Performance Differences Between Victor Soundboxes
By Paul C. Edie
There has been a great deal of discussion as to which type of soundbox provides better sound quality and which has lower distortion. This article will present performance data of three popular Victor soundboxes: Concert, Exhibition and No. 2.
As most readers know, the soundbox is the structure that holds the needle and converts its vibration into an acoustic (sound) wave, which is subsequently propagated down the tonearm and out the horn. While soundboxes continually evolved over the years, they are all variations of a simple fulcrum design. The needle vibrations are coupled through the fulcrum assembly or "linkage", (needle, thumbscrew, pivot point and stylus bar) into a diaphragm that covers the open end of the tonearm. The diaphragm vibration excites the air molecules in the tonearm, generating sound waves. The linkage acts as a mechanical amplifier to increase the amplitude of the vibrations transferred to the diaphragm, which ultimately increases the amplitude of the sound wave.
The amount of mechanical gain provided by the stylus linkage assembly depends on the soundbox design. Most soundboxes have a static gain ranging from 2 to 4. A gain of 3 implies that 0.001-inch of motion at the stylus is translated to 0.003-inches of motion at the diaphragm. The static gain can be roughly approximated by measuring the distance from the pivot point to the center of the diaphragm (along the stylus bar) and dividing it by the distance from the pivot point to the stylus tip. But this measured gain is true ONLY for static conditions, which are not applicable when a record is played. In other words, if we were to put a constant lateral force on the stylus and measure the displacement at both the stylus and diaphragm, the static gain conditions would hold true. However, when the motion becomes dynamic (vibrates back and forth), the static conditions no longer apply. The dynamic stiffness and resonant characteristics of each mechanical component in the soundbox causes the mechanical gain of the fulcrum to vary greatly as a function of frequency (these topics are discussed in the article on Loud and Soft Tone Needles). These variations can cause a significant difference in the frequency response of different soundboxes.
Of course, in evaluating the response of these soundboxes, one must consider not only the gain of the mechanical linkage, but also the response of the diaphragm element itself. The larger diaphragm utilized in the No. 2 Soundbox certainly provides a greater area from which to generate the acoustic wave, but some of this is advantage is offset by the need to move a greater mass (heavier diaphragm) than is required with either the Concert or Exhibition models. The larger diaphragm is also less stiff, potentially causing irregular vibration "patterns" on the diaphragm surface. All vibrating objects exhibit some natural vibration patterns, or "mode shapes", but flimsy structures can result in patterns which are severe and irregularly distributed, causing uneven frequency response, distortion or other acoustic anomalies.
Some new instrumentation was used to make needle-to-diaphragm response measurements. The intent was to vibrate the stylus with a stable controlled signal, rather than to use a record as the vibration source as was done in some previous studies. A small excitation shaker was configured to couple vibrations into the stylus as shown in Figure 1. This shaker was driven by an amplified signal source. The shaker was attached to a small impedance head that measures the acceleration and force levels the shaker is applying to the device under test. The impedance head was then attached to a loud tone stylus tip via a small spot weld and a rigid coupling fixture. The stylus was then conventionally attached to the soundbox via the thumbscrew; the same stylus and fixture combination was used for all subsequent tests.
Figure 1. Stylus Shaker and Fixture Attached to a Victrola 111. This allowed controlled vibration signals to be applied to the soundbox for analysis purposes.
In addition, a Polytec Laser Vibrometer was used to make vibration measurements. This device allows vibration measurements to be made at virtually any point simply by aiming a laser beam, and does not result in the mass-loading effect that can decrease the accuracy of high frequency measurements. The laser beam could also be swept across surfaces to measure changes in the vibration as a function of position. Data was acquired on several different samples of each soundbox model, all in excellent condition. Data was averaged over many sample sets to minimize scatter.
In order to quantify the diaphragm response characteristics quickly, the shaker was driven with a "swept sine" input signal. A swept sine is a pure tone which begins at a low frequency and automatically sweeps upward at a controlled rate, and results in a sound similar to what would get by using a slide whistle. The laser vibrometer was programmed to incrementally move across the diaphragm surface, monitoring a full frequency sweep at each individual measurement point. Thus, these plots represent the vibration response averaged over the surface of the diaphragm, using a constant-amplitude swept sine signal input at the needle tip. This testing served to quantify the response of the total soundbox system (the linkage combined with the diaphragm). This is an indicator of how accurately and how strongly the needle vibrations are transferred into sound-wave-producing vibrations at the opening of the tonearm. The use of the swept sine technique also allowed an accurate measurement of the Total Harmonic Distortion (THD) produced by the vibrating diaphragm, indicating the amount the reproducer alters the pure input signal and distorts the sound.
Swept sine response plots appear in Figure 2. All 3 soundboxes exhibit peak output in the 890 to 1300 Hz range. The evolutionary improvements in high frequency response can be clearly seen. The Concert Soundbox has significant roll-off at frequencies above 1700 Hz. The Exhibition is somewhat better, and the No. 2 is fairly flat up to 3500 Hz. Interestingly, as the high frequency response improves, the frequency response below 300 Hz. becomes worse, although this is probably not very significant, as most acoustic-era recordings have little recorded content below about 200 Hz.
Figure 2. Swept Sine Plots of Soundbox Response. Top to Bottom: Concert, Exhibition, No. 2. These plots show the overall surface-averaged response of the
diaphragm, indicating how much of the needle vibration is actually translated into potential acoustic energy at the opening of the tonearm. The vertical scale is
logarithmic, with gain ratios (needle-to-diaphragm) varying from 0.1 to 10.0. The horizontal bar across the center represents unity gain, meaning that the
diaphragm surface is moving at the same amplitude as the needle. The horizontal scale is linear.
The No. 2 soundbox had a significantly higher overall (THD) distortion, averaging around 6.3% over all frequencies. The Exhibition averaged 4.2% and the
Concert 4.9%. The larger diaphragm is less stiff than the other two designs. As noted earlier, this lack of stiffness can cause non-linear response and thus more
distortion. It is important to note that measurements of actual acoustic output of the soundboxes were not made, since these
soundboxes couple into very different horn and tonearm configurations. The effect of different tonearms and horns will have a strong effect on the measured acoustic frequency response.
This would result in an "apples and oranges" test that I felt would have muddled the direct vibration comparisons. Thus, the data presented here represents the
potential acoustic energy that is coupled into the tonearm opening.
Finally, variations in dynamic stiffness of soundbox components can cause a similar variation in the effective dynamic mass of the stylus. Dynamic mass refers to the amount of "resisting force" that the stylus applies against the record at different frequencies. For example, a low dynamic mass would mean that the resisting needle force is low; the needle and linkage are acting as a lightweight element and will follow the groove movements easily. A high dynamic mass will act like a heavy element, and will cause the needle to fight the groove movement, resulting in higher distortion when records are played. Much of the effective dynamic mass at the stylus tip is caused by the soundbox, since it is the soundbox that supports the needle; the needle must work against the resistance of the soundbox elements in order to vibrate. Ideally, the design should maintain a low dynamic mass across all frequencies in order to minimize distortion.
Technical Note: Some readers may be familiar with the term "compliance" as a measure of stylus resistance. Compliance is defined as "displacement/force" while dynamic mass is defined as "force/acceleration". Dynamic mass was chosen for this study to avoid integration errors in mathematically converting measurement units from acceleration to displacement.
The plots of dynamic mass as a function of frequency are shown in Figure 3. The Concert soundbox has a surprisingly smooth curve, meaning that it applies relatively even forces to the record grooves. The Exhibition's mass curve is also reasonably smooth, but the No.2 exhibits a large mass increase at high frequencies (could this also be a partial cause for the No.2's higher distortion measurements?). Over the years, I have heard several people comment that Exhibition soundboxes perform better than No. 2 soundboxes when used with VE (electrical) recordings, and seem to exhibit less distortion. The data supports this, as the lower dynamic mass (higher compliance) of the Exhibition at high frequencies would create less needle resistance to the increased groove modulation of VE recordings. The Concert Soundbox should also track VE recordings better, but the reduced high frequency response will decrease the sound quality.
Figure 3. Plots of Dynamic Mass Measured at Stylus Tip. Top to Bottom: Concert, Exhibition, No. 2. These plots show the variations of dynamic mass as a function of frequency. The vertical scale is logarithmic, with ranging from .001 lb/g to 1 lb/g. The horizontal scale is linear.
In conclusion, it would seem that the Exhibition soundbox has somewhat better overall vibro-acoustic characteristics than the other models, but by a relatively small margin. It certainly exhibits less distortion than the others, has a reasonably flat frequency response and an evenly distributed dynamic mass characteristic. The No. 2 Soundbox seems subjectively louder to most people, which is due to its increased high frequency response. I suspect that another cause for the higher volume perception is in part due to the larger diameter tonearms that are used with most No.2 Soundboxes (Victor increased tonearm diameter around 1917). Perhaps the larger diaphragm provides a better acoustic impedance match to the "fatter" tonearm. This is material for a future study. I also detect a slightly higher level of "harshness" when listening to No. 2 Soundboxes vs. Exhibitions; this may be indicative of the higher distortion levels at the diaphragm.
Overall, the quantifiable differences between the three models were generally less than I had expected, showing that the evolution of an effective soundbox design was indeed a slow process, and that newer designs did not always improve all aspects of acoustic performance. Future experiments will cover tonearm evaluations, comparative acoustic horn measurements, and response characteristics of Orthophonic reproducers.
My appreciation to Mr. Tom Gaines of Toledo, Ohio and Mr. Tony Delia of Warren, Michigan for allowing me access to their fine phonograph collections for measurement purposes.