It seems that “gold-sputtered” is a buzz word for condenser microphone diaphragms nowadays. Is this just good marketing or is it a necessary part of condenser microphone design?
Why are condenser microphone diaphragms gold-sputtered? Condenser mic diaphragms are often gold-sputtered since they need to be electrically conductive (to hold a charge across the capsule) but are typically made of non-conductive Mylar material. Gold is conductive and resistant to corrosion so it is used, albeit sparingly, on condenser diaphragms.
In this article, you’ll read more about microphone diaphragms, gold as a material, and the history of gold-sputtered diaphragms.
The Gold-Sputtered Condenser Diaphragm
The term “gold-sputtered” has become a selling point for many condenser microphone manufacturers. However, the truth is that there isn’t really anything special about gold-sputtered diaphragms. In fact, they are the norm.
Some condenser microphones have diaphragms made of extremely thin metal foil.
However, the vast majority of condenser diaphragms are now made with Mylar, a trademark name for BoPET (biaxially-oriented polyethylene terephthalate), a type of polyester film.
The thin metal foil is electrically conductive and typically works well as a diaphragm. Mylar, however, has become to go-to material for condenser (and dynamic) mic diaphragms for its light weight and improved flexibility and durability. The only issue is that Mylar, by itself, is not conductive. This is where gold comes in.
Sputtering is a technique for applying a molecular layer of atoms to a surface. When gold-sputtering a Mylar diaphragm, this thin layer of gold makes the diaphragm electrically conductive and, therefore, an effective part of the condenser capsule.
Gold-sputtered diaphragms are the norm for true FET condensers, tube condensers, and back electret condensers.
Why Must Condenser Microphone Diaphragms Be Conductive?
Condenser microphone capsules are designed just like parallel-plate capacitors. The diaphragm makes up the movable front plate and there is a stationary backplate.
The backplate is typically made of solid brass, which is an alloy of copper and zinc and is conductive.
With a conductive gold-sputtered diaphragm, the parallel-capacitor is able to hold an electric charge.
This charge remains constant. As the diaphragm moves, the distance between the plates varies and changes the capacitance of the capsule. This, in turn, causes an inversely proportionate AC voltage signal to be outputted.
This charge is provided via phantom power, DC biasing voltage, external power supplies, or by electret material. Without a conductive diaphragm, the capsule would not hold any charge and the condenser microphone would not function.
So we know that gold-sputtered Mylar is a better material for condenser diaphragms than thin metal foil but why are condenser mic diaphragms sputtered with gold rather than cheaper and more conductive metals like silver or copper?
The main reason is corrosion and oxidization. Gold is the least reactive of all metals in terms of corrosion and oxidization. Gold does not react with oxygen and will not rust or tarnish. This means that the gold-sputtered diaphragm will remain conductive and maintain its form for much longer than diaphragms that utilize copper, silver, or aluminum.
Why do condenser microphones need phantom power? Condenser mics do not necessarily need phantom power but do require some form of DC biasing. This voltage is used to power active components such as impedance converters (FETs), vacuum tubes and printed circuit boards. It also provides [non-electret] capsules with the permanent charge they need.
For more info on microphones and power, check out the following My New Microphone articles:
Do Microphones Need Phantom Power To Work Properly?
Do Microphones Need Power To Function Properly?
How do condenser microphones work? Condenser mics are transducers that work on electrostatic principles. A condenser capsule is a parallel-plate capacitor that holds a permanent charge. The diaphragm (front plate) moves with sound pressure. This alters the capsule’s capacitance and causes an inversely proportional AC voltage (mic signal).