The Art of Palladium - Woofer Design
Transducer Design
Low Frequency Driver (Woofer)
LF Driver Features
• 1 ½” Diameter – Flat-wire “low mass” aluminum voice coil
• 0.72” (18mm) linear peak-to-peak high excursion design
• High linearity magnet structure containing 3 different Neodymium magnets
• High Temperature N35SH Neodymium magnets
• Two Faraday aluminum shorting rings – flux stabilizing rings on each side of the gap
• Reverse half-roll low density foamed rubber surrounds
• High air-flow – low restriction die-cast aluminum basket
• Hybrid Aluminum/ROHACELL® cone
The P-39F uses three 9” (225mm) diameter low frequency drivers. This custom woofer utilizes a low mass hybrid aluminum/ROHACELL®/Kevlar cone with an over hung voice coil design, ensuring there is always an equal amount of coil winding in the magnetic gap regardless of the excursion of the driver. Even during high excursion, the driver remains linear and low in distortion. A cut-away of the woofer is shown below.
The cone is constructed of an outer aluminum skin and inner Kevlar skin bonded to a lightweight rigid core of ROHACELL®. This material is commonly used in high performance vehicles and aircrafts. This unique closed cell polymethacrylimide (PMI) foamed plastic core adds increased rigidity to the cone while minimizing ringing and maintaining low mass. The sandwich constructions results in an extremely stiff but light cone allowing the woofer to operate as an ideal piston throughout the woofers pass band. This translates into much less coloration giving an incredibly natural and realistic sound. A three part high intensity neodymium N35SH magnet design is used in the 9” woofer with a main magnet plus two supplementary magnets, placed above and below the main magnet, to ensure linearity, reduce stray magnetic energy, and provide intense field strength in the voice coil gap. The field strength of the neodymium magnet structure has an equivalent magnetic field strength of an 80 ounce ceramic magnet, with only a combined weight of 13 ounces. A magnetic Finite Element Analysis (FEA) of the motor structure is show to the right. Additionally, dual aluminum Faraday rings are also located inside the motor structure and on the pole piece. Faraday rings, also known as shorting rings, are used for four reasons:
Minimize unwanted inductance.
Minimize inductance change with voice coil position.
Reduce flux modulation caused by the magnetic field generated by the voice coil.
Increase heat dissipation.
These all result in reduced distortion and compression and lead to improved dynamic capabilities. The effect of the aluminum rings can be seen in the graph below sowing the variation in electrical inductance with voice coil position.A superior motor structure is complemented by the design of an efficient voice coil. The 1 ½” voice coil is wound with a flat-profile low-mass copper clad aluminum wire. Flat-wire allows for efficient packing of the wire when layered on the voice coil resulting in an increase in the Bl. The Bl is the product of the magnetic flux density (B) and the length of the coil in the magnetic gap (l). This increased Bl directly correlates to an increase in the sensitivity of the woofer. The flat wire voice coil/motor structure provides 0.72” (18 mm) peak to peak linear excursion. Below is a graph showing the difference in BL(X) with and without the additional bucking magnets in the motor
Another unique feature about this woofer is the surround. The surround is made of half-roll low density foamed rubber and inverted to allow for minimum diffraction. The low density foamed rubber helps to reduce the moving mass while maintaining surround integrity under high excursion and box pressures. The flat-sided shape was created to maximize the linear excursion while increasing the radiating area. Special care has also been taken in the design of the driver suspension which, along with the surround supplies the restoring force to keep the driver moving linearly about the rest position. It is important that the restoring force is symmetrical in the forward and backward motion of the woofer cone. The graph shows the measured stiffness vs displacement for the P39-F 9”woofer compared to an inferior design.
The cast aluminum woofer frame made specifically for this model employs narrow spokes which are grooved to provide maximum surface area in their minimal profile. This assures maximum heat transfer without any reflection of the back wave of the driver that would otherwise color the sound. A Finite Element Analysis (FEA) of the heat transfer of the aluminum basket is shown below. The aluminum frame is designed to acct as a heat sink for the woofer motor, conducting heat generated by the voice coil away and thus reducing the effects of thermal compression, translating into greater dynamic capabilities.