Cone Overtravel Protection

This web page shows a scheme to sense the motion of the 15" speaker cone and put a limit on the speaker current, so as to avoid damaging the cone and voice coil due to excessive motion. (Remember, this is a $225 speaker!)

Proposal: CAD Engineering at A.R. Johnson Magnet School takes up a project, designing and 3-D printing a plastic bracket for the black sensor in the following image. Time frame: Fall 2015, so that the bracket is ready to use a month before the 2016 Career Day, when Mr. Engelbrecht will offer an audio-system demo during Career Day, emphasizing many engineering aspects of audio systems. Contact: 706 421-6044 johnenge@earthlink.net Students may operate the demo if there is time to rehearse.

Above: this 15" speaker is heavy, at 15.8 pounds. The cone is able to move +-.5 inch before distortion gets too much. Beyond about +-.8", damage happens: cone rupture, or smashing the voice coil into the bottom of the magnet gap. Other info: this subwoofer normally handles audio program material up to 80Hz. Mr. Engelbrecht provides a custom audio amplifier that takes the overtravel sensor signal and limits current. The black sensor is a slotted, infrared transmission sensor that has a light beam. The paper flag has a window, 1" high. When the light beam is going through the window, things are OK. Beyond plus-or-minus 1/2", the beam gets interrupted, which is the signal to limit current. The photo shows a temporary mount, a steel wire. A 3-D printed bracket would be a much better way to mount the sensor.

Photo above is a close-up of the sensor and flag. The flag is taped onto the cone. This sensor actually is a dual sensor, two LEDs and two phototransistors, 8 pins in all. Only one set is needed for the overtravel sensing. Mr. Engelbrecht will provide a printed circuit board to solder the sensor onto, and the PCB will have mounting holes for the 3-D printed bracket.

Photo above shows how the flag follows the cone down, when Mr. Engelbrecht presses the cone down about .2". When an audio program puts current peaks into the speaker, the window in the flag will travel so far that the infrared beam will be interrupted.

A side view. The profile of the rim is complex, Mr. Engelbrecht will provide a profile for input to CAD. During audio peaks, the cone travel upward is enough that the "surround" of the cone may poke up above the rim. Therefore, the bracket must have a certain clearance over the rim, as the temporary steel wire shows.

The sensor is removed from the rim to show that the infrared light beam goes from LED to phototransistor. The window in the flag is shown here with transparent tape covering the window, so there are no edges to catch in the sensor's slot.

Closeup of the sensor.

If A.R. Johnson magnet school can take on this bracket project, Mr. Engelbrecht will provide some sensors soldered to printed circuit boards, plus a profile of the rim of the speaker & the shape of the cone. He will also visit the class with the speaker so students can see the hardware.

Here are more engineering questions about the 3-D printed bracket.

Vibration of the sensor (because the woofer's cone produces a lot of vibration!) is not desireable. A stiff bracket is needed. How strong is the PLA or ABS plastic?

A tapered design of the bracket makes it stiffer: wider at the speaker rim, tapering down near the black sensor. But large cross-section means a long printing time.

There are eight mounting holes in the speaker rim, to mount the speaker into an enclosure. Can one of these holes help secure the bracket to the rim?

On the other hand, the back side of the rim needs to press against the enclosure (a flat panel with a 13.75" round hole), so the best design of bracket doesn't even wrap around to the back of the rim. So, maybe the bracket should mainly attach to the flat mounting panel.

An open question that awaits the completion of the amplifier: how violent is the motion of the cone, considering that the flag has to stay attached to the cone? The max freq for the cone is 140Hz. The flag is going to have a resonant frequency, if it is below 140Hz the flag may pop off if it isn't glued on (tape may not be strong enough to hold flag onto cone). You can't drill a hole in the cone, that voids the warranty. Hot glue is a possibility, or wood glue, or epoxy, or some other glue. Experimenting with a small, cheap speaker is going to be needed.

The flag must not be heavy, that would unbalance the cone & make the voice coil scrape inside the magnet gap, destroying the voice coil. That is why I assume the flag is paper. But this is open to consideration. Maybe a translucent plastic drinking straw would work, & be stiffer. The window in the flag would be made by slipping in opaque paper strips "above and below." If a cylindrical flag (a straw) is used, it can be guided through the sensor by threading it through a 1/3"-long, slightly larger diameter collar, the collar being fastened to the sensor's gap.

The flag's window can be an "inverse," the infrared light is blocked during normal cone motion & the light gets across when motion is beyond 1/2".

The sensor may have a large optical "cross section," and this maybe should be narrowed down by "masking." This would make a thinner, lighter flag workable.

The flag can be "captured" on the open side of the sensor by fastening a little C shape of wire or plastic, to capture the flag & keep it from tilting out of the sensor's slot.

There are other ways to sense cone motion, other than a flag. On the underside of the cone, there is enough perforation of the steel basket to shine a red laser 10" across the basket & let the cone motion interrupt the laser. Or maybe just a high-brightness LED would work. But you really need cone position detection on both push-out & suck-in motion of the cone. This would favor a bright LED & two phototransistors, which I have. This is actually an attractive proposition, and it still needs plastic bracketry to position the LED & both phototransistors. For demo purposes, a visible flag is much prefered on the top, but the backside detectors require no flag, & no gluing of a flag to the cone.

Drawing out the previous paragraph, there are other alternatives. Push-out sensing can be done by interrupting a light beam going 15" across the speaker rim, with a 1/2"-tall paper "bump" glued onto the "surround" of the cone. This would be much lighter-weight than a flag. Suck-in sensing can be by the back of the cone interrupting a light beam shooting between holes in the steel basket.

Don't let all these alternatives hold you back. Engineering is all about thinking of alternatives & choosing the best.

A reasonable question: doesn't the flag & sensor interfere with the enclosure that houses the speaker? Answer: The intended use of the speaker is to demonstrate in schools how a subwoofer works in an audio system. A typical, two-cubic-foot enclosure will not be used, instead the speaker will be mounted in a cardboard panel & set into a doorway, achieving an excellent "infinite baffle" enclosure. The speaker will be wide open for viewing, also with a strobe light to freeze the cone motion. (A metal screen with large perforations will give mechanical protection to the speaker.) The paper flag & overtravel sensor will have plenty of room.

Question: is this overtravel protection scheme used in commercial woofers? Answer: Mr. Engelbrecht doesn't know that it is used commercially, this is just a prudent way to protect an expensive speaker.

Question: is this speaker the best one for demos? Answer: Anything Audio in Martinez stocks only one 15" speaker, this one, JL Audio 15W0v3. Even though this speaker is designed for use in a small enclosure (in autos), the Q_ts of the speaker indicates that it may be fine for large infinite baffle. The X_max spec for the speaker is .5" (plus & minus). Without the air pressure of a small enclosure, this speaker requires only about 6 amps peak to achieve X_max. This is about 65 watts, far below the 500 watt limit of the speaker. The 500 watt rating shows how inefficient small enclosures are.

Question: is Mr. Engelbrecht an audiophile? Answer: no, Mr. Engelbrecht likes to hear the lowest frequencies in audio tracks, down between 20Hz & 40Hz, and the only ways to do that are headphones & large speakers. To share the experience, Mr. Engelbrecht is building a custom audio system with many controls, & has a CD from IASCA, their competition sound reference CD.

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