The Rise of Intrinsic Acoustic Design

Zackery Belanger

Modern architectural acoustics has been advancing for more than a century. There are still gaps in our knowledge that manifest as uncertainty in the design process — even when clients are brave, design teams are formidable, and budgets are ample. The field mixes science and art, and unsurprisingly has a strong polarization: a body of scientific knowledge advancing one step at a time, and a varying stock of buildings touched by the experienced ears of practitioners. The two share often but political structures can inhibit important shifts in perspective. This essay describes an idea with the potential to transform the way we design rooms, and is intended to start a conversation that is long overdue. It is neither suited to scientific publication, nor derived from what my ears hear. I apologize for neither.


Imagine floating, surrounded by nothing. There is air to breathe but no ground or surfaces of any kind, no gravity. You clap your hands a single time. A perfect impulse of sound is created, a spherical wavefront that expands evenly in all directions, passing your ears and continuing away from you, never to return. This is the driest possible acoustic environment: anechoic, no reflections.

A large diaphanous sphere slowly appears around you. You’re at its center and clap again. It’s made of rigid material so finely perforated with holes that the wavefront from your clap passes through it unimpeded and never returns. It is also anechoic, acoustically transparent, the same as the nothingness described above.

The perforations close and the ghostly sphere becomes solid. It is perfectly smooth and infinitely rigid, and you’re still at its center. You clap again and the spherical wavefront expands and passes your ears as before, but this time it reaches the spherical boundary, every part striking simultaneously. It reflects back to the center where it passes through and expands again, only to reflect yet again. A cycle is initiated that would repeat ad infinitum — an expanding and contracting spherical wavefront — if you and the air around you didn’t dissipate it. The spherical enclosure has the minimum possible surface area and longest possible dissipation time.

It’s quiet again. The sphere that surrounds you starts to morph. Six regions of it flatten into planes: right, left, front, back, below, and above. Ninety degree corners form where they intersect. Call the planar surfaces walls, a floor, and a ceiling if you like; you’re in a rectangular room. You clap and again the wavefront encounters the surfaces, but this time the reflections are spread out it time and space, no longer focused back to the center. You hear the sound decay as its energy is mixed evenly throughout the room. This takes a notably long, but finite amount of time.

Now the planar surfaces begin to distort into peaks and valleys, on which small scale textures form. Layers of geometry pile on each other in a fractal-like manner across the surfaces of the room. The scale of the change is such that you can make it out visually from your vantage — the smallest details are a few millimeters and the largest a few meters. You clap again, the sound energy strikes the surfaces, and is dispersed in many directions. This increase in surface area has quickened the decay.

In a final and dramatic act, the enclosure surfaces again morph, this time violently at smaller scales. The total surface area of the enclosure spikes as bits of surface are pulled and stretched, tiny cavernous pores open and form a deep network of interconnected struts jutting at all angles. It has an airy porosity, like an open-cell foam. You clap again and the surfaces swallow the wavefront. Nothing is returned — an anechoic chamber, zero decay time, just as with no enclosure at all.


In the gedankenexperiment above the morphing enclosure moves from anechoic to infinitely long decay time, and continuously back to anechoic. The local surfaces change continuously from transparent to reflective to diffusive to absorptive without discrete jumps — all with increasing surface area.

Acoustic surfaces do not belong in separate categories. Reflectors, diffusers, and absorbers are named as such because they do these things particularly well, but they are only limiting glimpses of a continuum. Acoustic treatment is assembled of instances of intense performance plucked from the continuum of design possibility, and only needed to balance the vast regions of surface area neglected in the design process. A bright light is needed if you only have one. Acoustic properties are intrinsic in the geometry of rooms and objects, whether intended as acoustic or not, and we need to start designing like it.

Place a traditional acoustic reflector, diffuser, and absorber next to each other and the increase in surface area becomes apparent, as does the increasingly destructive effect each has on incident sound. Place the world’s best concert halls next to the world’s worst and you’ll notice a breadth and balance of curves and geometric scales in the former, a mix of simplicity and complexity. Even large flat reflective plaster surfaces exhibit, when inspected closely, variations imparted by the hand that provide a modicum of diffusion. Melamine foam is an acoustically absorbing material and melamine sheets are acoustic reflectors. Geometry is a continuum in acoustics; it cannot be a coincidence that a room of very small surface area has a long decay time and vice versa.

Put more specifically, it seems clear that local surfaces are integral to room geometry, and the further a room deviates from a sphere of equal volume, the more quickly it destroys the wavefront within.

Consideration of true geometry means understanding all scales that matter to sound, from large gestures of room shape down to the pores of brick, the filaments of textiles, and the minuscule struts of foam. We do not yet have a mastery over the measurement and fabrication of geometry at the smallest scales, but we soon will.

Acoustic performance, of course, is more complicated than geometry alone. Materials resonate, and we’re only beginning to understand the mechanisms by which the brain interprets sound. But when faced with immense complexity. an understanding of idealized conditions is critical. They inform important decisions and give rise to new approaches. Acousticians have poked and prodded like archaeologists at existing concert halls for a long time. There is much to be learned from the built condition, but we’ve placed sensors on baseballs to understand projectile motion without understanding the simple conditions familiar to the student of physics: the point mass, frictionless flight, and consistency of gravity with height. In search of greater certainty in the design of concert halls, we need a better foundation to successfully and consistently design the complex. Enclosure geometry is fundamental in room acoustics, and material properties modify enclosure geometry performance in complicated ways.

The geometric deviation of rooms could be quantified using a parameter like sphericity, which is a measure geologists use to assess the history of a pebble tossed by the forces of nature. The more round it is, the more violent and active its history. Sphericity varies from 0 to 1 and is calculated simply as the surface area of a sphere divided by the surface area of the enclosure in question, both being of the same volume. The sphericity of an empty spherical room is 1 and the sphericity of an anechoic chamber is near 0. Rooms with any shape, clutter, and combination of acoustic “treatment” would locate somewhere on the spectrum between the two. The exposed surface area of furniture, fixtures, objects, people, vents, reveals and crevices — anything that contributes to the shape of the enclosed volume — would be included.

Microphones and ears will always be needed to assess rooms, but they are probably insufficient without a deeper understanding of the connections between room geometry and sound. Many things would come into focus, including the role of ornament, furniture, and people in the way our spaces sound. Room shape, including the shape of furniture and objects within, can be designed for acoustic performance. Sphericity would be a gorgeous unifying acoustic parameter for a condition that is more connected than we currently admit.


The seeds of this essay were planted at Kirkegaard Associates from experience on new and renovated concert halls, including EMPAC at Rensselaer in Troy, NY and Royal Festival Hall in London, UK. The research began in Chicago with a grant from the Graham Foundation for Advanced Studies in the Fine Arts, in collaboration with architect Julie Flohr. An M.S. degree at Rensselaer shed light and underscored the resistance of the field to new paths of inquiry. The setbacks were countered with a Researcher-in-Residence position at EMPAC where, with the help of Johannes Goebel, the concepts matured. Rare is the place that understands the importance of pace and failure in innovation. Recent developments were carried out at 53 3rd Street in Troy at the late Edward Coolidge’s tragically short-lived incubator.

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