Quiz - Physics - Sound

[Mike1967]

Aghh I new it had to be a or d, I took a punt and was wrong lol

 

[NAM001]

Actually all answers are wrong.
The speed of sound in a liquid, c, is given by the square root of the bulk modulus (aka stiffness coefficient) K divided by the density ρ, as described by the Newton-Laplace equation:
View attachment 581401
Hence increasing the density makes the sound speed TO DECREASE!
The sound speed in water is larger than in air because water is much more stiff than air (which is highly compressible), not because water is more dense...

Dont let semantics and details ruin the intent of the question, Speed of sound in steel is nearly 4+ times higher than water. would you say steel is not more dense than water.

 

[NAM001]

As a retired submarine sonarman,,,,, some of this very entertaining.

this sounds like the argument that ,,,,,hot electric stove element did not cause the blister the fluid in the body going to he skin surface did.

 

[Pedro Burrito]

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?

I randomly look through my exams and pick a question. There is a technique for taking PADI exams ... do not over think them! Each question is taken from the Encyclopedia of Diving or other PADI book and the correct answer is usually verbatim from the text. The tests drive my wife, who is an Intensive Care Nurse and Special Education Teacher, crazy because she will think about each question and every possible answer while I just memorize the answers.

 

[Angelo Farina]

Dont let semantics and details ruin the intent of the question, Speed of sound in steel is nearly 4+ times higher than water. would you say steel is not more dense than water.

The sound speed in steel is higher than water because steel is much stiffer, not because it is heavier. The weight (density) works AGAINST the speed of sound!
The material with the higher speed of sound is the "resonance pinewood" employed for building musical instruments, such as Stradivari violins.
While in steel the speed of sound is around 6000 m/s, in resonance pinewood it can go up to 7000 m/s. Pinewood is a very light wood (density is less than 500 kg/m3, so it floats significantly, as water is 1000 kg/m3), but it is very stiff.
The lighter, the higher the speed of sound.
It is stiffness which boosts the speed of sound, while density (weight) reduces it...

 

[Angelo Farina]

I randomly look through my exams and pick a question. There is a technique for taking PADI exams ... do not over think them! Each question is taken from the Encyclopedia of Diving or other PADI book and the correct answer is usually verbatim from the text. The tests drive my wife, who is an Intensive Care Nurse and Special Education Teacher, crazy because she will think about each question and every possible answer while I just memorize the answers.

So these tests evaluate memory, and not the real understanding... That is crap.
At my exams at the university, I usually reject students who answer "by memory" and do not show a true understanding of physics..

 

[jonhall]

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.

Clarifying my response, I was only referring to the example that Angelo gave about an approaching boat and the Doppler effect. My thinking is that a diver underwater would have an idea of direction based on sound as the boat nears and then gets further away from the diver. Not true?

 

[Angelo Farina]

Clarifying my response, I was only referring to the example that Angelo gave about an approaching boat and the Doppler effect. My thinking is that a diver underwater would have an idea of direction based on sound as the boat nears and then gets further away from the diver. Not true?

Not true, unfortunately. The pitch change is truly minimal, for two factors: first the speed of the boat is limited (not like an ambulance on the highway). And then the speed of the boat is a very minimal fraction of the speed of sound, as the latter is much larger than in air.
Conclusion: no significant Doppler effect telling you if the boat already passed through your position and is going away...
You can evaluate this only by the reduction in noise level, when the boat is already at a significant distance.

 

[Angelo Farina]

Coming back to pitch change, the speed of sound has another interesting effects for scuba divers. Many tech divers use helium either in Heliox or Trimix mixtures. The speed of sound in Helium is very high (because of its SMALL DENSITY, again), it is around 965 m/s in normal conditions of pressure an temperature. So 2.5 time larger than in air.
This causes a significant increase of the pitch of your voice, as the pitch is mostly due the main resonance of your tracheal duct. The higher the speed of sound, the larger the wavelength, the higher the resonance frequency of a duct of a given length.
In reality the effect of breathing a gas with an higher speed of sound is much more complex, read here for a better explanation:
Why Does Helium Change the Sound of Your Voice?

 

[Jcp2]

Speaking of directionality, I read that people with the masks that have the ear muff attachments for easier equalization report that they get some sense of sound directionality returned underwater.