The Secret Life of Plastic Film Diaphragms
Voice Coil Focus The Secret Life of Plastic Film Diaphragms PEEK High-Performance Film Diaphragms and Cones for Headphones and Speakers
This article for Voice Coil magazine explores the use of high-performance film diaphragms for headphones and speakers. Having worked as a consultant for multiple material science companies involved in research for the loudspeaker industry, Mike Klasco is in a position to share real-world knowledge about the use of these plastic-based formulas in the most critical component for sound quality, which is the diaphragm.
The diaphragm is the most critical component in sound quality. This month, we take a closer look at Polyether ether ketone (PEEK) — a colorless organic thermoplastic polymer. PEEK is possibly the best of the thermoplastic resin films for their specification and application to microspeakers, soundbars, earphones, and headphones, and also from tweeters to the finest ribbon planar transducers. Thermoplastic films, from polyethylene terephthalate (PET) to PEEK, can be formed when the temperature of the plastic reaches above its glass transition temperature (Tg), changing from a glassy to a rubbery state—becoming pliable and moldable. After forming to the net shape, the polymer film becomes glassy again upon cooling below Tg. The diaphragm material and shape are the most significant contributors to speaker sound quality. The cost for extruded polymer films start at less than $4/lb. for PET (bottle plastic), which dominates most earphone and headphone diaphragm applications. This is the lower end of plastics and while it works, it is no match for better products. As PET (polyesters) requires higher weight than PEEK to achieve shape integrity, PET diaphragms have a lot of unnecessary additives.
Types of Polymer Films There are a half-dozen plastics that can be used as polymer films for diaphragms used in earphones, headphones, soundbars, smart speakers, laptops/tablets, and esoteric components such as ribbon planar drivers and air motion transformers (AMT). As mentioned, the most common is PET. First synthesized in the 1940s, PET was invented independently by both DuPont (Mylar) and ICI Films (Melinex). Because it is a cheap and a workable material, it has omnipotence in audio. The problem is that as with all thermoplastic resins, it softens when it gets hot. And, PET loses its mechanical integrity or Tg (glass transition) early on at about 78°C (172°F). The melting point Tm is 250°C (482°F) and while it is appealing to diaphragm forming factories to have such a low Tg and high melting point, this wide tolerance enables fast sloppy forming. Because of all of this, PET should be limited to entry-level products. Yet even in products with modest audio expectations, if exposed to high temperatures, a product’s diaphragm is going to melt. In this article, we will discuss the importance of precision extrusion processing for resins in the production of films. The main message is that as with all cheap materials, it is handled as such. PET is a commodity material and high-speed processes are used with resulting film thickness variations typically exceeding 10%. The resulting diaphragms formed from variable thickness films not only vary in weight and — a far more serious result—the formed-edge compliance is not uniform. Dr. Wolfgang Klippel’s team has published a number of Audio Engineering Society (AES) papers about their work on edge compliance and have found that surround edge variation is the dominant contributor to single-point suspension diaphragm rocking. If your company has the budget and wants a high-end product, you probably wouldn’t use PET. Even 30 years ago, there were upgrade options such as PEI (polyetherimide). Originally developed by General Electric (GE) and branded Ultem, the entire GE plastics group was acquired in 2007 by Saudi Basic Industries Corp. (Sabic). Twenty years ago, PEI began to lose its speaker industry market share to DuPont’s improved polyester film called polyethylene naphthalate (PEN). Through acquisition of polyester specialist ICI Films in the UK and its Kaladex PEN film, followed by a joint venture with Teijin, Dupont Teijin PEN Teonex film entered the market and gained acceptance. Aside from PEN being a higher performance polyester compared to PET, pricing is competitive to PEI, and Teijin’s extrusion process was an improvement over PEI (Ultem). Even PEI’s 1 mil (25μm) film was not tight enough for high-yield thermoforming, while Teonex offered consistent film extrusion. A couple of the leading pro sound speaker manufacturers developed popular and respected single- and dual-diaphragm ring radiator drivers using PEN films. But there is often a side effect with PEN, its forming temperature is quite close to its Tm (melt point), and challenged the capabilities of thermoplastic film forming industries. When the polyester resins group of industry giant ICI was acquired by Dupont, the original ICI management and engineering team knew there was a proprietary and superior film technology completing development in their lab. There was a management buyout and Victrex emerged in 1993 focusing on the polyarlyether ketone (PAEK) polymer family, most notably PEEK (polyetherether ketone)-based products, which includes a PEEK film product line, marketed as APTIV branded films, that were launched in 2007.
Relative to PEI and PEN, the PEEK-based APTIV film is pricier, but the combination of high-temperature performance, outstanding fatigue resistance, lightweight, mechanical strength, durability, chemical resistance, wear, and abrasion resistance, good flammability rating without the use of flame retardants, and low moisture absorption all combined with ease of processing made PEEK film a no-brainer for audio engineers to use in their next-generation products. Beyond the resin material characteristics, Victrex developed PEEK films for the audio market by investing in its own next-generation film extrusion plant with high consistency down to films of 3μm thickness (25μm = 1 mil inch).