There are two basic ways of reducing unwanted sound. The rage today seems to be Active Noise Control (ANC) or Noise Cancellation. A second method is through Passive Noise Isolation.
Active Noise Cancelling (ANC), like in many of those headphones today creates an effect called Destructive Interference. A noise canceling speaker attenuates noise by emitting a sound wave that is opposite or antiphase of the original sound. These combined waves create a third wave or a Destructive Interference Wave that is fed to your ear. “ Noise-cancelling headphones work on this principle. They detect the sounds coming into the ear and produce sounds with equal volume but with the peaks and troughs reversed, resulting in near silence.” [i]
ANC headphones generally use algorithms, along with battery-operated analog and digital circuit assemblies to analyze incoming waveforms or background noise. The algorithm will then phase shift or invert the polarity of the original signal effectively reducing the volume of the perceived noise. A headphone microphone hears the noise, the circuitry combines the signal, and a transducer delivers the Destructive Interference signal to your ear reducing the volume of the noise.
Passive Noise Control (PNC) is always the first line of defense against unwanted noise in a top-quality design. The combination of sound-absorbing, rigid, and geometric materials to reduce sound levels is the base of all noise reduction designs.
While active noise cancellation is sound reduction using a power source, PNC cancels noise by reducing and isolating the sound using highly engineered sound-insolating materials. Some may argue the advantages of ANC over PNC or vice versa.[ii] Each may have its place.
In the studio or for musicians PNC provides overall frequency dampening without the hiss or electronic noise of ANC. For travelers, PNC provides overall dampening of the highs and lows throughout the entire frequency spectrum. The ANC being used in loud industrial settings can be highly effective, but only for a selected frequency.
In 2010, Direct Sound received a US patent 7,853,034 B1[iii] for Ambient Noise Isolation Headphones having a layered dampening structure. Used in their EX29 Plus Extreme Isolation headphones, this patent allows the user to substantially attenuate ambient noise levels and focus on the electronic audio presented by the headphone speakers. The unique materials and geometric shapes combine to reduce high-frequency acoustic energy and control boomy base low frequencies.
I’m sure my hearing is different from yours. For my choice, PNC headphones, physically blocking and Isolating the noise going to my ear seems to be best. I get a much cleaner true sound from my headphone speakers without the Destructive Interference. My experience with ANC has been mostly positive but it seems searching for batteries or a charger takes up most of my usage. (not much fun on a flight).
Some quick science:
Sound is a wave consisting of alternating periods of pressure and compression. We measure the frequency of sound waves or pitch in a unit called hertz (Hz). Humans have a range of hearing between 10-20,000 Hz which is different than many animals. For example, dogs have a range from 20 – 40,000 Hz. A dolphin has a range from10,000 – 150,000 Hz.[iv]
When we hear something, we are sensing the vibrations in the air. The vibrations enter the outer ear and cause our eardrums to vibrate. Attached to the eardrum are three tiny bones. The hammer, anvil, and stirrup. These bones amplify the vibration before sending it to the auditory nerve. [v]
[i] https://www.sciencelearn.org.nz/resources/2816-sound-wave-interference [ii] https://journals.sagepub.com/doi/pdf/10.1260/0263-09126.96.36.199 [iii] https://patents.google.com/patent/US7853034B1/en?oq=7853034 [iv] https://www.nasa.gov/specials/X59/science-of-sound.html [v] https://www.scienceworld.ca/resource/sound/