Sound, Infrasound and Ultrasound

How Sound Waves Work

Sound is produced when an object vibrates and transfers mechanical energy into a surrounding medium, such as air, water or a solid material.

These vibrations create disturbances that propagate through the medium as acoustic waves.

Unlike electromagnetic waves, which can travel through a vacuum, sound requires a material medium for its transmission.

Understanding the physical properties of sound helps explain how acoustic energy travels, how it interacts with objects and how it is detected by the human auditory system.

Frequency — How Quickly a Sound Wave Vibrates

Frequency describes the number of complete oscillations occurring each second and is measured in hertz (Hz).

A frequency of 100 Hz represents 100 oscillations per second, while a frequency of 1,000 Hz represents 1,000 oscillations per second.

For audible sounds, frequency is closely related to the perception of pitch.

Lower frequencies generally produce lower-pitched sounds, while higher frequencies produce higher-pitched sounds.

Acoustic frequencies are commonly divided into three broad categories:

- Infrasound: Below approximately 20 Hz.

- Audible sound: Approximately 20 Hz to 20,000 Hz.

- Ultrasound: Above approximately 20,000 Hz.

These classifications reflect conventional human hearing limits. They do not mean that all sounds within the audible range can be heard equally well, or that acoustic energy outside this range can never be detected under particular conditions.

Wavelength — The Distance Between Wave Cycles

Wavelength describes the distance occupied by one complete cycle of a wave.

It depends on the frequency of the sound and the speed at which it travels through the medium.

The relationship is expressed as:

Wavelength = Speed of Sound ÷ Frequency

In air at approximately 20°C, sound travels at around 343 metres per second.

At this speed:

- A 20 Hz sound has a wavelength of approximately 17.2 metres.

- A 100 Hz sound has a wavelength of approximately 3.43 metres.

- A 1,000 Hz sound has a wavelength of approximately 34.3 centimetres.

- A 20,000 Hz sound has a wavelength of approximately 1.7 centimetres.

Longer wavelengths are associated with lower frequencies, while shorter wavelengths are associated with higher frequencies.

Wavelength influences how sound interacts with obstacles, openings and surrounding environments.

Amplitude — The Size of a Sound Wave

Amplitude describes the magnitude of an oscillation.

For sound travelling through air, it can refer to the size of the pressure variations produced by the acoustic wave.

Larger pressure variations generally correspond to greater sound intensity, although perceived loudness also depends on frequency, duration and the sensitivity of human hearing.

Amplitude and frequency describe different properties.

A low-frequency sound can have a large amplitude, while a high-frequency sound can have a small amplitude.

The frequency of a sound therefore does not, by itself, determine how loud it will be.

Sound Pressure and Decibels

Sound pressure describes the variations in pressure produced as an acoustic wave travels through a medium.

These variations can be measured using microphones and other acoustic instruments.

Sound pressure levels are commonly expressed in decibels (dB), using a logarithmic scale.

For airborne sound, measurements often use a reference pressure of 20 micropascals, conventionally associated with the approximate threshold of human hearing at frequencies where the ear is most sensitive.

An increase of 20 dB corresponds to a tenfold increase in sound pressure amplitude, while an increase of approximately 6 dB corresponds to a doubling of pressure amplitude.

However, a change in sound pressure level does not translate directly into an equivalent change in perceived loudness.

Human hearing responds differently across frequencies, particularly at very low and very high frequencies.

Sound measurements may also use frequency weighting, such as dB(A), to account for aspects of human auditory sensitivity.

For infrasound research, unweighted or specialised low-frequency measurements may be more informative than conventional A-weighted readings.

How Sound Travels Through Different Materials

The speed of sound depends on the physical properties of the medium through which it travels.

Approximate examples include:

- Air at 20°C: 343 metres per second.

- Fresh water: 1,480 metres per second.

- Steel: Around 5,900 metres per second for longitudinal waves, depending on the material.

These differences arise from the density and elastic properties of the materials.

In gases and liquids, sound commonly travels as longitudinal pressure waves, in which particles oscillate in the same general direction as the wave propagates.

Solids can also support transverse elastic waves, in which particle motion occurs perpendicular to the direction of propagation.

When sound encounters a boundary between different materials, some acoustic energy may be reflected, transmitted or absorbed.

These interactions are important in architectural acoustics, medical ultrasound, underwater detection and industrial inspection.

Reflection, Diffraction and Resonance

Sound waves exhibit several physical behaviours that influence how they propagate.

Reflection occurs when acoustic energy encounters a surface and travels back into the original medium. Echoes are a familiar example.

Diffraction describes how waves spread around obstacles or through openings. Longer wavelengths can produce substantial diffraction around objects that are relatively small compared with the wavelength.

Resonance occurs when a system responds strongly to stimulation near one of its natural frequencies.

Resonance can affect structures, mechanical equipment, musical instruments and enclosed spaces.

These phenomena are especially relevant when examining low-frequency sound, which can have wavelengths comparable to or larger than the dimensions of buildings and other structures.

The Difference Between Acoustic and Electromagnetic Waves

Although both acoustic and electromagnetic waves can carry energy and information, their physical mechanisms differ.

Acoustic waves involve mechanical disturbances propagating through matter.

Electromagnetic waves consist of oscillating electric and magnetic fields and can propagate through a vacuum.

This distinction is particularly important when examining auditory phenomena.

Ordinary sound reaches the inner ear through mechanical vibrations, while the microwave auditory effect involves a different process in which appropriately pulsed radiofrequency energy produces small thermoelastic pressure waves within tissue.

The resulting auditory sensation involves mechanical stimulation of the auditory system, but the initiating energy is electromagnetic rather than an ordinary airborne sound wave.

Understanding these differences provides a foundation for examining the properties of infrasound, ultrasound and other forms of auditory perception.

How the Human Auditory System Detects Sound

Human hearing is a complex biological process involving the detection of mechanical vibrations, their conversion into electrical signals and the interpretation of those signals by the brain.

Although hearing is commonly associated with sound travelling through the air, auditory perception ultimately depends on activity within the nervous system.

The auditory system consists of several interconnected structures that detect acoustic energy, analyse its properties and transmit information to specialised regions of the brain.

Understanding this process helps explain how humans perceive pitch, loudness, location and complex sounds such as speech and music.

The Outer Ear — Collecting Sound

The outer ear consists primarily of the visible external ear, known as the pinna, and the external auditory canal.

The pinna helps collect sound and contributes to the identification of its direction, particularly through the way its shape modifies incoming frequencies.

Sound waves travel through the auditory canal until they reach the tympanic membrane, commonly known as the eardrum.

The changing pressure of the acoustic wave causes the eardrum to vibrate.

These vibrations are then transferred to the mechanical structures of the middle ear.

The Middle Ear — Transmitting Mechanical Vibrations

The middle ear contains three small interconnected bones known as the auditory ossicles:

- Malleus, commonly called the hammer.

- Incus, commonly called the anvil.

- Stapes, commonly called the stirrup.

These bones transmit vibrations from the eardrum to the oval window of the inner ear.

The middle ear also helps match the mechanical properties of airborne sound to those of the fluid-filled cochlea.

Without this process, a substantial proportion of incoming acoustic energy would be reflected rather than efficiently transmitted into the inner ear.

The movement of the stapes at the oval window creates pressure disturbances within the cochlear fluids.

The Cochlea — Converting Vibrations into Neural Signals

The cochlea is a spiral-shaped structure within the inner ear that plays a central role in hearing.

It contains fluid-filled compartments and a specialised sensory structure known as the organ of Corti.

When vibrations enter the cochlea, they generate travelling waves along the basilar membrane.

Different regions of this membrane respond most strongly to different sound frequencies.

Higher-frequency sounds generally produce their greatest response near the base of the cochlea, while lower-frequency sounds produce their greatest response closer to the apex.

This spatial organisation is known as tonotopy.

It provides an important biological mechanism through which the auditory system distinguishes different frequencies.

Sensory Hair Cells and Electrical Signalling

The organ of Corti contains specialised sensory receptors known as hair cells.

These cells have bundles of microscopic projections called stereocilia.

Movement within the cochlea causes mechanical deflection of these bundles, opening mechanically sensitive ion channels.

This changes the electrical potential across the hair-cell membrane.

Inner hair cells release neurotransmitter onto nearby auditory nerve fibres, initiating patterns of neural activity that convey acoustic information towards the brain.

Outer hair cells perform an additional function by actively modifying the mechanical response of the cochlea.

This process, often described as cochlear amplification, improves sensitivity and frequency selectivity.

The conversion of mechanical movement into an electrical cellular response is known as mechanotransduction.

The Auditory Nerve — Carrying Information to the Brain

The auditory nerve forms the auditory division of the vestibulocochlear nerve, also known as cranial nerve VIII.

It carries electrical impulses generated by auditory nerve fibres from the cochlea towards the brainstem.

These signals contain information about acoustic frequency, timing and intensity.

Individual nerve fibres respond preferentially to particular frequency regions of the cochlea, preserving aspects of its tonotopic organisation.

The timing and patterns of neural activity also contribute to the perception of pitch and other sound characteristics.

However, the auditory nerve does not transmit a complete sound recording in the manner of a digital audio cable.

Instead, the nervous system represents acoustic information through distributed patterns of electrical activity.

The Brainstem and Auditory Pathways

Auditory nerve fibres first reach the cochlear nuclei within the brainstem.

From there, information travels through several interconnected pathways involving structures such as the superior olivary complex, lateral lemniscus and inferior colliculus.

These regions contribute to different aspects of auditory processing.

For example, the nervous system compares information arriving from both ears to help determine the direction of a sound source.

Differences in arrival time and sound intensity between the ears provide important cues for locating sounds.

Auditory information subsequently reaches the medial geniculate body of the thalamus before being transmitted to the auditory cortex.

The Auditory Cortex — Interpreting Sound

The auditory cortex is located primarily within the temporal lobes of the brain.

It contains regions involved in analysing acoustic features and supporting the perception of complex auditory information.

Neural processing within the auditory cortex contributes to the recognition of speech, music and environmental sounds.

However, auditory perception is not confined to a single brain region.

Understanding spoken language, recognising familiar voices and interpreting the emotional significance of sounds involve interactions between multiple neural networks.

The brain combines incoming sensory information with attention, memory, learning and expectations.

Consequently, hearing involves both the detection of acoustic energy and the interpretation of neural information.

Bone Conduction and Alternative Routes to the Inner Ear

Ordinary airborne sound typically reaches the cochlea through the outer and middle ear.

However, vibrations can also reach the inner ear through the bones and tissues of the head.

This process is known as bone conduction.

Bone-conduction hearing demonstrates that mechanical stimulation of the cochlea does not always require sound to enter through the external auditory canal.

Bone-conduction devices use this principle to transmit vibrations through the skull, allowing auditory information to reach the inner ear through an alternative mechanical pathway.

Auditory Perception and the Microwave Auditory Effect

The distinction between airborne sound and alternative forms of cochlear stimulation is particularly relevant to the microwave auditory effect.

In this phenomenon, appropriately pulsed radiofrequency energy can produce rapid, extremely small temperature changes within absorbing tissue.

These changes generate thermoelastic pressure waves that can propagate through tissue and stimulate the auditory system.

The resulting sensation may be perceived as clicks, buzzing or other auditory impressions, depending on the characteristics of the pulsed signal.

Historical experiments also demonstrated that encoded information, including speech, could be recognised through this phenomenon under particular experimental conditions.

Importantly, this mechanism does not require conventional airborne sound to travel from an external loudspeaker into the ear.

Nevertheless, the established explanation involves mechanically generated pressure waves and auditory reception rather than direct transmission of thoughts into the brain.

The Importance of the Auditory System in Acoustic Research

The human auditory system provides the biological foundation for understanding sound perception.

Its sensitivity varies across frequency, intensity and duration, while different forms of stimulation can produce distinct auditory experiences.

These characteristics are particularly important when examining infrasound, ultrasound, bone conduction and radiofrequency-induced auditory phenomena.

Understanding how the cochlea and brain process auditory information also helps distinguish measurable physical mechanisms from unsupported explanations of perception.

Infrasound — Frequencies Below Human Hearing

Infrasound refers to acoustic waves with frequencies below approximately 20 hertz (Hz), the conventional lower boundary of human hearing.

These waves are produced by numerous natural processes, industrial activities and mechanical systems.

Unlike electromagnetic radiation, infrasound consists of mechanical pressure disturbances that require a material medium, such as air, water or solid matter, to propagate.

Although infrasound is commonly described as sound below the audible range, sufficiently intense low-frequency acoustic signals can sometimes be perceived by humans.

The ability to detect these frequencies depends on sound pressure level, individual hearing sensitivity and the characteristics of the acoustic environment.

The Physical Properties of Infrasound

Infrasound is characterised by relatively long wavelengths.

In air at approximately 20°C, where sound travels at around 343 metres per second:

• A 20 Hz wave has a wavelength of approximately 17 metres.

• A 10 Hz wave has a wavelength of approximately 34 metres.

• A 1 Hz wave has a wavelength of approximately 343 metres.

• A 0.1 Hz wave has a wavelength of approximately 3.43 kilometres.

These long wavelengths influence how infrasonic waves interact with buildings, terrain and other obstacles.

Low-frequency acoustic waves can diffract around objects that would substantially obstruct shorter-wavelength sounds.

Under suitable atmospheric conditions, infrasound can also propagate over very large distances.

However, propagation depends on factors including frequency, atmospheric temperature, wind patterns, source characteristics and background noise.

Long wavelengths do not mean that infrasonic energy travels without attenuation or passes through every material without obstruction.

Natural Sources of Infrasound

Infrasound occurs throughout the natural environment.

Important sources include:

• Earthquakes: Seismic activity can generate low-frequency atmospheric pressure disturbances.

• Volcanic eruptions: Explosive volcanic activity can produce powerful infrasonic signals detectable at considerable distances.

• Ocean waves: Interactions between ocean waves generate persistent low-frequency acoustic signals, including atmospheric microbaroms.

• Severe weather: Thunderstorms, tornadoes and other atmospheric disturbances can produce infrasonic energy.

• Avalanches and landslides: Large movements of snow, rock or soil can generate detectable low-frequency pressure waves.

• Meteors: Large meteoroids entering the atmosphere can produce infrasonic signals through shock waves and atmospheric disturbances.

Scientists use specialised instruments to detect and analyse these signals.

Infrasound monitoring can therefore provide valuable information about natural events that may occur far beyond the range of ordinary human hearing.

Human-Made Sources of Infrasound

Human activities also generate low-frequency acoustic energy.

Examples include large engines, industrial machinery, ventilation systems, turbines, heavy vehicles and certain forms of construction equipment.

Explosions, rocket launches and aircraft operations can produce substantial low-frequency acoustic components.

In many practical environments, infrasound occurs alongside audible low-frequency noise rather than as an isolated acoustic signal.

The distinction matters because human responses to a source may involve audible sound, vibration, infrasound or a combination of these factors.

Identifying the responsible mechanism requires measurements of the actual acoustic environment.

Ultrasound — Frequencies Above Human Hearing

Ultrasound refers to mechanical acoustic waves with frequencies above approximately 20,000 hertz (20 kHz), the conventional upper boundary of human hearing.

Like ordinary audible sound and infrasound, ultrasound consists of mechanical vibrations travelling through a material medium.

However, its higher frequencies and shorter wavelengths allow ultrasound to interact with materials and biological tissues in ways that are particularly useful for medical imaging, industrial inspection, precision measurement and neurological treatment.

Importantly, ultrasound is not limited to imaging the brain. Focused ultrasound is already used clinically to treat selected neurological conditions, while other forms of ultrasonic stimulation have demonstrated the ability to influence neural activity without deliberately damaging tissue.

These capabilities have established ultrasound as an important technology in modern neuroscience and medicine.

The Physical Properties of Ultrasound

Ultrasonic waves obey the same fundamental physical principles as other forms of sound.

Their frequency determines the number of oscillations per second, while their wavelength depends on the speed of sound within the transmitting medium.

In water, where sound travels at approximately 1,480 metres per second:

• A 20 kHz wave has a wavelength of approximately 7.4 centimetres.

• A 100 kHz wave has a wavelength of approximately 1.48 centimetres.

• A 1 MHz wave has a wavelength of approximately 1.48 millimetres.

• A 10 MHz wave has a wavelength of approximately 0.148 millimetres.

Higher-frequency ultrasound can provide greater spatial resolution when imaging small structures.

However, acoustic attenuation generally increases with frequency, limiting the depth to which high-frequency ultrasound can penetrate many materials.

This creates an important engineering trade-off between imaging resolution and penetration depth.

How Ultrasound Is Generated

Ultrasound is commonly generated using piezoelectric transducers.

Piezoelectric materials change shape when an electrical voltage is applied.

When an alternating electrical signal is supplied to a suitable transducer, the material vibrates and produces acoustic waves.

The same principle can operate in reverse.

Incoming mechanical vibrations can generate electrical signals, allowing piezoelectric devices to detect returning ultrasound echoes.

This makes piezoelectric transducers particularly useful in medical scanners, sonar equipment, industrial inspection systems and certain neurological stimulation devices.

Other ultrasound-generation methods also exist, including capacitive micromachined ultrasonic transducers and electromagnetic acoustic transducers for specialised applications.

Reflection, Transmission and Acoustic Impedance

When ultrasound encounters a boundary between two materials, part of its energy may be reflected while the remainder is transmitted or absorbed.

The amount reflected depends partly on the difference in acoustic impedance between the materials.

Acoustic impedance describes the relationship between acoustic pressure and particle motion within a medium.

Large differences in acoustic impedance can produce strong reflections.

This principle is fundamental to ultrasound imaging, where returning echoes provide information about boundaries between tissues.

It also explains why medical ultrasound examinations commonly require coupling gel.

The gel reduces air gaps between the transducer and skin, improving acoustic transmission into the body.

Medical Ultrasound Imaging

One of the most established applications of ultrasound is diagnostic medical imaging.

Ultrasound scanners transmit short acoustic pulses into the body and detect echoes reflected from internal structures.

By analysing the timing and strength of these returning signals, computer systems construct images of tissues and organs.

Diagnostic ultrasound is widely used in:

• Pregnancy and fetal development examinations.

• Cardiac imaging, known as echocardiography.

• Abdominal and pelvic investigations.

• Musculoskeletal examinations.

• Blood-flow measurements using Doppler ultrasound.

• Image-guided medical procedures.

Unlike X-ray imaging, ultrasound does not use ionising radiation.

However, ultrasound still transfers mechanical energy into tissue, so appropriate exposure limits and established clinical safety practices remain important.

Focused Ultrasound and Medical Treatment

Ultrasound can also be used therapeutically.

Focused ultrasound systems concentrate acoustic energy within a selected region of tissue.

Depending on the frequency, intensity, duration and delivery method, this energy can produce different biological effects.

High-intensity focused ultrasound can generate localised heating sufficient to damage or destroy targeted tissue.

This approach is used in selected treatments involving uterine fibroids, prostate disease and certain neurological conditions.

Magnetic resonance-guided focused ultrasound, commonly abbreviated MRgFUS, combines ultrasound treatment with MRI-based anatomical targeting and temperature monitoring.

This enables clinicians to concentrate energy within carefully selected tissue regions while monitoring the procedure.

Importantly, therapeutic ultrasound can also operate at lower intensities that are intended to influence biological activity without producing permanent tissue destruction.

These different approaches have become important areas of neurological research and clinical development.

Parkinson's Disease and Essential Tremor

One of the most significant established neurological applications of focused ultrasound involves the treatment of movement disorders.

MRI-guided focused ultrasound is used clinically to treat selected patients with essential tremor and Parkinson's disease.

These treatments direct concentrated ultrasound energy towards carefully selected brain structures involved in abnormal movement.

For example, focused ultrasound thalamotomy targets part of the thalamus to interrupt circuits associated with tremor.

Other procedures investigated or authorised for selected Parkinson's disease symptoms involve different targets within movement-related brain circuits.

The US Food and Drug Administration has authorised specific focused ultrasound procedures for essential tremor and Parkinson's disease, although indications and eligibility differ between treatments and jurisdictions.

These procedures demonstrate that ultrasound can produce precise, clinically useful changes within the human brain without requiring a conventional surgical incision through the skull.

However, lesion-producing focused ultrasound treatments are not equivalent to non-destructive neural stimulation.

The former deliberately create a permanent therapeutic lesion, whereas the latter aim to modify neural activity without destroying the targeted tissue.

Transcranial Ultrasound Neuromodulation

A separate and increasingly important area of neuroscience involves the use of focused ultrasound to influence neural activity.

This technology is commonly described as transcranial ultrasound stimulation or transcranial focused ultrasound neuromodulation.

Experimental systems direct controlled acoustic energy through the skull towards selected regions of the brain.

Unlike high-intensity treatments that deliberately destroy tissue, low-intensity neuromodulation generally aims to produce temporary changes in neural activity.

Human and animal studies have demonstrated that ultrasound can influence measurable neural responses under controlled experimental conditions.

Researchers have reported changes in cortical excitability, sensory processing, motor responses and selected cognitive or behavioural measures.

The physical mechanisms are still being investigated, but proposed contributors include mechanical effects on cell membranes, mechanically sensitive ion channels and other processes through which acoustic energy interacts with nervous tissue.

These findings establish that ultrasound can interact with the central nervous system and modify aspects of neural activity.

However, the reliability, precision and clinical usefulness of particular stimulation protocols vary, and many applications remain experimental.

Ultrasound and Alzheimer's Disease

Focused ultrasound is also being investigated in Alzheimer's disease research.

One important approach involves temporarily increasing the permeability of the blood–brain barrier.

The blood–brain barrier regulates the movement of substances from the bloodstream into brain tissue.

In certain experimental procedures, focused ultrasound is combined with intravenously administered microbubbles.

When exposed to appropriately controlled ultrasound, the microbubbles oscillate and can temporarily increase blood–brain barrier permeability within a targeted region.

Researchers are investigating whether this technique could improve the delivery of therapeutic substances to the brain.

Studies have also examined its potential relevance to amyloid-related pathology and other processes associated with Alzheimer's disease.

Human research has demonstrated the feasibility of targeted, temporary blood–brain barrier opening under controlled conditions.

However, this does not yet establish that the technique provides a proven treatment for Alzheimer's disease or reliably prevents cognitive decline.

Ultrasound Research into Other Neurological Conditions

The ability of ultrasound to influence neural activity has encouraged research into a wider range of neurological and psychiatric conditions.

Investigated applications include:

• Epilepsy: Studying whether focused ultrasound can influence abnormal neural activity associated with seizures.

• Depression: Investigating whether stimulation of selected brain networks can modify symptoms or relevant neural responses.

• Anxiety: Examining potential effects on brain circuits involved in emotional processing.

• Chronic pain: Exploring whether targeted neural stimulation can alter pain-related processing.

• Disorders of consciousness: Investigating stimulation of structures such as the thalamus and its relationship to responsiveness.

• Schizophrenia: Early investigations into whether ultrasound neuromodulation can influence selected neural circuits and symptoms.

• Neurological rehabilitation: Exploring possible applications involving motor recovery and neural plasticity.

These research programmes demonstrate that ultrasound-based neuromodulation is an active and expanding scientific field.

However, the existence of clinical trials or experimental results does not mean that every proposed application has become an established medical treatment.

Evidence must be assessed separately for each condition, stimulation protocol and clinical outcome.

Ultrasound and Lucid Dreaming Technology

An emerging application involves investigating whether ultrasound stimulation can influence brain activity associated with sleep and dreaming.

Lucid dreaming is a state in which a person becomes aware that they are dreaming while the dream continues.

Researchers have investigated the neurological characteristics of lucid dreaming, including activity within brain networks associated with awareness, attention and self-reflection.

The neurotechnology company Prophetic has been developing wearable devices intended to use transcranial focused ultrasound to influence brain activity during sleep.

Its proposed technology involves delivering controlled ultrasonic stimulation to selected brain regions, with the aim of increasing the likelihood of lucid dreaming.

This represents a real technological development based on the established ability of focused ultrasound to interact with neural tissue.

However, an important distinction remains between demonstrating that ultrasound can modulate brain activity and demonstrating that a particular device can reliably induce lucid dreams.

As of October 2026, independent, peer-reviewed evidence establishing reliable lucid-dream induction through Prophetic's devices has not been verified here.

The technology should therefore be described as an emerging application rather than an established clinical or consumer capability.

Nevertheless, it illustrates how ultrasound neuromodulation is being investigated beyond conventional medical treatment, including potential applications involving sleep, awareness and conscious experience.

Can Ultrasound Influence Human Perception?

Scientific research has demonstrated that ultrasound can influence the activity of the human nervous system under particular experimental conditions.

Transcranial stimulation studies have reported changes in neural responses and selected perceptual, cognitive and behavioural measurements.

These findings are significant because they establish that externally generated mechanical energy can interact with neural tissue and influence aspects of brain function.

The effects depend on the location of stimulation, acoustic parameters, tissue characteristics and the physiological state of the nervous system.

Some experimental results involve sensory processing, while others concern motor activity, attention or other measurable functions.

However, demonstrating a change in neural activity does not automatically establish the ability to generate arbitrary thoughts, memories or detailed sensory experiences on demand.

Such capabilities would require independent experimental demonstrations with clearly defined outcomes.

The established conclusion is that ultrasound can interact with and modulate neural activity, while the range and precision of achievable effects remain dependent on the particular technology and supporting evidence.

Ultrasound Targeting and the Human Skull

Delivering ultrasound through the skull presents significant engineering challenges.

Bone can reflect, absorb and distort acoustic waves.

The skull's varying thickness, density and geometry can therefore affect the location and intensity of ultrasound reaching the brain.

Researchers use several approaches to improve targeting.

These include acoustic modelling, medical imaging, specialised transducer arrangements and electronically controlled phased arrays.

Phased-array systems can adjust the timing of signals emitted by multiple transducer elements to concentrate acoustic energy within a selected region.

Such techniques are important for both clinical focused ultrasound procedures and experimental transcranial neuromodulation.

They also illustrate why ultrasound interaction with the brain requires careful consideration of acoustic propagation and exposure conditions.

Ultrasound in Industrial Inspection

Ultrasound is widely used in engineering to investigate the internal structure of materials without destroying them.

This approach is known as ultrasonic non-destructive testing.

A transducer introduces acoustic pulses into a material, and returning signals are analysed to identify internal boundaries or defects.

Applications include:

• Detecting cracks in metal components.

• Inspecting welded joints.

• Measuring material thickness.

• Examining composite structures.

• Monitoring corrosion in industrial equipment.

• Inspecting aircraft and other safety-critical components.

Ultrasonic inspection is particularly valuable because it can reveal internal defects that are not visible from the surface.

Sonar and Underwater Acoustic Sensing

Ultrasound is also used in underwater sensing and navigation.

Sonar systems transmit acoustic signals through water and analyse returning echoes to identify objects, measure distances or investigate underwater environments.

Some sonar systems operate at ultrasonic frequencies, although sonar can also use audible and lower acoustic frequencies.

High-frequency sonar is particularly useful for detailed imaging over relatively short distances.

Applications include seabed mapping, underwater robotics, obstacle detection and the inspection of submerged structures.

The use of acoustic rather than electromagnetic signals is important because radiofrequency waves generally experience substantial attenuation in conductive seawater.

Audible Effects from Ultrasonic Sources

Although ultrasound lies above the conventional human hearing range, ultrasonic equipment can sometimes produce audible effects.

These may arise from nonlinear acoustic interactions, mechanical vibration, subharmonics or other processes that generate energy within the audible frequency range.

For example, parametric loudspeakers use intense modulated ultrasonic beams and nonlinear propagation effects to produce audible sound with relatively directional characteristics.

Such systems demonstrate that ultrasonic technology can be used to generate audible information.

However, the resulting audible sound should not be confused with direct perception of the ultrasonic carrier frequency.

The physical mechanism producing the audible component is an essential part of the explanation.

Ultrasound and the Microwave Auditory Effect

Ultrasound and the microwave auditory effect are distinct physical phenomena.

Ultrasound consists of mechanical acoustic waves generated and transmitted through a material medium.

The microwave auditory effect begins with the absorption of appropriately pulsed radiofrequency electromagnetic energy.

Rapid thermoelastic expansion produces small pressure waves within tissue, which can stimulate the auditory system.

Historical research demonstrated that the microwave auditory effect can produce recognisable auditory sensations and convey encoded information, including speech, under particular experimental conditions.

Ultrasound-based neuromodulation operates through a different mechanism.

It involves directing mechanical acoustic energy towards neural tissue to produce measurable biological effects.

Both fields demonstrate that physical energy can interact with biological systems in scientifically measurable ways.

However, the capabilities established for one technology should not automatically be attributed to the other.

Each mechanism requires its own experimental evidence.

The Importance of Ultrasound Research

Ultrasound has progressed far beyond its traditional role in diagnostic imaging.

It is now an established technology in selected neurological treatments, including focused ultrasound procedures for movement disorders.

Research has also demonstrated that lower-intensity ultrasound can influence neural activity without deliberately destroying tissue.

These developments have expanded investigations into Alzheimer's disease, epilepsy, psychiatric conditions, pain, sleep and other aspects of nervous-system function.

Emerging technologies intended to influence dreaming and conscious awareness illustrate further possible applications, although their effectiveness requires continued independent validation.

Ultrasound therefore represents an important connection between acoustics, neuroscience, medicine and advanced computing.

Understanding its demonstrated capabilities, underlying physical mechanisms and remaining limitations provides a scientific foundation for evaluating both established treatments and future developments in neural technology.

Comparing Infrasound, Audible Sound and Ultrasound

Infrasound, audible sound and ultrasound are all forms of mechanical acoustic energy. They differ primarily in frequency, but that difference influences their wavelengths, how they travel through materials, how they are measured and how they interact with biological systems.

The conventional divisions are useful for describing human hearing, but they are not absolute boundaries between fundamentally different physical phenomena. All three categories follow the same general principles of wave propagation, reflection, absorption and transmission.

Frequency Ranges and Human Perception

Infrasound describes acoustic frequencies below approximately 20 hertz (Hz). At sufficiently high sound pressure levels, some infrasonic frequencies can be perceived as low-pitched sound, pressure or vibration. The conventional 20 Hz boundary is not an absolute threshold for every individual or listening condition.

Audible sound is commonly described as ranging from approximately 20 Hz to 20,000 Hz. Human sensitivity is not uniform across this range and typically declines at very low and very high frequencies. The upper limit also tends to decrease with age and exposure history.

Ultrasound refers to frequencies above approximately 20,000 Hz. These frequencies lie above the conventional upper limit of human hearing, although ultrasonic equipment can sometimes generate audible by-products through nonlinear interactions or mechanical vibration.

Frequency and Wavelength

Wavelength is the distance occupied by one complete wave cycle. For a given medium, wavelength decreases as frequency increases.

In air at approximately 20°C, where sound travels at about 343 metres per second, a 10 Hz infrasonic wave has a wavelength of about 34.3 metres, a 1,000 Hz audible wave has a wavelength of about 34.3 centimetres, and a 40,000 Hz ultrasonic wave has a wavelength of about 8.6 millimetres.

These examples illustrate why lower-frequency waves can diffract around comparatively large obstacles, while higher-frequency waves can support finer spatial resolution in suitable sensing and imaging systems.

Wavelength must always be calculated using the speed of sound in the relevant medium. The same frequency has a different wavelength in air, water and biological tissue.

Propagation Through the Environment

Infrasound can travel over substantial distances under favourable atmospheric conditions. Long wavelengths and atmospheric refraction help explain why specialised monitoring networks can detect some volcanic eruptions, explosions and other energetic events far from their sources. Actual transmission still depends on source strength, terrain, atmospheric conditions and attenuation.

Audible sound propagates through air, water and solids, with its behaviour influenced by frequency, distance, obstacles and material properties. Everyday speech, music and environmental sounds involve combinations of frequencies rather than single pure tones.

Ultrasound is particularly useful for examining relatively small structures because its shorter wavelengths can provide greater spatial detail. Higher frequencies generally experience stronger attenuation in many materials, creating a trade-off between resolution and penetration depth.

How the Body Detects Acoustic Energy

Ordinary hearing begins when mechanical vibrations stimulate the cochlea of the inner ear. Sensory hair cells convert mechanical movement into electrical activity, which travels through auditory pathways to the brain.

Low-frequency acoustic exposure may also produce bodily sensations involving pressure or vibration, particularly at higher intensities. These experiences should be distinguished from auditory perception itself and from vibrations transmitted directly through floors or structures.

Ultrasound is different in an important respect: its medical and neurological applications do not depend on a person hearing the ultrasonic frequency. Carefully controlled acoustic energy can interact directly with tissues, including neural tissue, through physical mechanisms investigated in experimental and clinical research.

Established Uses of Infrasound

Infrasound is used in atmospheric and geophysical monitoring. Specialised sensors detect pressure waves generated by volcanic eruptions, large explosions, meteors, severe weather and ocean-wave interactions.

The Comprehensive Nuclear-Test-Ban Treaty Organization uses infrasound stations within its International Monitoring System to help detect and characterise atmospheric events.

These applications rely on measuring acoustic signals rather than producing effects in the human nervous system.

Established Uses of Audible Sound

Audible sound supports speech, music, warning systems, communication technologies and many forms of environmental sensing.

Clinical hearing tests use controlled acoustic signals to investigate auditory sensitivity. Hearing aids and other assistive devices process sound to improve access to acoustic information.

Audible-frequency sound can also be used in specialised research and therapeutic settings, although the evidence for any particular treatment depends on its mechanism and clinical results.

Established Uses of Ultrasound

Ultrasound has a particularly wide range of established applications. Diagnostic scanners use returning echoes to image organs, tissues, blood flow and developing fetuses. Industrial systems inspect materials for cracks, corrosion and other internal defects. High-frequency sonar supports underwater imaging and measurement.

In medicine, focused ultrasound can also deliver therapeutic energy to a selected target. Depending on the procedure, this may involve heating and destroying tissue or producing other controlled biological effects.

Ultrasound and the Central Nervous System

Focused ultrasound is an established clinical technology for selected neurological procedures, including treatments for essential tremor and certain Parkinson's disease symptoms. Some procedures use MRI guidance to create precisely targeted lesions in movement-related brain circuits.

Separate research into low-intensity transcranial ultrasound has demonstrated that acoustic stimulation can modify measurable neural activity under controlled conditions without deliberately creating a lesion.

Investigated applications include sensory and motor processing, depression, epilepsy, chronic pain and disorders of consciousness. Focused ultrasound combined with microbubbles has also been studied for temporarily increasing blood–brain barrier permeability, including in Alzheimer's disease research.

These examples establish that ultrasound can interact with the brain and central nervous system. However, clinical approval for one procedure does not establish the effectiveness of other proposed treatments, and experimental changes in neural activity do not automatically demonstrate precise control over complex thoughts or experiences.

Sleep and Lucid-Dreaming Research

Developers have also investigated ultrasound-based technologies intended to influence neural activity during sleep. Prophetic, for example, has described wearable technology designed to explore whether focused ultrasound can increase the likelihood of lucid dreaming.

Lucid dreaming occurs when a sleeping person becomes aware that they are dreaming. The proposed application is scientifically distinct from established focused-ultrasound treatments for movement disorders.

The existence of a device-development programme should not be confused with independent clinical proof of reliable lucid-dream induction. Its claimed effectiveness requires appropriate controlled studies and reproducible results.

Comparing Biological Effects

The biological effects of acoustic exposure cannot be predicted from frequency alone. Relevant factors include acoustic pressure or intensity, duration, waveform, focusing, the medium, the tissues involved and the route by which energy reaches the body.

For example, ordinary environmental infrasound, a loud audible tone, diagnostic ultrasound and focused ultrasound neuromodulation involve very different exposure conditions.

Similarly, low-intensity ultrasound neuromodulation and high-intensity lesion-producing focused ultrasound should not be described as the same treatment merely because both use ultrasonic waves.

Distinguishing Acoustic and Radiofrequency Mechanisms

Infrasound, audible sound and ultrasound are mechanical waves requiring a material medium. Radiofrequency signals are electromagnetic waves and can propagate through a vacuum.

The microwave auditory effect begins when appropriately pulsed radiofrequency energy is absorbed in tissue, producing small thermoelastic pressure waves that can stimulate the auditory system. This differs from transmitting an ordinary acoustic wave through air or using an ultrasound transducer to deliver mechanical energy.

Historical microwave auditory research demonstrated recognisable auditory sensations and the reception of encoded information, including speech, under particular experimental conditions. Those findings should be distinguished from ultrasound-based neural stimulation, which has its own physical mechanisms and evidence base.

Why These Distinctions Matter

Grouping acoustic technologies only by whether their frequencies are audible can obscure important differences in how they operate.

Infrasound is particularly useful for monitoring large-scale environmental events. Audible sound underpins ordinary hearing and acoustic communication. Ultrasound supports high-resolution sensing, diagnostic imaging, established medical treatments and active research into neural modulation.

The most reliable way to evaluate any claimed effect is to identify the energy source, frequency, exposure conditions, target tissue, demonstrated outcome and quality of supporting evidence.

Together, these principles provide a foundation for understanding the connections between acoustics, auditory perception, neuroscience and emerging technologies.