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