Speakers: Parts is Parts 2
Styles, Dome Tweeter Variations and a Balanced Drive Tweeter.
In this article series about speaker parts we explore the tweeter, a class of transducers designed to reproduce the upper range of audio frequencies. In the previous article we covered a bit of tweeter history, discussed the cone tweeter, and introduced the common dome tweeter. In this article, we will explore dome tweeter variations and introduce the balanced drive tweeter.
Dome Tweeter Dispersion
Dome tweeters generally have better dispersion than cone tweeters, but many factors come into play. Even dome tweeters have many variations including: soft and hard domes, back chambers, phase plugs, voice coil attachment (at the peripheral or modally driven), and magnetic structure topology.
First, let’s explore the speaker diaphragms' “effective radiating area” and how this consideration impacts the dispersion of tweeter cones and domes.
Simply measuring the tweeter diaphragm’s diameter often does not directly correlate to the transducer’s dispersion. When a tweeter’s diaphragm works as a perfectly rigid piston, the theoretical would more closely track the reality. But, this is not always the case.
The designer can intentionally try to stiffen the diaphragm assembly to increase its piston characteristics or intentionally design for the decoupling and flexing of the diaphragm. Soft dome tweeters are designed to blow with the breeze, while metal dome tweeters stand tall and stiff in the storm. In the case of hard domes, there are many ways to stiffen the diaphragm.
The diameter can be reduced, but this complicates many other design aspects. In general, smaller dome tweeters provide wider sound dispersion at the highest frequencies. However, smaller dome tweeters have less radiating area, which limits their output at the lower end of their range.
They also have smaller voice coils, which limit their overall power output. Another way to increase stiffness is to have the diaphragm with a deeper contour (e.g., a deeper dome or cone). As with anything else, too much will get you into trouble. A higher Young’s modulus (stiffness) of diaphragm material can be selected. It is possible to switch from a polyester like Mylar (a molten polyethylene terephthalate or PET) to a higher stiffness polyester like Teonex (a polyethylene naphthenate or PEN). In the case of hard domes, you can switch from aluminum
The topology of the voice coil-to-dome attachment can also be changed (e.g., a modally driven voice coil). Many inverted dome tweeters have used modal drive, which is where the voice coil is attached at the diaphragm’s half diameter. By driving the dome where its first break-up mode is physically located (halfway up the diaphragm), you can shift the break-up frequency up an octave. This is especially practical for inverted dome tweeters.