RRPPGG SSPPEECCIIAALLTTYY AABBSSOORRBBEERRSS DDrr.. PPeetteerr DD’’AAnnttoonniioo DDrr.. PPeetteerr DD’’AAnnttoonniioo RRPPGG DDiiffffuussoorr SSyysstteemmss,, IInncc.. RRPPGG DDiiffffuussoorr SSyysstteemmss,, IInncc.. Absorbers: Parametrix NRC 1 0.8 t n e i c i f f e o 0.6 C n o i t p r o s 0.4 b A 0.2 0 0 50 100 150 200 250 300 350 400 450 500 Frequency, Hz/10 Acoustic Absorbers Helmholtz Resonators ∅=2a D Perforated sheet t Porous absorbent d Rigid backing Surface Impedance [ ] z = r + j ωm − ρc cot(kd ) 1 m The acoustic mass or imaginary The resistance or real term is associated with phase term, which is associated change or resonant frequency with energy loss 2p k = the wavenumber in air; l d the cavity depth; m the acoustic mass per unit area of the panel; r w the angular frequency = 2p f the density of air, and c the speed of sound in air Resonant Frequency At resonance, the imaginary term goes to zero w m = 2p fm = rc cot(kd ) The cavity size is much smaller than the acoustic wavelength, i.e. kd<<1, so that cot(kd)→1/kd c r f = 2p md This is the basic design equation for resonant absorbers, i.e. Helmholtz, Membrane and Plate resonators Helmholtz Acoustic Mass/Unit Area 2 È ˘ 2 ’ rD 8n Ê t ˆ rD t m = t + 2d a + 1+ = Í ˙ Á ˜ 2 Ë ¯ 2 p a w 2a p a Í ˙ Î ˚ - The last term in the equation is due to the boundary layer effect, and ν is the kinematic viscosity of air. This last term is often not significant unless the hole size is small, say sub-millimetre in diameter. - δ is the end correction factor (not allowing for mutual interaction), which to a first approximation is usually taken as 0.85 and derived by considering the radiation impedance of a baffled piston. Other more accurate formulations exist. - t’ includes the end correction factor When Your Project Calls for CMU Finish RR AA DD Acoustical CMU Acoustical Properties Slotted/Unsealed Slotted/Sealed Unslotted/Unsealed Unslotted/Sealed Transmission Loss Absorption Coefficient High School Auditorium