PDX_28_Acoustics Part 3 Flashcards
A unit of sound of sorption, 1 ft.² of 100% absorbing material is one unit
Sabin
The ratio of the sound intensity absorbed by the material to the total intensity reaching the material. Varies with the frequency of sound/material
Coefficient of absorption (A)
The persistence of sound in an enclosed space after the source of stopped. Continuous reflection.
Reverberation
The time it takes the sound level to decrease 60 dB after the source is stopped producing sound
Reverberation time
The return of soundwaves from a surface
Reflection
Soundwave that is been reflected with sufficient magnitude in delay so that its heard as a sound as distinct from that transmitted directly
Echo
Random distribution of sound from a surface that occurs when the surface dimension is equal to the wavelength of the sound striking it
Diffusion
The bending of a soundwave around an object or through an opening
Diffraction
Tendency of a system to vibrate at increasing amplitude at certain frequencies
Resonance
Result of multiple reflections and enclosed space in a short period of time
Persistence
Reduction of sound energy levels through the use of material that absorbs reverberant sound and block airborne sound because of their high sound transmission loss factor.
Example: acoustical panels, foam curtains, fibrous material.
Sound insulation
Minimizing sounds transmission from one area of the building to another through building design.
Example: location mechanical rooms, partition detailing, using dampers/pads/insulating materials
Sound isolation
Process of dissipating sound energy by converting it to heat
Sound absorption
Indicates a soundwave on the opposite side of the wall
Wall vibration
Room who’s boundaries absorb all of the sound
Free Field Road (Anechoic Room)
Phenomenon which occurs when sounds in a room that are within a narrow band of frequencies tend to sound louder and sounds of other frequencies
Room resonance
Surface area of room
x Absorptivity of surfaces
Sabins
(Total surface area of Barrier or component Between rooms, S) x (Coefficient of absorption, a)
Acoustical Absorption A (sabins)
(Total sabins, sound observing units, A)
/ (Total service area in the room, S)
Noise reduction coefficient
0.05 (the speed of sound) x
[ (Room volume, V)
/ (total acoustical absorption at that frequency, ∑A) ]
Reverberation time (Tr)
*speed of sound depends on material
Low-frequency control usually requires allowance for:
Thicker partitions or more space to apply detailing that absorbs low-frequency sounds.
A sized furred panel @ certain distance from wall to absorb low-frequency energy + reflecting mid & high frequency energy:
Panel Resonator
A large airspace filled with absorptive material and a sized small opening to absorb specific low frequency range:
Cavity resonators (Helmholtz)
Ex. A CMU block, with narrow slit opening into the cavity of block
Tipping point of reflective and absorptive in regards to ‘coefficient of absorbency’ occurs @ value:
0.2
Less is reflective.
More is absorbing.
Changing room reverberation characteristics WITHIN the room effects noise level in adjoining spaces:
Minimally effect on adjoining spaces
Absorptive materials are usually better at reducing the transmission of:
High frequencies better than low frequencies
The effectiveness of absorptive materials dependents on:
The type of material, the method of installation, the area, rather than the thickness
The absorptivity per sqft of given surface varies 0 to 1:
- 0, all sound reflected
1. 0 Sabin, all sound absorbed
Offices might have reverberation times between
0.3–0.6 seconds
Very reverberant spaces often called:
Spaces with short reverberation times called:
‘Live’ = long reverberation
‘Dead’ = short reverberation
Concert halls might have recommended reverberation times between:
1.6–2.1 seconds
Two measures intended only for open office situations where speech is the primary sound concern:
Articulation class (AC) Articulation index (AI)
Gives a rating of system component performance does not account for masking sound
Articulation class (AC)
Measure the performance of all the elements of a particular configuration working together:
articulation index (AI)
Ceiling absorption, wall partitions, space dividers, background masking systems, furniture, HVAC systems, light fixtures
Critics the intelligibility of speech for a group of talkers and listeners and gives a result in a single number rating:
Articulation index (Ai)
Ranges from 0.00, complete privacy
To 1.00, no privacy
Ai 0.30 =
No privacy
Each doubling of the amount of absorption in a room results in:
Noise reduction of only 3 dB
Reduced reverberation time by 50%
The average absorption coefficient for a room should be at least:
0.2
Typically above 0.5 is not desirable or economically justified.
The amount of absorption of a porous type of sound absorber (EX: fiberglass, mineral wool) is dependent on:
Material thickness, density, porosity and orientation of fibers
Ceiling treatments are best for ___,
Wa treatments are best for____.
Ceiling treatments @ large rooms
Wall treatments @ small rooms
To reduce mechanical noise from vibrating device in direct contact with structure:
– Resilient mountings & flexible bellows
– Flex connections between ducts/pipes/equipment
– Maximize distance between diffusers in adjacent spaces
–locate noisy equipment away from quiet spaces
To limit noise at plumbing systems:
– Use expansion valves & FlexLoop connections to reduce rattle
– Seal pipe penetrations through walls & floors w/ flexible packing
Limit noise with building materials by:
– Use fibrous materials – Deep air space w/in wall cavity – Stagger studs – Seal openings in wall – Avoid back/back wall outlets – Sound attenuation above headers
