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Subsonic

Subsonic refers to any speed or flow condition below the speed of sound in a given medium, typically air, where the Mach number is less than 1.^[1] In standard atmospheric conditions at sea level and 15°C, the speed of sound is approximately 340 m/s (1,116 ft/s or 761 mph), though this value varies with temperature, altitude, and humidity.^[2] This regime contrasts with supersonic speeds, which exceed Mach 1 and produce shock waves, and is fundamental to understanding fluid dynamics without significant compressibility effects.^[3] In aeronautics, subsonic flight predominates for most civil and general aviation aircraft, operating below approximately Mach 0.8 to avoid transonic complications like drag rise.^[4] Airflow in this regime behaves as incompressible for low subsonic speeds (Mach < 0.3), allowing simpler aerodynamic models based on Bernoulli's principle and enabling efficient lift generation through wing designs optimized for steady, attached flow.^[5] Notable examples include commercial airliners like the Boeing 737, which cruise at Mach 0.78, prioritizing fuel efficiency and passenger comfort over higher velocities.^[6] Beyond aviation, subsonic principles apply in ballistics and acoustics. Subsonic ammunition, with muzzle velocities under 343 m/s, minimizes sonic cracks for suppressed firearms, enhancing stealth in tactical applications.^[7] In audio engineering, subsonic filters attenuate frequencies below 20 Hz to prevent subwoofer distortion from inaudible rumbles, ensuring cleaner sound reproduction.^[8] These applications underscore subsonic phenomena's role in engineering disciplines requiring controlled, low-velocity dynamics.

Physics and Aerodynamics

Definition in Fluid Dynamics

In fluid dynamics, subsonic flow refers to the motion of a fluid, such as air or water, where the velocity at every point is less than the local speed of sound in that medium, corresponding to a Mach number less than 1.^[9] This regime is characterized by the absence of shock waves, allowing pressure disturbances to propagate upstream and in all directions relative to the flow.^[9] In subsonic conditions, flows can often be approximated as incompressible when the Mach number is sufficiently low (typically below 0.3), meaning density variations are negligible and the fluid behaves as if its volume does not change under pressure, simplifying analyses in aerodynamics and engineering.^[10] The Mach number, denoted as M, is defined as the ratio of the flow velocity v to the speed of sound a in the fluid:

M = \frac{v}{a}

This dimensionless quantity arises from principles of wave propagation: the speed of sound a represents the velocity at which infinitesimal pressure disturbances, or acoustic waves, travel through the medium, derived from the linearized equations of fluid motion where small perturbations propagate at a = \sqrt{\gamma R T} for an ideal gas, with \gamma as the specific heat ratio, R the gas constant, and T the temperature.^[9] When M < 1, the flow velocity is slower than this propagation speed, enabling disturbances to influence the entire flow field isentropically without abrupt discontinuities.^[9] The distinction between subsonic and higher-speed regimes emerged in the early 20th century through foundational work in compressible flow theory. German physicist Ludwig Prandtl, along with his student Theodor Meyer, developed the first systematic theories of supersonic shock waves and flow patterns in 1908, explicitly differentiating subsonic flows—where smooth, elliptic partial differential equations govern the behavior—from supersonic regimes, which involve hyperbolic equations and shock discontinuities.^[11] This framework, building on earlier shock wave studies by Ernst Mach in the late 19th century, established subsonic flow as a domain free of shock waves, in contrast to transonic flows (near Mach 1, with localized supersonic regions and weak shocks) and supersonic flows (Mach > 1, featuring strong oblique or normal shocks that abruptly alter flow properties).^[11]

Speed Regimes and Mach Number

In aerodynamics, flight speed regimes are classified based on the Mach number (M), which is the ratio of an object's speed to the local speed of sound. Subsonic flow occurs when M < 0.8–0.9, where airflow over the object remains entirely below the speed of sound, allowing for relatively straightforward aerodynamic predictions using incompressible flow approximations.^[9]^[12] As speeds increase into the transonic regime (M ≈ 0.8–1.2), mixed subsonic and supersonic flow regions develop, leading to complex shock wave interactions and a significant rise in drag.^[13] Supersonic flow begins above M > 1.2, characterized by attached shock waves and wave drag, while hypersonic flow at M > 5 introduces additional effects like high-temperature chemistry and ionization in the boundary layer.^[9]^[12] The speed of sound (a), which defines the Mach number threshold, varies primarily with temperature in an ideal gas and is given by the formula:

a = \sqrt{\gamma R T}

where \gamma is the specific heat ratio (approximately 1.4 for air), R is the gas constant for air (287 J/kg·K), and T is the absolute temperature in Kelvin.^[14] This equation derives from the isentropic relations in compressible flow, showing that a scales with the square root of temperature, independent of pressure or density for ideal gases.^[14] In dry air at standard sea-level conditions (T ≈ 288 K or 15°C), a ≈ 340 m/s, though it reaches about 343 m/s at 20°C; thus, subsonic speeds are typically below 295–308 m/s (depending on exact temperature).^[15] Temperature decreases with altitude in the troposphere (lapsing at about 6.5 K/km up to 11 km), reducing a to around 295 m/s at 11 km (stratosphere base), which lowers the absolute subsonic speed limit to roughly 256 m/s despite similar Mach criteria.^[16]^[17] Above the tropopause, temperature stabilizes or increases, stabilizing a near 300 m/s in the lower stratosphere.^[16] Near M = 1, the transition from subsonic to supersonic flow involves the critical Mach number (M_crit), defined as the freestream Mach number at which the first local region of supersonic flow (M > 1) appears on the object's surface, often forming supersonic pockets accelerated by adverse pressure gradients.^[13] These pockets terminate in shocks, causing boundary layer separation, drag divergence, and buffet effects that mark the onset of transonic challenges; for typical airfoils, M_crit ranges from 0.6 to 0.8, depending on thickness and camber.^[13] In real-world conditions, such as at cruising altitudes around 10 km where a ≈ 299 m/s, the subsonic limit (M < 0.8) corresponds to true airspeeds below approximately 239 m/s, illustrating how environmental variations shift operational boundaries for aircraft.^[16]^[17]

Characteristics of Subsonic Flow

In subsonic flow regimes, where the Mach number is below approximately 0.3, the incompressible flow approximation becomes valid, allowing density variations to be neglected in the governing equations.^[18] This simplification leads to the incompressible Navier-Stokes equations, which eliminate strong compressibility effects and reduce the complexity of solving for fluid motion by assuming constant density throughout the flow field.^[19] As a result, analyses of subsonic flows often employ these equations to model pressure, velocity, and viscous forces more tractably, particularly in low-speed engineering contexts.^[20] A defining characteristic of subsonic flow is the propagation of pressure disturbances at the speed of sound, which exceeds the flow velocity, enabling these disturbances to travel upstream and adjust the airflow smoothly around an object.^[21] Unlike in supersonic regimes, this allows information about the object's presence to propagate ahead, preventing the formation of abrupt shock waves and resulting in a more gradual pressure distribution.^[22] Consequently, the flow remains elliptic in nature, with disturbances influencing both upstream and downstream regions, fostering a balanced adjustment without discontinuous jumps.^[23] Aerodynamic drag in subsonic flow arises primarily from skin friction and form drag, with the total drag force given by the equation D = \frac{1}{2} \rho v^2 C_d A, where \rho is fluid density, v is velocity, C_d is the drag coefficient, and A is the reference area.^[24] This quadratic dependence on velocity reflects the linear increase in drag with the square of speed, as kinetic energy dissipation through viscous shear (skin friction) and pressure differences due to shape (form drag) dominate the force balance.^[25] In typical subsonic conditions, skin friction accounts for a significant portion of the drag on smooth surfaces, while form drag prevails for bluff bodies, together comprising the parasite drag without contributions from wave drag seen at higher speeds.^[26] The boundary layer in subsonic flow develops from the surface outward, initially as a laminar region where viscous effects dominate and flow remains orderly, but it often transitions to turbulent flow downstream due to instabilities amplified by Reynolds number and surface roughness.^[27] This laminar-to-turbulent transition enhances momentum transfer and mixing, typically occurring at critical Reynolds numbers around $10^5 to $10^6 based on local length scales, and can significantly alter drag characteristics by increasing skin friction. Separation patterns emerge when adverse pressure gradients decelerate the boundary layer to near-zero velocity near the wall, causing the flow to detach and form recirculation zones, which are common in subsonic flows over curved or decelerating surfaces and lead to elevated form drag.^[28] Preventing or delaying separation through favorable pressure gradients or surface modifications is key to maintaining attached flow and optimizing performance.^[29]

Applications in Engineering

Subsonic principles underpin the design of aircraft operating below the speed of sound, enabling efficient lift generation without the complications of shock waves. Commercial jetliners, such as the Boeing 737-8, typically cruise at Mach 0.775, where airfoil-shaped wings exploit Bernoulli's principle to create lower pressure above the wing surface compared to below, producing lift while avoiding shock-induced drag that would arise in transonic regimes.^[30] Propeller-driven aircraft, like general aviation planes, operate at even lower subsonic speeds, relying on similar wing designs to maintain smooth airflow and maximize aerodynamic efficiency for shorter-range flights.^[31] Wind tunnel testing plays a crucial role in validating these designs under controlled subsonic conditions. Low-speed facilities, capable of simulating airflow up to Mach 0.3, allow engineers to measure forces on scaled models, optimizing wing profiles and control surfaces for real-world performance without the need for full-scale prototypes.^[32] NASA's 14- by 22-Foot Subsonic Wind Tunnel exemplifies such infrastructure, supporting tests for high-lift systems during takeoff and landing phases.^[33] Beyond aviation, subsonic flow principles inform streamlined designs in automotive and marine engineering to reduce drag. In automobiles, teardrop-like body shapes minimize pressure drag by promoting attached airflow over the vehicle, with studies showing drag coefficients as low as 0.25 for optimized sedans, enhancing fuel efficiency at highway speeds well below Mach 0.3.^[34] Similarly, ship hulls are contoured to streamline water flow, reducing frictional and wave drag in hydrodynamic regimes where practical speeds remain subsonic relative to water's speed of sound (approximately 1,480 m/s), as evidenced by hull form optimizations that cut resistance by up to 20% in planing vessels.^[35] The application of subsonic aerodynamics traces back to historical milestones, beginning with the Wright brothers' 1903 flight of the Wright Flyer, which achieved an airspeed of 34 mph (15.2 m/s)—firmly subsonic—and covered 852 feet in 59 seconds, demonstrating controlled powered flight through basic wing lift.^[36] This foundational achievement evolved into modern airliners over the 20th century, with advancements in wing aspect ratios and materials enabling efficient transcontinental travel at Mach 0.8, as seen in the progression from biplanes to swept-wing jets post-World War II.^[37]

Audio and Acoustics

Definition in Sound Waves

In the context of sound waves, the term "subsonic" is occasionally used informally to describe infrasound, defined as acoustic oscillations with frequencies below the lower limit of human audibility, typically less than 20 Hz.^[38] These waves propagate as longitudinal pressure disturbances through elastic media such as air, water, or solids, where particle motion occurs parallel to the direction of wave travel.^[39] Infrasound contrasts with the audible frequency range of approximately 20 Hz to 20 kHz and the ultrasonic range above 20 kHz, both of which involve higher-frequency pressure variations perceptible or generated differently.^[38] Physically, infrasound is generated by large-scale phenomena producing significant displacements, such as volcanic eruptions, earthquakes, or explosions, which create low-frequency vibrations capable of exciting the medium over extended areas.^[40] The fundamental physics of infrasound propagation follows the linear acoustic wave equation for small-amplitude pressure perturbations:

\frac{\partial^2 p}{\partial t^2} = c^2 \nabla^2 p

where p represents the acoustic pressure, c is the speed of sound in the medium, and \nabla^2 is the Laplacian operator.^[41] This equation applies particularly well to infrasonic frequencies, where long wavelengths—often exceeding hundreds of meters—dominate, allowing the assumptions of linearity and uniformity to hold despite the low rates of oscillation. Due to their extended wavelengths, infrasonic waves exhibit pronounced diffraction around obstacles like terrain features or structures, enabling them to bend and spread beyond line-of-sight paths.^[42] Additionally, infrasound experiences minimal atmospheric attenuation compared to higher frequencies, facilitating long-distance travel with little energy loss from absorption or scattering. This distinguishes infrasound from aerodynamic contexts, where the term "subsonic" denotes fluid speeds below the local speed of sound, underscoring a shared nomenclature but unrelated physical principles.

Subsonic Frequency Range

In audio and acoustics, infrasonic frequencies (sometimes informally called subsonic) are defined as sound waves below the lower limit of human audibility, typically ranging from 0.001 Hz to 20 Hz.^[43] In engineering contexts, the infrasonic band is more narrowly classified as 1–20 Hz to focus on perceptible low-frequency vibrations.^[44] The International Organization for Standardization (ISO) 226 outlines hearing thresholds starting at 20 Hz, where sensitivity drops significantly, requiring sound pressure levels around 80–100 dB for detection at the boundary of audibility.^[45] Infrasonic frequencies arise from both natural and artificial sources. Natural origins include geological events like earthquakes and volcanic eruptions, atmospheric phenomena such as severe weather and lightning, and oceanic processes like wave swells.^[43] Artificial sources encompass industrial machinery, explosions, and aerodynamic effects like sonic booms from aircraft.^[46] The wavelength of infrasonic waves is calculated using the formula \lambda = \frac{c}{f}, where \lambda is wavelength, c is the speed of sound, and f is frequency; for instance, at 20 Hz and standard conditions (c \approx 343 m/s), the wavelength is approximately 17 m.^[47] Due to their long wavelengths, infrasonic waves propagate efficiently over large distances with minimal attenuation. Detection of infrasonic frequencies requires specialized instruments, as standard microphones are ineffective below 20 Hz due to limited sensitivity. Microbarometers are commonly used, measuring minute atmospheric pressure variations induced by these waves.^[48] Environmental factors influence infrasonic wave propagation through variations in the speed of sound, given by c \approx 331 + 0.6T m/s, where T is air temperature in °C; higher temperatures increase c, thereby lengthening wavelengths and altering travel paths.^[49]

Subsonic Filters in Audio Engineering

In audio engineering, a subsonic filter is a high-pass filter applied to subwoofers and low-frequency audio systems to attenuate frequencies below the audible range, typically below 20-30 Hz, preventing damage from inaudible rumble or excessive cone excursion.^[8] These filters, often with a slope of 12-24 dB per octave, reduce power demands on amplifiers and improve clarity by eliminating non-audible energy that can cause distortion. Common cutoff frequencies are set around 25-35 Hz, adjustable based on enclosure type (e.g., ported vs. sealed) and music genre. For example, in car audio or home theater systems, engaging a subsonic filter at 30 Hz helps maintain driver integrity during bass-heavy playback without affecting perceived sound.^[50]

Effects on Human Hearing and Equipment

Infrasonic frequencies, typically defined as those below 20 Hz, are generally imperceptible to the human ear as audible sound and are instead experienced as physical vibrations or pressure sensations in the body, particularly at elevated sound pressure levels exceeding 70-80 dB.^[51] This tactile perception arises because the human auditory system loses sensitivity rapidly below 20 Hz, with thresholds requiring intensities over 100 dB for detection as low as 1-2 Hz, often manifesting as chest resonance or unease rather than tonal hearing.^[43] Prolonged exposure to such high-level infrasound (>100 dB) has been associated with subjective reports of discomfort, including nausea, dizziness, and fatigue, though physiological mechanisms remain debated and evidence for direct causation is inconclusive.^[52] The notion of a specific "brown note"—an infrasonic frequency purported to induce involuntary defecation through bowel resonance—has been widely debunked, with scientific investigations finding no verifiable acoustic trigger for such effects despite high-intensity exposures up to 115 dB.^[53] Regarding broader health impacts, claims of "wind turbine syndrome" linking infrasound from turbines to symptoms like headaches, sleep disruption, and vertigo have been examined in multiple reviews, including a 2003 assessment by the UK Department for Environment, Food and Rural Affairs, which concluded that while some individuals report annoyance from low-frequency noise, there is limited evidence of direct physiological harm or a distinct syndrome attributable to infrasound at typical environmental levels.^[54] Subsequent epidemiological studies on wind farm exposures, including reviews as recent as 2024, reinforce this, attributing most reported effects to audible noise annoyance or nocebo responses rather than infrasound alone.^[55] In audio equipment, infrasonic frequencies pose significant challenges due to the natural roll-off in frequency response of conventional loudspeakers, which typically exhibit a steep decline (often 12-24 dB per octave) below 20-30 Hz, limiting their ability to reproduce such content without distortion or excessive cone excursion.^[56] This necessitates specialized subwoofers designed for extended low-frequency extension down to 15-20 Hz or below, often incorporating high-excursion drivers and ported enclosures to handle the demands without damage, alongside subsonic filters for protection.^[57] Measurement of infrasound in standard audio gear is further complicated by calibration inaccuracies in microphones and analyzers, where defects like perforated diaphragms can introduce errors exceeding 20 dB in the 1-20 Hz range, underscoring the need for dedicated infrasonic instrumentation.^[58] While humans have limited sensitivity to infrasonic frequencies, certain animals exhibit remarkable adaptations for their detection and use in communication. Elephants produce and perceive infrasonic rumbles between 14-35 Hz that propagate over distances exceeding 10 km through the ground and air, enabling coordination of family groups across savannas.^[59] Similarly, baleen whales such as blue whales generate calls as low as 12-25 Hz that travel hundreds to over 1,000 km in ocean waters, facilitating long-range social interactions and navigation in deep-sea environments.^[60]

Uses in Music Production

In music production, subwoofers play a crucial role in bass enhancement by reproducing low frequencies while incorporating high-pass filters, often called subsonic filters, to eliminate subsonic rumble below 20 Hz, thereby preventing damage to speakers and improving overall clarity. These filters, often set at 30 Hz, remove unnecessary energy that can cause distortion or excessive cone excursion in subwoofers, allowing producers to focus on audible bass content around 20–60 Hz for a tighter mix. Tools like high-pass filters in digital audio workstations (DAWs) enable precise control, ensuring that subsonic content does not compromise playback on consumer systems.^[61] Genres such as dubstep and electronic dance music (EDM) intentionally synthesize infrasonic elements below 20 Hz to create tactile impact, evoking physical sensations in listeners through vibrations rather than audible sound. For instance, Skrillex's track "Bangarang" features frequencies below 30 Hz during bass drops, contributing to the genre's signature intensity and immersion. Similarly, dubstep producers like Kryptic Minds employ sub-bass extending below 20 Hz in tracks such as "Six Degrees," enhancing the "bassweight" effect that defines the style.^[62] Recording techniques for low frequencies have evolved significantly, with studios isolating bass elements to capture clean subsonic content without interference from room acoustics. In the 1970s, disco production relied on analog multitrack recorders and synthesizers like the Minimoog to layer prominent bass lines, emphasizing rhythmic low-end drive in tracks by artists such as Chic, though limited by vinyl mastering constraints that attenuated extreme lows. This foundation transitioned to modern digital plugins, such as FabFilter Pro-Q, which allow precise high-pass filtering and EQ sculpting to isolate and enhance sub-20 Hz tones during mixing and mastering.^[63]^[64] In live sound reinforcement for concerts, particularly EDM events, infrasonic arrays consisting of specialized subwoofers deliver sub-20 Hz frequencies to immerse crowds in visceral bass experiences, using stacked configurations for even low-end distribution across venues. Systems from manufacturers like L-Acoustics integrate these arrays to handle the dynamic range of infrasound-heavy sets, providing physical impact without overwhelming midrange clarity. However, such setups must balance intensity with hearing protection standards, adhering to guidelines recommending sound levels below 100 dB for prolonged exposure and earplugs offering 18–25 dB attenuation to safeguard attendees from noise-induced hearing loss.^[65]^[66]

Other Contexts

Subsonic Ammunition

Subsonic ammunition consists of cartridges engineered to achieve muzzle velocities below the speed of sound, approximately 343 m/s (1,125 fps) under standard atmospheric conditions, thereby preventing the supersonic crack that would otherwise occur during flight.^[67] This design is particularly effective when used with firearm suppressors, as it minimizes overall noise by eliminating the bullet's sonic boom while the suppressor handles muzzle blast.^[68] Common examples include standard .45 ACP loads with 230-grain bullets, which typically exit the muzzle at around 250 m/s (820 fps), inherently subsonic without modification.^[69] Similarly, specialized 9mm subsonic variants, such as those using 147-grain projectiles, are loaded with reduced powder charges to stay under the sonic threshold. Modern examples include the .300 AAC Blackout cartridge, designed for effective subsonic performance in suppressed firearms.^[70] The physics of subsonic projectiles emphasizes maintaining kinetic energy and flight stability at reduced speeds, where aerodynamic drag behaves differently from higher velocities. Heavier bullets are employed to compensate for the lower velocity, as kinetic energy scales with the square of speed but linearly with mass, allowing subsonic rounds to deliver comparable impact despite slower travel.^[71] At subsonic speeds, bullets experience steady drag without the sharp increase seen in transonic transitions, but they require rifling with sufficient twist rates to impart gyroscopic stability and prevent yaw or tumbling, especially given the increased mass of these projectiles.^[72] Subsonic ammunition offers key advantages in noise reduction for suppressed firearms, making it ideal for special operations where stealth is paramount, as the absence of a sonic crack significantly lowers detectability.^[73] However, it suffers from drawbacks such as reduced effective range—often limited to under 100 meters—and compromised terminal ballistics, where lower velocities can hinder bullet expansion and penetration in defensive or tactical scenarios.^[74] Historically, subsonic ammunition emerged during World War II for covert warfare, with the British Special Operations Executive developing the Welrod pistol in the mid-1940s as a bolt-action suppressed weapon chambered in subsonic .32 ACP or 9mm rounds to enable silent assassinations at close range.^[75] Post-war advancements refined these concepts for modern military and law enforcement use, including widespread adoption of 9mm subsonic loads in suppressed pistols and submachine guns for urban operations and hostage rescue.^[76]

Subsonic Communication Systems

Subsonic communication systems utilize low-frequency signals, typically in the infrasound range below 20 Hz or extending into very low frequency (VLF) radio bands, to enable data transfer in challenging environments such as underwater and atmospheric media. These systems leverage the superior propagation characteristics of subsonic waves, which experience minimal attenuation over long distances compared to higher frequencies.^[77] In underwater applications, subsonic acoustic signals have been pivotal for military surveillance, exemplified by the Sound Surveillance System (SOSUS), a network of passive hydrophone arrays developed by the United States Navy during the Cold War. Deployed across the Atlantic and Pacific Oceans, SOSUS detected submarine noise in the low-frequency band of approximately 25-200 Hz, with prominent peaks around 100 Hz, allowing for long-range tracking of Soviet vessels at distances up to thousands of kilometers.^[77]^[78] The system originated in the late 1950s as a response to escalating submarine threats and remained highly classified until its declassification in 1991, after which its role in monitoring noisy diesel and nuclear submarines was publicly acknowledged.^[79] Atmospheric subsonic communication often employs VLF radio waves in the 3-30 kHz range. These electromagnetic waves can penetrate seawater to depths of 10-40 meters, facilitating one-way broadcasts to submerged submarines without requiring surfacing.^[80] These signals are modulated using techniques such as frequency-shift keying (FSK) or minimum shift keying (MSK), where the carrier frequency is shifted by small amounts (e.g., ±50 Hz) to encode binary data, enabling reliable transmission over global distances via ground-based antenna arrays.^[81]^[82] The primary advantages of subsonic communication systems include exceptional long-range propagation due to low attenuation in water and earth-ionosphere waveguides, supporting strategic messaging across oceans without line-of-sight limitations.^[80] However, these benefits come with challenges, notably very slow data rates—typically limited to 300 bits per second or less—necessitating data compression and restricting content to essential commands rather than high-bandwidth information.^[83] In modern applications, infrasonic sensors have expanded subsonic communication principles to civilian uses, such as earthquake early warning systems that detect precursors in the 0.01-28 Hz range generated by seismic events, providing seconds to minutes of advance notice through global monitoring networks.^[84] Similarly, these sensors monitor wildlife by capturing infrasonic rumbles from species like African elephants, which propagate up to several kilometers for social coordination and can inform conservation efforts by tracking population movements and behaviors in remote habitats.^[85] In 2025, the U.S. Navy awarded a $49.5 million contract to modernize submarine communication systems, enhancing VLF capabilities for improved reliability and integration.^[86]

References

[1]
SUBSONIC Definition & Meaning - Merriam-Webster
1. of, relating to, or being a speed less than that of sound in air 2. moving, capable of moving, or utilizing air currents moving at a subsonic speed.
[2]
Lightning Strike
May 13, 2021 · On a standard day at sea level static conditions, the speed of sound is about 760 mph, or 1100 feet/second.
[3]
From Subsonic to Hypersonic Flow - Spartan College of Aeronautics ...
Nov 29, 2023 · Subsonic flight refers to speeds below Mach 0.75. In this regime, the airflow around the entire aircraft is below the speed of sound.
[4]
Subsonic Speed - an overview | ScienceDirect Topics
Subsonic speeds refer to velocities that are below the speed of sound, where aircraft performance can be compared using the range factor M (L/D).
[5]
[PDF] Chapter 5: Aerodynamics of Flight - Federal Aviation Administration
The speed regimes of flight can be grouped in three categories: low- speed flight, cruising flight, and high-speed flight.
[6]
FlyBy - Beyond supersonic? Defining the 4 speeds of flight - Boom
less than about Mach 0.8. Subsonic aircraft include ...
[7]
WHAT'S SUBSONIC AMMUNITION? - Magtech
Apr 18, 2023 · Subsonic is a term that relates to the speed of a bullet traveling through the air, measured at the muzzle, at less than the speed of sound.
[8]
What Is a Subsonic Filter | Arendal Sound
A subsonic filter is a component on your subwoofer that reduces the intensity of notes which come through at lower frequencies. It decreases the amplitude ...<|control11|><|separator|>
[9]
Mach Number
The Mach number appears as a similarity parameter in many of the equations for compressible flows, shock waves, and expansions.
[10]
Compressible Flow vs Incompressible Flow in Fluid Mechanics
Aug 11, 2023 · Compressible flow is a flow that changes in density under pressure, whereas incompressible flow does not. A good indicator is Mach Number.
[11]
Research in Supersonic Flight and the Breaking of the Sound Barrier
This work was expanded to include oblique shock waves by the famous German aerodynamicist, Ludwig Prandtl and his student Theodor Meyer at Göttingen University ...
[12]
Compressible Aerodynamics Home
May 13, 2021 · Flight less than the speed of sound is called subsonic, near the speed of sound is transonic, greater than the speed of sound is supersonic, and ...
[13]
[PDF] advanced transonic aerodynamic technology
The Mach number for the onset of such conditions is called the critical Mach number, thus transonic flow at subsonic freestream. Mach numbers is called ...Missing: transition | Show results with:transition
[14]
Speed of Sound Derivation
Air is a gas, and a very important property of any gas is the speed of sound through the gas. ... a = sqrt (gamma * R * T). Guided Tours · Button to Display ...
[15]
Speed of Sound
The speed of sound in air depends on the type of gas and the temperature of the gas. On Earth, the atmosphere is composed of mostly diatomic nitrogen and oxygen ...
[16]
Speed of Sound
The speed of sound depends on the temperature, and temperature changes with altitude in a very complex way. Engineers have created a mathematical model of ...
[17]
Earth Atmosphere Model - Metric Units
The speed of sound depends on the temperature and also decreases with increasing altitude. The pressure of the air can be related to the weight of the air ...
[18]
Mach Number - an overview | ScienceDirect Topics
In general, the flow with Mach number less than 0.3 can be considered as an incompressible flow and the flow with Mach number larger than 0.3 should be treated ...
[19]
[PDF] Solution methods for the Incompressible Navier-Stokes Equations
Solution methods include pressure-based and density-based approaches, pressure correction schemes, and explicit/implicit schemes. Pressure is derived from mass ...
[20]
[PDF] 1 Incompressible viscous fluid flow. The Navier- Stokes equations.
This section summarizes the Navier-Stokes equations for incompressible viscous fluid flow. An incompressible fluid has divu=0, which is the continuity equation.
[21]
Gas Dynamics & Supersonic Flow - Aerodynamics for Students
Mach Number is a measure of the relative importance of compressibility effects for a given flow. Propagation of a Source of Sound. A point source of sound will ...Missing: principles | Show results with:principles
[22]
[PDF] Fascinating World of Shock Waves - Indian Academy of Sciences
Since the information transfer can take place through acoustic disturbance, there will be no shock waves in the subsonic flow regime. The propagation of dis-.
[23]
Subsonic Flow - an overview | ScienceDirect Topics
In subsonic flow, both upstream and downstream pressure disturbances may influence the flow at the location of interest, while in supersonic flow only upstream ...
[24]
Drag Equation | Glenn Research Center - NASA
Jul 1, 2025 · The drag equation states that drag D is equal to the drag coefficient Cd times the density rho (ρ) times half of the velocity V squared times the reference ...Drag Coefficient · Lift Coefficient · Reference Area
[25]
Form Drag - an overview | ScienceDirect Topics
Form drag is defined as the net drag force that arises from differences in pressure distribution over a body in viscous flow compared to an ideal inviscid ...
[26]
[PDF] MEASUREMENT OF AND TURBULENCE SUBSONIC SPHERE ...
In this regime about 2/3 of the drag is due to skin friction and 1/3 is due to pressure or form drag. The inertial terms in the Navier-Stokes equations are ...
[27]
Boundary Layer Flows – Introduction to Aerospace Flight Vehicles
Boundary layers may begin as laminar but often transition to turbulence at some distance downstream, depending on the Reynolds number, surface conditions, and ...
[28]
Boundary Layer Separation - Richard Fitzpatrick
Boundary layer separation is an important physical phenomenon because it gives rise to a greatly enhanced drag force acting on a non-streamlined obstacle.
[29]
[PDF] Flow Separation and Reattachment
Separation of boundary layers in general occurs when the velocity of the boundary layer “struggling” against an adverse pressure gradient becomes nearly zero, ...
[30]
[PDF] SUbsonic Single Aft eNgine (SUSAN) System Integration Analysis ...
Cruise speed, Mach. 0.775. EM efficiency. 0.97. Takeoff altitude, m. 0. Engine P/W ... thrust value taken from a similarly sized aircraft, the Boeing 737-8 [8].
[31]
Subsonic Wings - for How Things Fly
(Remember, this relationship between pressure and speed is called Bernoulli's principle.) The higher air pressure below pushes the wing up—lift.
[32]
Wind Tunnel Testing - Boeing Global Services
The tunnel is capable of speeds of hover to Mach 0.3, with a dynamic pressure range of 0 to 160 pounds per square foot.
[33]
Capabilities & Facilities | Research Directorate - NASA
Jul 3, 2024 · 14 x 22 Foot Subsonic Wind Tunnel · National Transonic Facility (NTF) · 0.3 Meter Transonic Cryogenic Tunnel (0.3M TCT) · Unitary Plan Wind Tunnel ...
[34]
[PDF] Aerodynamics of Road Vehicles
For both airships and aircraft, streamlined shapes were developed which lowered drag significantly, thus permitting higher cruising speeds with any given ( ...
[35]
A review on the hydrodynamics of planing hulls - ScienceDirect.com
Jul 1, 2024 · In this paper, we holistically review scholarly studies on the subject, discuss research challenges and opportunities ahead.
[36]
1903-The First Flight - Wright Brothers National Memorial (U.S. ...
Oct 10, 2025 · Into the 27-mph wind, the groundspeed had been 6.8 mph, for a total airspeed of 34 mph. The brothers took turns flying three more times that day ...Missing: subsonic | Show results with:subsonic
[37]
The Decade After - Wright Brothers Aeroplane Company
The Wright brother's best flight on December 17, 1903 covered only 852 feet at a speed of about 34 mph.Missing: subsonic | Show results with:subsonic
[38]
Negative Effect of High-Level Infrasound on Human Myocardial ...
Infrasound is the extension of the audio spectrum, when the frequency falls below 20 Hz. As a result, it shares much with the audible spectrum, but with some ...
[39]
[PDF] Measuring Infrasound from the Maritime Environment - CDIP
Infrasound waves are longitudinal acoustic pressure waves. Infrasound pressure fluctuations for sources of interest are small compared to the ambient pressure.
[40]
[PDF] ° MD PREBEDING PAGE BLANK NOT FILMED
The formula is applicable for all sound waves from the low infrasonic frequency of f = 0.01 Hz (wavelength X = 34 km) through audible frequencies, f a. 1000 Hz, ...
[41]
[PDF] Computational Acoustics Principles - Bluefield Esports
The Acoustic Wave Equation. The acoustic wave equation is the cornerstone for many computational acoustics analyses. In its simplest form for a homogeneous ...<|control11|><|separator|>
[42]
[PDF] detection, source location, and analysis of volcano infrasound
Sound waves will diffract around a barrier, such as topographic obstacles, with the amount of the diffraction related to source-receiver position and the ...
[43]
What is infrasound? - ScienceDirect.com
Infrasound, in its popular definition as sound below a frequency of 20 Hz, is clearly audible, the hearing threshold having been measured down to 1.5 Hz.
[44]
(PDF) What is infrasound? - ResearchGate
Aug 7, 2025 · Infrasound, in its popular definition as sound below a frequency of 20 Hz, is clearly audible, the hearing threshold having been measured down to 1.5 Hz.
[45]
Hearing at Low and Infrasonic Frequencies - Noise and Health
The threshold of hearing is standardized for frequencies down to 20 Hz, but there is a reasonably good agreement between investigations below this frequency. It ...
[46]
Infrasound and low frequency sound | FPS Public Health - Belgium.be
Jan 12, 2016 · Besides natural sources of infrasound and low frequency sound like volcanoes, thunder and earthquakes, there are also artificial sources: ...<|control11|><|separator|>
[47]
Wavelength To Frequency - The Speed Of Sound | Brüel & Kjær - HBK
Wavelength of Sound In Air · Wavelength of a sound in air at 1 Hz: 340 m · Wavelength of a sound in air at 20 Hz: 340 m / 20 = 17 m · Membrane motion.
[48]
Infrasound monitoring - CTBTO
The microbarometer measures changes in the air's micropressure that are produced by infrasonic waves.Missing: methods | Show results with:methods
[49]
The Speed of Sound - The Physics Classroom
The speed of sound in air is dependent upon the temperature of air. The dependence is expressed by the equation: v = 331 m/s + (0.6 m/s/C) • T. where T is ...
[50]
Introduction to the Special Issue on Low Frequency Noise
Despite the general understanding that infrasound is not audible, it is possible for humans to perceive infrasound if the sound level is high enough, although ...
[51]
Infrasound, human health, and adaptation: an integrative overview ...
Sep 12, 2013 · A vibrating sound (~20 Hz) is perceived and becomes louder when the fists are tightened (muscles are tense). This frequency band appears to be ...
[52]
Is there really a noise that makes you poop yourself?
Oct 5, 2021 · The 'brown note' is a myth; scientists have found no evidence that a sound frequency causes uncontrollable pooping.Missing: debunked | Show results with:debunked
[53]
[PDF] Low Frequency Noise Report 2003 - National Wind Watch
Low frequency noise causes extreme distress to a number of people who are sensitive to its effects. Such sensitivity may be a result of heightened sensory.
[54]
Health effects and wind turbines: A review of the literature
Sep 14, 2011 · The authors concluded that results of their study suggest that there should be no adverse public health effects from infrasound or low frequency ...
[55]
https://science.feedback.org/review/no-evidence-show-infrasound-from-wind-turbines-harmful-human-health/
[56]
https://hsuresearch.com/blogs/how-tos/the-ultimate-guide-to-choosing-a-subwoofer
[57]
Calibration and testing of measurement devices at infrasound ...
The study has shown that errors of around 20 dB are possible. Since typical uncertainties are of the order of tenth of decibels for instrument calibration and ...
[58]
(PDF) Chapter 3.2 - Elephant infrasounds: long-range communication
Aug 7, 2025 · Infrasound in the range of 1 to 20 Hz may be generated and/or detected by elephants over distances in excess of 10 km.
[59]
How Far Can Blue Whales Hear? - IFLScience
Apr 20, 2024 · It turns out that “more than a decent level” doesn't even cut it; blue whales can hear sounds emitted by other whales up to 1,000 miles (1,600 ...
[60]
https://www.iflscience.com/how-far-can-blue-whales-hear-73888
[61]
25 Infrasonic Tracks That Underscore the Value of a High-End ...
Sep 2, 2025 · These UK dubstep pioneers created what you can only describe as fearsome sub-bass assaults that extend well below 20Hz, while maintaining ...
[62]
Music Technology of the 1970s: A Timeline | Pitchfork
Aug 25, 2016 · Looking back at the '70s innovations that shaped how people created and listened to music throughout the decade and beyond.<|control11|><|separator|>
[63]
Best filter plugins 2025: Our favourite frequency shapers - MusicRadar
Jan 30, 2023 · In this buyer's guide, we round up the best filter plugins on the market so you can enjoy the most creative effects used in music production today.
[64]
https://www.musicradar.com/news/best-filter-plugins
[65]
How loud are festivals and when is it harmful?
May 24, 2022 · To prevent hearing damage at a concert or festival, you should wear earplugs with an attenuation of 18 to 25 decibels. Our universal Alpine ...
[66]
Understanding Subsonic Versus Supersonic Ammunition - SilencerCo
Jul 24, 2024 · Subsonic ammunition is designed to travel below the speed of sound, which is 1,125 feet per second. By staying under this threshold, it avoids ...
[67]
Subsonic vs Supersonic Ammo: Quiet Shooting & Bullet Performance
Oct 16, 2024 · Heavier Projectiles: Subsonic rounds typically use heavier bullets, which naturally travel slower. Reduced Powder Charge: Less propellant is ...
[68]
https://www.silencershop.com/blog/subsonic-ammo-vs-supersonic-ammo
[69]
Best 9mm Subsonic Ammo Options - Silencer Central
Feb 15, 2022 · The most common target load for 9mm bullets is 115 grains, but a heavier bullet of 147 grains will be subsonic. The other way is to reduce the ...
[70]
Everything You Need to Know About Subsonic Ammo | Field & St
As a general rule, heavy-for-caliber bullets work best because they do a better job of retaining a subsonic round's reduced energy downrange. However, if ...<|control11|><|separator|>
[71]
Subsonic Ammo - Essential Knowledge for Shooters
Jan 19, 2024 · This is because heavier bullets, which are common in subsonic loads, need a faster spin to maintain a stable flight trajectory. The Case of 8.6 ...
[72]
Advantages and disadvantages of subsonic ammunition
Aug 15, 2023 · Subsonic ammunition refers to cartridges that are designed to have a muzzle velocity below the speed of sound (approximately 1125 feet per ...
[73]
Comparing Subsonic vs. Supersonic Ammunition - Silencer Central
Apr 7, 2021 · This is because a subsonic bullet is traveling slower than the speed of sound. As a result, it does not break the sound barrier, so it won't ...
[74]
The 'Welrod' Pistol: A Silent Arm For The SOE - American Rifleman
Feb 22, 2020 · The Welrod was born in the creative workshops of Station IX, part of Britain's Special Operations Executive (SOE) It was invented by a one of ...
[75]
The Welrod Assassin's Pistol - SilencerCo
Aug 27, 2022 · The Special Operations Executive (SOE) developed the Welrod for just this purpose: killing enemy sentries or high value personnel silently at close range.
[76]
Origins of SOSUS | Commander, Undersea Surveillance
It was believed that low-frequency spectrum analysis would do to the detection of the submarine threat what the magnetron had done for radar in 1939.Missing: Cold 1991
[77]
The Cold War: History of the SOund SUrveillance System (SOSUS)
Jan 14, 2022 · The SOSUS system was very successful in detecting and tracking the noisy diesel and then nuclear Soviet submarines of the Cold War.Missing: 1-100 1991
[78]
Canada and SOSUS | The Canadian Encyclopedia
Nov 2, 2021 · The mission was highly classified throughout the Cold War and only declassified in 1991. SOSUS became part of the Integrated Undersea ...Missing: 1-100 | Show results with:1-100
[79]
Very Low Frequency (VLF) - United States Nuclear Forces - Nuke
VLF uses digital signals at 3-30 kHz to communicate with submerged submarines, providing global coverage and seawater penetration.Missing: shift keying
[80]
[PDF] vlf, lf, and mf communications - Navy Radio
The VLF transmitter presently transmits single channel teletype via. FSK (frequency shift keying) but the transmitter has the capability and occasionally.
[81]
[PDF] VLF/LF Submarine Communications - Nuclear Information Service
The current mode at Aguada is reportedly Minimum Shift Keying [MSK] which makes maximum use of transmitter power and frequency spectrum by using a +/-50 Hz ...
[82]
Deep secret – secure submarine communication on a quantum level
Dec 5, 2013 · The VLF and ELF frequencies only offer a very low bandwidth: VLF supports a few hundred bits a second while ELF sustains just a few bits each ...
[83]
Infrasonic Earthquake Detectability Investigated in Southern Part of ...
Jan 29, 2021 · In this study we will focus on all the possibilities for infrasound detection generated from earthquakes using KUT sensor network and International Monitoring ...Missing: wildlife | Show results with:wildlife
[84]
Establishing the fundamentals for an elephant early warning and ...
Sep 4, 2015 · Elephants make extensive use of powerful infrasonic calls (rumbles) that travel distances of up to several kilometers. This makes elephants well ...