Sound on the moon?

[Count Zero]

Aren't the astronaut radios on VOX?  If so, how much extraneous noise is needed before the Vox starts picking it up?

I can't tell you the exact amount of sound energy that must be present for the Vox to "engage", but I can tell you that commercial models a few years ago and used extensively in and out of my profession differs dramatically.  I'm not sure if it is the brand of mic or the amount of use.  There were times that I had to "blow" into the mic to get it to engage or the first word likely never got transmitted, at the very least the first syllable. NASA may have had top line equipment for Apollo and thus you might have a "whack" when hammering in core tubes etc.

Many times when I hear the CapCom say something to the astronauts, I hear it repeated 3 seconds later.  I assume that I'm hearing the received audio coming from the speakers inside the astronaut's helmet, and getting re-transmitted by the helmet mike..  If that's loud enough to trigger the vox, I have no trouble believing that raps from a hammer can too.

[MBDK]

Many times when I hear the CapCom say something to the astronauts, I hear it repeated 3 seconds later.  I assume that I'm hearing the received audio coming from the speakers inside the astronaut's helmet, and getting re-transmitted by the helmet mike..  If that's loud enough to trigger the vox, I have no trouble believing that raps from a hammer can too.

The DOD ones I used during my career would not pick up the noisy ventilation system(s) around us, but a heavy breath, or the brush of your face against the mouth piece, could often make some peculiar sounds that would sometimes be reminiscent of a thud or thwack type of noise.

[MBDK]

I should add that they did occasionally have to be adjusted by technicians to provide the preferred background/communications balance.  I am certain such adjustments, if they used a similar system, for Apollo could have been accomplished by the flight crew, mission control, or both.

[JayUtah]

The maker is Plantronics, a name still very much known today in the field of headsets.

[Glom]

Did the mics have a squelch?

[ka9q]

A "squelch" is a receiver feature, not a transmitter feature. There's VOX (voice activated switching) which I believe was actually in the LM (or LRV) relay system, not in the PLSS -- but I'll have to check this. If that's the case then the two astronauts heard each other continuously, and only the signal to earth required the level to exceed a threshold. That threshold was set by a knob in the LM (or LRV).

There's a fair amount of cabling inside the suit between the PLSS and headset; I believe there were at least two microphones for redundancy. There had to be a preamplifier somewhere; at least the cabling between the microphone and preamp would be somewhat sensitive to electrical noise.

[ka9q]

OK, I've looked through the handbook again. As far as I can tell, there is a VOX circuit on the inputs to the S-band and VHF transmitters on the LM and in the LCRU, but not in the PLSS. So the astronauts heard each other continuously during an EVA.

Note: in space and mobile communications where relays are involved, we comm engineers use the term "forward link" to refer to the entire signal path to the mobile station and "return link" (or "reverse link") to the entire signal path from the mobile station. This is distinct from "uplink" and "downlink" that refer specifically to unidirectional satellite relay links. The forward and return links would each consist of their own uplinks and downlinks.

Here's how the return link worked:

The LMP transmitted audio to the CDR (only) by VHF FM on 279.0 MHz. Their audio was combined in the CDR's PLSS and transmitted by VHF AM on 259.7 MHz to the LMP's PLSS (so he could hear the CDR) and to the LM or LCRU so it could be relayed to earth on 2282.5 MHz (LM) or 2265.5 MHz (LCRU). The relay in the LM or LCRU had a VOX in the line between the 259.7 VHF receiver and the S-band transmitter so we'd hear dead air unless one of them spoke loudly enough to trip it. But the astronauts could in principle whisper to each other and we wouldn't necessarily hear it. During the Apollo 11 EVA this return link VOX in the LM was set too low and that's why their voices often broke up. But they could hear each other fine.

The forward link from the LM or LCRU was VHF AM on 296.8 MHz to separate receivers in the astronauts' PLSSes (they each had separate volume controls). There was also VOX in this path between the S-band uplink receiver and the forward VHF transmitter in the LM or LCRU, plus squelches in those PLSS AM receivers so that the astronauts would not be bothered by uplink noise when Capcom wasn't talking (or if the relay link to the PLSSes was weak).

When I figured this part out a while ago this resolved a question I'd had for some time, which was how interference was avoided between the two VHF AM forward link transmitters on the LM and LCRU, both of which transmitted on 296.8 MHz. The two S-band uplink receivers heard the same signal from earth on 2101.8 MHz (=221/240 * 2282.5 MHz), but Capcom could choose which of two subcarriers to use for forward link voice. The LM listened to 30 kHz, the LCRU to 124 kHz. Since their VHF AM forward link transmitters were VOX keyed, they wouldn't both transmit at the same time (unless Capcom transmitted on both subcarriers at the same time, but they wouldn't have any reason to.)

[ka9q]

Many times when I hear the CapCom say something to the astronauts, I hear it repeated 3 seconds later.  I assume that I'm hearing the received audio coming from the speakers inside the astronaut's helmet, and getting re-transmitted by the helmet mike..

That's correct. It was written up as an anomaly in one of the mission reports but I don't think there was anything that could be done about it.

With the full duplex hot mike (non-VOX) link between the two astronauts, I did think about the possibility of a mutual feedback loop. Only the acoustic path through the LMP's headset would be required to complete a loop since the CDR's PLSS electronically mixed the LMP's voice with his own and transmitted it the LM or LCRU -- and back to the LMP so the LMP could hear the CDR. I guess the round trip gain was kept low enough to prevent that.

[raven]

This phenomena  was used by Italian high school students to measure the distance to the moon, fascinatingly enough, including measuring its eccentricity.

[ka9q]

I'm surprised they could do this, since the path between Houston and the moon wasn't direct but went by way of one of the ground stations, usually by satellite, which would add its own delay.

[raven]

I'm surprised they could do this, since the path between Houston and the moon wasn't direct but went by way of one of the ground stations, usually by satellite, which would add its own delay.

Yeah, they mention those delays in the paper.

[BDL]

Revisiting this one for a bit, I’m still not sure what happened or what may have made those noises at 1:42-1:51 or with the lid closing/opening. Everything else is easily explainable to me.
I wonder if there’s any way to know? I’m having a little bit of trouble understanding. I hope I’m not asking for too much.

I think it’s likely just a coincidence and those sounds were just the coming from inside the EVA suit itself, but I’m not really sure. Thanks.

[MBDK]

Revisiting this one for a bit, I’m still not sure what happened or what may have made those noises at 1:42-1:51 or with the lid closing/opening. Everything else is easily explainable to me.
I wonder if there’s any way to know? I’m having a little bit of trouble understanding. I hope I’m not asking for too much.

I think it’s likely just a coincidence and those sounds were just the coming from inside the EVA suit itself, but I’m not really sure. Thanks.

Noises in the VOX microphones can be made by even slightly harder breaths, such as any extra exertion, like hammering - it all depends on the astronaut's mouth proximity to the mic, and it WOULD be closer when he leans forward in his suit to observe some of the tasks he was working on.  Also, the mics at Houston control were open and could make similar sounds just by being adjusted, tapped with a pen, or exhaling close to the mic.  As they were voice activated, and designed to filter out background noise, close proximity of the mouth was also required in order to be clear and audible, and often resulted in inadvertent brushing of the lips or chin up against the mic, causing all sorts of odd sounds.  I know because I used those type of systems for years during nuclear refueling operations.

[BDL]

Revisiting this one for a bit, I’m still not sure what happened or what may have made those noises at 1:42-1:51 or with the lid closing/opening. Everything else is easily explainable to me.
I wonder if there’s any way to know? I’m having a little bit of trouble understanding. I hope I’m not asking for too much.

I think it’s likely just a coincidence and those sounds were just the coming from inside the EVA suit itself, but I’m not really sure. Thanks.

Noises in the VOX microphones can be made by even slightly harder breaths, such as any extra exertion, like hammering - it all depends on the astronaut's mouth proximity to the mic, and it WOULD be closer when he leans forward in his suit to observe some of the tasks he was working on.  Also, the mics at Houston control were open and could make similar sounds just by being adjusted, tapped with a pen, or exhaling close to the mic.  As they were voice activated, and designed to filter out background noise, close proximity of the mouth was also required in order to be clear and audible, and often resulted in inadvertent brushing of the lips or chin up against the mic, causing all sorts of odd sounds.  I know because I used those type of systems for years during nuclear refueling operations.

Thank you very much. I guess those sounds were just coincidence. Did these sound happen often throughout the Apollo missions? It seems like they would, but I haven’t watched a lot of the Apollo footage (which I’m ashamed to admit).

[MBDK]

Thank you very much. I guess those sounds were just coincidence. Did these sound happen often throughout the Apollo missions? It seems like they would, but I haven’t watched a lot of the Apollo footage (which I’m ashamed to admit).

Absolutely.  Here is the first random video I looked at -

Just going halfway through it, and only noting discernibly audible sounds distinctly apart from the voices, I heard noises at 0:14, 0:55, 0:59, 1:16, 1:20, 1:30, 1:35, and 1:40.  That is where I stopped, as you get my drift.  Better ears than mine may hear more.

NOTE:  Edited to remove extra "may" in last sentence.