UAD Powered Plug-Ins Manual v5.2 - Just Music

4. Blend the absorption properties of the two materials by dragging the Blending
bar. The relative amount of each material, expressed as a percentage,
appears above their respective pop-up menu. Drag the Blending bar to the
right to emphasize the first material, and drag it to the left to emphasize the
second material. To use only one room material, drag the Blending bar so
the material is set to 100%.
About the
Materials
Some materials absorb high frequencies and reflect low frequencies, while
other materials absorb low frequencies and reflect high frequencies. This
characteristic is determined by the material surface and density.
Fiberglass, for example, absorbs high frequencies. When high frequencies
strike fiberglass they bounce around inside the fibers and lose much of their
energy.
At a thickness of 100%, fiberglass rolls off the high frequencies, a little bit
each millisecond. After a while the high frequencies dissipate and the low frequencies
linger. If we were to take fiberglass and increase its thickness to
+200%, the high frequencies would roll off even faster. At +200%, this high
frequency decay happens at twice its normal rate, producing a very heavy reverberant
tail. At -200%, a very “sizzly” late field is created.
Some materials, such as plywood, naturally absorb low frequencies while reflecting
high frequencies. Since plywood is usually very flat with little surface
texture to capture high frequencies, high frequencies tend to be reflected. At
+100%, the reverberation produced is very sizzly and increasingly bright. At
-100%, it is very heavy.
Keeping this in mind, if you look at the graphics in the material control panel,
you can get a sense of how chosen materials, material blend, and thickness
will affect the decay rate as a function of frequency. Hard materials that have
lots of small cavities (Brick, Gravel, Plaster on Brick) and soft materials (Carpet,
Grass, Soil) tend to absorb high frequencies. Flat, somewhat flexible materials
(Heavy Plate Glass, Hardwood, Seats) tend to reflect high frequencies.
Marble is the one material that tends to uniformly reflect all frequencies.
You probably noticed the artificial materials the top of the Materials menu.
These are materials designed to have predictable behavior and can be very
handy for achieving a desired reverberation preset when you know what decay
rates you desire. All these materials preferentially absorb high frequencies;
they give the selected decay time at low frequencies, and a much shorter
decay time at high frequencies. The frequency in each graphic is the transi-
UAD Powered Plug-Ins Manual - 262 - Chapter 31: RealVerb Pro