The 19 Hertz (Hz) frequency is often linked to feelings of dread or unexplained phenomena. This frequency sits at the edge of the acoustic spectrum, where sound transitions from being heard to being felt by the human body. Understanding whether 19 Hz is truly dangerous requires examining the science of sound intensity and human physiological response. The danger is not inherent in the frequency itself, but rather in the sound pressure level, or amplitude, with which it is delivered.
Defining Infrasound and the 19 Hz Frequency
Sound is measured by its frequency, expressed in Hertz (Hz), which is the number of pressure waves passing a point per second. The typical human ear perceives sounds ranging from 20 Hz up to 20,000 Hz. Infrasound is the term for acoustic energy that falls below this conventional 20 Hz threshold, placing 19 Hz directly within this low-frequency range.
While 20 Hz is widely cited as the lower limit of hearing, perception does not suddenly stop there. For a person to hear an infrasound wave, the intensity must be significantly higher than for an audible tone. For instance, a sound at 20 Hz requires an intensity of around 90 decibels (dB) to be perceived. As the frequency drops lower, the auditory threshold rises dramatically, meaning the sound must be much more powerful to be perceived.
At these low frequencies, the sound waves are so long that they are often perceived as a physical pressure or vibration rather than a distinct tone. The human body senses these powerful vibrations through specialized receptors in the skin and internal organs. This means the sound is essentially felt rather than heard with the eardrums.
The Myth and Reality of 19 Hz Bio-Effects
The notoriety of 19 Hz stems largely from a theory involving the human eyeball, suggesting that this frequency is close to its resonant frequency. This theory gained traction after scientist Vic Tandy investigated a supposedly haunted laboratory. He discovered that an extractor fan was generating a standing wave of approximately 18.98 Hz.
Tandy hypothesized that this low-frequency wave caused his eyeball to vibrate, leading to the visual illusion of a “grey blob” or ghostly figure he saw in his peripheral vision. While the eyes and other body parts do have natural resonant frequencies, the danger is often overstated. The sound intensity required to cause actual physical harm or sustained visual disturbance is extremely high and rarely encountered in normal environments. The claim that 19 Hz alone causes panic or dread is based on this single historical anecdote rather than widespread scientific evidence.
Physiological Responses to Low-Frequency Sound
Exposure to intense low-frequency sound in general can elicit measurable physiological responses. At high amplitudes, infrasound causes whole-body vibration because its long wavelengths easily pass through and resonate with internal organs and tissues. This physical interaction can manifest as a sensation of pressure in the chest or head, even if the sound is not consciously heard.
Exposure can also negatively affect the vestibular system, the part of the inner ear responsible for balance. High-intensity infrasound is linked to symptoms like dizziness, vertigo, and motion sickness. The body reacts to this unwelcome physical stimulation with a stress response, even if the sound is below the auditory threshold.
Chronic exposure to low-frequency noise, even at moderate levels, has been associated with health complaints. These reported effects include headaches, fatigue, anxiety, and sleep disturbances, which are consequences of systemic stress. Prolonged exposure can also lead to changes such as increased blood pressure and decreased heart rate variability.
Common Sources and Regulatory Safety Thresholds
Infrasound is a pervasive part of the environment, generated by both natural and man-made processes. Natural sources include large-scale phenomena such as volcanic eruptions, earthquakes, ocean waves, and strong winds. Man-made sources often stem from systems that move large volumes of air, like industrial ventilation systems, air conditioning units, and diesel engines.
The wind turbine is a common source cited in public discussion, generating low-frequency noise as its blades interact with the air. However, the mere presence of 19 Hz is not a measure of risk; intensity, measured in decibels (dB), is the determining factor for danger.
To cause direct physical harm, infrasound must be present at extremely high sound pressure levels. Pressure exceeding 186 dB is required to potentially damage the eardrum. Occupational safety guidelines recognize that exposure up to around 65 dB, even for prolonged periods, is considered safe. Therefore, the acoustic energy required to make 19 Hz a genuine threat is far greater than what is typically encountered in residential or normal occupational settings.