Last week’s article discussed the technical design challenge of our FLT shallow compact woofer products. This weeks article will discuss in more detail the solution we used in engineering this shallow compact woofer.
Design of the Solution
The engineering challenge posed by the approach outlined in the previous section was to come up with a replacement for the spider, with a suspension element fitting into the cylindrical space surrounding the voice coil…that, in fact, cannot be attached to the voice coil. The solution was to design a set of six vertically oriented spring elements. Each spring element would be attached on one end to the lower surface of the cone, and on the other end to the inside surface of the frame, adjacent to the motor. Six elements were selected to provide for an abundance of counterbalancing forces for any type of rocking mode, and to average out any part-to-part variation amongst the springs.
Various designs and materials were considered. For example, classical metal coil springs were considered, and discarded, as the springs would end up being too stiff, and we were concerned the springs would fatigue and fracture under power test conditions. Finally, we settled on a half-folded part, heat-formed out of specially-selected fabric, as the solution which would work. This part would unfold and fold as the cone moved up and down. Because of the shape and function, we named the part a “Plica”, based on the Latin word for a fold or folded part. The figure below illustrates the Plica™ suspension system placed into the FLT transducer, specially designed to be a shallow compact woofer.
The specifics of the Plica design were examined in detail by means of mechanical finite element analysis. Stiffness, linearity and pull-up, and stability were all examined. Once the initial computer-based design was done, it was time for the prototyping phase. Extensive effort then began on refinement of the overall performance and the assembly technique. Extensive power testing was also conducted…it is not every day we introduce an entirely new type of transducer suspension element, and we wanted to be certain that the products would be reliable.
Once all this work was completed, we had our brand-new FLT100 product, pictured below! It performs out a little past 2 kHz, as you can see from the graph below, which means it can be crossed over to many of our high performance, low resonance tweeters. Current specifications for the FLT drivers are on the Tymphany website.
Now that we have achieved our goal of making a shallow woofer, how can this new product be used in loudspeaker systems?
Typically, there are three ways that loudspeaker systems are constructed:
• Sealed cabinets
• Vented cabinets
• Sealed cabinets with passive radiators
Sealed cabinets are rather easy to understand; there is no air path from the outside of the cabinet to the inside of the cabinet, and sealing off the back of the driver from the front allows for the production of bass frequencies. Vented cabinets have such an air path, through what is called a port, which is essentially a long tube of air; the length and cross-sectional are of the port are important parameters in determining at which frequency the port’s air resonates, leading to the production of more bass sound if selected correctly. Passive radiators play a similar function; they are transducers without motors, resonating in response to the pressure generated by the woofer, and producing low frequency bass if it is designed correctly. Passive radiators are smaller than the ports which would be used to produce the same amount of bass output…but, they are more costly in many cases.
The graph below illustrates the predicted systems performance of the FLT100 product in its 4 ohm variety, when placed into a small 1.25 L volume sealed enclosure. The bass performance of the driver in this small enclosure starts to fall off below 200 Hz, mirroring that of the same driver in an infinite baffle measurement condition. This is happening because of the mechanical damping in the transducer’s suspension system (surround and Plica). Thus, it can be seen that there are not significant performance losses associated with the placement of this driver in such a small cabinet.
For comparison, the graph below compares the predicted systems performance of the same FLT100 driver, placed into the same 1.25 L cabinet, but this time sealed with a passive radiator (same compliance and radiating area as the FLT100, and with a moving mass of 35 grams). The bass performance between 100 Hz and 200 Hz is distinctly improved.
Now, selection of the desired cabinet volume and the desired system alignment (sealed, ported, or passive radiator, and associated parameters to achieve a desired frequency response performance) are up to the engineer designing the system. These graphs are just illustrations of what is possible. The 1.25 L volume was selected for a specific reason, however: given the geometry of the driver, the dimensions of the 1.25 L enclosure would be roughly 35 mm deep, by 120 mm wide (the woofer width, plus a little space to add a grille), by 300 mm long…truly a shallow and small systems solution for bass production.