95Chapter 5
Small Room Acoustics
by Jeff Szymanski
5.1 Acoustical Treatment Overview.............................................................. 95
5.2 Acoustical Absorption...................................................................... 95
5.2.1 Absorption Testing..................................................................... 96
5.2.1.1 Reverberation Chamber Method....................................................... 96
5.2.1.2 Impedance Tube Testing Methods..................................................... 98
5.2.1.3 Other Absorption Testing Methods..................................................... 98
5.2.1.4 Absorption Ratings................................................................. 99
5.2.1.5 Interpreting Test Results............................................................ 100
5.2.2 Porous Absorbers..................................................................... 100
5.2.2.1 Mineral and Natural Fibers.......................................................... 101
5.2.2.2 Acoustical Foams................................................................. 103
5.2.2.3 Acoustical Tiles................................................................... 105
5.2.2.4 Spray and Trowel Applied Treatments................................................. 105
5.2.2.5 Carpet and Draperies............................................................... 105
5.2.3 Discrete Absorbers.................................................................... 106
5.2.3.1 People and Seats.................................................................. 106
5.2.3.2 Acoustical Baffles and Banners....................................................... 106
5.2.3.3 Other Furnishings and Objects....................................................... 107
5.2.4 Resonant Absorbers.................................................................... 107
5.2.4.1 Membrane Absorbers.............................................................. 107
5.2.4.2 Helmholtz Resonators.............................................................. 109
5.2.4.3 Perforated Membrane Absorbers...................................................... 109
5.2.4.4 Slat Absorbers.................................................................... 110
5.2.5 Bass Traps........................................................................... 111
5.2.6 Applications of Absorption.............................................................. 112
5.2.6.1 Use of Absorption in Reverberant Spaces............................................... 112
5.2.6.2 Use of Absorption in Nonreverberant Spaces............................................ 112
5.2.7 Subjective Impact of Absorption.......................................................... 113
5.2.8 Absorption and Absorption Coefficients of Common Building Materials and Absorbers.............. 113
5.3 Acoustical Diffusion...................................................................... 113
5.3.1 Diffuser Testing: Diffusion, Scattering, and Coefficients....................................... 115
5.3.2 Mathematical (Numerical) Diffusers....................................................... 116
5.3.3 Random Diffusion..................................................................... 117
5.3.4 Applications of Diffusion............................................................... 118
5.4 Reflection and Other Forms of Sound Redirection............................................... 118
5.5 Electronic Treatments..................................................................... 119
5.6 Acoustical Treatments and Life Safety........................................................ 119
5.7 Acoustical Treatments and the Environment................................................... 120
References................................................................................. 121