March 01, 2013
| by Igor Levitsky
This has a very thin, pleated aluminum foil diaphragm between two magnetic poles to the left and to the right. The diaphragm is very fragile and about 20 to 30 microns thick. It’s about 2 to 3 inches long and 1/2-inch wide. It looks like a ribbon, and the diaphragm is a current conductor.
Signal current passing through the diaphragm makes it vibrate. Ribbon tweeters have been known for extremely airy and transparent sound. The absence of voice coil inductance, its mostly linear magnetic field and its extremely light diaphragm all contribute to exceptional sound quality.
Ribbon tweeters are expensive, they require transformers for impedance matching and their aluminum ribbon is fragile and prone to sagging with time, which may result in response deterioration. Some very high-end speakers use ribbon tweeters and well-known companies such Aurum Cantus, Fountek and Raven manufacture them.
Air Motion Transformer (AMT)
Using a folded thin film diaphragm with aluminum conductors that are formed similar to an accordion squeezebox, this tweeter’s diaphragm is placed between opposing magnets. When signal current is passed it starts oscillating in the plane of the diaphragm with folds contracting and expanding and thus squeezing air in and out.
AMT drivers are relatively complex and expensive to manufacture, and in the case of units with extended operating range down to 1kHz, they are bulky. They do possess all the advantages of transducers with light diaphragm and flat voice coils with negligible inductance, producing a very transparent, natural sound.
Planar Ribbon (planar-magnetic, ribbon, magnetostatic, isodynamic)
Planar ribbon tweeters have been experiencing a surge of popularity in the high-performance market. New Neodymium magnets and high-tech PEN and polyimide (Dupont’s brand names Teonex and Kapton) materials are facilitating new designs with high sensitivity, robust field operation and cost-effective manufacturing. In planar ribbon tweeters, a thin diaphragm consisting of foil/film laminate with etched flat conductors is stretched and positioned between high-energy magnetic systems.
Planar ribbon drivers possess all the benefits of thin film “zero” inductance tweeters, including electrostatic. They produce transparent, natural sound and are relatively easy to manufacture at a reasonable cost. They also offer high sensitivity up to 103dB/1m/1W with some pro devices, and they can be made small without sacrificing output.
Planar ribbon tweeters do not require any matching transformers or wall outlet power. Their diaphragm is very robust and weather resistant. Unlike other thin film tweeters planar ribbons’ form factor ideally suits contemporary trends in the loudspeaker market and are very design and application “friendly.”
Traditional electrostatic speaker company Martin Logan, for example, uses planar ribbon tweeters from BG Radia for its “alternative thin film” line of speakers for home theater and architectural applications. There are also several companies developing and manufacturing planar ribbon drivers and speakers: BG Radia, SLS Audio, Wisdom Audio, ADAM Audio and Hi–Vi Research to name a few.
Speakers need amplifiers, so learn more about home theater receivers here.
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