# How to find the power of a wave

1. Sep 21, 2015

### CubicLogarithms

A bit of background knowledge first. This term I have to do a presentation on the sonic screwdriver from doctor who. I have to investigate weather it is possible in this universe and prove/disprove it by using established physical theories. Right now I am looking to find the power of a sound wave. I have found the frequency, period and amplitude of the wave is there any formula that can give me the power/intensity of the wave from the frequency, period and intensity. Also if you have any other ideas on how to prove/disprove the sonic that would be great.

Thanks

2. Sep 21, 2015

### nasu

You need to know the amplitude of the wave in order to find intensity. If you want the power, then you need the area of the sound beam.
You will also need the properties of the medium (density and speed of sound).

I have no idea what is this sonic screwdriver.

3. Sep 21, 2015

### davenn

not a Sci Fi fan huh

4. Sep 22, 2015

### nasu

Used to be. But a different set of it.
Different country, different time.

Well, maybe not different time. But I was not even aware that Dr Who is classified as SciFi.

5. Sep 22, 2015

### davenn

Oh my Gosh .....
it would almost be the original TV SciFi and probably the longest running one with the many reincarnations of the Dr. over the last 50 + years
of ongoing new seasons/episodes

The Tardis was the best time machine ever, externally looked like a a standard police/public phone call box
Inside it was huge

inside with a more modern look ...

as a kid I grew up on Dr Who, having the crap scared out of me with the Darleks chasing and zapping people with their "death rays"

I could never understand why people didn't escape from them just by running up stairs ??
as the Darleks could only go on flat surfaces LOL

and a small video clip ....

Dave

Last edited: Sep 22, 2015
6. Sep 22, 2015

### HallsofIvy

In later shows, the Daleks could move up a few feet into the air, allowing them to climb stairs.

7. Sep 22, 2015

### nasu

For me SciFi was mostly books rather than movies.
The only shows I watched as a child and will qualify as SciFi were "Planet of the giants" and "Lost in space".
I was a big fan of both. :)

8. Sep 22, 2015

### Henryk

Ok, I love sci fi too, but actually you can get numbers.
Power density of a sound wave (power per area) is given by

Q = p * V, where P is sound wave pressure amplitude and V is velocity amplitude

Pressure and velocity are related to via acoustic impedance

p = R * V

For air and plane waves, R is around 400 Pa s/m
That gives you
Q = $p^2/R$

At 120 dB, that is when you are almost going deaf, the sound pressure is around 20 Pa, so the power density at that level will be around $\frac {20^2} {400} =1 W/m^2$
Your screwdriver can't be 1 x 1 m in dimension, most likely something like 10 x 10 cm, that gives the area of 0.01 $m^2$
If you want to power it by a sound that makes you almost deaf and can actually collect the energy with 100 % efficiency, the most you get is 10 mW.
Not really enough to power anything mechanical.

9. Sep 23, 2015

### davenn

I know we have hijacked the thread ... It wasn't really going anywhere anyway LOL
probably needs to be moved to Sci Fi or general discussion

yup those 2
think it was Land of the Giants, tho ...... http://www.bing.com/search?q=land+of+the+giants+tv+show&src=IE-SearchBox&FORM=IENTSR&pc=U270
remember ... Voyage to the Bottom of the Sea ?
the original Battlestar Galactica ( and the much newer one)
The Time Tunnel

Star Trek .. of course
All the Gerry Anderson series ( puppet and actual actor ones) --- Thunderbirds, Captain Scarlet, Joe 90, Space 1999, UFO
Through the '70's in my teens and into the '80's my 20's I read masses of good sci fi by many of the well known authors

in the early 80's, I remember when "The Hitchhiker's Guide to the Galaxy" came out in books, then radio series then finally on TV
The TV series only covered the first 2 books ... The Hitchhiker's Guide to the Galaxy and .... The Restaurant at the End of the Universe
there were 2 more books ... So Long and Thanks for all the Fish and ... Life, the Universe and Everything
The whole series was somewhere between comical and silly but I enjoyed it as it was such a big change from my years of serious sci fi

over the last ~12 months I have been watching many of these old TV programs on youtube .... wonderful time reminiscing

There's next to no new scifi progs on TV these days .... like the good old westerns ( The High Chaparral, Bonanza etc) they are slipping into history

Dave