Finding a bike helmet

I wanted to start riding my mountain bike again. One thing had been holding me back -- my old helmet would not work with my CI. The external headpiece is in a location on my head right where just about all bike helmets have a strap assembly. I could not come up with any way to modify this area without compromising the helmet's function and safety.

After a bit of online research I found a helmet made by Fox Racing, model name "Transition," that does not have any strap components in the area of the CI. The price of $50 is reasonable for a bike helmet.

Transition helmet on foxracing.com

It looks in style like a combination between a skateboarding and BMX helmet. I found some favorable reviews noting that it is lighter and better ventilated than similar models.

My idea was to remove a small area of the inner shell at the location of my CI magnet. My goal was to make three main improvements:

1. Safety: A recess in the shell at the CI headpiece location should lessen the impact on the headpiece and internal implant if the helmet takes an impact in that area.

2. Comfort: Eliminate the pressure of the helmet on the CI headpiece

3. Function: Prevent the CI from cutting in and out as the exterior CI headpiece gets moved by normal helmet jostling.

I found one at a local store, and after checking that the inner shell was accessible for some cutting at the general area of interest, I purchased it.

Donning the unmodified helmet confirmed that it put a lot of pressure on my headpiece. It would be uncomfortable and unsafe to wear.

NOTE: MODIFYING A PIECE OF SAFETY EQUIPMENT IS NOT RECOMMENDED BY THE MANUFACTURER. DOING SO VOIDS ALL WARRANTIES AND COULD COMPROMISE THE OVERALL SAFETY OF THE HELMET. DO SO ONLY IF YOU UNDERSTAND AND ACCEPT THE RISKS.

Here's an overlay photo that shows the area in question in my case:

STEP 1: Mark the location

I had my wife use a small piece of white tape to mark the location of the headpiece while I put the helmet on and off several times.





I then marked the interior to match the location.



STEP 2: Excavate inner core for headpiece

I used the tip of a 3/8 inch bit in a standard drill to slowly grind away an area about 1.5 inches in diameter and about 3/16 inch deep.



After donning the helmet and checking that the location was correct, I deepened the excavation to about 3/8 inch and rounded the edges slightly, and made a small bevel in the location of the headpiece's wire attachment.



STEP 3: Excavate for implant (if necessary)

Since I have an AB implant, the implant is about the size of two quarters placed next to each other. I also removed about 3/16 inch in the area of that location which is toward my ear.



I finished it off by smoothing the surface and edges of the excavation by simply rubbing the area with my thumb. The material is relatively soft.




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The helmet fits and feels very comfortable after making the modifications. I was also pleased to find that the helmet curves quite closely to my processor over and behind my ear. Not too close to hit it, but close enough to keep it from popping off easily.

And, as an added bonus, my little boy looks like Dark Helmet in Spaceballs when he puts it on :)

To AGC, or not to AGC

Way back at my last mapping appointment, I had my backup processor loaded with some test programs so I could have a listen to different settings that are available. I finally got around to trying the first one out. The only difference between it and my typical one is that AGC (Automatic Gain Control) is disabled in the test program. AGC is by default active when programming the Harmony processor, so any of you Harmony users most likely use AGC all the time.

AGC is defined by Advanced Bionics as "A dual loop gain and compression system. One loop adjusts the gain to soft level input while the second loop attenuates transient loud input. The dual AGC automatically adjusts patient sensitivity for optimal audibility and comfort." And "AGC automatically optimizes gain and compression to the listening environment. Disabling AGC results in peak clipping and may be situationally preferred by some listeners."

I spent a few hours switching back and forth between my normal program and the test.

Pros (AGC OFF):

• No pumping. Pumping is a side effect of loud transients (e.g. dishes clattering) causing other concurrent continuous sounds to vary in volume. It is really only a minor annoyance when AGC is ON.
• Transient peaks don't squash other sounds. When AGC is ON, loud bursts reduce overall volume quickly, with a several hundred millisecond release time delay. Other concurrent sounds, such as the voice of someone one is listening to, are momentarily lowered in volume which can affect understanding.
• More natural sound in a setting without wide-ranging levels.
  

Cons (AGC OFF):

• Transient peaks distort easily. The importance of this affect depends on the source. The distortion of a rapid transient peak, like a hand clap, is easily ignored. But with a loud talker at close range, the distortion reduced intelligibility.
• Music quickly turns to a distorted messy mush of sound as volume increases. Awful.
  
• Soft sounds are not made louder. Quiet speech and other desirable soft sounds are harder to hear well.
  

Conclusion:

I don't think I will be using an AGC OFF setting very often. AGC handles a wide range of situations very well, with only a couple of shortcomings. I think it might be most useful in a controlled sound environment in which there are occasional transient bursts causing pumping when AGC is ON -- such as watching TV while one's child is banging on toys :)

I am almost certain that AGC as implemented in the AB Harmony processor is a full-band process. It compresses the whole signal. I think a future improvement would be to make it a multi-band process, which is what is used in most hearing aids. The benefit is much less pumping and squashing since only narrow bands are normally affected.

I will see if I can find any situations where disabling AGC results in enough improvement to garner a permanent slot in my processor.