Performance Differences Between "Loud" and "Soft" Tone Needles
Author: Paul C. Edie
In a recent discussion with a fellow phonograph collector, the topic of needles came up. After playing a record with a "loud tone" needle, and then with a "soft tone" needle, we both certainly agreed that soft-tone needles produced a "mellower" sound, but was the overall volume really that different? My perception has always been that loud-tone needles were indeed louder, but after some serious critical listening, I began to question what the measurable differences really were. Thus, I set out to determine what made a soft-tone needle sound "soft" and a loud-tone needle sound "loud".
To many, this might seem to be a trivial problem. Loud needles are bigger and produce louder sounds! But if we play a record first with a loud-tone needle, and play the same record again with a soft-tone needle, the motion of the tips of both needles are identical at each point along the record. The record alone (not the needle) determines how far and how fast the needle tip must move….needles are literally forced to comply with the record grooves. It is the needle design which causes the volume differences.
I have heard many incorrect theories on the causes of volume differences between needles. One popular misconception is that the thicker loud needles "track" the record grooves better, and thus produce more volume. This is not the case, as poor tracking simply adds distortion to the sound as the needle rattles around in the groove. Another idea states that the volume increase from a loud needle is due to its higher weight (mass). Newton's Law states that Force = Mass x Acceleration. Thus the assumption is that the higher mass of the loud needle being accelerated and decelerated by the record grooves would produce a higher force on the soundbox diaphragm, thus increasing the overall volume. From a random sample of 26 soft and loud needles from various manufacturers, I measured an average mass of 0.0025 ounces for soft-tone needles, and 0.0063 ounces for loud-tone needles. But this theory falls apart, since the needle is only a very tiny fraction of the mass of the entire needle-to-soundbox link (including the thumbscrew, pivot mount, stylus bar, linkage components, etc.). All these elements must be included in the total mass of the link acting on the diaphragm; the tiny 0.0038 ounce difference between the two types of needles is completely overshadowed by the total mass of the other components in the linkage. Any force or volume differences due to the tiny added mass of the loud needle would be acoustically indistinguishable.
The only other possible difference between soft-tone and loud-tone needles would be due to the variation in their rigidity. The thicker needle would tend to bend and flex less under the motion of the record grooves and would transmit more energy to the diaphragm. Actually, this is what is occurring, but is an oversimplification of the process. Without going into technical derivations, the real differences in acoustic performance between the needles can be attributed to the variation in "dynamic stiffness". Stiffness is a measurement of how much a cantilevered object bends as a constant force is applied. However, if the applied force is dynamic, that is, vibrating back and forth, the stiffness will also vary as a function of the applied frequency (see the enclosed box for definitions of italicized terms). For simple cantilevered structures (like a needle clamped by a thumbscrew), dynamic stiffness typically decreases as frequency is increased; thus at high frequencies, the needle flexes more. It absorbs the high frequency vibration from the record (through its flexing) and thus, less high frequency vibration is coupled into the soundbox. Thicker objects usually have higher dynamic stiffness at high frequencies than thin objects, and this characteristic can easily be mathematically modeled. A computer model of the two needle designs shows that both needles have identical stiffness characteristics at low frequencies (e.g. reproduction of low notes), but the thicker "loud-tone" needle is much stiffer at high frequencies (above approximately 1,500 Hz.) and does a far better job of coupling high frequency vibrations from the record to the soundbox diaphragm. According to the model, what we are hearing as "more volume" from loud-tone needles is simply better high frequency reproduction.
To prove this hypothesis, some data was taken on a recently restored Victrola XI with a #2 soundbox. Tiny accelerometers (to measure vibration) were placed on the needle thumbscrew and on the soundbox diaphragm (as shown in Figure 1), and two instrumentation microphones were placed approximately 3 feet from the front of the horn. To assure that a statistically meaningful sample of recordings was represented, data was acquired by averaging the results from 10 complete Victor 18000 series records, all of which were in excellent condition. Each record was played in its entirety; first with a soft-tone needle, and then repeated using a loud-tone needle. Needles were replaced after each playing. Data from the microphones and accelerometers was digitized and recorded on DAT tape. It was then sorted into "loud needle" and "soft needle" data files and fed to a Hewlett Packard 35670A Signal Analyzer for averaging and signal analysis.
Figure 1 Photograph of accelerometers mounted on Victrola XI. The two accelerometers monitored vibration levels on at the thumbscrew and on the diaphragm. Two microphones (not shown) were used to measure acoustic output
The results matched the mathematical model very closely. The soft needles produced an average overall sound pressure of 90.0 dBA at the microphones, while the loud needles produced an average sound pressure of 93.4 dBA. A spectral plot of this data for both needles appears in Figure 2. The overall difference in average volume between these needles is just over 3 dBA, which is rather small and would not be particularly noticeable if not for the much greater high frequency content of the loud needle. Note on the curves of Figure 2 that the sound amplitude in the 400-1250 Hz. range is virtually identical for both needles. These "low tones" are reproduced at equal volumes by both needles. It is only above 1250 Hz that any differences are obvious. The loud-tone needle produces 5 to 10 dBA higher output than does the soft-tone needle, but only in the range above 1250 Hz. The response of both needles drops off severely above 3500 Hz. and below 200 Hz., which are the high frequency limits of the record and reproducing mechanism. While this increase in volume due to the stiffer needle is limited to only the higher frequencies, and does not greatly effect the overall average sound pressure level, the 1250-3500 Hz. range is where our hearing sensitivity is greatest, and thus the subjective perception is of a much sharper tone and significantly higher volume. This is somewhat equivalent to turning up the treble control on a modern stereo without adjusting the volume; the overall sound coming out of the speaker increases only slightly, but we perceive a louder and sharper sound since the high frequencies are emphasized. The sharp peaks in the range of 800 to 1800 Hz. are the natural frequencies of the linkage, reproducer and tone arm/horn cavity, and these characteristics are partially responsible for the "classic" acoustic phonograph sound.
Figure 2. Spectral plot of average sound pressure level for 10 Victor recordings. Frequency is plotted on the horizontal axis and sound pressure level (in decibels) on the vertical axis. The two needles provide very similar responses up to 1250 Hertz (shown by the hash mark on the bottom of the plot). Above 1250 Hertz, the loud tone needle exhibits 5-10 dBA greater output than does the soft tone needle.
It is important to note that those portions of records which have
considerable musical content in the 1250-3500 Hz. range (e.g. harmonics of
higher-pitched instruments such as trumpets, violins, etc.) will be considerably louder when the loud-tone needle is used. On the other hand, some of the records tested had little
musical content in the 1250-3500 Hz. range, and exhibited less than a 1 dB overall difference between the loud and soft needles, which is
acoustically insignificant. But on average, when the total volume of an typical record is averaged over its entirety, the overall volume is not
significantly higher when a loud-tone needle is selected; the higher frequencies are simply being emphasized.
Since the needle vibrations are the source of the phonograph’s acoustic output, it is logical to assume that the same needle-dependent high frequency response is present at the needle itself. Analyzing the vibration motion of the needle support (measured at the thumbscrew) and the on the soundbox diaphragm, this same trend appears. The loud needle is much stiffer in the 1250-3500 Hz. range, and allows approximately 2-3 times more high frequency vibration to be coupled from the record into the soundbox than does the soft needle. For example, the displacement of the average groove-induced vibration measured at the thumbscrew for an 800 Hz. tone segment on Paul Whiteman’s "Whispering" was measured as 62 microinches (0.000062 inches) for both the soft and loud-tone needles. However, a 2000 Hz. tone on the same record vibrated the thumbscrew an average of 8.4 microinches with a loud tone needle, while the soft needle moved it only 4.9 microinches. Since the average vibration-induced motion of the needle tips is identical for both types of needles (the same segments of the record was averaged for both needles) this difference in motion between the needle tips and thumbscrew at 2000 Hz. has to be attributed to the lower dynamic stiffness of the soft-tone needle. Again, the soft-tone needle is flexing more at the higher frequencies, and thus less vibration energy is arriving at the thumbscrew.
Since the motion of the needle at the thumbscrew is what is ultimately coupled into the soundbox diaphragm, it would be expected that a similar motion response as a function of frequency would be seen on the diaphragm. This was confirmed through the measurements. The diaphragm vibration displacement averaged 197 microinches for the same 800 Hz. segment with both needles. At 2000 Hz., the diaphragm averaged 19.8 microinches with the loud needle, and 7.1 microinches with the soft needle. As an observation, the stylus linkage mechanism provides a mechanical advantage (motion gain) of approximately 3 from needle to diaphragm (measured as 197 microinches/62 microinches). This will be the topic of a future article, as each soundbox design has different mechanical gains and different natural frequencies of components, which give each model its own characteristic sound.
Of course, this data is representative of only the #2 Soundbox, and is based on using the Victrola XI as a baseline. Other soundboxes and machines with different horns would have different results, but the primary differences in the performance of the loud and soft needles remains essentially the same no matter what machine is used. The stiffer needles provide improved high frequency response only, and do not increase the volume of the lower frequencies.
Finally, it is interesting to note a major difference in the performance of record labels. When 10 Columbia series A3000 recordings were averaged, the overall volume using the soft needle was reduced to 82.5 dBA, which is just slightly more than half the subjective volume of the Victor records. Likewise, the needle vibration at the thumbscrew was also reduced proportionally. Columbia certainly recorded at much lower levels. Other than the differences in overall amplitude of the recordings, the same general response and motion characteristics were seen for both Columbia and Victor. In other words, the frequency content of both labels is about the same. Several Regal recordings showed significant volume differences between loud and soft tone needles which is attributed to the fact that these recordings had a much "brighter" sound; more recorded information was present in the high frequency range, which was emphasized very effectively by the loud tone needles.
Readers who have playable needles of other configurations (with a mass in the center of the needle, for example) are encouraged to contact the author so that additional data can be gathered.