Any room or hall used for recording or broadcasting needs to have a reasonably flat response; that is, that there should be no pronounced reverberation at a specific frequency. Whilst the work of Sabine provided architects with objective guidelines, the entire subject should be regarded as a Black Art! Indeed it has only been with the coming of Acoustic Modelling that more accurate predictions can be made about how a hall or studio will actually sound prior to its construction. The characteristic will depend on the size, shape and materials used in the construction.
It is relatively easy to provide any required absorption at higher frequencies with carpeting and curtains, as a quarter wavelength thickness of absorbing material will absorb that frequency and above (the ¼l of 10kHz is about 1/3”). The lower frequencies are more difficult to deal with and, if a room has merely HF/MF absorption, there will be a pronounced LF (and often audible) honk which, while normal people will “filter” this out and ignore, the dumb animal - the microphone - will hear very clearly! This is especially so where the room is used for speech.
The BBC solution came with the design of “membrane” absorbing boxes in which a relatively small box (typically 22” x 22” x 8-12” deep) closed on one of the large sides, has a membrane of (classically) roofing felt enclosing the opposite side and rock wool within the box. The membrane side is covered and protected by perforated hardboard, the fewer holes per sq. foot giving greater reflection at HF. By using a variety of sizes and percentage of perforations, together with MF and HF absorption from furnishings, it is possible to treat a studio so that it has a relatively flat and neutral response.
In many of the older civic and concert halls built prior to the advent of recording and broadcasting and never envisaged for such a purpose, wood panelling was often used as a decorative finish. These were constructed and fixed by hand and the panels were frequently different in resonant frequency. These were rudimentary wide band membrane absorbers and one of the reasons for many of these venues becoming ideal for recording. Many examples could be quoted: Watford Town Hall, now re-named the Watford Arena, Walthamstow Town Hall and the Kingsway Hall in London, the Victoria Hall in Hanley, Staffordshire, Dewsbury Town Hall and Ossett Town Hall in Yorkshire. In Europe, the outstanding examples are the Concertgebeouw in Amsterdam and the Musikverein in Vienna. Also to be noted are some of the modern concert halls, built with a century of acoustic knowledge, which have turned out to be poor! See also Royal Festival Hall (roof and Assisted Resonance System) Brangwyn Hall, Membrane Absorbers, Helmholtz Resonators.