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Basics of the Acoustic Phonograph


This page gives a very brief overview of the operation of the acoustic phonograph. Readers who are new to this hobby are encouraged to consult some of the references listed in the Resources section for complete information on the workings of acoustic phonographs. This brief section is mainly intended to show the basic acoustic phonograph system and to establish the terminology that I will be using in articles and discussions.

Function of electrically amplified phonographs (many models, 1925 and beyond) are not covered in this section.

All Victor machines of the acoustic era were based on the Berliner flat disc design. The discs revolved at 78 RPM and used the lateral (side-to-side) cut method. Some other manufacturers, including Edison, used cylinders or discs with a vertical cut wherein the needle moved up and down to reproduce the acoustic signal (see diagram at right). Only the lateral cut medium will be discussed in these articles. As the needle tracks the grooves, vibration is mechanically coupled into the soundbox, which consists of a thin diaphragm of mica or (later) aluminum. The diaphragm vibrates and provides a large surface area to vibrate the air molecules into the hollow tonearm. Thus, mechanical energy is converted into acoustical energy. The air molecule vibration is routed through the tonearm and into the horn, which directs the soundwaves into the listening environment.

A hand-wound, spring-powered motor was utilized to spin the turntable on most Victor machines. Depending on model (and price), from one to four spiral-wound springs were used in the motor. A simple mechanical governor provided a stable drive system. Electric motors became an option on Victrolas around 1913, but the more common spring drive was used in most models through the 1920's. It wasn't until around 1928 that electrically powered phonograph motors became very commonplace. Even many of the early radio-phono combination sets of the mid-1920's used a spring-wound motor, with batteries providing DC power for the radio's electronics.

Early phonographs used an external horn. External-horn Victors varied considerably in design detail in early years, evolving in sophistication as more was learned about effective transfer of vibro-acoustic energy from the disc to the surrounding room. The earliest designs had an integral horn and soundbox structure (photo at left). In these models, the horn's neck was attached directly to the soundbox housing, and the entire horn assembly moved along with the needle as the record was played (some of the horn's weight was supported at a pivot point, allowing the soundbox/horn assembly to follow the record grooves). Thus, the record groove had to pull along the mass of the entire system (needle, soundbox and some of the horn's weight). This mass loading resulted in loss of frequency response, reduced volume, and premature record wear. Al
so, the need to counterbalance the horn's weight became a problem as horns became bigger and more complex.

Eventually, a "rigid" tonearm design was developed in which only the tonearm and soundbox moved along the record while the horn remained stationary (photo at right). This was accomplished by using a simple pivot joint between the tonearm and the horn itself. This design served to decouple the mass of the horn from the soundbox and tonearm. Subsequent improvements to tonearms resulted in a gradually expanding diameter (taper) from soundbox to horn, which further improved the system efficiency by providing a better acoustical impedance match. Soundboxes also evolved to improve their performance. These topics will all be discussed in depth in the technical articles. 

 


With the development of the internal-horn Victrola in 1906, the major change was the inversion of the tonearm elbow to route the sound into the horn which was now inside the cabinet (photo at left). This resulted in a much less intrusive piece of furniture, and the use of integral doors in front of the horn's outlet allowed the listener to adjust the volume. The internal horn was smaller and was "squared off", which resulted in reduced volume when compared to the external horn models, but the public preferred the appearance of these newer machines, and they soon outsold their earlier counterparts. Soundbox improvements continued to be made, but the basic design configurations remained the same.
 

With the introduction of the Orthophonic Victrola in 1925, many of the key design parameters were improved. Increased compliance of the stylus was achieved through use of a ball-bearing needle support system, with magnets added to improve alignment and accuracy. An increase in stiffness of the soundbox diaphragm (now made of aluminum) improved the overall response characteristics and reduced the dynamic non-linearity that was present in the previous mica diaphragm design. But most importantly, elemental acoustic impedance matching equations were applied to the tonearm and horn, greatly increasing both the volume and frequency response of the system. The resultant sound was markedly improved. 

An analysis and resulting comparative performance of many of these evolutionary designs are discussed in the Technical Articles Section. The key point to remember, however, is that the basic overall principal of operation remained the same from the earliest models until electrical pickup came into common use in the late 1920's.

The picture below shows a typical later-vintage Victrola soundbox and tonearm, with the key components labeled.




Soundbox and tonearm assemblies of a Victrola XI

The basic reproduction process is as follows:

The stylus tracks the grooves in the record which contain the acoustical signal. This signal source consists of a lateral displacement of the groove itself. As the record spins and the stylus tracks the groove, this lateral movement is translated into stylus motion (vibration). This motion contains both the frequency and amplitude information of the audio signal (picture at right). Many different stylus designs were available over the years, with varying degrees of thickness and shapes. As a rule a "loud tone" stylus was of a thicker and heavier design than was a "soft tone" stylus. A thick stylus has higher stiffness, and does a better job of coupling of high frequency vibrations into the soundbox, resulting in a subjectively higher volume. 

The stylus is held in place by the thumbscrew, which serves to hold the stylus tightly in the stylus bar assembly (diagram at left). The lateral vibration of the stylus is coupled to the center of the diaphragm, which moves the column of air in the tonearm.  The stylus bar assembly serves to couple the vibrations from the stylus itself to the soundbox diaphragm. Note that a significant mechanical advantage is provided through this system. The relatively small motions of the stylus itself are increased significantly due to the leverage across the pivot point (the overall magnitude of this advantage is dependent on the type of soundbox and the frequency content of the signal), resulting in a much larger displacement at the center of the diaphragm. This is a variation of the simple mechanical lever concept. The support springs serve to keep the leverage mechanism in place and yet allow sufficient dynamic motion to be coupled from stylus to diaphragm. Many other variations of this basic design appear in early phonographs.

The diaphragm's vibration serves to convert the mechanical energy (vibration) into acoustical energy. It is essentially a membrane-type "piston" which excites the column of air in the tonearm itself. This oscillating column of air in the tonearm is called a plane progressive acoustic wave. However, since the diaphragm itself is highly flexible and is held firmly in place by the gasket at its edges, the center will move a great deal and the edges will move very little. Thus, the motion is not equally distributed over the entire diaphragm, resulting in some degree of distortion in the sound wave. The greater the distortion, the poorer the reproduced sound. 

The sound wave moves through the tonearm and into the horn, which serves to match the acoustic impedance of the tonearm to the listening room. The concepts of acoustic impedance and the utilization of tapered structures (tonearms and horns) are covered in detail in the technical articles. Suffice it to say that different horn and tonearm tapers have different impedances, resulting in different efficiencies. The lower the efficiency, the less the sound volume produced in the room, and the poorer the low frequency response. Many types of horn designs and materials have been used over the years, resulting in a great variation of sound qualities and volumes.

One of the key problems in the design of acoustic phonographs was in designing a "smooth-flow" tonearm. Ideally, the tonearm and horn should consist of one continuous constant and unbending taper from soundbox to the end of the horn itself. However, the practicalities of making a phonograph both reasonable in size and convenient to use mandated that certain compromises be made. The sharp bends at the connection between the tonearm and the horn, as well as the U-shaped gooseneck at the soundbox tube all contribute to reduced acoustic efficiency and added distortion. The technical struggles to optimize these designs will be discussed in the technical articles.

The "Technical Articles" will discuss the various designs and configurations of these components, and will detail the technical development of the designs as well as to show vibro-acoustic data taken from a variety of machines. The effects of various stylus, stylus bar, diaphragm, horn and tonearm designs will be detailed, along with supporting data. Readers who are unfamiliar with acoustical engineering concepts and terms are encouraged to read the "Introduction to Vibro Acoustics" page under the heading "Technical Articles on the Victrola Phonograph".

 

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