A P100 Mask Intercom

COVID-19 PPE taken to the next level. Anticipated to be good against aerosol transmission.

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This is for all you worker bees out there who already own a respirator. A P100 mask should provide the wearer greater security against infection by respiratory pathogens than a cloth mask (see: and it doesn't fog up your glasses. It is made of rubber and hard nylon and seals to the face without leaks if the straps are correctly adjusted and the height of the mask on the face is correctly adjusted. A suction test and a pressurization test are needed each time the mask is put on. However, you can’t talk and be understood while wearing one. You have to remove the mask to talk–precisely the highest-risk situation, and talking seems unavoidable in social situations. I therefore fitted a home-built intercom box to a P100 that I already had and put an electret mic inside. This project cost me about $30 US for the main items.
  • 1 × P100 mask medium, 9200, McCordick
  • 1 × black project box Hammond 1593K
  • 1 × grey project box Hammond 1593J
  • 1 × speaker Mode 61-650-1
  • 1 × electret microphone Mode 72-160-1

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  • Post-submission Field Notes

    David Matthew Mooney10/05/2020 at 18:49 0 comments


    I have a tendency to lean in to my listener if I am not being understood while using the intercom, which breaks social distancing rules. Turning down the volume usually works better because that gives less distortion. I have to remind myself that the intercom is not a bullhorn, and it is not karaoke, it is just a voice replacement and it doesn't have to be any louder than my normal voice.


    I am treating all condensate that forms inside the P100, which is plenty, as if it were full of SARS-CoV-2. Options seem to be: after use, drain mask with due care, blot the inside, dispose of the wet paper towel carefully, spray mask inside and out, and hands, with A) 70% isopropanol, or B) 99% isopropanol.


    We have integration!

    And the first field test thereof: A trip to the local cop shop by public transport to file a crime report ("No incident should be considered too small to report."), but that is a story for another day. My thoughts:

    1) Intercoms are not magic. It seems that even the commercial units have limitations. When using this intercom, you need to speak up and you need to speak slowly and clearly. I understand from reddit that Walmart employees should do the same.

    2) Now that the cold weather is here, condensate is rapidly forming on my cloth mask, making exhalation difficult. When that happens, I just slide the mask up or down to put a dry spot over the end of the exhalation tube. Five minutes later, it's time to shift it again. This is only an issue in cold but crowded places like bus shelters, where I can't simply remove the outer mask.  We'll see what a January in this town throws at me.


    The tube for the exhalation valve kept falling out, so I fixed it as shown. The white blob is putty, which I added to prevent any air leakage. (The blob on the near side cannot be seen in this shot.) The wire is 20-gauge, tin-plated copper (header wire). Holes were drilled with a 3/64" twist drill. I am solving the understandability problem and the exhalation-filtration problem on separate masks, and will eventually integrate the results.

    When my doctor saw me wearing the dual mask arrangement, he said, "I like your mask." Twice.

    I got a COVID-19 exposure alert on my phone this month and had to go and get tested, then self isolate until the results were ready. Result: Negative. Thanks to Bill & Sal for bringing me groceries.

    I have tried 3 different cloth masks with the exhalation tube. Two were OK but the third offered too much resistance to airflow. One needed a rubber band to cinch it around the tube, two didn't.


    False alarm. People can only understand me while I wear a P100 mask with the intercom switched off if I am saying something they hear every day, such as "Bag with that," or "Medium, double-double." If I try to say something unusual, such as "What do you think of Tolkien?" I am likely to hear: "Couldn't say, I've never toked."


    Lately, it seems that persons who deal with the public occupationally can understand me OK with the intercom switched off. How could I be so mistaken? I have clear memories of people telling me I sound like I'm speaking Mandarin if I tried to talk (English) while wearing a respirator. I now feel like a fool but I will be looking into this phenomenon. 


    The photos show how I am filtering the exhalation air from a P100. There is obvious backpressure on exhaling, but that just pressure-tests the mask + new filter with every breath. It's not disabling; I can climb 2 flights of stairs without stopping or getting winded. Moderate coughs can be filtered if the straps are tight, but not violent coughs or sneezes, which leak up the sides of the nose. Fortunately, I don't cough or sneeze a lot.

    10/19/2020: The gallery photos have been updated to show the new, tripartite intercom design.


    A) I have decided to redesign the project to mechanically separate...

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  • 1
    Build details for mask intercom

    The speaker is protected from disinfection solutions by plastic film taped inside the enclosure, and the speaker is clamped onto the film using mirror brackets bent to fit in a drill-press vise, to provide further mechanical stabilization. (I disinfect the speaker and the mask with 70% isopropanol spray.) The white strap on the speaker box is a creative use of straps sold for holding the corners of unfitted sheets on mattresses. The other straps come from a spare P100 mask. (At the start of the project, I had 3 identical P100s, all bought before the pandemic.)

    The microphone board is secured inside the mask with two stainless bolts that also serve as the signal conductors. There are 3 nuts on each bolt, all stainless. The first secures the external brass connection lug, the second seats on the white nylon mask insert with a stainless lock washer underneath, and the third is an acorn nut that seats on the internal signal lugs, which I place on top of the mic board. I also include a rubberized-fiber washer under the board to prevent short circuits. No messy protective coatings were applied except around and under the microphone capsule; I am putting all my faith in stainless steel.  However, I am not quite satisfied with the amount of mechanical stability this arrangement of the nuts gives me. The alternative seems to be an additional nut, to place a nut just above and below the hard nylon insert to clamp it tight, but this would have required somewhat longer bolts, which I had difficulty finding.

    When drilling the holes in the mask for the microphone bolts, there is a risk of the rubber part wrapping itself around the drill bit and coming away in a big chunk. Not good, considering what we are about here. This happened to me when drilling from the inside out but not when drilling from the outside in. The trick may be to go slow and keep the rubber pressed firmly to the nylon insert while drilling, which is not an issue if drilling from the outside in. It's a female dog to fully separate the rubber and nylon parts so they can be drilled/punched separately. Remember to leave sufficient flat area around the hole to seat a future nut, especially inside, and don't leave any turnings between the rubber and nylon parts.

    The mic is protected from humidity by a 1.5" square of two layers of film cut from a sandwich bag and held in place with a 7/16" O-ring. Before the film was put on, I glued a 5/16" dia × 1/16" thick spacer ring (improvised from a small split ring sold for keychains) on top of the microphone capsule with tire-repair rubber cement. Without this spacer, reproduction quality is unsatisfactory. After pushing the O-ring all the way down, I pulled it back up a little to give the film some slack; this also seems to help with sound fidelity.  

    The battery is lithium-ion type, 9V. It connects to the intercom box with an adapter that plugs into a barrel jack on the box. The wires of this adapter tended to break at the point where they enter the 9-V clip, so I reinforced this spot with epoxy and a zip tie. The power seems to be good if I can hear a harsh noise burst on the "s" and "sh" sounds. (I understand that this is caused by the unavoidable nearness of my mouth to the mic.) If you do not hear this, try a new battery; this can markedly improve sound fidelity. 

    A problem with the P100 is that there is no filtration on the exhalation valve, so others are not protected if the wearer is infected by SARS-CoV-2 in spite of taking unusual precautions. You can mitigate this by wearing a second mask over the P100. The second mask is of fabric and hooks around your ears per convention. Make sure the cloth mask covers the exhalation valve. The project log has more to say on this.

    The cabling is mechanically polarized to prevent mix-ups between the mic and speaker connections. The mic accepts 2.5 mm phono plugs and the speaker 3.5 mm. A small, removable adapter connects the external signal lugs on the mask to a sub mini jack. This arrangement was a safety precaution. If something snags one of my cables, I want the cable to pull away instantly and not take my head with it. All plugs are therefore straight, not right-angled. 

    I made the 3', 2.5 mm phono cable by cutting a female-to-male, 6', 2.5 mm cable in half and putting a 2.5 mm plug on the male side. Insist on a mono plug: stereo is too hard to solder. 2.5 mm technology seems to be going obsolete; poor selection in  stores and Digikey. An alternative would be to make one patch cable male-to-male and the other female-to-female, all 3.5 mm.

    To make the little brass adapter (pictured), I removed the red insulation from a quick disconnect and unrolled the cylinder inside to make a flat square. After fabricating the brass bracket, I soldered it to the square, taking great care to get the angle right. I pre-tinned both pieces, then clamped them between heat insulators held in a drill-press vise, then reheated with my fattest tip and added rosin-cored solder until everything flowed. Then I filed off the excess solder to make it pretty. My heat insulators were dremel abrasive disks, but pieces of circuit board would have been smarter.

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