I believe Nicholas Bergh is working an a paper about this subject. From discussing this with him, my understanding is there were some purely mechanical adjustments responsible for shaping the recording curves in the early WE disc cutters. 


Date:    Tue, 5 Mar 2019 15:55:10 +0000
From:    Karl Miller <[log in to unmask]>
Subject: Re: Recording Equalization

 I am reminded of a lengthy conversation I had with John Eargle. John said that he had discussed this with many of the "old guys." John said that they all agreed that prior to RIAA, EQ was subjective. "Whatever sounded best." As Dennis has suggested it is a challenge and, ultimately, in my experience, somewhat subjective. 
When I was teaching my class, I used to have the students listen to, and compare, restorations of the same recording done by different individuals...for example, a Mark O-T versus a Ward Marston version. Sure, one does not know the condition of the discs/pressings each guy used, but the differences in the results could be startling. Then you have someone like Andrew Rose who uses software to analyze the spectrum of a modern recording of a particular work and then apply that to his restoration of an old recording of that work. To my ears, one loses the acoustic of the original recording. For example, his restorations of the stuff recorded at Eastman, sound nothing like the distinctive acoustic of their hall. 
Not doubt this is common knowledge in this email list...we apply the notion of EQ to the electrical process. However, when I think about it, I am reminded that a form of "EQ" was a part of the acoustic process. For example, it is known that, especially in the case of pianists, they were expected to adjust the dynamics of their playing, in different parts of the frequency range, to suit the particular qualities of the reproducers of a specific manufacturer. 
Karl
 
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