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Author
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Topic: Mercury capsule cabin audio whining sound
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MOL
Member Posts: 102
From: Los Angeles, CA
Registered: Oct 2004
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posted 03-29-2019 10:17 PM
   
I have always wondered what specifically caused the very noticeable whining sound heard in the background of the onboard audio on all the Mercury flights. If you want to sample what I am referring to please play track 3295 here, and listen to the period starting at 6:44. This is the VOX audio directly from Alan Shepard's microphone recorded onboard MR-3. If anyone has an answer please let us all know! Thanks. |
oly
Member Posts: 971
From: Perth, Western Australia
Registered: Apr 2015
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posted 03-30-2019 03:16 AM
If Shepard had his helmet visor closed during the mission, the sound recorded from his VOX (voice operated transmit switch [XMIT]) channel is most likely feedback or signal interference from an electrical accessory within the capsule, and unlikely to be direct sound from within the capsule.It sounds very similar to interference caused by an electrical inverter within older aircraft. Given the design of the system, recording radio signals via many long distance radio and wired relays, the sound could originate from many sources. In addition to this, the majority of radio receivers would have the signal gain adjusted to be sensitive to any sound, and interference from atmospheric conditions and static electricity. There is also the quality of old magnetic audio recordings to consider. |
MOL
Member Posts: 102
From: Los Angeles, CA
Registered: Oct 2004
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posted 03-30-2019 09:27 AM
Thanks for the reply. Note that this tape is the onboard recording — there is no atmospheric noise, etc. on Shepard’s voice since this is direct from the microphone to the tape onboard. When you hear CAPCOM speaking to him then yes that has the noise coming through the capsule’s receiver. I think there is another answer pointing to something electrical within the spacecraft. Maybe the inverter you mention? |
David Carey
Member Posts: 802
From:
Registered: Mar 2009
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posted 03-30-2019 01:50 PM
I like Oly's Inverter theory. Mercury used a 400Hz AC inverter and the principle/fundamental tone of the noise you mention is (to my ear) on pitch to a 400Hz sine wave. Can also hear the same 400Hz noise in MA-6 onboard tape recordings. Acoustic pickup of the recorder's drive train hum and/or electrical noise from drive motor commutators are additional possibilities but the 400Hz tone suggests microphone pickup of inverter noise in the space craft or possibly bleed-through of AC inverter rail noise to the 24V DC battery supply and circuitry used for the tape recorder. |
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