notes | acoustics.01

These are notes from Computational Acoustics in Architecture and Architectural Acoustics, they have been pulled due to their relevance in either generating form, materiality, or layout of space(reflection/absorption). They outline some specific limits that we might be able to work within; audience within 50′-0″ of performer, shell no larger than 30′-” from front of stage, airspace below performance stage, reflection above performance stage and ceiling, walls, and back wall of audience area. Seating should also be reflective. Absorption will mainly be around the stage, floor, back wall, and upper walls of stage. Absorptive materials will also need to be placed behind any reflective material if an air space exists between the material and structure of the/a building. Since we’re creating an acoustically enhanced condition for an event, a possible siting strategy for the large event would benefit from an urban condition with high surrounding walls for sound waves to bounce off of. Foam reflectors could be sprayed onto the side of the buildings to redirect sound back onto the audience or out into the city. The reflectors should never be concave, but flat or convexed shape, so that leaves us geometrically speaking with either parabolic forms giving us convex shapes, or low polygon geometry to give us flat reflectors. The main principle behind absorption is the transfer from sound energy to heat energy. Areas where we would like absorption of sound waves, double walls can be created, and to create resonance chambers, the double will can split apart to form an air gap within a wall. A further iteration of this process will allowed the interior wall to be milled out, leaving portions of the interior wall with very thin layers of foam. As sound waves hit these areas the vibrations in the foam turns into heat energy. In an urban condition where noise is not desired, layers of air and foam can build up to create ultra dense areas of absorption.

Also thinking about event spaces, to begin the process of bringing context into the project, an acoustical analysis of the site is performed to identify sound levels(dB) and direction/intensity into and from the site. These areas will receive a build up of foam structure to reflect or absorb these sounds before they enter into the performance area. Secondary points will come from point source of speaker and the space of intended audience(seating). Surrounding buildings and city infrastructures(such as roads) will create a bounding box around the performance area for which the growth of foam can occur.

Im working on a couple diagrams of acoustic areas of performance area, a diagram of the process mentioned above, and some generic tests of geometry.

Absorption:
Each time a sound ray hits an absorbing wall its energy is reduced by a factor (1-x), where x = the absorption coefficient (see absorption coefficient materials table-polyurethane foam).
Helmholtz resonance is the phenomenon of air resonance (the tendency of a system to oscillate at larger amplitude at some frequencies than at others) in a cavity wideband absorption is achieved by glossy materials, such as mineral wool, either fully exposed of behind perforated panels. Narrow frequency ranges are best absorbed by acoustic cavities Helmholtz resonators.
Absorption – physical phenomena resulting from the dissipation of sound energy into heat energy.

Sounds absorption coefficient: the ability of a material to absorb sound energy depends on the frequency and the angle of incidence of sound waves. (see material absorption coefficient table) The sound absorption of a surface area S obtained by multiplying its sound absorption coefficient by its area. The total sound absorption of all surfaces of the enclosure can be calculated by the equation…()

Sound Abosorbing Materials:

Porous Sound Absorbers: Their structure is granular or fibrous and the skeleton can be either rigid or elastic. The absorption of these materials is greatly influenced by the thickness of the material and the distance between it and the rigid surface behind.

Resonant Sound Absorbers: The simple kind can, at the same time, subdivide into the helmholtz type, membrane or panels type. The associated resonators can be in series and in parallel (usually with circular or grooved perforations).
Mixed Sound Absorbers: A combination of both Porous and Resonant.

The presence of an air space between the porous material and a rigid impermeable wall increase the sound absorption at low frequencies.

Single Resonator – Good absorbers at low frequencies. Use for medium and low frequencies.
Perforated Panels, Cylindrical or Grooved – The greater the thickness is (of the air space), the more noticeable is the increase in sound absorption at low frequencies.

Speech – strong overhead reflection preferred.
Flat or convex shaped, never concave or coffered.
Shell Design – Should not be more than 30-40 feet deep from front edge of stage to face of wall.

Air space below performance space is preferred: air space below performance space – 24”

Absorption surfaces – ceiling above and behind reflective surfaces, upper portion of side or rear wall, and on either side of the stage.

Walls near the performer should be angled or splayed to enhance projection and prevent “flutter echoes” at the stage.

Reflections:

Sound Diffusion
Ceiling reflecting phase grating based on…
Schroeder Diffusers/diffusion – the efficacy by which sound energy is spread evenly in a given environment. a perfectly diffusive sound space is on that has certain acoustic properties which are the same any where in the space.
Alternative or complimentary to absorption because they do not remove sound energy. The goal of absorption is the turn sound energy into heat energy. Quadratic Residues form a periodic sequence, p=17…1,4,9,16,8,2,5,13.

Number theoretic reflection phase based grating on successive quadratic residues modulo. The prime #17, the pattern repeats with a period of length of 17 and scatters frequencies over a range of 1:16, corresponding to four musical octaves.
In a closed area, direct sound can reach any listener in a time of interval from 20-200ms, depending on the distance between the sound source and the listener.
early reflection: 80-100ms
Ground noise; reflections; <50ms
25dB; speech recognition

In order to improve intelligibility, the early reflections can be enhanced by placing reflective materials on the surfaces that produce them and by providing high sound absorption elements for the surfaces that generate delayed reflections.

Impulsive response areas
Depends on two factors
First, geometry – numbers of reflections, temporal distribution absorption coefficient of reflectors – surfaces.

The “send end” of the room (i.e. the stage) should be acoustically hard.

Walls and ceilings where the audience sits should be hard so they can reflect sound, unless absorptive treatment is needed to eliminate problematic reflections, or focusing or to reduce reverberation time(RT) for a particular programmatic need.

Spaciousness – Side and rear wall reflection, enhanced by protusions and angled sidewalls.

Speech: reflection: 50-80ms after direct sound.
sound travels 1120ft/s in SI units, 333m/s

Prime seating locations should do not exceed 50’-0" from source of sound.

Outdoor sound drops off at 6dB each time the distance from a source is doubled(inverse square law).
Outdoors: sound ceases when the source stops.

Reverberation time is directly proportional to the volume of space and inversely to the units of absorption (Sabins) in ….: RT = kV/a k=.049(ft3)


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