The Sound of Stealth...

With sound technologies like 3D sound, directional sound and others, can stealth warplanes be identified? Do they have any particular sound to them? Or is the sound of their engines the same as other warplanes?

We know that Sound waves take approximately 5 seconds to travel 1 mile. Using this information, it is possible to measure one's distance from a lightning bolt.

Physics of sound

Sound can propagate through compressible media such as air, water and solids as longitudinal waves and also as a transverse waves in solids (see Longitudinal and transverse waves, below). The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel with the sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the average position of the particles over time does not change). During propagation, waves can be reflected, refracted, or attenuated by the medium.[4]

The behavior of sound propagation is generally affected by three things:

• 
  
  
  • A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium.
  • The propagation is also affected by the motion of the medium itself. For example, sound moving through wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.
  • The viscosity of the medium also affects the motion of sound waves. It determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

When sound is moving through a medium that does not have constant physical properties, it may be refracted (either dispersed or focused).


Spherical compression (longitudinal) waves
The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.

Longitudinal and transverse waves

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation. Additionally, sound waves may be viewed simply by parabolic mirrors and objects that produce sound. [5]

The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium.

Sound wave properties and characteristics


Sinusoidal waves of various frequencies; the bottom waves have higher frequencies than those above. The horizontal axis represents time.
Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:

• 
  
  
  • Frequency, or its inverse, the period
  • Wavelength
  • Wave number
  • Amplitude
  • Sound pressure
  • Sound intensity
  • Speed of sound
  • Direction

Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m to 17 mm. Sometimes speed and direction are combined as a velocity vector; wave number and direction are combined as a wave vector.

Transverse waves, also known as shear waves, have the additional property, polarization, and are not a characteristic of sound waves.

Speed of sound
U.S. Navy F/A-18 approaching the sound barrier. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (see Prandtl-Glauert Singularity).
The speed of sound depends on the medium that the waves pass through, and is a fundamental property of the material. The first significant effort towards the measure of the speed of sound was made by Newton. He believed that the speed of sound in a particular substance was equal to the square root of the pressure acting on it (STP) divided by its density.

This was later proven wrong when found to incorrectly derive the speed. French mathematician Laplace corrected the formula by deducing that the phenomenon of sound traveling is not isothermal, as believed by Newton, but adiabatic. elastic modulus, c = velocity of sound, and

= density. Thus, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density.

Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h; 767 mph) using the formula "v = (331 + 0.6 T) m/s". In fresh water, also at 20 °C, the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph). The speed of sound is also slightly sensitive (a second-order anharmonic effect) to the sound amplitude, which means that there are nonlinear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array).

Perception of sound

Human ear
A distinct use of the term sound from its use in physics is that in physiology and psychology, where the term refers to the subject of perception by the brain. The field of psychoacoustics is dedicated to such studies.

The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound frequencies between approximately 20 Hz and 20,000 Hz (20 kHz),[9] Both limits, especially the upper limit, decrease with age.

Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz, but are deaf below 40 Hz. As a signal perceived by one of the major senses, sound is used by many species for detecting danger, navigation, predation, and communication. Earth's atmosphere, water, and virtually any physical phenomenon, such as fire, rain, wind, surf, or earthquake, produces (and is characterized by) its unique sounds.
Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. A square of this difference (i.e., a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of this average provides a root mean square (RMS) value. For example, 1 Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm

Pa) and (1 atm

Pa), that is between 101323.6 and 101326.4 Pa. As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or Lp is defined as

where p is the root-mean-square sound pressure and

is a reference sound pressure. Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level.

Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level matches perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.
http://en.wikipedia.org/wiki/Sound
Sound - Wikipedia, the free encyclopedia

Aeroacoustics

Aeroacoustics is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces. Noise generation can also be associated with periodically varying flows. A notable example of this phenomenon are the Aeolian tones produced by wind blowing over fixed objects.

Although no complete scientific theory of the generation of noise by aerodynamic flows has been established, most practical aeroacoustic analysis relies upon the so-called aeroacoustic analogy,proposed by James Lighthill in the 1950s while at the University of Manchester. whereby the governing equations of motion of the fluid are coerced into a form reminiscent of the wave equation of "classical" (i.e. linear) acoustics.

The modern discipline of aeroacoustics can be said to have originated with the first publication of Sir James Lighthill in the early 1950s, when noise generation associated with the jet engine was beginning to be placed under scientific scrutiny.

Lighthill's equation

Lighthill rearranged the Navier–Stokes equations, which govern the flow of a compressible viscous fluid, into an inhomogeneous wave equation, thereby making a connection between fluid mechanics and acoustics. This is often called "Lighthill's analogy" because it presents a model for the acoustic field that is not, strictly speaking, based on the physics of flow-induced/generated noise, but rather on the analogy of how they might be represented through the governing equations of a compressible fluid.

The first equation of interest is the conservation of mass equation, which reads


where P and v represent the density and velocity of the fluid, which depend on space and time, and D/Dt is the substantial derivative.

Next is the conservation of momentum equation, which is given by


where P is the thermodynamic pressure, and is the viscous (or traceless) part of the stress tensor from the Navier–Stokes equations.

Now, multiplying the conservation of mass equation by v and adding it to the conservation of momentum equation gives


Note that v x v is a tensor (see also tensor product). Differentiating the conservation of mass equation with respect to time, taking the divergence of the conservation of momentum equation and subtracting the latter from the former, we arrive at


Subtracting , where is the speed of sound in the medium in its equilibrium (or quiescent) state, from both sides of the last equation and rearranging it results in


which is equivalent to


where is the identity tensor, and : denotes the (double) tensor contraction operator.

The above equation is the celebrated Lighthill equation of aeroacoustics. It is a wave equation with a source term on the right-hand side, i.e. an inhomogeneous wave equation.
http://en.wikipedia.org/wiki/Aeroacoustics
Aeroacoustics - Wikipedia, the free encyclopedia

Can this Aeroacoustics bring any new anti-stealth technology to existence?
I think there are some possiblities with sound intercepting radar like gear and ground sensors and sound descriminating software. Since the engines of a stealth fighter will be generating sound through the air, either at subsonic or supersonic speeds.

Any thoughts?

Anti Stealth Technology does exist........Mr @gambit was once talking abt some unique sound waves.

This is very old technology:

Sound Mirrors: Before Radar, Hearing Was Believing, page 1

It is not about enhancing human Hearing it is more of some ground and airborne gear that detects engine sound in particular, either the sonic boom or else coming from the engines .

Acoustics on the 21st Century Battlefield

Acoustics on the 21st Century Battlefield

Acoustic stealth. Although sound moves too slowly to be an effective locating signal for antiaircraft weapons, for low-altitude flying it is still best to be inaudible to ground observers. Several ultra-quiet, low-altitude reconnaissance aircraft, such as Lockheed's QT-2 and YO-3A, have been developed since the 1960s. Aircraft of this type are ultralight, run on small internal combustion engines quieted by silencer-suppressor mufflers, and are driven by large, often wooden propellers. They make about as much sound as gliders and have very low infrared emissions as well because of their low energy consumption. The U.S. F-117 stealth fighter, which is designed to fly at high speed at very low altitudes, also incorporates acoustic-stealth measures, including sound-absorbent linings inside its engine intake and exhaust cowlings.

Stealth Technology - Visual stealth, Infrared stealth, Acoustic stealth, RADAR stealth

..
Another method of physical detection is worthy of mention. Although widely used in WWII, it seems acoustic signature analysis has fallen out of favor in recent decades. While most stealth aircraft are very quiet during approach, the authors first hand experience with an over flight by a B2 bomber indicates this is certainly NOT the case as the aircraft was departing. This observation may not appear to be useful, until you consider the situation depicted in figure 6.


Figure 6

Two acoustic sensors (1 & 2) are sequentially triggered by over flight of the stealth aircraft. Since the distance between acoustic sensors 1 and 2 is known, the time interval between triggers of sensors 1 and 2 yields the velocity of the stealth aircraft. Knowing the aircraft velocity, and the distance between sensor 2 and the countermeasure weapon, allows the weapon to be triggered in advance of stealth aircraft over flight. When employed at a natural choke point such as a long narrow valley, or an artificial choke point such as the mid point between two conventional search radars, the utility of the tactic becomes self evident. A typical countermeasure weapon would consist of multiple mortar launched shells, containing small metal fragments dispersed by a high explosive charge, directly in the flight path of the oncoming stealth aircraft. This countermeasure system has the added advantage of being completely passive, and therefore undetectable by the stealth aircraft.

Countermeasure deployment:
The effectiveness of stealth countermeasures and tactics discussed herein can be enhanced by the careful selection of deployment locations. Since radar and infrared stealth DO NOT mean complete absence of detectable signature, but only greatly reduced signature. If follows that stealth platforms will chose penetration and egress routes that either avoid close approach to traditional detection systems (search radars, etc.) or use natural terrain features to reduce the chance of detection. These facts can be used to advantage in choosing deployment locations for stealth countermeasure systems. As an example, conventional search radars are generally deployed with a slight overlap at the limit of the detection range. This area of overlap, near the limit of detection range is where a stealth platform will naturally chose for it's penetration and egress corridor(s), and represents an ideal location for stealth countermeasure deployment. Long sinuous valleys that terminate near high value targets and act as natural barriers to radar would represent another useful penetration and/or egress route for stealth platforms, and therefore another logical deployment location for stealth countermeasures.

Stealth naval platforms will employ the hide-in-plain-sight strategy. Since their radar signature will be comparable to that of a small fishing boat, the use of a conventional marine search radar, coupled with advance phased array techniques (1.2.1) will serve to uncover the "shark among the minnows". A conventional search radar would be used to locate targets, each of which would then be examined with the phased array system. Since the (phased array) radar pulse used to paint the target is not in phase, standard ECM suites will fail to detect it, and the stealth platform will not know it's true nature has been exposed.

Stealth Countermeasures. The Billion Dollar Boondoggle by Steven J. Smith

The evolution of anti-stealth detection
Even with the huge investment being poured into stealth aircraft, the technology hardly provides fighters with the power of invisibility. Technologies, both highly advanced and surprisingly low-tech, exist that are able to spot stealth aircraft.

That much was proved back in 1999 when a US F-117 stealth ground-attack aircraft was shot down over Yugoslavia, having been spotted by long-wavelength radar after its radar signature was briefly raised when its bomb-bay doors opened.

The availability of information on counter-stealth technologies remains low, but it's clear that adaptations of generations-old radar technology have the potential to turn the invisible visible.

Very high frequency (VHF) and ultra high frequency (UHF), pioneered in the 1940s, is still used today for long-range aerial surveillance. These frequencies, as explained by Arend G. Westra in a 2009 issue of Joint Force Quarterly, can confound stealth techniques by operating on decimetre to metre-long wavelengths.

The meeting of wavelengths between radar and aircraft causes resonation between the two, significantly raising an aircraft's reflection in the radar spectrum, making it much more visible. VHF radar has been incorporated into the Russian military's 1L119 Nebo SVU, its first VHF-band active electronically steered array (AESA); although detailed analysis of this vehicle-mounted array, Russian sources report it has achieved excellent results in spotting stealth aircraft.

"Adaptations of generations-old radar technology have the potential to turn the invisible visible."
Passive radar is another well-established, relatively inexpensive technology that has potential against LO aircraft. This system uses multiple transmitters of opportunity to collate data, estimating aircraft positions by calculating the intersection of the receiver-to-target bearing and the bistatic range ellipse. In the past, these estimates have been too inaccurate to be useful, but modern advances in signal and digital processing, along with the availability of sophisticated, low-cost hardware, make passive radar a viable way to detect stealth targets. Passive radar systems in the defence market include Lockheed Martin's Silent Sentry passive coherent location (PCL) system.

The game-changing advance for passive radar technology will be the ability to identify targets as well as track them, allowing passive radar to integrate with surface-to-air missile defence systems.

While the concurrent development of the latest generation of stealth-enabled fighters constitutes an arms race in itself, this race sits within a wider technological contest between stealth and counter-stealth technologies and techniques. Governments around the world are pouring investment into stealth aircraft development programmes, but it remains to be seen if these costly paragons of modern military hardware will end up undone by the evolution of comparatively modest radar systems

Stealth technology and the counter-stealth response - Airforce Technology

Most modern armies have some ongoing work in battlefield acoustic sensors, with no one country having a dominant capability. The U.K. and France offer strong capabilities related to seismic sensors and Israel provides unique opportunities in acoustic sensors. Current efforts in acoustics include adaptive beamforming algorithms, sound cancellation techniques, and neural network algorithms for target identification. Israel has been developing advanced helicopter detection, sniper, and mortar location systems based on acoustic sensing. The United Stateshas been conducting joint exercises with the Israeli Army and future cooperation will provide potential solutions to acoustic propagation problems, long–range target detection algorithms, and detection in the presence of wind and platform noise.

Click to expand...

17. Sensors

IIRC, the Israëli's developed a system by which to locate helicopters based on their noise.
http://www.rafael.co.il/Marketing/186-931-en/Marketing.aspx

See also Helispot


Google: "helicopter detection" + sound.
You'll find a number of interesting reads (papers), including by Dutch TNO research institute


I say: use ECM, IR and sound sensors and apply sensor fusing techniques.

Helicopters are much noisier, fly at lower attitudes and are slow compared to jets. But the technology used for helicopter sound detection can be built upon to detect engine sound of supersonic jets, although with much different parameters.

Sniper detection systems are another avenue: triangulating the sonic boom of rifle bullets. The technology is relatively similar to the artillery sound ranging since about 1916, radio direction finding, radar receiver, sonar receiver and passive radar warning/direction finding.

Sound detaction against aircraft
# MILITARIZED ARCHITECTURES /// Military Concrete Sound Mirrors on Der Spiegel | The Funambulist

Sound ranging - Wikipedia, the free encyclopedia