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Etymotic Mc5 AccuChamber Side Branch Diagram; Acoustic Sound Tuning

post #1 of 21
Thread Starter 

http://g-ecx.images-amazon.com/images/G/01/electronics/detail-page/AccuChamber_large.jpg

post #2 of 21

What a complex piece of engineering! Thanks for posting it up.

post #3 of 21
Thread Starter 

One thing I don't quite understand is the acoustic side branch, particularly the L2 R2 C2 symbols, ect. Anyone have an idea of what they are? Seems like a circuit diagram, but I don't think it's a crossover.

post #4 of 21

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.

 

 

Quote:
Originally Posted by stevenswall View Post

One thing I don't quite understand is the acoustic side branch, particularly the L2 R2 C2 symbols, ect. Anyone have an idea of what they are? Seems like a circuit diagram, but I don't think it's a crossover.

post #5 of 21
post #6 of 21

No. MC5 isolate several times better than ER20.
 

Quote:
Originally Posted by Nachash View Post

Mmh, they isolate like an er20?.

post #7 of 21
Thread Starter 

Thank you ClieOS for the explanation, greatly appreciated!

 

Right now I am listening to them with Monster SuperTips (yeah, Monster, I know...) I wonder if tube 3 tailors the sound much, an the Supertips are not as constrained on the tip as the tri-flange is.

post #8 of 21
Thread Starter 
This video immediately reminded me of Etymotic's dampening side branch:
post #9 of 21
Quote:
Originally Posted by ClieOS View Post

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


Edited by UltraBird - 4/12/13 at 3:01am
post #10 of 21

When it comes to impedance networks, I like this explanation:

 

Resistance (electrical), friction (mechanical) and viscosity (acoustic) all oppose their driving forces (voltage, force, pressure) in a linear fashion.

 

Inductance (electrical), mass (mechanical), and inertance (acoustic) are all ways of storing kinetic energy. They respond to changes in motion (current, velocity, volume "current"). Can act as low-pass filters.

 

Capacitance (electrical), compliance (mechanical), and compression (acoustic) are all ways of storing potential energy. They respond to changes in the driving force (voltage, force, pressure). Can act as high-pass filters.

 

Quote:

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) 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.

 

 

Are you referring to the "inductance" marked L2 here, or the whole rest of the sound tube? The way they caption the picture makes it very confusing.

post #11 of 21

When I refer to “inductance”, I’m referring to L2, which is the acoustic pipe connecting the main canal to the acoustic side branch. Apparently (I know a bit about electronics but I’m learning about acoustics as I go), pipes impede transmission of high frequency sound pressure oscillations.

This acoustic mechanism is totally different than the mechanism for electrical inductance, which is ultimately a relativistic effect (due to length contraction of the spacing of stationary nuclei in the moving electrons’ reference frame), but the analogy is very useful for engineering purposes.

post #12 of 21
Thread Starter 

Thanks for the further clarification... I haven't checked the thread in a while, but it's great to see it's getting some use.

post #13 of 21

This is really impressive. It would be even more interesting to see a comparative frequency response diagram, showing how all this actually shape the sound. Are the effects minimal or really worth the effort? Maybe it's just another gimmick?

post #14 of 21

post #15 of 21
Thread Starter 

Hmm... Best compare it to their flagship I think. (Can you say bland, because the SE535 can! (Not that it was terrible, and I may like the odd sound if I got used to it, but it didn't click for neutrality for me, or for being musical and enjoyable... Oh, and it's like $300 ~$500.))

It does seem like the chamber is doing smoothing though. The spikes are pretty mild.

 

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