Thanks for the answers. I have contacted Ray Rayburn.
Indeed it is sadly not widely used anymore. The way it is measuring the acoustic response of a loudspeaker, with
a real phase detector and a tracking filter, provide`s me the best response of the loudspeaker alone with the highest SNR`s possible.
Today`s analyzers are off course much faster and have more possiblity, but doesn`t always provide the correct answers.