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I'm seeking advice from the group as to the correct (or recommended) bass
turnover frequency for Orthacoustic transcriptions.

I note that this has also been called Orthocoustic (Audio Cyclopedia by
Tremaine) and Orthoacoustic (many users.) A U.S. Patent

http://www.freepatentsonline.com/2286494.pdf

...from 1940 refers to Orthacoustic. Furthermore, I have several 16" NBC
lacquers from the late 1930s boldly rubber-stamped as Orthacoustic.

A Google search of available literature, as well as reference books in my
collection, make various claims as to the turnover frequency. Common claims
are 700 Hz and 500 Hz, as well as several ranging from 400 to 1000 Hz. There
is no consistent recommendation that I can find in the manuals for various
preamplifiers which have variable playback equalizations.

When I studied filters in the 1960s, it was common to refer to the turnover
frequency of an equalization curve as that point at which the curve deviated
from a reference point by 3 dB. A common reference point for audio filters
was 1 kHz.

The curve displayed in the above referenced patent is insufficiently
detailed to estimate the 3 dB point. It makes reference to 500 Hz, but, in
my opinion, that is not the 3 dB point as shown.

The "Orthocoustic" curve in the Audio Cyclopedia by Tremaine (1st edition,
1959) on page 431, figure 13.99, shows a 3 dB point at about 700 Hz,
referenced to 1 kHz. The curve in the Radiotron Designer's Handbook (4th
edition, 1953) page 730, figure 17.15, is insufficiently detailed for my
weak eyes to discern, but it may be 3 dB at 500 Hz.

I should say that I believe that the correct equalization curve is that
which yields accurate sound reproduction on a calibrated loudspeaker system,
but that is of course highly subjective.

Thanks for any help!

--Kevin Mostyn