@MAB You were right. I found another pressure measurement of the Rice-Kellogg speaker from that era which was in pascals. I converted both to dB SPL and they lined up fairly well.
This turned into a good excuse to explore the FR of speakers from a century ago. I'm limited to on-axis measurements, since this is all that's available outside of a few cases. Sources are at the end.
First, take a modern speaker. I'll use it throughout for comparison
Genelec S360A, released in 2018. Exceptionally flat on-axis. 250mm (10") direct radiator woofer and 25mm (1") tweeter.
Next is the earliest measurement I could find. A 1921 measurement of what is assumed to be a common balanced armature moving iron speaker. These drivers were typically used in radios and telephones. I have no details about this speaker's construction. Back then, the diaphragms could be unloaded when used in headphones, or loaded using horns, cylinders or discs in various forms.
From the same source, a 1922 improved version of the above. Surprisingly flat, when considering what frequency response looks like in most cases. The improvements, measurement technique, circumstances and accuracy are unknown.
Now 1923.
1924. Increased bandwidth, but the response seems to be almost entirely composed of resonances.
Now 1925. All of these measurements, by the way, come from a JP Minton in an addendum to the article in which Rice and & Kellogg introduced their speaker and covered the existing speaker technologies of the time.
Below is a picture of the Rice-Kellogg direct radiating ("hornless", as the article calls it) speaker, whose advantages are clear in the following chart. The second trace is from article that started this thread. They line up well enough. It's unclear what causes the deviations.
What's really interesting is that the addendum also has comments by Kellogg, responding to Minton, which include a polar plot!
A few years later, the Schlenker speaker was introduced 1928. This is an unusual 757mm (29.8") direct radiating speaker. The motor is deliberately mounted off-center to reduce breakup. The diaphragm is a stretched metal alloy called duralumin. I'm really impressed.
The 1920s Western Electric 15-A horn driven by a 555 receiver. This speaker is so well-known that certain boutique manufacturers are still making versions of it today.
https://www.resistormag.com/features/western-electric-horn-systems-time-travel-with-tim-gurney/
Work by Kolbrek and Dunker produced contour plots (polar maps) of the WE 15-A horn.
For comparison, the Genelec S360A plots:
https://www.spinorama.org/speakers/Genelec S360/ASR/asr/SPL Horizontal Contour.html
https://www.spinorama.org/speakers/Genelec S360/ASR/asr/SPL Vertical Contour.html
Sources
- 1921-1925 FR measurements. Rice, Chester W and Kellogg, Edward W. Notes on the development of a new type of hornless loud speaker. Journal of the American Institute of Electrical Engineers. Vol. 44, No. 9. New York September 1925.
- 1928-29 FR measurement of Rice-Kellogg speaker. Jahrbuch des Forschungs-Instituts der Allgemeinen Elektricitäts-Gesellschaft. Erster Band. 1928-1929, Berlin 1930.
- 1929 measurement of the Schlenker speaker. Wilson, P and Webb GW. Modern Gramophones and Electrical Reproducers. Cassell & Co. London 1929. Found in Kolbrek, Bjorn and Dunker, Thomas. High Quality Horn Loudspeaker Systems: History, Theory and Design. Kolbrek Elektroakustikk. 2019.
- Measurements of the WE 15-A. Kolbrek, Bjorn and Dunker, Thomas. High Quality Horn Loudspeaker Systems: History, Theory and Design. Kolbrek Elektroakustikk. 2019.
