Before continuing the discussion, I just want to thank JF for his excellent questions. It is valuable to our film to engage in technical discussions with people who know their way around the issues. I truly value your opinions, knowledge and curiosity. Our film is meant to establish truth. If anyone here, or if any of the professionals we are speaking with currently unearth anything that calls our conclusions into question before or after we release the film, I will report that back to the community accurately. I would not have a film if it weren't for encountering this forum's discussion of Alan Bean's hammering sounds on Apollo 12. Thank you for tolerating what must seem unbelievable to many of you.
Continuing from my previous reply, let's look at your analysis a bit closer, JF: "This diagram shows that they add the signals from the EVA suits and the LM mike
." As stated previously, there is no separate "LM microphone", there is only an LM microphone jack
in the audio center. When the astronauts are in the LM, headsets are connected to the audio centers via the Comm Cables in the umbilical cord that connects all of their suit monitor data communications and voice transmissions:"The astronauts headsets are used for all voice communications...The headsets are connected to the audio panels by separate umbilical cables. These cables also contain wiring for the biomedical sensors in the constant-wear garment...The audio center is the assimilation and distribution point for all spacecraft voice signals."
[emphasis added. info quoted from http://www.apollosaturn.com/asnr/p173-188.htm
This makes clear that the "headsets are used for all voice communications". There are no other microphones.
So we've discussed that the cabin was depressurized upon EVA making the inner LM subject to vacuum conditions. And we've established that the only voice communications possible were through astronaut headsets. Now I want to discuss in detail the following two excellent questions raised by JF.
Anyway, its hard to figure out why the "hit" of the LM would cause a sound on the voice loop. One possibility is that somehow there is a open mike on the voice loop in the LM. Not sure this is possible or how to deduce this. How many headsets were in the LM and could they have been "on"?
Let's discuss and determine the amount of headsets on board Apollo 15. At launch, Commander David Scott, Lunar Module Pilot James Irwin, and Command Module Pilot Alfred Worden wore their "Comm Carriers" (aka Snoopy Caps) on board for launch. According to the Apollo 15 Stowage List, there were also 3 Lightweight Headsets stowed at launch in section A8 of the CSM. Here is the official stowage list. https://www.hq.nasa.gov/alsj/a15/a15stowage.pdf
No other headsets/microphones were on board. That makes a total of 6 microphones available in the CSM at launch. That list also informs us that no other headsets were stowed in the LM at launch.
The same list also tells us that 2 of the 3 lightweight headsets were to be transferred later to the LM for use by Scott and Irwin. However, the lightweight headset assigned to Worden malfunctioned upon attempted use in the CSM, so he kept Irwin's Lightweight Headset while Irwin took the broken one to the LM, and it never got used on the mission, causing Irwin to rely on the Snoopy Cap in the LM. Scott used his Lightweight Headset in the LM extensively. The broken headset was jettisoned to oblivion with the LM. See the Apollo 15 Mission Report https://www.hq.nasa.gov/alsj/a15/ap15mr.pdf
So there were 3 working microphones/headsets on the LM. At the time of the incident in question, when Irwin and Scott were outside the LM during the first EVA using their Snoopy Caps, there was 1 functional Lightweight Headset in the LM. The question above by JF also mentions the possibility of an "open mic" in the LM during the EVA. His next question also raises the issue as follows:
This diagram shows that they add the signals from the EVA suits and the LM mike. It just seem unprovable that the LM mike was open or closed. I doubt it was always open.
From "APOLLO EXPERIENCE REPORT - VOICE COMMUNICATIONS TECHNIQUES AND PERFORMANCE" NASA TN D-6739
JF states that the issue of whether there was an open mic in the LM seems to be "unprovable". Because the inside of the LM was in a vacuum, I believe the issue to be moot. But with one extra functional mic/headset on board the LM at the time of the suspect audio event, I will take a moment to explain why it is provable that there was no open mic in the LM.
Both the Snoopy Caps and Lightweight Headsets connected to the LM Audio Centers through the "Comm Cables" contained in the LM umbilical cords. The Apollo 15 LM Lunar Surface Checklist tells us - at "Page 2-10, Suit Donning" - that both the LM Comm Cables and the Headsets were stowed prior to EVA.
Page 2-10 - https://www.hq.nasa.gov/alsj/a15/a15sur037.gif
(see attached below) Apollo 15 LM Lunar Surface Checklist - https://www.hq.nasa.gov/alsj/a15/surface15.html
Also on Page 2-10, it states that the "Audio CB" is closed at the end of that checklist. That refers to the Audio Circuit Breaker, which starts the Page 2-10 checklist open, but ends in the closed position.
Additionally, Page 3-1 of the Lunar Surface Checklist - "Cabin Prep EVA-1" - states: "Stow All Loose Items Not Required For Eva" - https://www.hq.nasa.gov/alsj/a15/a15sur039.gif
The A15 Stowage List tells us that the Lightweight Headsets were stowed in section F18 of the LM - https://www.hq.nasa.gov/alsj/a15/a15stowage.pdf
So with the LM Comm Cables and Lightweight Headsets stowed, there was no possibility of an open mic during EVA.