Frequency Absorption Characteristics of a Variety of Materials

In order to understand a topic fully, it's important to deviate sometimes and uncover bits of data that are not necessarily contributing toward our physical training or implementation, but instead, add to our overall understanding of a subject.  Whether or not you actively utilize this information during your echolocation practice is not important, however it is important to understand and acknowledge it.

In this case, I would like to take a look at how different materials absorb different frequencies.  This will help you to understand how softer items, like upholstered furniture, might differ from doors and how doors differ from concrete.  The numbers listed in this chart are the "absorption coefficients" of these materials.  

To give you an idea of how it works, carpet, the first item on the list has an absorption coefficient of 0.01 for 125Hz frequencies.  That means that it will only absorb 1% of a tone at a frequency of 125Hz.  It will absorb 2% of a 250Hz tone, 6% of a 500Hz tone, 15% of a 1kHz tone, 25% of a 2kHz tone and 45% of a 4kHz tone.  

It becomes easy to tell that some materials absorb a good percentage of some signals, but reflect a good percentage of other signals.  Remember that any percentage of the signal that is not absorbed is reflected.

The reason we start learning echolocation using hard flat objects is because of the fact that, as you see below, things like glass only absorb 2-3% of most frequencies and are therefore very reflective, or responsive.  As opposed to things like upholstery and people which absorb at least 25% of most frequencies, and up to half of some of the higher frequencies.

Material 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 Benches (wooden, empty) 0.1 0.09 0.08 0.08 0.08 0.08 Benches (wooden, fully occupied) 0.5 0.56 0.66 0.76 0.8 0.76 Theater seats (wood, empty) 0.03 0.04 0.05 0.07 0.08 0.08 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 occupied) 0.6 0.74 0.88 0.96 0.93 0.85 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, unpainted) 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 Drapery (10 oz/yd2, 340 g/m2, flat against wall) 0.04 0.05 0.11 0.18 0.3 0.35 Drapery (18 oz/yd2, 612 g/m2, pleated 50%) 0.14 0.35 0.53 0.75 0.7 0.6 Performated metal (13% open, over 50mm(2") fiberglass) 0.25 0.64 0.99 0.97 0.88 0.92 Wood tongue-and-groove roof decking 0.24 0.19 0.14 0.08 0.13 0.1 People-adults 0.25 0.35 0.42 0.46 0.5 0.5 People-high school students 0.22 0.3 0.38 0.42 0.45 0.45 People-elementary students 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

Source: http://www.sae.edu/reference_material/pages/Coefficient%20Chart.htm