Saturday, May 18, 2013

So quiet, a pin drop sounds like a plane taking off...

This has been a hell of a week. Some folks came in from out of town, most notably Dr. Pat Walter from TCU, who came in to give a lecture on how to take better shock and vibration measurements. Pat draws a pretty good crowd, about 30-40 people from different industries who are all interested in dynamic measurements. Among these was an old friend of mine, Andy, and some of his colleagues from Los Alamos National Lab. I was between hanging out with them, and dealing with some other boring, non-work related stuff all week, hence no posts. Sorry folks, that will happen from time to time...

Anyway, I'm going to steer a bit off of the lesson plan just a bit (And actually this will fit in nicely with sinusoidal amplitude, which I'll be talking about next) and talk about an application. Yes, I know that it's about time, but the fundamentals must be learned before we can actually understand the applications and all of the cool things in dynamics.

Orfield Labs in Minneapolis, MN had Eckel Noise Control Technologies design and build what Guinness World Record's calls the quietest place on Earth. It's an anechoic chamber (an meaning not, and echo meaning a sound reflection) within two more nested rooms. The nested rooms isolates the anechoic chamber from the outside world to such an extent that the background noise level of the room with nothing in it except the measurement equipment is -9.4 dBA (Decibels, A-weighted). The room is pictured here:

I know a lot of folks have heard the term dB thrown around a lot, so let's talk about what that actually means:

The decibel is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities.(Wikipedia)

So, simply put, a dB is an indication that what's being measured is in relation (a ratio) to something else. In power quantities (watts and volts^2), an increase in +10 db means a times 10 increase in the actual number. However when dealing with the root of power quantities like pascals, volts, m/s^2... (pretty much everything that isn't watts or volts^2), an increase of 20 is needed for a times 10 increase in the quantity, usually an RMS (Root, Mean, Square) level.

Since a dB is a ratio, or the quotient of two numbers, and the quantity we are measuring is the dividend, what's the divisor, or the reference of the ratio? In each case it can be different, but for sound pressure level, the reference is the lower threshold of human hearing, which is 20uPa. This means that a person with absolutely perfect hearing cannot distinguish an acoustic pressure variation smaller than 20uPa. This, then, is 0 dB. 

The threshold of human hearing isn't a very good means of comparing sound on a day to day basis. Human breathing creates about a 10-20 dB sound pressure level. Normal conversations have a sound pressure level of between 40 and 60 dB. Most automobiles are between 60 and 80 dB. Hearing damage starts to occur at around 85 dB. 

What this means for the anechoic room at Orfield Labs is that when no other stimulus is occurring within the room, the sound level is more quiet than what a human can perceive. In fact, the background noise level in this room, at -9.4 dBA means that the sound pressure is about 1/3 of the pressure we could perceive. In a room like this, a person can hear a lot of things about themselves that they couldn't normally hear. Your heart beating, the gurgling of your stomach, creaks in your joints, and even the blood rushing through your own head. It can be said that when you enter this chamber, you become the sound...

So... Why build a room like this? What's the purpose? I'll let you know soon!