see also:

• sound dB meters
• sound recorders
• microphones
• audio and music:
  • Music
  • audio cables, adapters, bluetooth speakers, karaoke for camping
  • digital audio workspaces (DAWs) / VST synthesizers
  • battery powered speakers for synths and PA
  • video and music streaming services in Australia
  • physical keyboard synthesizers
    • Roland XV-88 keyboard synth
    • Korg Nautilus keyboard synth

• sound is the acoustic energy waves from vibrating particles at frequency ranges human can hear
• sound intensity is usually measured in decibels (dB) - see sound dB meters
• 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
  • NB. shock waves from blasts are super-sonic as they travel faster than sound have much higher amplitudes and different physics

• 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 ¹⁾
        • 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

• a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximum amplitude response in the system
• all systems, including molecular systems and particles, tend to vibrate at a natural frequency depending upon their structure - when there is very little damping, this frequency is approximately equal to, but slightly above, the resonant frequency
• small periodic forces that are near a resonant frequency of the system have the ability to produce large amplitude oscillations in the system due to the storage of vibrational energy
• resonance can occur with all types of waves not just acoustic waves (ie. electromagnetic, nuclear magnetic, etc)
• resonant systems can be used to generate vibrations of a specific frequency (e.g., musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies (e.g., filters)

• boom box speakers are optimized to produce high intensity lower frequency sounds at audiences
  • they generally have an array of speakers aligned for various purposes:
    • straight pattern for speaker at audience head height
    • C pattern for audience sitting on steps going upwards
    • J pattern for speaker above audience
    • reverse J pattern for speaker below audience
• the physical perception (other than via hearing) of music and other sound waves such as “chest thumping feeling” of loud bass music generally maximally occurs at 60-80Hz
• sub-woofers generally are optimized for producing sound outputs at around 37-45Hz
• “sub woofer wearable vests” mainly for virtual reality gaming
  • eg. https://www.woojer.com/products/vest-4 1-250Hz outputs

• explosive or impulsive events, like a hand clap or a percussive impact, displace air or other fluids abruptly, forming a sound wave

• this is the striking of an object with another object or particles to make that object vibrate which then vibrates the air surrounding it to create sound waves
  • eg. piano, drums, other percussion instruments
• the factors determining the main frequency and its harmonic frequencies are:
  • vibrating surface properties:
    • the size (diameter and thickness) of the membrane or bar has a direct effect: larger or thicker surfaces vibrate at lower frequencies, while smaller ones vibrate at higher frequencies.
    • material properties—density and elasticity—strongly influence frequency and tone
    • tension or tightness significantly changes pitch; higher tension leads to higher frequencies
  • instrument body and air cavity
    • the shell material and construction impact resonance, timbre, and sustain
    • the size of the air cavity affects low-frequency responses and the sustain of the sound; larger cavities boost lower frequencies - see below for Helmholtz resonance
    • in some instruments, the interaction between membrane and air inside the cavity alters the frequencies produced, especially for lower tones
  • vibrational modes:
    • when struck, the surface generates several vibration modes (“harmonics” or overtones) with frequencies set by complex relationships (not simple multiples of the fundamental, unlike a string)
    • the overtones’ distribution contributes to the perceived pitch and timbre;
    • modes depend on where and how the instrument is struck
  • environmental and other factors
    • temperature, humidity, and material aging can change physical properties and thus shift frequencies
    • damping materials or attachments affect decay time and the presence of overtones

• sound is produced when air flows through or around a structure and sets it vibrating (e.g., vocal folds, reeds, air columns)
• the interaction between airflow and vibrating surfaces produces varying pressures (compression and rarefaction), translating the energy into audible sound

• this is the sound made when air is forced in and out of opening(s) to a resonant air chamber
• eg. when you blow air into a ocarina - the frequency of the sound it makes is determined by how many holes are open
• eg. the response curve of stringed instruments such as guitars and violins consists of a series of Helmholtz resonance modes associated with the size and shape of the resonance cavity (harmonics of the fundamental cavity mode), as well as vibration damping from absorption by the resonance cavity material
• eg. bass-reflex speaker enclosures - the compliance of the air mass inside the enclosure and the mass of air in the port forming a Helmholtz resonator
• eg. a piezoelectric disc acts as the excitation source, but relies on the acoustic cavity resonance to produce an audible sound
• eg. then driving a car and a rear window is partly open - this produces a low frequency throb due to the large car air volume
• eg. vehicle exhaust resonators are also used to reduce potentially loud engine noise where the dimensions are calculated so that the waves reflected by the resonator help cancel out certain frequencies of sound in the exhaust

resonant frequency (Hz) = (speed of sound in gas (343m/sec) / 2π) x sqroot (cross-sectional area of the neck of opening / (volume of cavity x equivalent length of the neck) )

• length of the neck appears in the denominator because the inertia of the air in the neck is proportional to the length.
• volume of the cavity appears in the denominator because the spring constant of the air in the cavity is inversely proportional to its volume