misc. notes re: light and sound ------------------------------- Date: Sun, 21 May 2006 15:54:34 -0400 (EDT) Subject: Misc. Notes re: Light & Sound > TUNING FORKS: lower freq. fork might gen. waves on water "512 C" = 512 cycles (C), not a pitch of "C" > SPEED GUNS (RADAR & LASER) Does radar gun have audible mode? (If so, then use to demo Doppler shift.) Yes! 35mph; 65mph. Operates in two modes: stationary mode (gun isn't moving); moving mode (gun is moving as well as target vehicle). Emits "V"-shaped beam; range ~300 ft. "Sees" fastest/biggest (biggest trumps fastest). Laser gun -- thin beam that pinpoints car; measures both speed and distance. salvage only notes re: Doppler shift from Phil's msg., "URGENT" (dated 18 MAY 2006) Google Search: police radar gun How Radar Detectors Work http://electronics.howstuffworks.com/radar-detector1.htm Radar gun http://en.wikipedia.org/wiki/Radar_gun > AUDACITY Misc. Notes re: freeware "Audacity" (equivalent industry std. software = ~$500.00) Click outside first track (in grey workspace); make new track. Play or save automatically overlays all tracks. laptops should have built-in mic; in Audacity, click record button (red dot)/stop button (red square) guidance re: distance betw. sound source and mic: as loud as possible w/o clipping (a VU meter of 6dB less than max is good vol.) Normal range of human speech: 300 - 3000 Hz Editing speech files: select segment of file; press "Z" key (shifts end-points of selection to "0 crossings"); cut-and-paste (or copy-and-paste) as necessary. == Date: Thu, 18 May 2006 08:59:30 -0400 From: Phil Wherry To: Walter Sanford Subject: Re: URGENT Walter, First of all, this sounds really cool - I'm really glad that you're going to get these demos; they'll illustrate the principles and will be interesting for your classes, I'm sure. Short summary: the radar and laser guns are directly measuring the speed of the tuning fork tines. The sound you hear from the tuning fork is just a byproduct of this motion, which sets up a pressure wave in the air. Longer version: The tuning fork works because the tines of the fork really are moving. The construction of the fork ensures that they'll move at a predictable frequency (which is why tuning forks always produce a particular note). The tuning fork does produce sound waves, but it does that by moving back and forth in the air, which sets up a pressure wave; i.e. sound. The metal fork is also reflective to both radio waves (radar) and light (laser). While the amplitude varies (it'll be at its highest right after the fork is struck, then will diminish to zero as the tines come to rest), the frequency is constant. Looking at the fork from the edge, this means that a tine will be moving toward you at speed X, then away from you at speed X, then back toward you at speed X. As the vibration decays, the distance the tine moves in each cycle will diminish, but each cycle will take the same amount of time--which means that speed X is constant throughout the decay process. (To be extra-precise, what's being measured is the peak speed of the tine; think about the motion of a pendulum--the peak velocity is as the pendulum bob crosses the zero marker. That velocity (but not the amplitude) will always be the same for a given pendulum length.) You know about the Doppler effect: a wave reflecting from a moving object is compressed or expanded depending on whether the object is moving toward or away from you. That's how radar and laser calibration works. In the case of radar, a beam of RF energy is pointed at a moving object. The motion of the object will shift the frequency of the RF signal up or down a little bit depending on whether it's moving toward or away from the radar unit. The difference in emitted frequency and received frequency tells you the speed. Laser works the same way; what's being measured is the difference in frequency between the emitted infrared laser beam and the reflection from the vibrating tine. As an aside, this is how the speed of celestial objects is measured: since things like hydrogen have distinct emission and absorption spectra, you can compare, say, the emission spectrum of a distant galaxy with the known emission spectrum of hydrogen. The characteristic spectral lines will be shifted up or down in frequency (blue shifting and red shifting, respectively), which tells us whether the object is moving toward or away from us--and how fast. Phil Walter Sanford wrote: > Phil, > Officer Goodley is visiting my classes today. He's going to demo both types of speed guns: radar; & laser. Here's my problem: radar is light; so is laser. Yet a tuning fork -- that produces sound waves -- is used to calibrate both types of speed guns. How is that possible?!? I don't wanna ask Officer G. -- in case he doesn't know the answer, it may embarrass > him. So I turn to you for help. - WBS == > One of the big questions re: sound I never had a chance to ask is why a > sound recording of say speech or music looks so chaotic, at least in > contrast with a recording of a single tone. 3. Speech and/or music looks chaotic because it's not a pure tone--it's a mixture of several tones, each of which may not be a pure sine wave to begin with. These waves interfere with each other constructively and destructively to produce the final waveform. I uploaded a new file (three-waves.wav) to your "extras" directory so you can see how this work. I made this file by producing three sine waves in Audacity (middle C, E above middle C, and G above middle C). Each wave was done at an amplitude of 0.3 (so that even with three peaks added together, the result won't exceed one [which would cause clipping and therefore distortion]). The resulting waveform is the sum of the component parts, and it looks much more complicated even though it's still relatively simple to produce. Also, I'm planning to order some tuning forks. Please take a quick look at the following URL to let me know which one(s) or set you think would be best. I'm leaning toward the "Concert Pitch Tuning Forks." - WBS http://www.sargentwelch.com/product.asp_Q_pn_E_WL3244C%5FEA_A_Concert+ > Pitch+Tuning+Forks_E_ 4. The tuning fork set you pointed out looks good to me! - PSW == Date: Tue, 09 May 2006 12:34:46 -0400 From: Phil Wherry To: Walter Sanford Subject: Re: What Do You Think? > O.K., let me see if I've got this straight: For every crest and trough > along the transverse wave or sine wave (and there are LOTS of VERY > CLOSELY-SPACED crests & troughs), you're saying the crest of a wave > represents a compression and the corresponding trough represents a > rarefaction, right? Correct. It's actually most correct to say that the crest represents one extreme (compression/rarefaction) and the trough represents the other, since you don't know (absent any other information) how the conversion from pressure to voltage works. > Similarly, the crests would represent maximum pressure/voltage; the > troughs minimum pressure voltage -- right? Most likely correct. It would be perfectly valid to construct a system, though, where the voltage drops as pressure rises; in practice, though, there's generally a direct mapping. Note: if you want to be completely correct here, there are some subtle differences between microphone types that impact what's actually being measured. Condensor microphones measure pressure, whereas the more common dynamic microphones actually measure velocity. At audio frequencies, these distinctions don't actually matter much, but they'd become more important if you were measuring absolute pressure or very low frequency signals. You don't need to worry about this, I suspect, but I can explain in more detail if it ever comes up. > IMO, the author of the lab is thinking the closely-spaced sine waves (that > result when you make a "puh" sound) are the compressions, while the places > in between are the rarefactions. If in fact that's the case, then much of > the lab is based upon a fundamental misconception that should be > corrected. Please advise before I make a fool of myself. > The plosive sounds ("P", "B", etc.) all start off with a single very large positive pressure release. You'll see a single huge spike on your graph if you're using high-quality instrumentation (and if you don't go past the measurement limits of your equipment). Closely-spaced peaks mean high frequency and nothing else. As an aside: the "P" and "B" sounds cause a lot of problems when using sensitive high-quality microphones. When you're out here to visit, I'll show you the "pop screens" that we use in order to diminish the effects of these large pressure peaks on sensitive microphones. > BTW, the pieces that you & I co-created worked wonderfully to make the > same points, only more clearly. Several kids (musician types) copied the > URL for "Audacity." Good stuff! Good to hear! Audacity is a pretty cool application; the fact that it's free just makes it that much better. Phil == Date: Mon, 8 May 2006 00:10:19 -0400 From: Phillip Wherry To: Walter Sanford Subject: Re: What Do You Think? By the way: here's a link to a photo of a microphone where you can clearly see the diaphragm that's displaced by the pressure wave to generate the voltage. http://www.neumann.com/img/photosGraphics/Zooms/U89i_Z.jpg Remind me when you're visiting the Institute and I'll show you an actual microphone where the sensing element is visible like this. Phil On May 7, 2006, at 11:08 PM, Walter Sanford wrote: Phil, FYI, I uploaded the pages from the Teacher's Resource Guide for the "Sound's Cool" lab activity. I'm concerned by the fact the TRG says repeatedly that sound graphs show compressions and rarefactions -- please correct me if I'm wrong, but they don't, right? FWIW, this wouldn't be the first time the TRG featured incorrect info; at least the MS science coordinator is responsive to suggestions for improvement. Frankly, I think the activity is a BIG MESS (needing LOTS of clarification/focus/revision); please take a look-see and let me know what you think (before I contact ISD). Thanks! - WBS http://www.wsanford.com/~wsanford/gr8ps/03_purple/08_sounds_cool/ == Date: Mon, 8 May 2006 00:03:20 -0400 From: Phillip Wherry To: Walter Sanford Subject: Re: What Do You Think? Actually, the graph does show the effects of compressions and rarefactions. Though there are a couple of different ways to do it, microphones work by converting the pressure wave into a voltage. So one way of looking at this is that the graph represents the pressure detected at a particular moment in time. So for a typical microphone, the maximum positive value measured corresponds to the highest compression, and the maximum negative value measured would correspond to the greatest rarefaction. The thing to remember is that once you've recorded data, you're looking at a graph of the sampled values rather than the pressure wave itself. It's fairly trivial to convert from the graph back into pressure waves (using a variable voltage and a speaker), though. Phil On May 7, 2006, at 11:08 PM, Walter Sanford wrote: Phil, FYI, I uploaded the pages from the Teacher's Resource Guide for the "Sound's Cool" lab activity. I'm concerned by the fact the TRG says repeatedly that sound graphs show compressions and rarefactions -- please correct me if I'm wrong, but they don't, right? FWIW, this wouldn't be the first time the TRG featured incorrect info; at least the MS science coordinator is responsive to suggestions for improvement. Frankly, I think the activity is a BIG MESS (needing LOTS of clarification/focus/revision); please take a look-see and let me know what you think (before I contact ISD). Thanks! - WBS http://www.wsanford.com/~wsanford/gr8ps/03_purple/08_sounds_cool/ == Date: Sun, 7 May 2006 01:00:10 -0400 From: Phil Wherry To: Walter Sanford Subject: Re: Sound Forge Equivalent > Good to chat with you this evening -- I got lots of good ideas for > "juicing up" the sound lab demos on Monday! Great to hear! Would you please remind me of the name of the freeware app you used to create the file, eight-notes.wav? Also, if you have time, then please send me some step-by-step directions re: how you created the file. I might like to create one or two additional files that play a single note. - WBS The audio editor is called "Audacity" and is available from: http://audacity.sourceforge.net/ Generating the audio is trivial; once you have the application open, just choose "Generate | Tone" and follow the prompts. Also, why are sound graphs called sine waves? Ignoring the fact that some sounds are square waves (I have NO IDEA how that's possible!), couldn't some sounds make either cosine or tangent waves? Translation: What's the relationship between sound and the sine function? - WBS The "sound graph" is actually called a "waveform." A sine wave is just one of many possible waveforms; it just happens to be a pure tone that's pleasing to the ear. It's called a sine wave because it's the waveform that results if you take the sine of an ever-increasing X value. Many natural processes (such as a vibrating tuning fork) produce waveforms that look like this. The cosine function could be used to generate a wave, but the waveform is the same shape as the sine wave so it doesn't have a different name. The tangent function has a number of values that are undefined (division by zero), so it's not really a good candidate for making a waveform. Square waves are a different type of waveform; Audacity can generate these too by selecting different options in the generate tone dialog box. You'll find that they have a distinctly different sound than the sine wave. You can also generate a white noise track in Audacity; that consists of a set of random samples, so there's no pattern at all to the waveform. If you zoom in enough with Audacity, you can see the individual samples (data points), so it's a powerful way to visualize things. It's instructive to open up an MP3 file and then zoom way in to see what it looks like; you'll see definite (though not pure) sine wave patterns throughout. Keep in mind that since Audacity is free, this is something you can have your students try either at school or at home without having to worry about software licenses! Phil == Date: Sat, 6 May 2006 21:49:26 -0400 (EDT) Subject: Sound Recording: Longitudinal- to Transverse Wave Conversion microphones convert pressure waves to voltage output of mic is voltage capacitor: two plates w. (nylon) insulator in between diaphram = flat condenser plate connect rod to diaphragm; add pen to rod. rod will trace sine wave on scrolling piece of paper! http://www.wsanford.com/~wsanford/gr8ps/03_purple/08_sounds_cool/extras/ eight-notes.wav (0:07 sec.) http://en.wikipedia.org/wiki/Piano_key_frequencies ~262 Hz (C4, middle C) - 523 Hz (C5) Rule of Thumb: sample at 2x max. frequency, e.g., 1200 samples/second practical application: Audio CDs 44,100 Hz = ~2x max. human hearing 44.1 KHz = ~2x max. human hearing == Date: Sat, 6 May 2006 20:55:50 -0400 From: Phil Wherry To: Walter Sanford Subject: http://en.wikipedia.org/wiki/Piano_key_frequencies http://en.wikipedia.org/wiki/Piano_key_frequencies