How does a needle (technically referred to as stylus) moving in the record’s groove generate sound? A quick recap:
- The stylus is rigidly attached to the cantilever. You will have to look closely to see the stylus tip while the cantilever is clearly visible.
- The stylus wiggles in the record’s groove (right and left channel) and through the cantilever in either a Moving Coil (MC) or a Moving Magnet (MM) cartridge.
- The magnet moves in the coil and induces electric current – this is the signal coming out of the cartridge wires and into the circuit that applies the correct amplification. The adjusted signal is then sent to the input of a preamp or power amplifier.
You can see the simplified schematics of MM cartridge (top) and MC cartridge (bottom) below:
Going back to the cantilever, its movement is controlled by a suspension system (yellow part above). The electrical energy retained during the signal generation/adjustment process is converted to mechanical wave and transferred into the cartridge.
The suspension in the previously mentioned (see Part 1) AT33PTG2 cartridge is quite soft. It limits unnecessary vibrations well, perhaps even too well as the sound lacks the dynamics achieved by DL103 (now this is powerful!). The DL103 is a stiff cartridge (low dynamic compliance) that requires a higher effective mass tonearm. Its heavy dynamics, however, comes with a side effect – an increased level of vibrations transmitted to the tonearm.
Mechanical Waves are waves which propagate through a material medium (solid, liquid, or gas) at a wave speed which depends on the elastic and inertial properties of that medium. There are two basic types of wave motion for mechanical waves: longitudinal waves and transverse waves. (Wikipedia)
I will return to these waves later.
On a macro level, you could compare a tonearm to a car (apologies for applying this simple logic):
- The bumpy road is the record’s groove. The wheel is our stylus. The shock absorber is the cantilever. The car is the tonearm.
- The car moves with constant speed.
- The job of the wheel and shock absorber is to reduce the effect of driving over rough ground (and improve driver’s perception of the road surface) without causing excessive vibrations to the car (think cobblestones).
Things to consider here:
- Selecting an appropriate suspension system (shock absorbers and wheels) for your car – it can’t be too stiff or too soft.
- If too stiff, the wheels will skip or slide over the road surface, not to mention an unwanted trip to the chiropractor.
- If too soft, the wheels will glide smoothly over bumps, but the driver will struggle to feel the road surface.
- Detecting and damping resonant vibrations in the car.
A similar connection can be found between the cartridge dynamic compliance and the resonant frequency calculated for tonearm effective mass. Example – Denon DL103 on a low mass tonearm sounds dreadful, lacking dynamics, energy, and bass. And conversely, a low mass cartridge can sound blobby on a high mass tonearm
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a periodically applied force is equal or close to a natural frequency of the system on which it acts. When an oscillating force is applied at a resonant frequency of a dynamic system, the system will oscillate at a higher amplitude than when the same force is applied at other, non-resonant frequencies.
Frequencies at which the response amplitude is a relative maximum are also known as resonant frequencies or resonancefrequencies of the system. Small periodic forces that are near a resonant frequency of the system have the ability to produce large amplitude oscillations in the system due to the storage of vibrational energy.
Resonant systems can be used to generate vibrations of a specific frequency (e.g., musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies (e.g., filters). (Wikipedia)
Going back to tonearm – the mechanical wave generated by moving cantilever travels through cartridge, headshell, main arm, scale, bearing pivot, and base (red arrows below).
Internal vibrations occur when trapped energy particles move back and forth within the turntable parts, transforming energy in the process. Some vibrations also originate from external sources (e.g., through the shelf upon which the turntable rests) or from the mechanical system itself, such as motor or bearing interacting with the record (blue arrows).
Ideally, we would want to keep tonearm vibrations to a minimum. This could be challenging as any tonearm will exhibit resonance at a specific frequency.
A bell is a good example of vibrating (resonant) system – when the bell is struck, the metal vibrates and generates mechanical energy.
The Newton’s Cradle is a popular device that shows the energy transfer between moving spheres and a group of stationery spheres. When one sphere at the end is lifted and released, it strikes the stationery spheres, transmitting mechanical wave through the stationery spheres that pushes the last sphere upwards. The middle spheres remain still as they immediately transmit the entire energy to the next sphere. This is exactly how BennyAudio tonearm was designed to work – the energy generated in the cartridge moves through and out of the tonearm and then gets converted into different energy output. Just as if you stopped the last sphere from moving back.
Resonance can be restrained by damping. As per the Law of Conservation of Energy, vibration damping does not destroy energy but only transforms it from one form to another.
There are various vibration damping methods used in turntables:
- Decoupling the tonearm from cartridge vibrations (an extreme example below, not sure what to say about it):
- Platter mats – various types of mats (soft, hard, thin, thick, conductive, non-conductive) are advertised as helping to dampen resonances and minimise other vibrations. In the next blog entry, I will show you a performance graph of one of the best platter mats in the world.
- Spacers – used to reduce vibration generated by the cartridge and transmitted to the headshell. When measured in the headshell, vibration appears to decay, but unfortunately it still exists in the cartridge.
- Headshell damping – using quite complex headshells where most of the unwanted vibration is dissipated away through the soft joint, e.g., Nasotec Swing Headshell.
- Tonearm pivot damping – usually using oil, specialist isolator rings, various material blends etc.
- Arm tube damping – typically with oil, often found in more expensive turntables.
- Transferring vibration for damping outside of the tonearm – using lead sheets or quartz sand application etc.
This is in line with the BennyAudio philosophy – transfer maximum energy out of the tonearm and then convert it into different energy type such as heat.
How do I approach this (in theory)? See Part 3 of the blog.