a couple of friends mine were talking about scifi movies and how (in the movies) you always hear sounds in space when actually (and obviously) you wouldn't.

which got one of my friends sayin that he in fact did hear hammer pings coming from one of the Apollo astronaut's hammer in a media file but not loud at all, quite faint, while taking a rock sample.

He's not a HB, he believes the astronauts did land on the moon with absolute certainity and he's convinced that he heard the hammer sounds.

Would the sound have traveled through the rock, the lunar surface, and the astronaut himself all acting as some kind of a sound conductor?

I think it's feasible.

If you read the Apollo Lunar Surface Journal transcripts, similar sorts of things seem to be reported, IIRC.

IIRC, this is mentioned in the ALSJ, but I can't find the place at the moment. I guess, the sound went from the hammer's grip into the suit/bones/air and from there reached the microphone.
We all know from SF stories where the asronauts bring their helmets into contact to be able to communicate without radio. Has this ever really been tried out or was an official contingency?

Harald

Ha, just found one location, in
http://www.hq.nasa.gov/alsj/a17/a17.sta2.html

143:29:03 Cernan: Now, I want to try to take this piece off first. (Pause as Gene hammers)

143:29:10 Schmitt: Pretty hard, isn't it.
[Hammering sounds - a soft "plock" - can be heard through Gene's suit. He takes seven whacks at the top of the boulder on the east end.]

[Cernan - "Although we can hear on the tape that the microphone in my suit was picking up the sound of my hammering, I don't ever remember hearing it. I could certainly feel it; and I've always contended that it's a very fine line between hearing noise and feeling noise. With that hammer, when you hit something the shock went through your whole body; but I'm not sure I ever heard the noise, probably because my ears were covered with the Snoopy helmet."]

[What seems likely is that, when Gene hits the rock, the hammer rebounds against the palm of the pressurized glove, creating a sound wave in the suit loud enough to be picked up by the microphone at Gene's lips. In brief, the suit acts like a drum.]

Interesting... a sound that the mike can pick up but the wearer of the mike can't hear.

It's conceivable that there was no sound at all in the audible range -- just a shock wave going through the suit. That could have been converted to an electrical signal in the mike by intermodulation or resonance effects. Not at all uncommon. (Survivors of Hiroshima who were very close to the explosion did not hear it; they experienced it as a flash-blast, while those further away remember it as a flash-bang. Not exactly the same situation, but suggestive.)

Maybe the next "Aliens" movie should have the tag line, "In space, you can't hear your own hammer."

Whenever I'm recording anything in my studio I have to be very careful not to rub against the mic cable. The slightest brushing travels easily up the cable to the mic. Also if you tap lightly on a guitar near the pickups they pick it up very well even if you can't hear it without the amp.

Anyway... it doesn't take much. It could have been the hammer-to-glove transmitting the sound, or maybe it was just another part of the suit rubbing or knocking together or brushing/tapping against the mic wire in time with the arm motion.
--Tommy

Good example, with the mike cables. There is an elaborate elastic webbing which acoustically isolates the microphone frame from the rod or stand it's mounted on. And for good reason. The stand rests on the floor and floors creak notoriously. Just shifting your weight during a recording would transmit an incredibly strong noise up the stand and into the mike. And so the mike is actually "suspended" by an elastic band. But the wire is not; it remains a purely mechanical connection to the mike frame.

Consider how a microphone works. A membrane is stretched between a frame, and some means are provided to measure the vibration of that membrane (typically by its motion in a magnetic field). Any sound in the room will produce sound waves which will want to induce a vibration in anything they touch. The microphone frame itself, having more mass and more inertial, is less affected than the membrane with its very low mass and its degree of mechanical decoupling from the frame. So the vibration of the membrane relative to the vibration of the frame is measured and encoded as an electronic representation of sound.

Now consider the reciprocal. What if the frame were to move suddenly or vibrate? The portion of the membrane attached to the frame would be compelled to move with it, but the membrane would be impeded by the stationary air around it. But the microphone is unable to distinguish between motion of the membrane relative to a stationary frame, and motion of the frame relative to a stationary membrane. So the membrane's inertial response to a motion of the frame is also recorded as sound.

This is important because it means you can record "sound" by jiggling the microphone even though someone in the room will not hear anything because it's not the air that's vibrating.

The astronaut's microphone was on his communications carrier, the black and white "Snoopy hat" that goes on his head under the fishbowl helmet. It will move with his body. Now go get a hammer and hit something solid with it, paying special attention to the motion of your upper body and its shock-absorptive response to your hammering. As you can imagine, there is considerable musculo-skeletal involvement. Hammer makers for centuries have been trying to figure out how to keep the shock waves of hammering from traveling up the hammerer's arm and into his shoulder, chest, and head.

Thus you have one potential source of induced sound -- the sharp motion of Cernan's head responding to the hammer blows.

And you have the other possible explanations, such as the suit acting as an acoustic waveguide from wrist to neck.

And also you have the latency of the accordion joints in the suit elbow, which may induce noisy airflow in the suit with any sharp arm motion.

Hehe... rock-n-roll science meets rocket science [img]/phpBB/images/smiles/icon_smile.gif[/img]

Here's another bit of evidence.

If you've heard a recording of the communications between mission control and the astronauts on the lunar surface, you'll remember they were plagued by a form of feedback in which the cap com's voice (heard by the astronauts) would be picked up and resent back to Earth, delayed by 2.6 seconds.

It was determined that this was due to mechanical coupling between the astronauts' earphone and mike. It wasn't supposed to happen; there was supposed to be enough isolation between the earphone and mike so that the audio relay in the LM would "gate out" the ground transmissions, and only transmit the astronauts' words. But unfortunately the isolation wasn't good enough. (The above applies to Apollo 11; the problem may have been reduced or eliminated on later missions - but the A-11 mission report isn't very optimistic that this could be done.)

Anyhow, it demonstrates one path for vibrations to reach the mike, other than through the air -- namely, right through the boom.