The research ”Laboratory and Field Studies on Infrasound and its Effects on Humans”—is a foundational paper by Swedish researcher Ulf Landström, published in the Journal of Low Frequency Noise and Vibration (Vol. 6, No. 1) in March 1987. It explores how infrasound (sounds below 20 Hz) impacts human perception and physiology, drawing from both controlled lab experiments and real-world field observations. This work has been cited in later studies on topics like fatigue, animal behavior, and even occupational health standards. I’ll break it down here, including the abstract, methods, key findings, reported effects, and conclusions, to give you a comprehensive overview. Note that while influential, this is from the late ’80s, so modern research has built on it with mixed results on infrasound’s health implications.

The Abstract in Full

The paper’s abstract sets the stage: ”In many working environments the total noise energy is dominated by infrasonic frequencies. Exposure to this type of low frequency noise has, by several authors, been correlated to different effects on human subjects. Recent laboratory experiments have thus indicated correlations between noise, perception and other effects on humans. Exposure to infrasound therefore appears to become a hygienic problem when the pressure level exceeds the threshold of perception. The present paper is a description of various laboratory experiments on perception and the physiological effects of infrasound. Subjects were exposed to pure tones and broad-band noise within the infrasonic range. Changes in perception, and physiological reactions, during different types of exposure are described. The paper also includes levels and frequency analyses of noise emanating from some of the most common infrasonic sources: buses, lorries, trains, railbuses, ships, helicopters, manoeuvre rooms and mills. The characteristics of the noise spectra are compared and related to the threshold of perception. Finally, field studies on physiological changes correlated to wakefulness in typical and authentic infrasonic noise environments are described.”

In essence, Landström argues that infrasound becomes a workplace health issue only when it’s perceptible, linking it to various effects while emphasizing the role of perception thresholds.

Methods: How the Research Was Conducted

Landström combined lab-based experiments with field measurements to provide a balanced view. Here’s the breakdown:

• Laboratory Setup: Tests occurred in a specialized pressure chamber (2.0 m x 1.6 m x 1.2 m) equipped with eight 50 W loudspeakers and two 100 W amplifiers to generate clean infrasound with minimal harmonics (first harmonics over 40 dB below the fundamental frequency). Background noise was kept low at 50-55 dB(lin) or below 50 dB(A). This setup acted like a Helmholtz resonator for precise control.

  • Participants: Included 10 deaf and 10 hearing subjects in some tests to compare sensory responses.
  • Exposures: Subjects faced pure tones (e.g., 6 Hz at 95-115 dB(lin) for 20 minutes; 16 Hz at 80-100 dB(lin)) and broad-band noise in the 2-20 Hz range. Some exposures were 10 dB above or below individual hearing thresholds. Additional comparisons involved low-frequency noise (42 Hz at 70 dB) vs. high-frequency (1000 Hz at 30 dB), and even EEG-regulated noise to check for brainwave synergies.
  • Measurements: Hearing and vibrotactile (body vibration) thresholds were assessed using sinusoidal frequencies. Physiological responses were tracked via EEG (brain waves) for wakefulness and EKG (heart activity). Exposure durations ranged from 20-30 minutes.

• Field Studies: Landström analyzed real-world infrasound sources like vehicles (buses, lorries, trains, ships, helicopters) and industrial sites (manoeuvre rooms, mills). He measured noise spectra and levels, comparing them to perception thresholds.

  • Exposures: In authentic environments (e.g., dumpers, helicopters, railway engines), participants experienced 30-minute sessions of noise alone, vibration alone, or combined stimuli, mimicking occupational conditions.
  • Focus: Emphasis on wakefulness changes in these settings, with annoyance ratings collected.

This dual approach allowed Landström to bridge controlled science with practical applications, referencing earlier works like Johnson (1980) on thresholds and Leventhall (1980) on effects.

Key Findings: What the Data Revealed

The study uncovered nuanced insights into infrasound’s detectability and impacts:

• Perception Thresholds: For pure tones, hearing thresholds were around 110 dB(lin) at 4 Hz, dropping to 90 dB(lin) at 20 Hz. Broad-band noise was detectable 1-5 dB lower. Vibrotactile sensations kicked in about 20 dB above hearing thresholds, with no difference between deaf and hearing people—suggesting it’s a body-wide response, not just auditory.
• Wakefulness and Drowsiness: Exposure above thresholds (e.g., 115 dB(lin) at 6 Hz or 100 dB(lin) at 16 Hz) reduced wakefulness in hearing subjects, as seen in EEG changes, but not in deaf ones. This points to the cochlea (inner ear) as the key mediator. Below thresholds (e.g., 95 dB(lin) at 6 Hz), no effects were observed.
• Frequency Comparisons: Low-frequency noise (42 Hz at 70 dB) promoted drowsiness, while high-frequency (1000 Hz at 30 dB) increased alertness. No strong synergy was found between infrasound and individual EEG frequencies.
• Real-World Sources: Noise from vehicles and mills often neared or exceeded perception thresholds, with combined noise-vibration exposures causing the most drowsiness and annoyance.
• Overall: Effects like reduced wakefulness were tied directly to perceiving the sound, aligning with prior theories but emphasizing no harm below audible levels.

Reported Effects on Humans

Landström’s work highlights a range of potential effects, but stresses they’re perception-dependent:

• Physiological: Reduced wakefulness and increased drowsiness (via EEG); potential links to fatigue in occupational settings. In field tests, combined infrasound and vibration led to higher annoyance and lower alertness.
• Perceptual and Psychological: Annoyance, body vibrations, and sensations like pressure. The paper notes correlations (from cited studies) to breathlessness, panic, anxiety, and even wakefulness disruptions—but Landström’s experiments focused more on drowsiness than acute panic.
• No Effects Below Threshold: Crucially, no physiological changes occurred when infrasound was sub-perceptible, countering some broader claims about ”silent” effects.
• Vulnerable Groups: Hearing individuals were more affected than deaf ones, underscoring the auditory pathway’s role.

Exposure levels mentioned include 80-115 dB(lin) at frequencies like 4-20 Hz, with durations up to 30 minutes in tests (or 2 hours daily in related contexts).

Conclusions and Lasting Impact

Landström concludes that infrasound poses hygienic risks in workplaces only when levels exceed perception thresholds, as that’s when annoyance, reduced wakefulness, and other effects emerge. He recommends larger studies on synergies (e.g., with vibrations) and highlights common sources like vehicles as potential concerns for workers. Overall, the paper supports the idea that effects are mediated by cochlear stimulation, not mysterious ”inaudible” mechanisms.

This research has influenced guidelines on low-frequency noise, though later reviews (e.g., by the WHO) have debated ”wind turbine syndrome” links, often finding psychological factors play a role too. If you’d like me to pull up related modern studies or compare it to ocean/forest/wind turbine contexts from our earlier chats, just say the word!