Quote:
Yeah, those symbol are meant to be confusing. However, if I have read the patent that Ety filed for MC5 correctly, L2 (coil) is basically an auxiliary air duct (side tube), R2 (resistor) is basically an acoustic damper (filter), and C2 (capacitor) is basically an acoustic capacitance (empty space)
The design is meant to absorb excess acoustic energy and tune the sound. Any unwanted peaks will penetrate over the damper from the side tune (while normal sound wave won't get pass easily due to by the damper's acoustic impedance) and get trap in the empty space and loses its energy. The overall result will be a smoother FR curve.
To expand a bit on this:
I don’t know how confusing the symbols are meant to be, but they are referencing an electronic circuit directly analogous to this acoustic cavity. The circuit shown is an inductor (coil L) in parallel with a resistor (zig-zag R) and a capacitor (parallel lines C), and a ground. This type of circuit (RLC) is very useful due to its frequency-dependent impedance properties.
The circuit analogy
Inductor (L):
Basically, an inductor is a coil of wire, which opposes rapid changes in current (more specifically, magnetic flux, ultimately due to special-relativistic effects). In other words, an inductor acts like a normal wire below a certain AC frequency, but starts behaving like an open circuit (think, no current flow) for high frequencies.
Resistor (R):
Most of you probably know what a resistor does. It’s just a piece of material that impedes the flow of electric current, requiring more electric force to push the same number of electrons in a given time period than a less resistive material.
Capacitor (C):
A capacitor is actually a little harder for me to explain in simple terms. Basically, imagine two conductors near to each other but unconnected. If you shove electrons onto one conductor, any electrons on the other will feel a repulsion. For an AC current, this behaves just like a plain old wire, because the push and pull of electrons on one conductor is mirrored in the movement of the electrons on the other. That is, unless your AC is at a low enough frequency. At a low enough frequency, you will accumulate enough electric charge on the first conductor to oppose any further accumulation of charge. The same thing happens on the other side, but for the opposite sign of charge (electrons bunched up on one plate will push electrons away on the other, causing a net positive charge to accumulate over there, since there aren’t enough electrons to cancel out the proton charges in the nuclei).
Thus, just as the inductor behaves like a brick wall for high frequencies and a plain old wire for anything else, a capacitor doesn’t allow low frequency AC currents to flow through it, but is transparent to mids and highs.
The acoustics
OK so now you know all about L, R, and C. What does this have to do with sound?
L and R:
Basically, an acoustic pipe behaves like an inductor/resistor combo for air, rather than electrons. It opposes high frequency sound pressure oscillations from flowing through it at all, and it impedes all airflow to a certain degree. I won’t try to explain why, since I don’t know enough about fluid dynamics or acoustics to give a good explanation, but it has to do with molecules knocking and rubbing together in the air and against the tube walls.
EDIT: after studying the diagram a bit more, it appears that Etymotic has put some sort of acoustic damping material across the “bottom” end of the branch pipe, which is what they are referring to as a resistor in the circuit analogy diagram. It will behave as described above, impeding the flow of sound pressure currents equally at any frequency.
C:
An acoustic cavity behaves like a capacitor. You can push a high frequency sound pressure oscillation through it just fine as the air in the chamber expands and contracts a bit to accommodate the changing pressure, but at low frequencies you can build up enough positive or negative pressure in the cavity during one half-cycle that it opposes any further change in pressure, stopping the sound pressure oscillation from propagating through the cavity.
Putting it all together
By connecting a narrow pipe to an acoustic cavity, the smarties at Etymotic have built a series RLC circuit for air. What does it do? Based on the above physics, I think it works like this:
At middle frequencies, the “inductor” (pipe) and “capacitor” (cavity) have little effect, leaving a “resistor” (pipe+damping material) to prevent all of the sound pressure in the ear canal from dissipating into the cavity.
At low frequencies, the capacitor effect closes off the cavity entirely, thus boosting the sound pressure in the ear canal relative to the middle frequencies. Likewise, the inductor effect closes off the cavity for high frequencies, again boosting pressure.
In conclusion:
It looks like Etymotic Research has used built a resonant acoustic circuit, similar to a “Helmholtz resonator” according to my reading, into their earphones, designed to boost sound pressure levels somewhere in the high and low end of the frequency spectrum (or, equivalently, reduce the pressure somewhere in the midrange). The exact frequencies where these effects dominate, as well as their extent, depend entirely on how they decided to tune the geometry of the pipe and cavity. Probably, they had some target frequency response curve and engineered this clever gizmo to match it as closely as possible.
For more on acoustic circuits and their electrical analogies: http://mysite.du.edu/~jcalvert/waves/acoucirc.htm