aect350 acoustics lecture 5 new

8/13/2019 AECT350 Acoustics Lecture 5 NEW

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Acoustics Lecture 5 Page 1 of 13

Lecture 5 Acoust ical Materials & Methods (cont.)

All materials have acoustical properties, some obviously much better than others. Mostmaterials used for building have inherent acoustical properties that are used for otherpurposes (wall & roof components, glazing, insulation, etc.), while other materials aremade specifically for acoustical purposes.

Three Sound Boundary Events:

1. Reflection Reflection can be thought of as sound bouncing off a material.The shape and the surface texture greatly affect the sound reflection. In apolished smooth, dense surface, sound strikes the surface with its angle ofincidence, i and reflects off the surface by an equal angle of reflection, r.This is referred to as pure specular reflection.

If the surface is rough and dense, the incoming sound bounces off the irregularsurface with a multitude of specular and diffuse reflections as shown below:

i r

Smooth, densesurface

i

Rough, densesurface

Specular Reflection

Diffuse Reflection

Transmitted

Sound

Incident Sound

ReflectedSound

Refracted andabsorbed sound

Material X-Sect.


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Sound can be focused into a point by shaping the surface into a concave form asshown below, or dispersed if striking a convex surface:

2. Absorpt ion Defined as the materials ability to dissipate sound energy intoheat or mechanical energy (i.e., vibration). Generally, porous materials are moreabsorptive than non-porous materials (i.e., carpeting versus sheet metal). Thesound absorption coefficient (alpha) is used to describe the materials abilityto absorb sound energy. Absorption can be thought of as friction that a soundwave must overcome in order to pass through the material. Big = absorptive.

Refer to lecture 4 notes for various values of sound absorption coefficient :

0.02 0.06 0.14 0.37 0.60 0.65


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3. Transmission The amount of sound that passes through a material that hasnot been reflected or absorbed. The opposite of attenuation. Usually the amountof sound energy transmitted through an object is small, comprising only about1% of the total energy (reflection & absorption account for the remainder).

Transmitted sound may be refracted, as shown below:

Transmission Loss (TL) is defined as the sound difference due to the physicalbarrier between two spaces, and is NOT attributed to other factors. Therelationship between noise reduction (NR) and TL is the following:

NR = TL 10 log S + 10 log A2

Where: NR = noise reduction, dBS = surface area of barrier, ft2

A2= Amount of acoustical absorption in receivingroom, ft2

If S = A2, then NR = TL, however, S and A2are rarely equal,but are usually close.

Refractedsound zone

Source

Sound can actually reflectoff of cantileveredbalconies into occupancy

Floor


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As mentioned earlier, transmission loss (TL) noise reduction (NR) andattenuation are all dependent upon the sounds frequency. The SoundTransmission Class (STC) is an average of TL from different frequencies overthe frequency ranges of 125, 250, 500, 1000, 2000 and 4000 Hz.

For illustrative purposes, all 3 materials have an STC = 40.Material A = Lead sheet

Material B = concrete blockMaterial C = wood stud wall w/ gypsum drywall

As can be seen, the materials have different levels transmission loss at differentfrequencies. In general, heavy materials such as concrete and masonry performbetter at lower frequencies than lighter weight materials.

0

10

20

30

40

50

60

63 125 250 500 1000 2000 4000

Frequency Hz

Material C

Material A

Material B

TL (dB)


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Attenuation:

Attenuation is the opposite of transmission. It can be thought of as the ability toreduce sound energy from one room to another, as shown below:

Table of Building Component Assemblies:

Source Room Receiving Room

Principlesound barrier

Sound Level

Sound LevelAttenuation


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Acoust ical Shielding:

Providing a partial-height barrier to diminish some sound energy between twospaces is considered acoustical shielding. The effectiveness of shielding isalmost totally dependent upon the materials absorptive qualities. An excellentexample is the use of movable partitions within an office. Their use is not veryeffective most of the sound energy makes its way over the barrier or isflanked. As a general rule, acoustical shielding must be within line-of-sightbetween the sound source and receiver in order to be at all effective.

Ineffective

Sound

Acoustical sh ielding iseffective for line-of-sight


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Special Acoustical Materials:

1. Acoust ical Metal Deck Similar to ordinary corrugated metal deck, exceptthere is additional fibrous acoustical material placed within the corrugations,as shown below. Acoustical metal deck is VERY effectivefor reducing noiseand reverberation is roof construction assemblies.

Coefficient ofabsorption


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2. Acoust ical Ti le Widely used as a material usually made of mineral orcellular fibers. Sold in panels ranging from 12 x 12 up to 24 x 48.

Acousticalceiling tiles

Acousticalwall tiles

Acousticalceiling tiles


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3. Acoust ical Soundproof Drywall Made by proprietary companies, (suchas Serious Materials), these panels are made of conventional gypsum andadditional sound-absorbing materials. Fairly expensive, approx. $40.00 per4x8 sheet.


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4. Acoust ical Foam Can either be open-cell (air can be blown into material

aect350 acoustics lecture 5 new

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