Phys 1240 Fa 05, SJP 3-1 (Haven't heard any feedback, + or -, about these lecture notes. If you think they're worthwhile, let me know. Alternatively, if you look at them and then think it's a waste of time, let me know that too!) This chapter focuses on "production of sound" (as versus propagation and perception). It's our first introduction to broad categories of instruments. Fairly qualitative and descriptive, it's got some useful concpets and vocabulary, and sets us up for more detailed study in a few chapters. (We're not looking to "zoom in" just yet!) The author tries to categorize sounds. This has some value, although frankly I think whatever categories you come up with, we can find interesting sounds that don't "fit" exactly. E.g., "natural" vs "artificial". We're going to be mostly studying artificial sounds (those produced on purpose, by us, for some reason, generally musical although it could be for medical or other research reasons) So, if Laurie Anderson records a buzzing bee and plays it in a concert, is that "natural" or "artificial"? What if she slows it down or distorts it to make sound cooler? Then there's "original" versus "reproduced". This has little to do with the sound itself, althoug we'll certainly be interested in the fidelity (honest, faithfulness) of recorded sounds. This'll come much later in the course! The author introduces "steady" vs "transient" sounds. Real waveforms are NEVER perfectly periodic: they start ("attack") and end ("decay"). The amplitude may be fairly constant while the sound is playing, or it may vary, fade, rise and fall... Last chapter when we looked at waveforms, we starting talking about issues of transience. If you look at figure 3.1 of the text, you see a note which is transient (it dies away after only about a dozen cycles), yet it DOES have a characteristic period... the period is the time for ONE little cycle to occur, not for the whole transient to die away. (In that figure, the period looks to be about 0.5 mseconds, which means a frequency = 1/Period = 2000 Hz. So you'll percieve a note of 2000 Hz, which lasts for about 5 ms. A "high click". (Does this make sense? Ask if you don't get it!) Of course all sounds are ultimately transient, but some are MORE transient than others! (Think of the difference between saying the letter "P" and saying the letter "E" while holding it... The former is definitely transient, the latter can be quite steady (for awhile!) How do we feed energy into these two letters? How do we sustain the "E"? This is something we'll want to talk about, to make sense of instruments (and distinguish e.g. percussion from wind) This leads us to considering "families" of musical instruments. Again, this can be quite arbitrary. Since this is a physics class, we'll try to look at categories that have powerful, physical commonalities. (like wind vs string) But even then, a flute is quite different from an oboe in the physics involved... In this chapter we'll do a quick survey of "percussion", "wind", and "string", and come back to all of them later in more detail. As the author points out, it can be quite vague which is which - is a piano percussion or string?Phys 1240 Fa 05, SJP 3-2 Percussion: Remember, ALL sounds arise from pressure waves in the air. A vibrating object (with some frequency f) will push back and forth on the air, making a sound with that frequency... If you whack a solid object, generally you put some energy into it, and it starts to vibrate. It may have ONE natural resonant frequency, but in reality, all real objects have multiple possible vibrations. You will generally excite them all when you hit it. Waves (disturbances) will travel back and forth across the object. If it's solid, they might travel THROUGH it, or they might be wiggles of the surface. For a drumhead, you will generally make a surface vibration, a disturbance of the surface that travels out in 2-dimensions, bounces off edges, passes through itself, and ultimately damps away. (See Fig 3.2. of the text). Large amplitude vibrations, as we've seen before, tend to move more air (make bigger "pressure amplitudes"), and thus sound louder. The more efficiently you can "couple" the wiggling surface to the air, the louder and clearer the sound will be. If the size of the wiggling object is small, you generally won't get much sound out, bigger surfaces move more air around (make louder sounds) All objects have internal friction, their vibrations die away. You can design instruments to vibrate for a long time (like my Tibetan bell!) or very short (e.g. a xylophone bar). It's a question of how efficiently you dissipate the vibrational energy into heat (or into other objects like the table or floor the instrument sits on) No bell can ring *forever* (because the sound waves themselves carry away some of the stored energy!) Tuning forks vibrate for a long time and are very quiet. You have to hold it right to your ear to hear it. They have relatively little losses, have a very well defined resonant frequency. If you DO want to hear them, you need to put them onto a bigger surface (a box works well) which can vibrate, and couple to the air better. That makes it louder... but it also means the sound becomes more transient, it dies away faster (because you're losing lots more energy to the produced sound!) Drums USUALLY don't have a very clear pitch, although you will have a sense of "high" or "low". There are several reasons for this. First (and most important), the shorter a sound lasts, the less well-defined the period is. (There's a sort of "mixup" in interpretation between the time for a cycle, and the time that the whole disturbance lasts..) Second, complex surfaces can produce more than one frequency at the same time (!) But the shortness of the sound really dominates the story. Even if you record a PERFECT sin wave, and play it back, if you only play one or two cycles (which might last only a tiny fraction of a second), you hear a little "click", and it's very hard to say what tone it is. There's some mathematics behind this as well as just "perception" issues - we'll come back soon and talk about why this is - what makes the "color", "quality", or "timbre" of notes. Whether an object sounds high or low is determined by a bunch of things, all of which lead to the "resonant frequencies" of the object. For a mass on a spring, we learned that the natural Frequency gets HIGHER if the object is stiffer. We also
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