Deep Dive: "High Level Sound"

On p. 189 of Bill Kaysing’s We Never Went to the Moon, he notes the following:

HIGH LEVEL SOUND
NASA has distributed a record which allegedly presents
the actual sounds of the Apollo 11 flight. In it we hear Armstrong
and Aldrin talking with the Mission Control staff at
Houston while they are descending to the surface of the
moon.

There is just one problem ... talking near a rocket engine
developing 10,000 lbs of thrust would be impossible since the
sound level of this engine is in the area of 140 to 150 db.
Voices could not be heard no matter what kind of amplification
was used.

Seems logical: rockets are really loud. We've all seen footage of rocket launches and you can hear them for miles. But when they're descending to the Moon, there's no air and Apollo engineers had really pushed the evolution of noise-canceling microphones.

Sound Requires a Medium (Vacuum Physics)

The most critical error in this claim is the assumption that the 10,000 lbs of thrust from the Lunar Module Descent Engine (LMDE) would create an ambient roar inside the cabin.

In an atmosphere: Sound is the vibration of air molecules. On Earth, a rocket engine is loud because it vibrates the surrounding air.
In a vacuum: Sound cannot travel through the vacuum of space because there are no molecules to vibrate. The "140 to 150 dB" noise level Kaysing cites refers to the external acoustic energy of the engine. On the Moon, that energy has no medium to carry it from the engine bell to the cabin walls.
Mechanical Isolation: The only way engine noise could reach the astronauts' ears is through mechanical vibration (conduction) through the frame of the Lunar Module. However, the engine was mounted on a gimbal system with significant structural damping, which prevented the high-frequency acoustic "roar" from vibrating the cabin air significantly.

Reference: The Apollo Guidance Computer: Architecture and Operation, Frank O'Brien (2010); Vacuum Physics and Technology, G.L. Weissler (1979).

Directional and Noise-Canceling Microphones

Even if there were significant background noise in the cabin, the Apollo communications system was specifically designed to filter it out.

The "Snoopy Cap": Astronauts wore a Communication Carrier Assembly (the black-and-white "Snoopy Cap") which featured dual noise-canceling microphones.
Close-Proximity Sensing: These microphones were designed to pick up sound only from a very specific distance (the astronaut’s lips). They used a "pressure-gradient" design, which means sound waves hitting the microphone from a distance (like background engine hum) would hit both sides of the microphone diaphragm simultaneously and cancel themselves out. Only the direct, high-pressure sound of the astronaut's voice from less than an inch away would trigger the signal.
Voice-Actuated (VOX) and Push-to-Talk: The systems were gated to ensure that low-level ambient noise did not trigger the transmitter.

Reference: NASA Technical Report: Apollo Experience Report - Communication System, (NASA-TN-D-7643).

Actual Cabin Decibel Levels

NASA actually measured the noise levels inside the Lunar Module during testing. During the descent, the cabin noise level was approximately 90 to 100 dB—roughly equivalent to being inside a loud factory or near a lawnmower. While loud, this is nowhere near the 150 dB "deafening" level Kaysing claimed.

Combined with the noise-attenuating properties of the astronauts' helmets and the noise-canceling microphones, clear communication was not only possible but expected.

Reference: Biomedical Results of Apollo, Section II: Environmental Factors (NASA SP-368, 1975).

The "Snoopy Cap"

The Communication Carrier Assembly (CCA), famously known as the "Snoopy Cap," was the critical interface for Apollo audio. It was designed to maintain clear voice communication in a high-noise environment by using hardware-based acoustic cancellation rather than digital processing.

The Design of the CCA (Snoopy Cap)

The CCA consisted of a Teflon fabric hood containing two earphones and two independent, redundant microphones. The microphones were mounted on flexible "booms" that placed the sensors directly in front of the astronaut's mouth, nearly touching the lips.

Pressure-Gradient Noise Cancellation

The microphones used in the Apollo missions were pressure-gradient (or "differential") microphones. Unlike a standard microphone that has a sealed back, a pressure-gradient microphone has a diaphragm that is open to the air on both sides.

How it cancels noise: When a sound comes from a distance (like the roar of an engine or the hum of life support), the sound waves hit both the front and the back of the microphone diaphragm at almost exactly the same time and with the same intensity. Because the pressure is equal on both sides, the diaphragm does not move, and no electrical signal is generated. This effectively "ignores" ambient noise.
How it captures the voice: Because the microphone is positioned less than an inch from the astronaut's mouth, the sound waves from their speech hit the front of the diaphragm with much higher pressure than they hit the back. This pressure difference causes the diaphragm to vibrate, converting the voice into a clear electrical signal.

The "Noise Gate" and VOX

To further ensure that mission control didn't hear a constant background hiss, the Apollo communication system utilized a Voice-Operated Switch (VOX) or "noise gate."

The system was calibrated to remain "closed" (silent) until the sound level at the microphone reached a certain threshold—a threshold only achievable by the direct, close-range pressure of a human voice. If the astronaut wasn't speaking, the microphone was essentially turned off, even if the engine was running.

Technical References:

NASA Technical Note D-7643: Apollo Experience Report - Communication System. This report details the specific microphone designs and the signal-to-noise ratios required for the lunar descent.
NASA SP-368: Biomedical Results of Apollo. Section II describes the acoustic environment of the spacecraft and the protective measures taken for astronaut hearing and communication.
Plantronics (Manufacturer): The MS50 headset was the basis for the Apollo hardware. Their archival documentation describes the evolution of the noise-canceling boom microphone used for the missions.

This combination of vacuum physics (preventing sound from traveling outside the craft) and pressure-gradient technology (canceling sound inside the craft) ensured that the audio recordings we hear today are of the astronauts' voices, not the machinery around them.