[ATTACH=CONFIG]190391.vB5-legacyid=8570[/ATTACH]
Three years ago McCauley R&D undertook a project to update the HX32 10” carbon fiber cone mid-range driver used in our MLA6 Line array product. The goal of this project was to improve the overall efficiency and remove two distortion issues marked as (1) and (2) in the frequency response graph. Obviously, this is far easier said than done, and it took a full two years before a production loudspeaker, the 8321, was ready to ship.
What we discovered during this process is that (1) is caused by the cavity resonance inside the space created by the basket and magnet structure. This was very easy to fix with a little simulation and prototyping. Running down the exact cause of (2) and then developing a solution was much more difficult. If you spend a lot of time looking at 10in speaker frequency response curves, then the distortion and corresponding SPL dip at (2) should be familiar. Looking at the posted data from a large selection of manufactures shows pretty much the same thing across the board.
After setting up a number of experiments, it was pretty clear that the distortion could be varied by changing the cone stiffness, the surround termination, or the coupling between the cone and the coil former. Because the HX32 was built around an extremely stiff carbon fiber honeycomb layup, it was very easy for modes to be excited. Unlike a normal paper speaker, which has a high loss factor, this unwanted energy was traveling easily through and radiating from the composite cone.
To solve the problem we designed a new composite cone body built from a polyurethane core sandwiched between dense but lightweight Kevlar fabric. Instead of using hard epoxy for bonding, we used a very lightweight UV resistant flexible epoxy. Unlike the previous cone body, the polyurethane core had a very high loss factor which brought the distortion down to an acceptable level 40dB below the fundamental, and the hole in the frequency response disappeared.
[ATTACH=CONFIG]190389.vB5-legacyid=8568[/ATTACH]
The reason the HX32 and the new 8321 were built with stiff composite cones, was to raise the frequency at which the pattern begins to collapse. Spherical measurements of both products showed that this was true compared to a standard paper cone loudspeaker. Although this extended pattern control was a requirement for their use in the MLA6, it is really not necessary in lower cost two or three way horn and driver products. The dealer cost for the 8321 is 600 USD which makes it unusable in a vast majority of products.
Recently we have been working on a new line of loudspeaker drivers which will draw on the lessons learned in the development of the 8321, yet whose cost will not break the bank. The first member of this new family is the 12” 8331, an update to our very popular but dated 6338 low-mid driver.
As it was with the 10in drivers, a vast majority of 12in drivers have a similar distortion issue in their mid-range response. For the 8331, instead of designing an expensive composite cone, we decided to try modifying the termination impedance of the surround. Instead of using a hard adhesive to attach the surround to the basket as is typically done, a flexible but strong adhesive was used which has an excellent bond strength with aluminum and good UV resistance. The use of this new adhesive only added 10 USD to the material cost of the loudspeaker and reduced the probability of error during the construction process.
The 8321 was our first speaker to use a dual voice coil motor. To improve the inductance, as opposed to using a copper shorting ring which might corrode, the 8321 has a 4” diameter, aluminum dual 0.20” underhung winding neodymium motor. Similarly, the 8331 uses a 4” diameter, aluminum dual 0.7” overhung winding, neodymium or ferrite motor. Changing to an overhung design does reduce the power handling but keeps the cost in check while providing the same linearity, and extending the low frequency response.
Two construction techniques are the hallmark of all the new 8000 series drivers. First, with the exception of the 8321, all the paper speaker cones are built with ultralight weight paper impregnated with a proprietary epoxy solution. The epoxy solution cures to an incredibly hard surface but also has a filler material which improves the internal damping of the cone. By reducing the weight of paper pulp used in the cone and treating it with the new epoxy solution, we end up with a paper cone which weighs less, and is at the same time stiffer, more durable and water resistant.
Second is the use silicone impregnated spiders. It has been common knowledge in the industry for quite a while that the use of silicone is spider layup improves the stability of driver parameters over time and increases the longevity of the driver.
The 8331 brings all of these technologies together into one, reasonably priced product, which should be ready to ship during the first part of 2014. The plan right now is to build a two way speaker-on-a-stick product with a 1.4” exit driver, possibly available with passive network, to pair with the M8, M5 or M55 subwoofer.
