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37.5kHz Ping – The Frequency of Hope

The world is awaiting the results from the 2 faint ping discovery from Indian Ocean. Families and rescue workers are on their virgil hoping to discover the believe crashed Malaysian Airline MH370. The plane has gone missing for more than 30 days now. Rescue time frame is getting smaller and the hunt gets even more intense. It’s all based on one simple fact that the battery on the Black Box would not last longer than a month under water. Every second, the window closing in.

What I find absolutely unnerving is that this rescue is based on a single hope, of a hydrophone (underwater microphone) towed behind a high tech vessel piercing through the rough ocean surface. It’s goal is to pick up one frequency from beneath the sea, which may give the living their long-awaited closure. This intermittent pinging from a Black Box, if discovered will unveil the story on what actually happened to the Beijing-bound flight. No one know for sure what happened. Let’s hope Australia’s Ocean Shield would make the discovery soon. But its only days or hours before the faint ping would stop.

Remember, it took rescue units 50 mins to find Dutch Prince Friso when he was buried in snow avalanche via his rescue audio beacon (possibly RECCO on skiing jacket). This time its a 72 foot long airplane ‘believed’ buried in the Idian Ocean floor. The task is gargantuan.

Below is a breakdown from L-3 on how audio beacons on blackbox work.

Underwater Locator Beacon
In addition to the paint and reflective tape, black boxes are equipped with an underwater locator beacon (ULB). If you look at the picture of a black box, you will almost always see a small, cylindrical object attached to one end of the device. While it doubles as a handle for carrying the black box, this cylinder is actually a beacon.


Photo courtesy L-3 Communication Aviation Recorders
A close-up of an underwater locator beacon

If a plane crashes into the water, the beacon sends out an ultrasonic pulse that cannot be heard by human ears but is readily detectable by sonar and acoustical locating equipment. There is a submergence sensor on the side of the beacon that looks like a bull’s-eye. When water touches this sensor, it activates the beacon.

The beacon sends out pulses at 37.5 kilohertz (kHz) and can transmit sound as deep as 14,000 feet (4,267 m). Once the beacon begins “pinging,” it pings once per second for 30 days. This beacon is powered by a battery that has a shelf life of six years. In rare instances, the beacon may get snapped off during a high-impact collision.

Table 1 illustrates that, under normal conditions, the existing pinger would not be detectable from the surface in depths exceeding 2km. A transponder, operating at the existing 37.5kHz but at a higher power, would be detectable from the surface in depths down to 5km in normal operating conditions. Alternatively, a transponder operating at 10kHz would be detectable from the surface down to full ocean depth under most operating conditions. A transponder-type beacon operating at the same 37.5kHz frequency but with 20dB greater peak acoustic power output (a factor of 100 in power ratio) is about twice the volume of the existing pinger beacon. A 10kHz transponder would be about four times the volume. See Table 1 below.

Table 1. Typical Detection Range performance


Maximum depth of beacon detection (km)

Normal conditions

Good conditions

Pinger: 37.5kHz (160.5dB re 1μPa)



Transponder: 37.5kHz (180dB re 1μPa)



Transponder: 10kHz (180dB re 1μPa)




God speed Ocean Shield. May the truth finally be told.


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