Wednesday, December 28, 2005

Speed of Sound

So we have all seen the basic formula for calculating the speed of sound through air. It is dependant primairly on air density. Well, it is obviously more complex and dependant on humidty and other simple factors. I have also read anecdotal stories in some physics books that it is dependant on the actual intensity of the sound itself (i.e. loud sound travels faster than quiet sound). I have never found a formula that takes into account the intensity of the sound. How loud is loud enough to observe a difference? I suspect it is somewhere around 130-140dB with very loud sounds 170-200dB traveling faster.

I have read that lould sounds are difficult to measure; they tend to blow out the human insturments for measurement and are often not sustanied long enough for the insturment to slew up to the full peak intensity. That is the measurement is limited by the slew rate of the insturment. The article I read was about high powered rifles and that they can get up into the 170dB range (ouch!).

I believe there was mention somewhere of the Navy using sonar up to 200dB and it actually killed some sea life by exploding their ear drums, maybe they know how the intensity of sound affects the speed of sound. Although the speed of sound in water is a very differnt thing than in air. I can see it now... (use a gruff voice) "Seaman give the ruskies one ping..." PING... "That should do it, they won't be able to hear anything for a while" That was probably the hight of humor duing the cold war, sneaking up an enemy sub and getting them with a nice loud (200dB) PING!

Anyway I have been distracted, other loud (read very loud) things include volcanos erupting, high caliber cannons, and Kali when I step on her. Can anyone find a formula or emperical data that details the speed of loud sounds versus quite sounds I have been curious about this for a long time but haven't found the data. Maybe the one of us who is at a University can get one of the physics grad students to take some data (ah to be able to expense a cannon to the University).

2 comments:

palegreenhorse said...

mmm i'll have to look at the library tomorrow (or is campus still closed tomorrow?) but for now you can read hyper physics' section on sound. specifically the loudness section. it is like university squashed into a very well laid out web page.

Dan said...

Some research has turned up a few interesting things about sound...

"Volcano eruptions may be the loudest commonly occurring sounds on Earth, at over 272dB"

Blue Whales can make sound as loud as 188dB.

The Krakatau eruption in the late 1800's was probably the loudest event in recent history and was heard over 3,000 miles away. This brings up another question about high intensity sound... is it directional? Does it travel at frequencies beyond human hearing and then shift into human hearing? Some lumber workers near Mt. St. Helens when it blew never heard it, yet people in Seattle heard the explosion...??? what's up with that.

All of the velocity of sound approximations are based on air being an ideal gas... well what if it's not an ideal gas? What if the intensity of the sound heats the air and shifts its velocity?

One other interesting note... apparently there exists a nasty little shrimp that can squirt water so hard it forms a caviation bubble that on collapse makes a pop somewhere near 200dB.