Quiz - Physics - Sound

[Angelo Farina]

The real problem is to locate where the sound is coming from. There are many cases where you would like to be able to do that:

• you hear the noise of a boat approaching, but the visibility is bad and you cannot understand if it is going to hit you while resurfacing (this is a big problem for free divers)
• one of your buddies is calling you, possibly for help, you hear him, but you cannot see him, and you do not know in which direction to go for reaching him
• you hear the sound emitted by some critter (for example, a lobster) and again you cannot locate it

As said, some systems have been developed for making it possible to locate sound sources underwater, either with binaural rendering over headphones or with visualisation of colour maps. This is actually one of the fields where I am doing research. And here you can see one of the results of my research. This is a video with "spatial audio" (Ambisonics) soundtrack. It is meant to be watched using an Oculus VR visor, or a smartphone with a Google Cardboard visor, and of course using headphones. However on a PC you can "look around" by panning the viewport with your mouse, and still the stereo rendering of the soundtrack should provide some decent spatial information:
This was recorded at the Panarea island in September 2019, and the bubbles emerging from the bottom are CO2 being released due to volcanic activity.
This is just a short initial recording for checking the hydrophone array, then the system was left on the sea bed for 24h, monitoring the noise coming from the CO2 bubbles (an indirect way of monitoring volcanic activity and possibly predicting eruption form the near Vulcano island).

 

[Pedro Burrito]

Spoiler: Answer

d. four / denser

 

[scubadada]

Thanks @Pedro Burrito another good question

 

[Couv]

Hi @Angelo Farina ,

I quite enjoyed finding the sound source with the help of my headphones. If it's not top secret, could you post a picture of the hydrophone array that made the above possible? TIA

 

[Angelo Farina]

Hi @Angelo Farina ,

I quite enjoyed finding the sound source with the help of my headphones. If it's not top secret, could you post a picture of the hydrophone array that made the above possible? TIA

Try watching the video with your smartphone and the Youtube app, you will be able to look around (and listen around) rotating the smartphone...
If you insert it in a VR visor, the effect is very similar to being really underwater.
Regarding the technology I am employing, look here below: you will find the paper I presented recently at the ICONS2018 conference and the Powerpoint presentation: Index of /Public/ICONS-2018

 

[ToneNQ]

.................. we cannot localise anymore where the sound is coming from..................

I'd agree it is far more difficult, but it is still possible to find things by sound. The process can be quite slow and tedious as we've lost the instinctive ability to do it, but it can be done. Turn your head left and listen, turn it right and listen, turn your whole body 90° and listen, approximate direction and swim 3-5m, start again. You can find an anchor chain clinking, or buoys/boats with water slapping on the hull.

A handheld piece of technology would be awesome.

 

[Angelo Farina]

Thanks @Pedro Burrito another good question

Thanks @Pedro Burrito another good question

Well, at least these questions give us the excuse for discussing physics topics related to diving, which is always useful for everyone. Thanks!
But considering how much we debated, I start thinking that PADI should get some help for fixing their questionnaires, as at least 2/3 of them contain some significant errors (as this one) or are badly formulated...
Or are you picking by purpose the questions you already know having problems, for stimulating us to find the faults?

 

[jonhall]

you hear the noise of a boat approaching, but the visibility is bad and you cannot understand if it is going to hit you while resurfacing

Foe this example, does the Doppler effect work as well underwater as it does above?

 

[tursiops]

Foe this example, does the Doppler effect work as well underwater as it does above?

No. The critical factor is the ratio of the object's speed to the speed of sound, which becomes the fractional change in the perceived frequency you hear. Larger speed of sound, smaller change in perceived frequency.

 

[TMHeimer]

Well, at least these questions give us the excuse for discussing physics topics related to diving, which is always useful for everyone. Thanks!
But considering how much we debated, I start thinking that PADI should get some help for fixing their questionnaires, as at least 2/3 of them contain some significant errors (as this one) or are badly formulated...
Or are you picking by purpose the questions you already know having problems, for stimulating us to find the faults?

How DARE you be a PADI basher! Kidding aside, I only get frustrated when the questions are obviously unclear for a slob like me. There are very few that I've seen. But, considering that these tests probably don't change much (such as school teachers' tests that vary from year to year), there should be NO unclear questions. Questions should not contain two very possibly correct answers. The several debates about questions where all answers could be argued as wrong are really of no consequence to me as a layman, as long as there is one clear answer for the layman. I found a couple of questionable questions on that SSI Science of Diving test, but that's maybe two out of 100. Same as PADI. I would guess that other than maybe computer questions instead of tables ones, the PADI OW exam most likely hasn't change much if at all since I took it in 2005. No excuse for unclear questions.