facility by adding thicker carpet, padded pews and a drop acoustic ceiling… All of a sudden, their once lively space feels “flat” and dull. Acoustic instruments are more anemic, less distinguishable and choirs have a difficult time blending and tuning. To “liven up” the space, replace sound absorptive materials with diffusers. For example, replace 1/3rd of the acoustic ceiling tiles with a combination of gypsum tiles and lay-in diffusers. These days, there are wide variety mid-range quadratic sound diffusers available for drop tile ceiling grids, as well as the more traditional barrel and pyramidal diffusers.
Sound diffusion can often seem a little mysterious compared to sound absorption. This is at least partly because sound diffusion is a more complex and multi-dimensional phenomenon compared to the more easily quantifiable sound absorption. However, sound diffusion is often times the missing piece of an acoustic puzzle: its benefits can help a bad room to sound good, or a good room to sound great!