174th Meeting of the Acoustical Society of America New Orleans, Louisiana, 4–8 December 2017
Session 1aSA: Standards in Structural Acoustics and Vibration
Methods for calculating flanking noise transmission VENEKLASEN ASSOCIATES 1711 Sixteenth Street Santa Monica, CA 90404 www.veneklasen.com
John J. LoVerde Principal
[email protected]
Wayland Dong Associate Principal
[email protected]
Introduction • Focus on structure-borne flanking (not leaks, holes, crosstalk through ductwork) • Flanking noise transmission is often important. • It is often considered to be difficult or impossible to calculate. • So instead we use some rules of thumb…
Some “Rules of Thumb” • With continuous floor slab (e.g. high-rise condominiums), field test of demising walls will not exceed about NIC 60, regardless of wall. – – – –
STC 50 wall = NIC 48 STC 60 wall = NIC 57 STC 65 wall = NIC 60 STC 70 wall = NIC 60
• In other words, the flanking path through the floor slab has an effective rating of NIC 60.
Wall Intersections • Avoid bridging of demising walls with perpendicular partitions. • Sometimes cannot be completely avoided (shear panels, structural posts, etc.) • How much worse is it?
There is a Standard: EN ISO 12354
Canadian Building Code • Canadian Building Code requires/allows calculation of ASTC using these methods. • NRC-CNRC Guide to Calculating Airborne Sound Transmission in Buildings • NRC-CNRC Wood-framed Flanking Studies • NRC-CNRC SoundPaths website http://soundpaths.nrc-cnrc.gc.ca/
Canadian Building Code • Canadian Building Code requires/allows calculation of ASTC using these methods. • NRC-CNRC Guide to Calculating Airborne Sound Transmission in Buildings • NRC-CNRC Wood-framed Flanking Studies • NRC-CNRC SoundPaths website http://soundpaths.nrc-cnrc.gc.ca/
Motivation • A project required quantitative estimates of airborne flanking transmission. • EN ISO 12354 provides a framework for performing the calculations. – Review basic concepts.
• Project involved large concrete assemblies – One of the details of the method involves the difference between lab and field performance. – The effect of dimension on in-situ sound isolation
Flanking Paths Definitions
• • • • •
Capital letter = source side. Lowercase letter = receive side. “F” and “f” for flanking. “D” and “d” for direct. Path “ij” generically, sum over all paths.
Flanking Paths • There is an Fd, Df, and Ff path at each junction. • Often only the Ff path is significant. • Typical room has 4 junctions, one at each edge of the separating specimen.
But How To Calculate? ISO 12354 Procedure • In building acoustics, the sound field are assumed to be diffuse. • Similarly, we assume that the structural vibration field is diffuse. • We use the transmission loss (TL) measurement to provide the necessary information on the assembly properties. • Direct airborne transmission: – Source sound pressure excites assembly – Sets up a diffuse vibration field in assembly – Vibrating surface radiates into receiving room.
But How To Calculate? ISO 12354 Procedure • Flanking: – Source sound pressure excites assembly – Sets up a diffuse vibration field in assembly – Vibration transmits through the junction from the source assembly into the receive assembly – Sets up a diffuse vibration field in receive assembly – Vibrating surface radiates into receiving room.
Calculation Method i =Source surface
j =Receive surface
Excitation • Transmission = excitation and radiation. • If you remove the radiation you have the excited vibration. • 𝜏= 2
𝑊𝑟𝑒𝑐 𝑊𝑠𝑟𝑐
• 𝑣 =
=
𝜌0 𝑐0 𝑣 2 𝜎𝑆
𝑝2 𝜏 4𝜌02 𝑐02 𝜎
𝑝2 𝑆 4𝜌0 𝑐0
Calculation Method Solving for vibration gives the excitation on the source surface: 2 𝑝𝑠𝑟𝑐 𝜏𝑖 2 𝑣𝑖 = 2 2 4𝜌0 𝑐0 𝜎𝑖 Vibration reduction at junction: 𝑣𝑗2 𝑑𝑖𝑗 = 2 𝑣𝑖 Radiation to receive space: 2 2 𝜏𝑖 𝜎𝑗 𝑆𝑗 𝑝𝑠𝑟𝑐 𝜏𝑖 𝑝𝑠𝑟𝑐 2 𝑊𝑗 = 𝜌0 𝑐0 𝑣𝑗 𝜎𝑗 𝑆𝑗 = 𝜌0 𝑐0 𝑑𝑖𝑗 2 2 𝜎𝑗 𝑆𝑗 = 𝑑𝑖𝑗 4𝜌0 𝑐0 𝜎𝑖 4𝜌0 𝑐0 𝜎𝑖 Flanking transmission coefficient: 𝜏𝑖𝑗 ≡ Since 𝑅 = −10 lg 𝜏
𝑊𝑗 𝑊𝑖
=
𝑊𝑗 𝑝2 𝑠𝑟𝑐 𝑆 4𝜌0 𝑐0
𝜎 𝑗 𝑆𝑗
= 𝑑𝑖𝑗 𝜏𝑖 𝜎
𝑅𝑖𝑗 = 𝑅𝑖 + 𝐷𝑉,𝑖𝑗 + 10 lg
𝑖
𝑆
𝜎𝑖 𝑆 + 10 lg 𝜎𝑗 𝑆𝑗
Enforce Reciprocity 𝜎𝑖 𝑆 𝑅𝑖𝑗 = 𝑅𝑖 + 𝐷𝑉,𝑖𝑗 + 10 lg + 10 lg 𝜎𝑗 𝑆𝑗 Require 𝑅𝑖𝑗 = 𝑅𝑗𝑖 gives: 𝑅𝑖 𝑅𝑗 𝑆 𝑅𝑖𝑗 = + + 𝐷𝑖𝑗 + 10 lg 2 2 𝑆𝑖 𝑆𝑗
Remarkably Simple Expression Apparent TL due to path ij
𝑅𝑖 𝑅𝑗 𝑆𝑠 𝑅𝑖𝑗 = + + 𝐷𝑣,𝑖𝑗 + 10lg 2 2 𝑆𝑖 𝑆𝑗 TL of surface i TL of surface j ij are typically Ff, Fd, Df.
Directionaveraged vibration reduction across junction
Dimensions
Heavyweight Assemblies • We focus on heavyweight, monolithic construction such as concrete. • In these assemblies, it is a good approximation that the structural fields are diffuse. • In other words, these assemblies have low internal damping. • That means that the vibration level depends strongly on the boundary losses, i.e., on the surrounding elements.
Structural Reverberation Time • Can define structural reverberation time in the same way as acoustical reverberation time. • Just as an acoustic diffuse field level depends on the absorption of the surfaces, the structural diffuse vibration field level depends on the losses at the edges. • Just like acoustic field, it can be measured or calculated. Structural RT
Acoustic RT
Internal damping
Air attenuation
Radiation
--
Edge losses
Absorption from surfaces
Effect of Structural Reverberation Time • Lower 𝑇𝑆 (higher edge losses) will reduce the vibration levels within the assembly. This will give higher transmission loss. • Analogous to: lower T60 (high absorption) will reduce the reverberant sound field in a room. • The transmission loss therefore depends on the surrounding elements. • For the flanking calculation, should adjust R based on the field conditions: 𝑇𝑆,𝑙𝑎𝑏 𝑅 = 𝑅𝑙𝑎𝑏 + 10 lg 𝑇𝑆,𝑠𝑖𝑡𝑢
Structural Reverberation Time Requirements for Laboratories • Even in laboratories, different edge conditions surrounding the test openings will give different results. • Current version of ISO 10140 gives requirements for labs: From E. Gerretsen
Effect of Slab Size on Transmission Loss 𝑇𝑆,𝑙𝑎𝑏 𝑅 = 𝑅𝑙𝑎𝑏 + 10 lg 𝑇𝑆,𝑠𝑖𝑡𝑢 • Acoustically, the T60 in a room increases with volume for the same wall absorption. • Structurally, 𝑇𝑆 will increase with specimen area for the same edge conditions. • Conceptually, for larger slabs edge losses become less important. That is, internal damping becomes a larger fraction of the total damping.
Effect of Slab Size on Transmission Loss • Larger specimen longer 𝑇𝑆 lower TL • We can measure or predict structural reverberation time. • Another way is to scale the transmission loss with assembly size.
Effect of Slab Size Dimensions on Transmission Loss
Effect of Slab Size Dimensions on Transmission Loss 8” concrete
Effect of Slab Size Dimensions on Transmission Loss
Lab value
Large area value
Lab specimen size
Effect of Slab Size Dimensions on Transmission Loss 70 65 60
Transmission loss (dB)
• Modify TL of large slabs • The lab value of the TL is 40-45 at 63 Hz. However, a value
