see also:

• massage guns
• Basics of ultrasound

• sound is the acoustic energy waves from vibrating particles at frequency ranges human can hear
• sound intensity is usually measured in decibels (dB)
• vibrations can make other objects vibrate and this becomes amplified if the frequency of the vibrations match the resonant frequency of the object (this can cause bridges to collapse)
• in general, the lower the frequency, the further it propagates
• the speed of the sound wave propagation depends on the medium - in air, it is 343m/sec

• we are all continuously exposed to relatively high levels of sound waves and vibrations of various frequencies
• these can induce stress and raised cortisol and cause secondary effects of these but relaxing sounds can reduce stress and cortisol levels
• loud higher pitched sounds will cause noise-induced deafness / hearing loss
• the intense localised vibrations and percussive energies from massage guns can cause local shear stress trauma including vertebral or carotid artery dissections, dislodge a carotid artery plaque if used on the neck, or blindness (if used near the eyes)
  • sound waves from music speakers at concerts or subwoofer vests (eg. Woojer vest 3) do not possess the localized mechanical shear stress required to physically “shake loose” a plaque from an arterial wall and there is no evidence they directly adversely affect the aorta - apparently, the broad haptic vibrations cannot penetrate the bony shield of the chest wall with enough localized physical pressure to stretch or tear an artery wall however there is some evidence they may reduce cardiac contractility (see below)
• chronic, long-term exposure to heavy industrial low-frequency noise (such as aviation workers or heavy machinery operators) has shown correlations with long-term endothelial dysfunction (blood vessel stiffness) and systemic inflammation over decades as well as increased risk of Dupuytren's contractures in those that use hand held vibrating machinery as an occupation.
• listening to music additionally has a range of CNS mediated effects unrelated to the direct sound wave / vibration effects
  • alter stress levels
  • modify autonomic nervous system activity
    • a small study showed that for the duration of listening to either classical music or rock music, aortic stiffness was reduced¹⁾

• mechanical waves which unlike electromagnetic waves, requires a medium (hence there are no acoustic waves in a vacuum and space is essentially acoustically silent)
• they transfer energy by causing vibrations and pressure fluctuations in the particles of the medium and create regions of compression and rarefaction
• they have characteristics such as acoustic pressure, particle velocity, frequency, amplitude, and intensity
• based upon their frequency they are described as:
  • ultrasonic
    • acoustic waves above 20,000Hz
    • dolphins use echolocation with ultrasonic clicks and whistles often reaching up to 150 kHz or more
    • bats use echolocation with ultrasonic frequencies often ranging up to 100 kHz or more
    • rats and mice use ultrasonic communication up to about 90 kHz
    • used in healthcare as ultrasound diagnostics and also in some therapeutic devices
    • these can heat up organic tissues if high intensity
    • ultrasonic cleaners, humidifiers or pest repellents obviously generate noise at these frequencies
    • some power drills generate noise in the 20-50 kHz range
  • sound waves - the maximum frequency range that humans can hear sound
    • 20Hz to 20,000Hz although the upper frequencies in particular decline as we become adults and age
    • cats can hear 48Hz-85,000Hz
      • cats have one of the broadest hearing ranges among land mammals. Their most sensitive range is about 500 Hz to 32,000 Hz. They can hear ultrasonic sounds which helps in hunting prey like mice.
    • dogs can hear 67Hz-45,000Hz
    • the heart beat sounds are in the range 10-250Hz
    • sonars on ships generally use 1kHz - 15kHz
    • explosion-generated waves, especially from shallow underground or surface detonations, commonly fall in the range of about 30 Hz to 300 Hz
  • infrasonic
    • acoustic waves below 20Hz which may be felt as a deep humming sensation
    • vibration of mechanical equipment inside enclosed spaces (like heating and ventilation systems)
    • wind turbines
    • trains, planes, rockets
    • opening the rear window in a car traveling at 100 km/h, exposes the passengers to levels of infrasound as high as 125 dBz
    • “sub woofer wearable vests”
      • eg. https://www.woojer.com/products/vest-4 1-250Hz outputs
    • although it is inaudible, it may cause adverse effects which have not been fully evaluated
      • high levels may cause headache, concentration deficit, mood change, depression, sleep disorders, pulsation and panic disorders
      • resonant frequency of the eye in vivo is 18-19Hz (various with intraocular pressure) and high levels at near this frequency could cause perceptual and physical visual artefacts and “ghosts”
      • potential adverse cardiac effects:
        • exposure to high levels of infrasound can reduce in vitro cardiac muscle contractile ability by around 10-18% after just one hour, impairing the heart’s pumping function ²⁾
        • animal studies suggest adverse effects on multiple organs from high level exposures
    • seismic waves
      • usually < 3Hz
      • pressure waves through earth generated by earthquakes or from large explosions (eg. nuclear, chemical, or volcanic events)
      • NB. in addition, large explosions also produce shock waves which are initially hundreds to thousands of Hz and may be propagated faster than speed of sound and these could cause serious blast injuries including ruptured ear drums