Passion for Sound – A Historical Perspective

Passion for Sound – A Historical Perspective - Part 1

I recently attended a sale of audio equipment mainly used at theatre and concert venues. There were many large loudspeaker cabinets and vintage loudspeaker transducers.

Buried in all of the equipment were two items that caught my eye. The first item was a large metal round enclosure that was covered in layers of dust and dirt. I wiped off the top of it and found a metal label that read “Radiola Loudspeaker RCA Model 100”. The round metal enclosure had two grill covers, one on the front and one on the back. I removed both and found a large paper cone facing out the front with the rear of the cone facing out the back. At the rear of the metal structure, supporting the cone, was a magnet, wires and a large round container that looked like a large capacitor.

The second item was a large piece of wood in the shape of a rectangular horn with the words “Starr Richmond, Indiana” printed on the mouth of the horn. There was another label with “The Singing Throat” on it. The wood was pretty dirty but yellow in color.

I made an offer and bought them both.

Upon investigation both items have significant acoustic relevance. Starting with the oldest item, the “yellow wood horn” is an acoustic horn from a Starr phonograph built around 1920. The Starr piano company entered the phonograph market along with a host of other companies all trying to compete with the Edison phonographs. Many phonographs at that time used external acoustic horns made of metal rising into the air. These horns served as an impendence matching device coupling the low sound pressure levels, created by the needle connected to a small sealed diaphragm, to the air in the room. In order to provide for better marketing in the home, many companies placed the phonograph platter, spring motor, “tone arm” and the horn in a cabinet. It’s rather ironic that this was a strategy to obtain more acceptance of sound equipment in the living room by the lady of the house!

Given the numerous companies trying to compete in this new market, the Starr Piano Company marketed their line of phonographs as having a special type of acoustical horn. They made the horn out of “seasoned long-leaf Adirondack silver grain spruce” and marketed it in many different ways all aimed at providing greatly improved sound. Some of the claims found in ads were:

“Music wood of centuries”

“The music wood used for famous Stradivarius and Cremona violins”

“….most perfect for conveying sound waves in original purity..”

And best of all

“…Vibrant, sweetly sensitive exquisitely sympathetic in tonal response”

They also claimed that the wood was the same wood used for their piano sound boards. Given that the wood looked good they did not cover it with a grill clothe on the front of the phonograph. They also made a smaller standalone speaker using the same horn material and design.

I could not find frequency spectrum specifications for the horn so I measured the impulse response of the horn by using a small tap close to the throat opening and measuring the output at the mouth. What I found was a cutoff frequency of about 500Hz with a -6dB/octave slope and a high frequency roll off at about 4000Hz. Making some assumptions about the throat and mouth areas and taper of the horn I calculated a cutoff frequency of about 400Hz. I also measured the impulse response of the entire wood structure by using a tap on the side of the structure and measuring the response 1m away. The response indicated two peaks at 700Hz and 1400Hz. Without removing a horn of a different material from another phonograph of this era it is difficult to determine if the “spruce wood” enhanced the sound of the horn.

The company did not stay in the phonograph business for very long but they deserve credit for thinking creatively about how to market a potential improvement in sound by using spruce wood.

As an important side note, the Victor Orthophonic Victrola released in 1925 was the first phonograph to play electrically recorded discs. This was considered to be a major step in sound reproduction. Due to the improved frequency range of the new discs, the designers needed to improve the frequency response of the horn. They accomplished this by using a new 9 ft exponential folded horn inside the cabinet. The advertised frequency response of the Orthophonic was 100Hz-5000Hz. The new Orthophonic claimed to set a new standard for sound reproduction. Listening tests conducted by Dave MacRunnel, a St Louis audiophile and owner of an Orthophonic, confirm that it does have improved sound over other phonographs of the era.

Now on to more Passion for Sound….the Radiola RCA Model 100 speaker

With the formation of RCA in the 1920’s and the development of the Radiotron vacuum tube, the Radiola tube radio was developed. Radio transmissions were made possible by post war developments of transoceanic radio stations and wireless radio services. Early radios were integrated with existing horn speakers but higher fidelity was sought after and eventually an electrodynamic cone speaker was developed by Rice-Kellogg. Using this type of speaker immediately allowed for small cabinets to be used. The Radiola RCA Model 100 speaker was the first permanent magnet speaker to be used with the new Radiola radios. It is a very interesting dipole design with the sound able to project from the front and the back. The resulting sound radiation pattern is two wide beams, one from the front and one from the back with a null or no sound pressure at the sides. This speaker can be modeled as a round transducer in a circular baffle. A transducer mounted in a circular baffle has a cutoff frequency due to the cancellation of sound waves from the back cancelling sounds in the front.

Measuring the diameter of the speaker and the equivalent circular baffle formed by folding back the sides of the round enclosure, I found that they form a ratio of 2:1. Using the 10 inch diameter of the cone and 24 inch diameter of the enclosure, the calculated cutoff frequency is 430Hz. Due to the high impedance (400-1000 Ohms) of the Model 100, I was not able to test it with a modern amplifier or radio. This speaker was much smaller than existing horn speakers, provided a better impedance match to the tube radios and its radiation pattern may have been an improvement over the horn speakers, but the frequency response was not an improvement.

The Model 100 was not on the market for very long and it was replaced by the model 100-A in 1927. The Model 100-A was a transducer mounted in a closed back enclosure eliminating the dipole radiation pattern. The same transducer was also used in larger cabinets which provided a larger baffle potentially providing a better frequency response.

Both the development of the Starr “spruce” horn and the RCA Model 100 (along with the Victor Orthophonic), were great early examples of engineers and marketers having a “passion for sound”.

These products led the way for improvements to transducers and loudspeakers from the 1930’s to today.