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!

-CMW

Friday, May 10, 2013

What goes into a sign?

Sine that is... a sinusoid... That is where we left off, correct? (Looks back at Up, Down, Up, Down...)

Let's go back a little bit first. What are sine (and cosine)? They represent a function where the output is the ratio of one of the two legs of a right triangle over the hypotenuse. You say, "This sounds a LOT like trigonometry," Well that's where a LOT of our math in engineering comes from. Sine is the ratio of the leg opposite the measured angle and the hypotenuse, which, when the angle is zero, the length of that side is zero. In this instance you really don't have a triangle, you have a line segment the length of the hypotenuse, but that's not a big deal. Maybe a picture is needed for this.
This is called the Unit Circle. It's made by changing the angle that the hypotenuse of a right triangle make with the x-axis, while keeping the radius one (hence unit). Looking at sine (the vertical component of the triangle we see that as we increase the angle, the ratio gets bigger until the angle measured reaches a right angle, 90 degrees, or pi/2 radian (we'll probably stick to radians a lot, since the math later will be much easier that way). At this point, sine begins to shrink again until and angle of pi, where it becomes zero. 

Now, on the lower half of the circle, we see sine grow negative, to a maximum of 1 at 3pi/2, and then shrink back to zero at 2pi, or 0 radians (same thing). If we were to plot each of these points as we went around the unit circle, we would get a plot of what we call a sinusoid. Wikipedia has a great graphic for this:

Now because the function's parameter, theta, is locked in to radians, we can adjust the scale of sine across other variables. This parameter is called the phase of the function. Commonly we use space and time variables (x,y,z, and t) to scale sinusoids across those domains. This is shown below:
The higher the frequency, the more cycles per duration of the domain the sinusoid goes through. In terms of acoustics, the higher the frequency, the higher the pitch of the sound we're hearing. For those music types, Middle C is about 264Hz. We'll talk about frequency more later. Because the shape of the sinusoid is being stretched over the domain we're working over (x,y,z, or t) we want to be able to measure that as well. If we measure the distance between consecutive peaks or troughs, we get the wavelength, often stated with the variable lambda:
Wavelength can bring on many discussions of speed of a wave in a medium (Speed of sound or Speed of Light), as well as a whole other realm of mathematics. Needless to say, we're going to hold off on that. Speed of sound will come soon enough.

Shifting a sinusoid can be done by adding a number to the phase, this then called a phase shift. Here's an example of that:
This will become very important later when we want to compare to signals, and when we want to talk about physical phenomena like resonance and electrical phenomenon like filtering.

I'm going to save talking about the amplitude of a sinusoid until next time, since there are a ton of ways to describe it, and getting the all straight is something that I know many people have a hard time with.

Sorry about the delay in getting this out, real life caught up with me this week, and I was super busy at work, which made me crazy tired when I got home, and therefore crashed early, when I should have been writing this...

Anyway, if this weekend isn't too nice, I'll try to get back to amplitude on Sunday! Cheers!

-CMW

Thanks to Wikipedia for use of all of these images. Without you, I would have had to generate them in MATLab. I believe that all of them are linked properly, so you can check out the various Wikipedia sites where they reside.


Thursday, May 2, 2013

Up, Down, Up, Down...

This is

y=sin(x), courtesy of Google.com. This is actually one of Google's cooler features, the ability to do simple plots from the search box. I highly recommend trying it out! 

No worries, touting the much awesomeness of Google is not why I'm here tonight.

The title describes it pretty well I think. There are many things that go up and down in our world: the light switch in your office, the bumps on the road, the hands on the clock (well if you just look at the motion of the tip of the hand, there's up/down there), and the function shown above. You don't just have to limit it to up/down, you can also say forward/back (like a playground swing), or left/right (like a pendulum and the tips of the hands of the clock, again).

If you think about sound, and one of the fundamental sources of sound, a speaker, you come to realize that the motion of the speaker is forward/back (depending on how you view it). The diaphragm of the speaker moves back and forth depending on the signal coming to it from the amplifier (Electrodynamics of a voice coil is another blog post...way down the road), and this in turn pushes the air molecules back and forth, creating a sound wave that propagates. This sound wave can reach someone's ear, where it can be heard (Might get to hearing at some point in the future, it's a bit of an interest to me).

So now we know what the motion looks like, how do we write that down? Typically words are great at describing fixed or static things, but nobody wants to type the position of a speaker diaphragm as it is moving. First, you would be writing until the object stopped moving. Second, no one can type fast enough while measuring the speaker diaphragm position in order to do it accurately. Third, even if you get the position correct, when was it correct? 

Luckily, us folks with a math background have functions! We can describe a wide variety of shapes, complex geometries, solids, curves, and surfaces using mathematical functions. The cool part, and the part I want to talk about on here a lot, is how to describe motion. This is, after all, the most fun with a piece of chalk! (If you know what that means, please feel free to comment! Guesses are welcome too!) (Nick M, you're not allowed to guess, neither are you Peter R, if you read this.)

I plotted sin(x), which is the vertical position of y, when y is the sin(x). This makes it valid for any two dimensional set of labeled axes. (Note I said labeled, this is a pet peeve of mine...) However this can be made just as valid for y=sin(.08*pi*t), where t is time, let's assume in seconds. (Yes, I know I added a parameter, I'll get there soon!) Let's also assume that y is something we can measure, like the position of the speaker diaphragm, in inches. This position can be measured with sometime like a laser used by a contractor to measure the rooms in a house, except really accurate, and it can measure it over a constantly repeated time span (Once per second... Ugh, Sampling, the topics keep piling up) This would look a little like this:



Here's the nifty part, we can use that measured function to replicate it, and in turn get an image of the original y=sin(.08*pi*t). In fact, we do it all the time! We measure tons and tons of complex things, record them, store them, transmit them, reconstitute them, and play them back (probably though a speaker). However in order understand the content of the things we measure, we have to compare them to some reference. Because the general motion of a lot of stuff is back/forth, left/right, up/down, a sinusoid is a great way to do this.

So what was with that extra bit I put into the sin() function above? I'll save that for next time...

In case you guys haven't noticed, I'm trying to do this Tuesdays and Thursdays, and I might sneak in a Sunday here or there, depending on if I'm home and how much other crap I have to do. I see people taking a peek, but still no comments (but a couple +1's on Google+). I know I'm pretty light on these topics (for some people...). If something gets too complex too quick, drop me a message on here and I can try to explain things better.

-CMW


Tuesday, April 30, 2013

The CAV...

A big hand for the folks down at Penn State, specifically the ones who work in the Center for Acoustics and Vibration. They put on a great spring workshop, which I just returned from (oh... about an hour ago). It was good to see my friend, Andrew Barnard, who is faculty down there, and to see what so many people in acoustics and vibration are up to.
I'm going to geek out a bit, because so much of the things I saw this past two days were so cool. One of the PSU researchers is working on ultrasonic helicopter rotor deicing. The cool thing about his setup is that he's proven it out. Here and here are a couple of Jose's papers, but the coolest thing isn't even in either of those papers. He build a rotor lab in a freezer... That's right, he can take data on a spinning helicopter rotor under conditions that will form ice. Not only that, he's also done all of the legwork to determine that the ice that forms in his lab is the same as what would form in nature! The dents in the walls of the safety cage where ice (and a couple of rotor blades, Jose admitted) made everyone on the tour know for sure that this lab isn't just for show.
A lot of other topics were discussed as well. Energy Harvesting/Structural Health Monitoring  (EH/SHM) was a pretty common topic. What this entails is using some sort of device to collect (usually vibroacoustic) energy  and powering a sensor or group of sensors. These sensors may in fact be for collecting vibroacoustic data, but they can also be used to measure fatigue, stress, or some other phenomenon on the particular structure of interest. A discussion panel about EH/SHM as well as Noise and Vibration Instrumentation (NVI) convened during the workshop.
I actually sat on the NVI panel, along with Andrew, Tom Gabrielson (who is one of the foremost minds of this time when it comes to instrumentation), and Wim Desmet from KU Leuven. We fielded questions ranging from the future of ultrasonic measurement instrumentation, to how to devise a high pressure transducer for very high tempertature in water (on a very small structure, something that each of us pretty much said would be really hard).
The first discussion this morning was also a very interesting topic, adaptive structures and noise control. George Lesieutre talked about how composite materials can be built such that they change shape (become stiffer) in one direction or another depending on if an electric field, magnetic field, or thermal transient is applied to the surface. George said that they were experimenting with such materials at the time, and that things seemed promising.
It was a very good couple of days, but now I'm exhausted. So I'm going to crash. I'll pick up on Thursday where I left off on last week's thoughts (Sinusoids... right?)

For those of you interested in checking out the stuff going on at the PSU CAV go here. In the next couple days all of the presentations will be posted, and you can get a better idea of what all went on.

Cheers!

-CMW

Thursday, April 25, 2013

So Noise...

I talked about it a little bit in the last post, but as my signal processing professor stressed, this is one of the most important definitions in signal processing, acoustics, vibrations, and image processing. That along with the reason sinusoids are studied (DOH, that's another blog entry) is the foundation of these sciences.

The definition I locked onto in the last post was that Noise was any unwanted sound/signal/vibration/data/image (plenty of unwanted images on the internet...). I also stated that this is a completely relative definition. In my lab, when I'm working on measuring some acoustic phenomenon, and my neighbor in the lab is listening to music through his ear phones so loud that I can hear it (with is horrific for his hearing) that music, to me, is noise, because it will be an unwanted artifact in my measurements. To him, it's part of how he focuses on his work, perhaps making him more productive... but also probably causing error in his measurements which makes it noise to him, as well!

It's also interesting that we redefine noise as the sound we might be looking for when trying to determine a problem with mechanical systems. For example: The noise my car is making when I hit the brakes. In the terms of that state of mind, when trying to diagnose that phenomenon, it now becomes the sound you're looking for rather than noise. In that case, if the engine is knocking, or an excessive amount of tire noise is present, those affects become noise, and the brake squeal becomes a sound phenomenon.

This means, by definition, noise is completely subjective based on the experimenter. Studies have been done for years trying to describe, in general, what noise is to the average person. The interesting thing is, that one of the things we typically call noise, namely white noise (defined as a random signal with a flat power spectrum), is actually perceived by most humans as a pleasant sound, the absence of which causes distress. This is why some people are uncomfortable when they leave the confines of a city and go out in the country. The ambient sound level, also called background noise, a very ironic term, is gone. These same people are very uncomfortable when in an anechoic room, because the only thing they have to listen to is all the stuff going on in their own bodies.

Humans, in general find blended sounds pleasing, and sharp sounds displeasing. Without getting too much into psychoacoustics (which is a real thing, go look on wikipedia), sudden sounds, impulsive noise, tonal noise, and poorly blended music all tend to have a negative affect on people. This is why sharp sudden tones (in the 1-4 kHz range, the most sensitive frequency range of the human ear), are used as warning sirens in buildings and on emergency vehicles.

However, to an experimenter, noise in your system is whatever you define it to be. You have to be very well versed in your system in order to diagnose this fact. If you're trying to work with an electric motor, and you've grounding issues, 60 Hz (or 50 Hz for those of you not in North America) is really bad. Not only is this going to be a frequency of interest, but it's also going to be a huge source of error in your measurements.

The last little bit of info I want to give tonight is this: Because noise is whatever the experimenter defines it to be, very few other people will be able to help you diagnose noise problems without you giving them a very well worded definition. The person who knows a set of data taken from a group of experiments is the person who took the data. This is why I prefer to take almost all of my own data, because then I can think backward and forward what all of the possible sources of noise might be. 

Tonight I'll leave you folks with a question... I spoke about sinusoids a couple times, why are they important? (Full circle... BWAHAHAHAHAHA!)

-CMW

Tuesday, April 23, 2013

Das Laute Lehrer

This whole, "Writing down what's on my mind," thing is somewhat of a new concept to me. I tend to mull over my thoughts in my noggin, and figure things out there. However, I've decided that I need a bit more of a creative outlet, so here I am!

Along with that creativity, I hope that people might learn something by reading this... And with a nod to the title of this post, you can imagine some of the things I might have to teach. No, it's not just a statement that I'm a loud man (you can ask my friends, it's a well known fact that most of them consider me the loudest person they know, to their chagrin), but the title speaks more to my studies of vibration and acoustics. I help design microphones for a living, but mostly I do a lot of measuring with many types of transducers. 

The window is open (because it's spring here, finally), and the traffic outside at this hour is pretty crazy (It's around 9PM). Some jerk at the traffic light down the road just felt the need to rev the engine to tell half of the apartment complex I'm in that he has a souped up motor. I hate to tell him that none of us care...

This leads to a statement I get from people a fair bit: That has to be a noise violation! How come nothing is ever done about this!

I am especially entertained to read the noise ordinance in the town I live. Specifically it is only mentioned in the zoning area, and then again with regards to specific things like howling dogs and drilling for foundations. Read below the definition of "Noise" per my village:

Noise. The sound-pressure level (SPL) as measured at the edge of a lot and which is produced by a mechanical, electrical or vehicular operation on the lot, where said lot is adjacent to a residential area, shall not exceed the average intensity of the street traffic noise in that residential area. No sound shall have objectionable intermittence, volume, beat, frequency or shrillness characteristics.

This isn't a very good definition for noise, but that's another blog entry. The one part of this statement I actually agree with is the last sentence stating, "No sound shall have objectional intermittence, volume, beat, frequency or shrillness characteristics." so I'll use that as my working definition of noise. To put it much more succinctly, Noise is any unwanted Sound.

Here's the next question, who determines unwanted from wanted sound? In the course of noise ordinance in a city, town, or village, the complainant. However this must be judged by another party, namely law enforcement, in order for some sort of reprimand to be administered. The problem is how does law enforcement determine if something truly is noise? How do you measure that?

Some communities (not mine, so I'm a poor example) define it very well. The definition above says that the SPL as measured at the edge of a lot (blah blah blah) shall not exceed the average level (Intensity is a completely different sound term, so I'm fixing it here) of the street traffic noise in that area. 

Here's the rub, that jerk I mentioned earlier, technically was part of the background noise, as defined by the noise ordinance of my town. Therefore, I can't submit a complaint against him, even if I could catch him.

There are a plethora of different ways this conversation can go from here, so I'm going to stop, and mull it over a bit, however I think the next things to cover are a couple of definitions. Not tonight however, I don't want to make these things so long that they are too boring, and I think leaving this as a bit of a cliffhanger (you should be asking, "Well what is a noise violation then?") will serve better in the long run.

-CMW