Sennheiser HD800 Part3: In-depth analysis #1

A ring radiator explained

When Sennheiser first introduced HD800 in 2008, they introduced a technology which has never been seen before in headphones: a ring radiator transducer. This is from Sennheiser themselves:

"A completely new type of ring driver, delivering the best ever simulation of spatial hearing by way of a curved sonic wave front. Superlatives in so many parts combined to create an overwhelmingly rich and detailed sound experience."

What makes this 56-mm wide dynamic transducer with a hole on it so special, compared to other conventional dynamic dome types? According to Eargle, the author of Loudspeaker Handbook,

"The typical soft dome acts as a unit at lower frequencies; however, at progressively higher frequencies, the mass of the dome gradually decouples so that at the highest frequencies the radiation is primarily from the voice coil itself. It in effect becomes a ring radiator at the highest frequencies."

As a voice-coil gets driven in a piston motion, the sound waves emitted from the inner/outer annuli inevitably interfere with one another. This can cause a phase cancellation / asymmetric radiation in the high frequency range, which substantially affects fidelity of sound, or "sound imaging", as some people say. With a hole in the center, the phase interference simply disappears. Since the manufacturer claims that the radiator delivers "natural sound field" and "spatial sound field experience", this interpretation is most likely correct. In order to further investigate the secret of a ring radiator, I asked a lot of people, and finally found a lead: Sennheiser's European patent, DE102007005620.

On their patent document, Sennheiser outline the working principle of HD800's driver in detail:

1. The principle of a ring radiator

2. Two conductive diaphragm surfaces insulated from one another, but connected to the coil.

3. An acoustic vent on the rear magnet ring

4. A curved acoustic baffle

1. An annular diaphragm with variable compliance

First of all, since the diaphragm has a hole on it, the inner/outer ridges are mounted on the chassis, while the voice-coil is located in between, where the stiffness is the greatest.

"The dynamic sound transducer...has a ring radiator with a vapor-deposited film (Duofol) to reduce the resonance frequency. Thus there can be provided a wideband transducer...Oscillation modes can propagate worse due to the enlarged periphery of the diaphragm. It is thus possible to achieve a regular amplitude and frequency characteristic."

It seems the ring radiator tames the high-frequency oscillation & frequency response altogether. Interestingly, instead of "the hole" at the center of the ring, Sennheiser's diaphragm technology with variable thickness (aka Duofol) plays much more important role in this "phase cancellation" issue:

"The fact that the diaphragm is not of a uniform stiffness means that the magnitude of the sound-emitting surface area depends on the frequency. At low frequencies a large part of the ridges oscillates homogeneously with the coil and thus represents a large sound-emitting area. If however the frequency is raised only a near region of the coil seat oscillates so that the sound-emitting area is reduced. Thus high frequency components can be correspondingly emitted. The upper limit frequency of the dynamic sound transducer is adjusted in this case outside the audible range."

Of course, as the stiffness of a center diaphragm increases, its normal modes will definitely be reduced, and thus the resonant frequency will be lowered consequently. And when this Duofol technology is combined with a hole in the middle, even greater oscillation control is achieved. 

2. Electrically-conductive diaphragm

To my surprise, HD800's diaphragm is actually aluminium(in a few Å thickness) & gold(in 2 kÅ thickness) vapor absort, connected to the voice coil separately. As a result:

"Thus there can be provided a dynamic sound transducer having a nomial resistance slightly greater than the nominal resistance of the coil."

 In addition, this can effectively increase the sound-emitting surface area, according to Sennheiser:

"If the active sound-emitting surface of the dynamic sound transducer is increased in size, shorter stroke movements are made possible for producing the sound signals, and that can reduce distortion...therefore involves the notion of reducing or avoiding distortion phenomena which occur, by the diaphragm surface area being increased, with an upper limit frequency being maintained. In addition there is provided a dynamic sound transducer having a reduced resonance frequency so that such a sound transducer can be used as a wideband transducer. A greater periphery relative to the cap-shaped transducer is provided to reduce the oscillation modes."

Again, by increasing the area and its stiffness via metal vapor absorption, a great control over distortion, resonance, and oscillation can be achieved. Eyes can be only deceptive; its coating is so thin!


3. An annular magnet venting

With the vented rear magnet, the diaphragm is directly connected to the outside, working independently from the rear acoustic cavity:

"It is thus possible to avoid in particular unwanted acoustic bouncing due to compression of the air cushion."

This will certainly boost the sub-bass frequency region nicely, but also introduce some amount of odd-harmonic distortion inevitably.

4. An ergonomically shaped housing

An acoustic baffle, also known as an enclosure, is to guide the sound wave emitted from a mounted driver in a certain way, so that the sound is directed as engineered. If poorly designed, the baffle can affect the sound in a harm's way. Sennheiser utilize a curved housing, in order to minimize any type of resonances & reflections that can derive from the baffle's geometry.

"..to control the acoustic path to the outside world and to the rear side of the sound transducer..There are therefore no unwanted acoustic effects due to an additional protective housing."

Any empirical proofs?

From the manufacturers themselves, a ring radiator has been fully explained. Theoretically, it is the best dynamic transducers in terms of electroacoustic performance according to the manufacturer; Low oscillation, low distortion, wide frequency range, and low phase interference. However, there is a problematic resonant peak at 6 - 7 kHz range when it is measured. What could have possibly caused that? Also, what does the hole exactly do to a conventional dome transducer?

In order to finally answer these questions, I obtained a 600-Ohm mylar micro transducer. This will be analyzed inside and out, including T-S parameters. And later will be punctured in the center for ring radiator simulation. The deviation between Sennheiser's official frequency response data and conventional HATS measurement data will be discussed as well.

Continued to part 3, In-depth analysis #2


References

[1] German Patent DE102007005620 "DYNAMIC SOUND TRANSDUCER, AND RECEIVER " granted to T.F. Till, K. Markus, G. Vladmir, H. Dirk, E. Heinz, M. Burkhard, G. Axel, M. Andre, assignors to Sennheiser Electronic, 2010 (Filed 2007).

[2] J. Eargle, "Loudspeaker handbook" 2nd Edition, (Springer, 1994).