Some recording tasks require close recording with no room ambience; "dead" or "dry" recording, in the colorful language of the trade. Alcatraz is as dead a space as we could make.
Deadening high-frequency sound is fairly easily done; as the wavelengths are short, absorbent materials can be applied to surfaces to a depth that determines the cut-off frequency. To deaden lower frequencies, this approach is impractical, as the absorbent material would need to be ever thicker to absorb the acoustic energy at long wavelengths. A low E on a bass guitar, for example, at 42Hz would not be affected by absorbent trapping unless it was a significant fraction of the 24-foot wavelength in depth.
To control low frequencies in Alcatraz, we have used a membrane absorber and a perimeter vent to couple the performance space with a dead air volume in the basement -- effectively increasing the air volume to almost double for low frequencies. Both of these measures have given Alcatraz excellent low-frequency performance, which is evident when recording drums, bass and heavy guitar.
A dry environment can also accentuate subtleties in the sound of wood-bodied acoustic instruments and increases the intelligibility of voice recording, for "...in a world..."-style movie-trailer narration.
A nice feature of a highly-absorbent dead room like Alcatraz is that little or no baffling is required to achieve a high degree of separation and isolation for amplifiers and instruments sharing the room.
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