24 May A Quiet Place: An Acoustical Review
A Quiet Place tells the story of a family in plight—the Abbotts—fighting for survival against killer aliens that are sightless, yet hypersensitive to impulsive sounds. These malleus’trosoties track their prey with acute echolocation, similar to the bio-sonar mechanisms observed in bats and whales. They’re fast, scary, and beyond belligerent.
So what happens when an acoustician hits the movies to see what all the hype is about? You get an acoustical survival guide for you and your loved ones. Keep reading to discover how effective the family’s survival techniques would be in the real world.
Pan to feet…
Acoustical Survival Technique #1:
Footfall Impact Noise Isolation
In the movie the family minimizes footfall noise by removing their shoes and covering all accessible walking areas with sand.
In the real world audible footfall noise is a common annoyance experienced in apartment buildings. Isolation of impact noise depends on the design of the floor and ceiling system. Subject to the design of the system, hardwood or tiled flooring finishes should be installed on a high-quality underlay. In addition to providing adequate overall mass to a system, other impact-isolation measures may include resiliently mounted ceilings with insulation in the ceiling cavity.
BKL’s Survival Advice: Try to avoid Michael Flatley’s Riverdance routine in buildings finished with old wooden floors. Creaking is caused by the relative motion of timber on nails rubbing against joists or diaphragms.
Acoustical Survival Technique #2:
Sound Masking and Speech Privacy
In the movie the Abbott family is at constant risk of murder-by-murmur. To communicate verbally, the family treks to a nearby waterfall, which provides loud, continuous noise that masks vocalization and improves speech privacy.
In the real world, for effective masking of speech, the spectra of the masking signal should contain energy over a broad range of frequencies (wide bandwidth). For example, if a sound masking signal reproduces energy in only high-frequency bandwidths, low-frequency components of speech would still likely be audible (and vice-versa). This is why many commercially available electronic sound masking systems use white noise, which contains equal energy across all frequencies. It’s also important for sound masking systems to emit continuous noise so that any impulsive (short duration) sound events are also masked temporally.
Generally speaking, the louder the sound masking signal is, the better the speech privacy. Other common sources of masking include background music and HVAC noise. This is often required in open-plan offices and places where confidential discussions take place, such as healthcare facilities and interrogation spaces.
BKL’s Survival Advice: Stay at the damn waterfall. Pack a generator, tent, stereo, and a few ‘Sabbath records. Turn the post-apocalyptic mess into a post-apoca-party.
Acoustical Survival Technique #3:
Noise Emission Mitigation
In the movie the Abbotts seek refuge in the basement of an isolated, albeit exposed, farmhouse (we know, right. You’d never have guessed…). To reduce the levels of noise egress, the family covers the hatched basement entrance with a mattress.
In the real world the mattress serves as a sound barrier and an absorptive panel. Sound energy from the underside of the hatch that impinges with the underside of the mattress is absorbed, and the residual sound energy that passes through the material is attenuated with what acousticians refer to as transmission loss.
Unfortunately, a mattress alone is unlikely to provide sufficient attenuation. However, installing thick porous insulating material, such as fibreglass or mineral wool, to a dense backing material reduces noise levels and reverberation times. Where practicable, this type of treatment should always be implemented as close to the noise source as possible. If the extraterrestrial ear’nemy is standing at a relative distance, atmospheric effects (air absorption, ground attenuation) and distance (approximately 6 dB decrease for every doubling of distance) provide further attenuation.
Reverberation and the “loudness” of internal spaces is commonly controlled with surface finishes: Highly reflective materials (polished wood, glazing) create live (and louder) sounding rooms with higher reverberation times. Highly absorptive materials (thick carpet, perforated ceiling tiles) create dead (and quieter) sounding rooms with lower reverberation times.
Acoustical screens and barriers can also be designed to reflect sound back toward a noise source, to diffract grazing wavefronts, and create acoustic shadows at nearby noise-sensitive receptors. Road barriers and mechanical louvres are used in the same way.
BKL’s Survival Advice: Seal it up. The acoustical path of least resistance includes all gaps and openings. If light and air can pass through a system, expect that noise will too.
Acoustical Survival Technique #4:
Audio System Design
In the movie the Abbotts’ daughter is deaf. The father fashions a hearing aid from assorted radio and electronic components. The transducer (in this case, a device that converts sound pressure level into an electrical signal) doesn’t work as anticipated, and instead of aiding the girl’s hearing, it creates a feedback loop whenever the predators are in close proximity.
In the real world audio systems should be designed so the input (e.g., microphone) and output (e.g., loudspeaker) do not directly feed into one another. When they do, the output signal re-enters the audio system, creating a loop of feedback squeal that generally gets louder and louder with each passing electronic cycle. Not fun.
Feedback can be managed with signal processing (noise suppression) and by selecting appropriate transducer directionalities. In brevity, noise suppression (or noise-gating) involves the processing of a signal’s envelope to only allow signal above a specified threshold (usually amplitude) to pass through the system. Any signal below this threshold is otherwise attenuated. This method is particularly effective when multiple microphone configurations occupy a space, such as in conference rooms or on drum kits.
Additionally, the polar pattern of a microphone (describing its sensitivity to directionality) and the spatial distribution of loudspeaker output should also be chosen so that microphones and speakers are oriented to avoid feedback. If you’ve ever experienced a high-frequency squeal from a local pub singer standing too close to stage monitors or heard a flight attendant struggle with safety announcements over a screaming loudspeaker, this is feedback.
BKL’s Survival Advice: While I don’t think anybody should be mashed into puddles of porridge by aliens for being too loud, these pinnae-less predators could be analogous to local bylaws, or future development conditions, and any reddish chunk-pools that once resembled an old friend could be akin to infringement fines, tinnitus, or even remedial renovation costs associated with poor acoustical design. Regardless of how you interpret the film, we can all agree that unwanted noise is something worth talking about.
If you have any architectural, environmental, or alien invasion–related acoustical enquiries, we can work with you to develop appropriate noise and vibration control solutions. Our team of professional engineers and acousticians has more than 150 years of accumulated experience in acoustics, and we’re supported by an extensive complement of specialized instrumentation and software. Get in touch in today!
Review by David Stepanavicius