While looking into Flight Data Recorders, I came across a part that has me very confused.?

Question

The Underwater Locator Beacon operates at a very high frequency of 37.5 KHz, this is confusing for multiple reasons.
1. This is not in the audible range of the Human Ear, meaning it will require special equipment to even detect it even if a vessel or diver is directly over it. This can prevent many additional resources from assisting in finding the Data Recorders, simply because they may not be able to detect it.
2. The higher a frequency, the higher the transmission loss of range (mostly by attenuation, or transfer into heat). This means VERY short detection ranges so short in deep water the transmission may not be detectable unless directly over top the beacon, and in poor acoustic conditions, not even then.
3. The higher the frequency, the more the sound will be refracted, and in many environments in the ocean, this refraction is towards the Deep Sound Channel axis(which is very deep), or towards the bottom, not many environments have acoustic conditions that refract sound upwards.
4. The source level is only about 160 dB//1 micropascal, this is the equivalent of 99 dB in the air, this is not very loud, especially for a source that has to fight through 1-3 above.
5. Modern Submarine distress beacons are low Frequency, audible to the human ear, are battery operated, very compact overall [compact enough to fit into a medium lunch bag if broken down and have long life spans (hundreds of hours).

Answer 1

The 37.5 kHz frequency was chosen because no natural sound are at that frequency; dolphin's echolocation clicks are between 40 kHz and 150 kHz.

The pings are designed to be detected up to 2 km away, which is adequate range for a systematic search.

Human hearing range is not even a remote consideration since electronic detectors will ALWAYS be used in such circumstances.

The refraction of the sound is also immaterial, as the detectors towed by ships would be lowered almost to the ocean bottom, since the emitter would be there, at the bottom.



I do not know about that submarine distress beacon, but may I suggest that a submarine in distress may not necessarily be at the bottom of the ocean, since it would not merely be in distress at that depth, but completely crushed? How are those activated and released anyway?



You see, you are confused because you deal with submarines. Aircraft are not submarines. Your expectations do not equally apply.

Answer 2

You misunderstand. 37.5 khz is the radio frequency, not audio.

Answer 3

Submarine guy Is U.L.F. still used and is that large grid still around. I know you know but maybe can't say.

Answer 4

Usually, the searchers have some idea where the plane went down, so the short range of the locator beacon is not an issue. in case of MH370, the plane was so far off course that by the time they figured out where it might have gone down, the beacons' batteries have died.

Answer 5

Kudos and a big salute for your 20 years of service.



Now, let's look at the problem from another angle. Only ONE jet airliner (MH370) has EVER gone missing and stayed missing on an intercontinental flight. Looking at JUST the past 5 years of international air travel, that is one out of about 150 MILLION trans-oceanic flights that has gone missing SO IT ISN'T EXACTLY A DIRE PROBLEM, and certainly not worth the HUGE COST and effort it would take to change the system based on 37.5Khz which works extremely well except in deep water immersion. It may sound a bit callous, but it is the most rational approach to take.



CASE DISMISSED.

Answer 6

Hi there is a simple answer to this around the world there are detection bouys used to pick up such devices. planted there under international agreement to detect objects for national security for all nations of the world.

there is a film called battle ship which has Rihanna in it as a naval officer this shows how the bouys can be deployed.

Answer 7

Having worked in a communications center where there were a scad of devices that used sonalerts, I can tell you short range is what is wanted. The farther it propagates the harder the task of locating it actually is.



In the case of flight recorders we know it is somewhere, maybe underwater, maybe there. I am certain if more power or a lower frequency would improve the results the change would already have been made.

Answer 8

Well, you could do with a little RF engineering. 37Khz is SLF and high enough to propagate a signal, poorly. Not acoustically.

If you really search,you will find locator beacons for black box and ELT operate at 120 to 240 KHz

Within range, easily detected through water or earth with direction finding equipment.

Acoustic location is a bit different, that is what makes sonar work, and = to 99Db in air would be painfully loud to the human ear.

Modern sonar can detect and locate some ping of that level well over1000 miles away.

20 year Master Submarine Sonar Technician,,,bull$hit.

Answer 9

-37.5 KHz is HORRIBLE for propagation, the loss is astronomical when crossing a layer. It is a short range frequency.



-Low frequencies are not good for RESOLUTION, but you are not looking for resolution you are looking for signal (Highest Signal to Noise Ratio), and the prorogation loss is lower for lower frequencies chiefly due to absorption (transfer of energy into heat, the higher the frequency the faster the transfer due to the shorter wavelengths). If you were doing an Active search I would agree, but this is a passive search for a bearing only, not to resolve an image



-You don't need an ear in the water, but an human audible hydrophone is much easier to use than one that needs a display/and or frequency shifting to detect the signal.



Please tell me how the rest of the statement is wrong (I am going to enjoy this). I would easily concede the ~160dB//1 micropascal may be (source is not the best that I got it from) but as for the rest of the underwater acoustics. I made a good career knowing it.



99dB sounds like a lot, but for an underwater source level (measured 1yd from source) it is not. In the first 3000ft, 60dB will be lost due to spreading loss (20*log(r2/r1)) and 10,000ft 70.5dB is lost. Starting from 160dB underwater, that drops to 90 dB at the 10,000ft in micropascal or 28.5dB for the airborne equivalent. That is NOT loud when searching for a source at depths with other interfering noises, this is not even factoring in scattering and absorption loss yet. When you plug that into a Passive equation for Signal Excess (SE=LS (Source Level)-PL(Prop Loss)-NL(Noise Level)+DI(Directivity Index)-RD(Recognition Differential)), 160-70-36+0-0=54. 54dB//upa means the source is not loud enough for human ear audible levels at 10000ft. [This used a Surface wind speed of 10-15 kts to cause 36dB in Noise, an omni-directional Hydrophone for 0 DI also resulting in a 0 RD, you are not going to gain so much in DI and RD even with good equipment to really change much, perhaps 6-9 dB?]



Snells law drives #3

#4 is the standard (61.5dB) conversion of underwater to airborne dB.

#5 Please, prove me wrong!

Answer 10

1. we don't go around with one ear in the water during a search mission. It takes equipment anyway. the frequency is selected for best propogation through water.



2. Wrong. low frequencies are not good for underwater search.



3. the reat of your statements are totally wrong.

Answer 11

20 years as a sonar technician and you don't get that the 37.5 is a radio frequency?

Answer 12

Gee, all this time engineers from all over the world thought that 37.5 KHz was a very effective frequency for detection and location. then you came along. I think they still do.