Wednesday, July 3, 2019

3D Printed Pneumatic Headphones for 32 Ch. Headcoil

A common complaint when using the 32 channel head coil is that there are few options available for delivering auditory stimuli to subjects. The inner diameter of the coil is considerably smaller compared to that of the 12 channel coil and limits the use of the standard Siemens pneumatic headphones that come with the 3T Tim Trio. Some current alternatives are to purchase an expensive electronic ear bud system or to crank up the volume for the speaker inside the magnet room and hope that the subject can hear at least some of what you're saying. For most researchers neither of these are adequate solutions, so I was tasked to come up with an alternative that fit the following criteria:
  • Integrates into existing pneumatic systems
  • Durable
  • Low cost
  • Slim profile
  • Comfortable
  • Easily reproducible
If you've read my last few posts then it's no surprise that I decided to use Tinkercad and our in-house 3D printer to come up with a suitable solution. A link will be provided at the bottom of the post for those wishing to print their own.

The Siemens Option

The default Siemens headphones work wonderfully...if you are using the 12 channel coil. But when using the 32 channel it's useless to all but the smallest of heads. The headband and thick headphone chambers simply do not fit inside the coil along with a subject's head and adequate padding. Thankfully pneumatic systems like this don't have many components, so I won't have to alter the design too drastically to come up with a solution that fits our criteria. 

Figure 1 - Siemens pneumatic headphones 
Figure 2 - Siemens tube connector

The 3D Solution

Keeping in mind the aforementioned criteria, I used the Siemens system as a starting point. Eliminating the headband and slimming down the profile of the headphone chamber instantly gave us the extra space required. Creating a ridge in the headphone chamber is the simplest design solution (see Note 1) as it provides a means to secure the cushion in place and slim down the design even further. I moved the tube connection to the bottom of the headphone for a more streamlined appearance and to make it easier to attach the "Y" connector. The original Y connector never posed a problem, but I tapered the holes just slightly for a tighter fit around the tubing and slimmed down the design to save on material cost.

Figure 3 - Slim 32 Ch headphone chamber design
Figure 4 - Y connector solid and transparent
For those a bit more interested in the design details, I used a pair of Bose headphone cushions as the template for the shape. They are similar in size and shape to the Siemens cushions, though any cushions would work fine. It's mostly a matter of personal preference so long as all the proper measurements are taken to ensure a snug fit. Same goes for the tubing, though for the sake of easy integration I'd recommend sticking with the 1/4" and 1/2" tubing that is standard for this system.

Final Print

Due to the simple nature of these components, the material and settings for printing them is largely up to personal preference. Though because they're likely to take a bit of a beating I printed them out of ABS with 2 mm layers, 95% infill and supports. 95% infill may seem like overkill, but since we're dealing with a pneumatic system you have to make sure there are no holes that could reduce the quality of the audio (See Note 2). Plus the pieces are so thin and small that you don't want to run the risk of them being too brittle for every day use.

Figure 5 - Components w/supports on build plate
After a few hours of printing I got the following results:

Figure 6 - Headphone chamber printed with ABS
Figure 7 - Y Connector printed with ABS
The tubing fits securely inside the connectors without the need of any extra adhesive, though you can use a bit of superglue to make sure it stays put (see Note 3). It takes some effort to insert the 1/4 inch tubing into the Y connector, but that makes for a better fit!

Since I used a personal pair of headphones as the template, replacement cushions were ordered that match the dimensions. They are of higher quality than the Siemens headphones and serve multiple purposes; providing comfort, support, and ear protection. The cushions come with a black mesh that I super-glued to the inside to cover up the headphone chamber. Since I measured the dimensions of the hard plastic ring that shapes the cushion earlier it should fit snug on the ridge. Once satisfied that the fit was correct I went ahead and used superglue again to secure it in place.

Figure 8 - Headphone with cushion and mesh
Figure 9 - Pair of headphones with cushions installed
When it came time to integrate these into the Siemens system we purchased the following connectors and inserted them into the tubing for our new headphones as well as the originals. Some modification of the Siemens headphone tubing is required (See Note 4), but now it only takes a quick push of the button to switch between multiple pneumatic options.  As you can see in Figure 11, the quick release connector allowed us to created a microphone using a funnel. The possibilities are endless!

Figure 10 -  Quick release connector
Figure 11 - Siemens headphones (A), slim 32 ch headphones (B) and funnel microphone (C) 

Final Fitting

With the headphones finished it is time to test them out! As you can see below, even with the headphones inside the coil there is still room for more padding on the side of the head if needed. But the standard headphones only fit inside when set to the smallest setting and adding extra padding is no longer an option (see Note 5).

Figure 12 - New headphones inside 32 ch coil
Figure 13 - Siemens headphones inside 32 ch coil
OK so they fit, but do they work? Of course they do! Pneumatic systems like this are simple and straightforward. So long as you secure all the connections and there are no holes in the tubing then you're good to go.

You can access the original TinkerCAD model and download it here. If you are using the same cushions and tubing that I mentioned earlier, then you should not have to alter the models much if at all (See Note 3). But if you decide to go a different route then you'll have to use TinkerCAD and do a bit more in-depth adjustments. Thankfully they have several basic tutorials that should be enough to get you going.


Note 1: I went through several iterations before going with this design. Some required cutting holes in padding, while others proved difficult for my 3D printer to complete. You can't make an omelet without breaking a few eggs, but for the sake of simplicity I chose not to include every step.

Note 2: PLA, ABS, PETG or just about any other material would be suitable. It's mostly a matter of personal preference, but durability should definitely play a factor in your decision. I would even suggest printing out a few pairs and keeping them handy in case something should happen

Note 3: All 3D printers are not created equally. You may need to use adhesive or alter the size of the tubing holes in the STL model if the parts don't fit snug after you print them.

Note 4: The modifications are simple: 1) Cut a length of tubing roughly a foot long from the end of the 1/4" thick tubing that connects to the foot of the bed. Leave it plugged into the foot of the bed. 2) Insert the larger quick connector piece (the one with the button on it!) into it. 3) Insert the smaller quick release connector into the long piece of tubing that connects to the headphones. 4) Repeat for every pneumatic system you want to integrate (in case you design your own headphones later on!)

Note 5: The polystyrene head is by no means meant to show that everyone will have as much room when they use the slim headphones. It is meant to showcase that there is more room than if attempting to use the standard Siemens headphones. 

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