Sound versus radio energy
I've been offline all weekend and a very quick peruse of the last couple of pages spurred me to produce the following "tome". Apologies if it is just repetition. I was a sonar specialist with the UK Nimrod force and offer the following to help those who still mix radio energy with sound energy...
Some people are still getting radio transmissions mixed up with sound transmissions. They are totally different but share some similar characteristics and terminology. The confusion clearly arises from these similarities.
The fundamental difference between sound and radio transmissions is that radio signals travel (as near as dammit) at the speed of light whereas sound travels at a speed some 880,000 times slower.
Radio transmissions are a form of radiation and require no medium, like air or water, in which to propagate (travel). Radio energy can be transmitted across space - think of pictures and telemetry beamed back to earth by Voyager, the Hubble telescope, the International Space Station, etc. Radio energy can propagate happily through the vacuum of space and through our atmosphere.
Sound, on the other hand, is a series of compressions and rarefactions of molecules. No molecules, no sound transmission or reception. In space, no one can hear you scream!
Sound can travel through air, most solids (listening for a train by putting your ear on a rail is NOT recommended - especially not the one carrying the electric current!!). Sound can travel through water. So too can radio energy, but it is so fiendishly attenuated that there are few practical uses for trying.
Speeds:
Radio energy, RADAR energy (derived from RAdio Detection And Ranging), light and other forms of radiation are all part of the electromagnetic spectrum. To most intents and purposes, they all travel at (rounded) 186,000 statute miles per second and we'll call it 300,000 kilometers per second for cash. Pilots are well accustomed to working with a multiplicity of units but forgive me for choosing metric, i.e. metres per second. Radio energy travels at 300,000,000 metres per second.
Sound! For the purpose of this post, I'll stick to air and sea water:
Sound in air, as any ATPL holder will tell you is calculated by: "Thirty-eight point nine four ROOT Tee!" This ancient mantra tells you that if you convert the air temperature from Celsius to Kelvins (i.e ADD 273), you get "Tee", the temperature on the Absolute scale.
Multiply "Tee" by 38.94 and you arrive at the speed of sound in air, in nautical miles per hour. That's knots to you!
e.g. Air at 15 degrees Celsius is 15+273. T = 288Kelvins.
The square root of T is 16.97056274847714
The speed of sound in this air is 38.94 X 16.97056274847714 = 661 KNOTS
= 340 metres per second. Compare this speed with that of radio!
The speed of sound in seawater is a bit more tricky to calculate. Our (Nimrod) equipment used data based on Wilson's Simplified 32 term Equation. Well why wouldn't we?
Whereas the temperature of air (at levels and densities in the atmosphere where conventional flight takes place) is the dominating factor by far, temperature, pressure and salinity of seawater cause significant variations in the speed of sound in that medium. If we accept that the criminally average speed of sound in seawater is 4900 feet per second, this gives us 1493 metres per second.
For what it's worth, to get sound to travel at 1493 m/s in air, the air would have to be 5,281 degrees Celsius or 5,554 degrees Absolute! RAF Kinloss on a sunny day, hey!!
Sound transmissions and radio transmissions share many properties and problems. One important shared characteristic is the general principle that higher frequencies travel shorter distances than lower frequencies. There are particles in the air and there are particles in the water that obscure our ability to see very far. Clouds and dust in the air spring to mind as do shoals of fish and particles of digested and excreted "stuff" in the water. The chief reason for this obscurity is the wavelength-to-particle ratio.
We tend to speak of "frequency" whereas, in the context of a signal's obscurity or inability to penetrate the medium, we should really be thinking in terms of "wavelength". Bear with me, I'll explain why in a short while.
Wavelength and frequency are directly related to each other. But they are GOVERNED by the speed of propagation of their transmitted signal. I say again: the speed of propagation is VASTLY different between radio waves and sound waves. The two should not be mixed up but in the heat of the moment....!?!
Wavelength is calculated from the following triangle. Cover the element you need and the structure gives you what to do with the other two terms. In this case, C = f X λ
C
f λ
Where C = the speed of the transmission in the stated medium. I'm using metres per second.
f = the frequency of the transmission in Hertz (cycles per second).
λ(Greek lambda) = wavelength. Your adding stick will churn it out in metres if you used metres per second for C
Radio, RADAR and visible light are all bands of the electromagnetic spectrum. To all intents and porpoises, use 300000000 (3, then eight taps on the zero key). If you do so, you are using metres per second.
For example, typical airborne weather RADARs work between 9GHz and 10GHz. Using 10 gigs (ten thousand million Hertz), the wavelength would be:
C
f λ
C = 300000000 metres per second, divided by 10000000000
λ = 0.03 metre or 3cm
Subwoofer sound at 20 Hertz in air in your living room at 23 degrees Celsius:
C in knots = 38.94 √T = 38.94 √(23+273) = 38.94 X 17.21= 670kts = 345m/s
Wavelength:
C
fλ
345
20 = 17.25 metres. Useful for checking your listening room's dimensions versus the space needed for one wavelength of sound! Or finding a null zone where direct sound and reflected sound from ceiling or floor might cancel each other out if in antiphase.
In seawater of average salinity (35 parts per thousand), a sonic locator beacon transmitting at 37.5 kHz:
1. For imperialists:
C = 4900 feet per second
frequency = 37500 Hz
Wavelength = 4900/37500 = 0.13066666666667 foot = 1.568 inches
2. For bass tenors (!) 4900 fps = 1493.52 metres per second
Wavelength = 1493.52/37500 = 0.0398272 metre = 40mm for cash
Signal in seawater at 9.5 kHz = 1493.52/9500 = 0.15721263157895 metre. 6 inches, give or take.
Signal in seawater at 1000 Hz; Wavelength = 1.49352 metres
Signal in seawater at 500 Hz; Wavelength = 2.987 metres
Signal in seawater at 100 Hz; Wavelength = 14.9352 metres
Sound transmissions, like electromagnetic transmissions, are prone to attenuation. The signal power and intensity diminish with range from the source.
Ordinarily an emission spreads from its source in a spherical pattern (i.e. in ALL directions). Transmitted power at the wavefront diminishes rapidly. Power loss = 20 log Range.
If conditions are conducive to the formation of a duct, the signal losses, being constrained in the vertical, spread cylindrically. The power lost at the wavefront is considerably less with spherical spreading than with cylindrical spreading. Power loss = 10 log Range.
The importance of thinking in WAVELENGTH rather than frequency.
Absorption and scattering. Any object roughly equal in size, or larger than the wavelength of the transmission will be in part absorbed by the object as heat energy. In addition, some of the signal will bounce off the object and be reflected from its surface. You see this with RADARs, whose wavelengths become equal to or shorter than large water droplets or large agglomerations of frozen water held aloft by updraughts. To some users these RADAR returns will be annoying because they shield what the user is looking for; this user would call this "clutter". To someone else, this same "clutter" is useful for illustrating weather to be avoided.
In air or water, sound wavelengths may be similarly absorbed and scattered by objects of similar size to the wavelength. If you were to try to cross a busy shopping mall at Christmastime, your progress would be much impeded by other angry shoppers of similar size and mass to you. Drive through those shoppers in a tank and your passage through them is much improved.
For a given patch of sea, absorption of sound in the sea is linear with range. Absorption will be increased by an increase in salinity, a decrease of temperature or an increase of transmitted frequency. Absorption varies approximately with the square of the frequency.
We tend to talk a lot about frequency whereas, in my opinion, the effects can be better conceptualised by reference to wavelength - hence my crude illustration about the shopping mall above!
The scattering of sound in the sea is also frequency (in fact think wavelength) dependent. Surface scattering varies directly with wave height, signal wavelength and angle of incidence of the sound wavefront.
Bottom scattering is dependent upon bottom roughness, particle size versus signal wavelength and angle of incidence of the sound energy.
Volume scattering is the name given to scattering of sound by whatever is in the water of comparable size to the sound's wavelength - or larger. Again this is frequency (wavelength) dependent. It depends on the mass of the object if the object is a solid - like a fish but bubbles are even more efficient scatterers of sound energy.
Layers in the ocean.
Without temperature measurement it will be impossible to state there is or is not a sound channel (duct) in the locality.
The ocean is often conceptualised into a three-layer model: the surface layer, the thermocline and the deep ocean layer.
The surface layer is isothermal - the same temperature. It is isothermal due to wave action mixing the water in the layer.
The deep ocean layer is also isothermal and in most oceans, is around 4 degrees celsius.
Separating the surface layer and the deep ocean is a layer called the thermocline, where the temperature decreases from that of the surface layer to that of the deep ocean layer.
The heat of the day can cause a transient thermocline in the surface layer and the seasons impose their own alterations.
The importance of knowing the thermal profile of the water is in determining how a sound may behave between the source and the receiver.
An increase in any of the following will produce an increase in the speed of sound in the water: temperature, pressure, salinity.
It's late and I can't be bothered giving you the mathematics of Snell's Law. But sound exhibits some properties of waves and some properties of rays in the way it propagates in a medium. It is subjected to changes in behaviour as it encounters boundaries: the sea surface, the boundary between the bottom of the surface layer and the thermocline, the boundary between the bottom of the thermocline and the top of the deep ocean layer and the seabed are all boundaries.
The signal path the sound takes may reflect off a boundary, pass through unimpeded or be refracted (i.e. change angle). The angle it meets a boundary is quite important.
Sound rays tend to be refracted away from depths of maximum sound velocity. As a result there often exists at these depths a region into which very little acoustic energy penetrates. Such a region is called a shadow zone.
The limiting ray is that ray which just grazes, or is tangent to, a boundary as just described. Any sound ray approaching the boundary at a more perpendicular angle than that of the limiting ray will be reflected, or pass through the boundary.
The Critical Angle ray is that which strikes a boundary at the steepest (most perpendicular) angle and still exhibits almost 100% reflection. Any sound rays striking the boundary at a more perpendicular angle than that of the Critical Angle ray will lose some significant reflection losses.
A sound, from its source, may undergo many alterations of course. But don't forget, all the while the sound is losing power with distance and being scattered and reflected by particles and objects in its path. And the higher the frequency, the shorter its wavelength and the greater the number of things in the water at or above its wavelength dimension to absorb the sound and/or reflect it away from the path it needs to travel to the receiver!
Alterations in the sound signal path may also occur near river mouths where the salinity is reduced by fresh water. There are also ocean fronts which can cause density discontinuities and also alter the signal path.
Another layer worth considering is the "Deep Scattering Layer". Like the ionospheric changes that occur in the D, E, F1 and F2 layers, the Deep Scattering Layer is on the move at sunrise and sunset. It rises to near the surface at sunset and descends to the deep at sunrise. Plankton rise as the sun sets. So too do the fish etc that feed on them and they all go back to the deep as the sun comes up. In the average ocean, the Deep Scattering Layer will be about 600 feet at night and 3000 feet by day.
Pontius Navigator has described sound channels and a peculiar type of transmission path called Convergence Zone in earlier posts. The difficulty I see in the path sound takes in the case of this suspected sonic locator beacon, is that the beacon is on the seabed, whereas any receiver could well be at least two boundary layers up. What with refraction and reflection of the ray, this adds a further difficulty in homing in on any such signal. Getting source and receiver in the same layer would be a huge help.