In this edition’s Voice Coil Spotlight, Aleksandar Aleksov (Ourthree, Inc.) introduces readers to a New Earphone Technology, which his company named Balanced Frequency Range. The solution he describes is optimized for wide application in in-ear headphones of various designs, and aims to balance the headphone’s frequency response without adding further active systems or digital EQ.
Next up, and appropriately for an anniversary edition, Mike Klasco (Menlo Scientific) writes about Subwoofers and Bass in a valuable Voice Coil Focus update. The article covers an extensive range of technologies, principles, and solutions for better bass in speaker design, starting with the fundamentals of the Iron Law that determines the two-out-of-three parameters that designers need to manage: extending lowbass reproduction, shrinking (enclosure) size, or higher (output) sensitivity.
The article details relevant applications in popular product categories such as soundbars, addresses the topic of cabinet stuffing and the major acoustical variables for different fiber types, before discussing specific solutions from companies such as Mundorf, Sound-Solutions, CarbonAir, Resonado Labs, Trulli Audio, Dinaburg, and Tectonic Audio Labs. Mike then dives into the potential of the Nuvoton and Klippel Controlled Sound (KCS) integrated solution in bass reinforcement.
For this month’s Acoustic Patents article, James Croft (Croft Acoustical), reviews a patent titled Loudspeaker with Passively Controlled Voice Coil Sections, assigned to Tymphany Acoustic Technology. Patent Number US11102575, awarded in August 2021 addresses a voice coil design and method for driving a voice coil of a loudspeaker suspended in an air gap. While providing a large number of variations of the basic concept, the idea is to make for more efficient use of coil turns due to having the most effective coil both activated and immersed in the magnetic gap at the correct signal levels. Unfortunately, the patent doesn’t detail the control system required for the invention to work, and that's precisely where this review offers a valuable perspective of potential applicability and theoretical potential advantages.
This edition will also please those readers who eagerly read Vance Dickason’s Test Bench characterizations every month. The first driver in Vance’s Test Bench is Purifi Audio’s PTT4.0X04-NFA-01 4” midbass, which is very similar to the PTT6.5X reviewed in Voice Coil July 2021, and includes the same constant Sd surround and low magnetic hysteresis motor design, also discussed in a dedicated article by the company’s founders. The distinct cone assembly for this driver consists of a flat profile paper cone made with a proprietary fiber mix, a concave dust cap that covers about 90% of the cone surface, plus the addition of a V-shaped edge reinforcing ring glued around the outside rim of the back side of the cone, which makes the cone edge extremely stiff, and is designed to dampen cone edge resonances. Compliance is provided by a unique NBR surround, which as in the PTT6.5X surround, deserves to be explored, motivating Vance to discuss it with Purifi’s co-founder Lars Risbo.
The motor for the Purifi PTT4.0X04-NFA-01 midbass consists of an FEA-optimized ferrite ring magnet with milled and tapered plates, plus a pole piece comprised of a neodymium pole magnet sandwiched between steel sections that serves to linearize the Bl. Driving the cone assembly is a 30mm (1.18”) diameter four-layer voice coil wound with round aluminum wire on a non-conducting fiberglass former, featuring alternating two- and four-layer sections on the length of the coil with varying widths. No doubt, one of the most detailed Test Bench characterizations on a product that truly deserves that degree of discussion.
The second Test Bench for this anniversary edition of Voice Coil is another very interesting new driver: Eighteen Sound’s AMT200P Air Motion Transformer and accompanying XMT200 6.5” waveguide. This driver from the company that remains a discrete brand, separate from B&C Speakers, is its latest entry into the high-frequency transducer category, specifically designed for the pro sound community. The Eighteen Sound AMT200P includes a rectangular throat with a Kapton diaphragm with a neodymium magnet structure, and is rated for a power handling of 90W AES (180W continuous) above 1.2kHz, with a sensitivity of 105dB, and a recommended crossover frequency of 1.2kHz at 48dB/octave. The AMT design also has a damped rear cavity that includes a black emissive coating and heatsink fins for cooling enhancement. While the AMT200P can be used as a standard planar device it can be also coupled with a dedicated constant directivity horn, the 90° x 20° XMT200, featured in the characterization.
Of course, there’s no lack of Industry Watch topics in this edition as well. The November 2021 edition of Voice Coil is available for download and already in the mail to print subscribers. Subscribers can go to - www.gotomyvcoil.com - to download this issue.
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