I got this chart off the web and it gives you an idea of how the different materials absorb sound at different frequencies.

Remember that full absorption is 1 whilst full reflection is 0

Absorption coefficients of common building materials and finishes
 Floor materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
 carpet 0.01 0.02 0.06 0.15 0.25 0.45
 Concrete (unpainted, rough finish) 0.01 0.02 0.04 0.06 0.08 0.1
 Concrete (sealed or painted) 0.01 0.01 0.02 0.02 0.02 0.02
 Marble or glazed tile 0.01 0.01 0.01 0.01 0.02 0.02
 Vinyl tile or linoleum on concrete 0.02 0.03 0.03 0.03 0.03 0.02
 Wood parquet on concrete 0.04 0.04 0.07 0.06 0.06 0.07
 Wood flooring on joists 0.15 0.11 0.1 0.07 0.06 0.07
 Seating materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
 Benches (wooden, empty) 0.1 0.09 0.08 0.08 0.08 0.08
 Benches (wooden, 2/3 occupied) 0.37 0.4 0.47 0.53 0.56 0.53
 Benches (wooden, fully occupied) 0.5 0.56 0.66 0.76 0.8 0.76
 Benches (cushioned seats and backs, 0.32 0.4 0.42 0.44 0.43 0.48
 Benches (cushioned seats and b 0.44 0.56 0.65 0.72 0.72 0.67
 Benches (cushioned seats and backs, fu 0.5 0.64 0.76 0.86 0.86 0.76
 Theater seats (wood, empty) 0.03 0.04 0.05 0.07 0.08 0.08
 Theater seats (wood, 2/3 occupied) 0.34 0.21 0.28 0.53 0.56 0.53
 Theater seats (wood, fully occupied) 0.5 0.3 0.4 0.76 0.8 0.76
 Seats (fabric-upholsterd, empty) 0.49 0.66 0.8 0.88 0.82 0.7
 Seats (fabric-upholsterd, fully occupie 0.6 0.74 0.88 0.96 0.93 0.85
 Reflective wall materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
 Brick (natural) 0.03 0.03 0.03 0.04 0.05 0.07
 Brick (painted) 0.01 0.01 0.02 0.02 0.02 0.03
 Concrete block (coarse) 0.36 0.44 0.31 0.29 0.39 0.25
 Concrete block (painted) 0.1 0.05 0.06 0.07 0.09 0.08
 Concrete (poured, rough finish, unpai 0.01 0.02 0.04 0.06 0.08 0.1
 Doors (solid wood panels) 0.1 0.07 0.05 0.04 0.04 0.04
 Glass (1/4" plate, large pane) 0.18 0.06 0.04 0.03 0.02 0.02
 Glass (small pane) 0.04 0.04 0.03 0.03 0.02 0.02
 Plasterboard (12mm (1/2") paneling on 0.29 0.1 0.06 0.05 0.04 0.04
 Plaster (gypsum or lime, on masonry) 0.01 0.02 0.02 0.03 0.04 0.05
 Plaster (gypsum or lime, on wood lath) 0.14 0.1 0.06 0.05 0.04 0.04
 Plywood (3mm(1/8") paneling over 31.7mm 0.15 0.25 0.12 0.08 0.08 0.08
 Plywood (3mm(1/8") paneling over 57.1mm 0.28 0.2 0.1 0.1 0.08 0.08
 Plywood (5mm(3/16") paneling over 50mm( 0.38 0.24 0.17 0.1 0.08 0.05
 Plywood (5mm(3/16") panel, 25mm(1") fi 0.42 0.36 0.19 0.1 0.08 0.05
 Plywood (6mm(1/4") paneling, airspace, 0.3 0.25 0.15 0.1 0.1 0.1
 Plywood (10mm(3/8") paneling, airspace 0.28 0.22 0.17 0.09 0.1 0.11
 Plywood (19mm(3/4") paneling, airspace 0.2 0.18 0.15 0.12 0.1 0.1
 Absorptive wall materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
 Drapery (10 oz/yd2, 340 g/m2, flat aga 0.04 0.05 0.11 0.18 0.3 0.35
 Drapery (14 oz/yd2, 476 g/m2, flat agai 0.05 0.07 0.13 0.22 0.32 0.35
 Drapery (18 oz/yd2, 612 g/m2, flat agai 0.05 0.12 0.35 0.48 0.38 0.36
 Drapery (14 oz/yd2, 476 g/m2, pleated 0.07 0.31 0.49 0.75 0.7 0.6
 Drapery (18 oz/yd2, 612 g/m2, pleated 0.14 0.35 0.53 0.75 0.7 0.6

Fiberglass board (25mm(1") thick)
 0.06 0.2 0.65 0.9 0.95 0.98
 Fiberglass board (50mm(2") thick) 0.18 0.76 0.99 0.99 0.99 0.99
 Fiberglass board (75mm(3") thick) 0.53 0.99 0.99 0.99 0.99 0.99
 Fiberglass board (100mm(4") thick) 0.99 0.99 0.99 0.99 0.99 0.97
 Open brick pattern over 75mm(3") fiber 0.4 0.65 0.85 0.75 0.65 0.6
 Pageboard over 25mm(1") fiberglass bo 0.08 0.32 0.99 0.76 0.34 0.12
 Pageboard over 50mm(2") fiberglass bo 0.26 0.97 0.99 0.66 0.34 0.14
 Pageboard over 75mm(3") fiberglass b 0.49 0.99 0.99 0.69 0.37 0.15
 Performated metal (13% open, over 50mm 0.25 0.64 0.99 0.97 0.88 0.92
 Ceiling material 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
 Plasterboard (12mm(1/2") in suspended 0.15 0.11 0.04 0.04 0.07 0.08
 Underlay in perforated metal panels (25 0.51 0.78 0.57 0.77 0.9 0.79
 Metal deck (perforated channels,25mm(1" 0.19 0.69 0.99 0.88 0.52 0.27
 Metal deck (perforated channels, 75mm(3 0.73 0.99 0.99 0.89 0.52 0.31
 Plaster (gypsum or lime, on masonary) 0.01 0.02 0.02 0.03 0.04 0.05
 Plaster (gypsum or lime, rough finish 0.14 0.1 0.06 0.05 0.04 0.04
 Sprayed cellulose fiber (16mm(5/8") on 0.05 0.16 0.44 0.79 0.9 0.91
 Sprayed cellulose fiber (25mm(1") on s 0.08 0.29 0.75 0.98 0.93 0.76
 Sprayed cellulose fiber (25mm(1") on 0.47 0.9 1.1 1.03 1.05 1.03
 Sprayed cellulose fiber (32mm(1-1/4") 0.1 0.3 0.73 0.92 0.98 0.98
 Sprayed cellulose fiber (75mm(3") on s 0.7 0.95 1 0.85 0.85 0.9
 Wood tongue-and-groove roof decking 0.24 0.19 0.14 0.08 0.13 0.1
 Miscellaneous surface material 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz
 People-adults (per 1/10 person) 0.25 0.35 0.42 0.46 0.5 0.5
 People-high school students (per 1/10 0.22 0.3 0.38 0.42 0.45 0.45
 People-elementary students (per 1/10 p 0.18 0.23 0.28 0.32 0.35 0.35
 Ventilating grilles 0.3 0.4 0.5 0.5 0.5 0.4
 Water or ice surface 0.008 0.008 0.013 0.015 0.02 0.025

RT60 relates to intelligibility. Diffractors reduce pronounced reflection by breaking up the sound wave before reflecting it back. This does not reduce reverberant energy, but does reduce echo spikes that may otherwise exceed -60db of direct, thus lowering RT60 and improving intelligibilty, but not necessarily improving the listening environment for music.