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ARCHITECTURAL ACOUSTICS

Any space needs to be designed with acoustics in mind; geometry, material, climate, fittings, and furnishings all impact on the way sound interact with a space, so acoustics is a critical consideration to make in architecture.

 

Architectural acoustics can be taken into account to help enhance the quality of music in a recording studio or concert hall, suppress noise in your home or office, and can also enhancing speech intelligibility in a classroom, a railway station, restaurant etc.

Truth is, some sounds are desirable and need to be enhanced but some are undesirable and need to be suppressed. To be able to do this, you have to understand architectural acoustics. This is defined as the study and science of the generation, propagation, and transmission of sound in buildings. Although it may sound like a foreign term to some, it permeates every aspect of our lives.

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3D acoustic modeling software like EASE Focus 3 is critical to visualizing the acoustic properties and reactions of speakers arrays in a theater or home.

Sources of sound, directivity, and vibration

Sound sources in indoor spaces can be very decisively controlled and manipulated, so characteristics and directivity patterns generated by each source are important considerations to make. Sounds are approximated (to a good degree of accuracy) as a single source or a combination of several sources. There are two types of sound source directivity that are useful in architectural acoustics:

 

1. Monopole (simple sound source)

A monopole sound source is described as a pulsating sphere generating spherical acoustical waves harmoniously in any surrounding medium that’s both homogenous and isotopic. Many practical sound sources are thought to behave like a monopole.

2. Dipole

 

A dipole, on the other hand, is directional, and what almost all speakers implement The resulting radiation patterns will be from both sides of the loudspeaker cone. This is what is referred to as a dipole sound source. A dipole source projects in the plane vertical to its axis of oscillation.

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Contour plots and particle motion vectors of monopole (A and C) and dipole projectors (B and D), where pressure magnitude is indicated by darkness of the contour band.

Fundamentals of architectural acoustics
 

There are several characteristics of sound and sound control that are of importance to acoustics in design:

 

Reverberation time
 

Wallace Clement Sabine, the man who put architectural acoustics on a firm scientific base pointed out that the most important quantity in determining the acoustic suitability of a room is reverberation time. Reverberations are reflected sounds from a sound wave created in a room. The time required for the intensity of these reflections to drop by 60 dB after the sound source has seized is what is known as reverberation time.

Design and analysis of any room acoustics begin with the reverberation time equation. And while there’s no ideal reverberation time value, there’s a range of acceptable values for each application.

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The Sabine Formula is used to determine optimal reverberation times in concert halls.

Sound distribution
 

Sound distribution defines how sound is propagated in a room depending on what the room is to be used for. Usually, there are a number of ways sound can be distributed:

sound can be distributed directly from the source to the receiver, reflected back to the receiver at an angle of incidence from a surface, diffracted to the receiver by bending or flowing sound through an object/ opening, diffused to the receiver by way of scattering, or echoed to the receiver.

 

Sound distribution in architectural acoustics is the reason why you cannot play certain music in certain places or even record a song in certain studios. It is manipulated to optimize space utilization, and – poorly controlled – can easily ruin a how acoustically “pleasing” a space is.

Sound absorption
 

Sound absorption is the loss of sound energy as a result of the sound coming into contact with an absorbent material. Absorption is used in architectural applications to reduce reverberation time. Sound absorbents can be categorized from A to E, with A being the most absorbent and E being almost reflective. However, there’s another way they are also categorized:
 

  • Porous absorbents-fibrous material or open-celled form

  • Resonance absorbents-have a mechanical/ acoustic oscillation system.

  • Membrane absorbents

 

Sound absorption is especially important in halls, schools, recording studios, theatres, cinemas, and other spaces.
 

Sound insulation
 

Sound insulation is the reduction in sound across a partition. The conventional office to office sound insulation is at 45 dB Dw. This means that if a person is speaking in one room (typical speech is 65dB), you would receive 20 dB (barely audible) of sound if you were in the adjacent room. In short, sound insulation describes the level of sound lost.

Sound insulation is measured by two parameters: Dw is used for sound insulation between rooms on a site, and the levels are usually specified by the client or various building regulations; the other parameter is Rw, which is the lab tested sound insulation of an element you want to use in your partition, wall, or floor. The conversion between Dw and Rw is complex, resulting in a 5-10 dB reduction between Rw and Dw figures.

 

Architectural acoustics measurement techniques
 

Various instrument arrangements can be used to measure architectural acoustics. This list by Bruel and Kjaer isn’t conclusive, however, in compliance with ISO 3382 and ISO /R 354, there are ways you can gauge a number of acoustic elements.

Reverberation time
 

The technique for measuring reverberation time is dependent on circumstances but the procedure is identical. There are four arrangements you can make:

1. Pistol shot method

2. Filtered noise method

3. Paper loop method

4. Digital frequency analyzer/ calculator method

Sound distribution
 

Designers and constructors of auditoria buildings attach special significance to sound distribution measurement as sound should be distributed as evenly as possible. To measure this, there are two ways: designing model techniques and using them to measure sound distribution or measuring directly in an existent room by installing a sound source and a sound level meter.

Sound absorption
 

The sound absorption of a material is difficult to measure and pinpoint as it largely depends on how the material is placed. In architectural acoustics, the sound absorption coefficient provided by the manufacturer is regarded as an average. However, there are three methods you can use to measure sound absorption:
 

1. Reverberation room method

2. Tone burst method

3. Standing wave method

Sound insulation
 

The basic instruments required in sound insulation measurements are the sound source itself in the source room and a microphone in the receiving room. You would also need amplifiers and filters. Measurements can either be field or laboratory measurements but for practicality reasons, the average acoustician is always in the field which is affordable.

Sound insulation testing can either be:
 

1. Airborne sound insulation

2. Impact sound insulation
 

Careful thought on the acoustic properties of proposed buildings should be given at the design stage. Combined with measurements on material samples and scale models, you can save time, effort, and cost that would later be incurred, deeming the field vital to architecture as a practice.


 

Select research and further reading:


Tanner, C. K, & Langford, A. (2003). The Importance of Interior Design Elements as They Relate to Student Outcomes.


Luo, J., & Gea, H.C. (2003). Optimal Stiffener Design for Interior Sound Reduction Using a Topology Optimization Based Approach. J. Vib. Acoust 125(3), 267-273

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