Phys 1240 Fa 05, SJP 5-1 Chapter 6: The ear, and its response (Loudness) This chapter has a lot of cool stuff in it. I'm not going to cover it all in lecture - if you're interested in the connection of physics and physiology and psychology, I encourage you to read it! I'll talk about the ear in class, at a level slightly simpler than section 6.1. Don't memorize the names of these various parts (unless you're into that kind of thing), what I want you to take away is a sense of the physical purpose of a few key elements, as outlined below 1) The OUTER EAR: is the channel and structures that help bring sound waves to the eardrum. This part of the ear ends at the eardrum - a thin membrane that vibrates when there is excess (or under-) pressure on the outside. (The inside, i.e. the middle ear, is suitably "separated" from the outer ear that changing room air pressure causes the eardrum to vibrate. ) 2) The MIDDLE EAR: Connects to the outside world through the Eustachian tube to your throat. (That's why swallowing, or holding your nose and blowing, can help equalize the pressure in your inner ear and the outside world when you're flying. When would you want to "blow", on takeoff, or on landing? Why?) The middle ear has this awesome collection of little "levers", three bones (hammer, anvil, and stirrup, colloquially) which transfer the vibrations of your eardrum into the inner ear. The principle of the lever is that you can use one to change a small force into a bigger force (the PRICE you pay is that the bigger force acts over a smaller distance - so you don't get any extra WORK out of it) The next "link" in the chain is the "oval window", which separates the middle ear from the inner ear. There's a second key piece of physics at work here: the oval window is very small, much smaller than the eardrum (20-25 times). So, you're applying even MORE force on an even SMALLER membrane, so we've really ratcheted up the pressure by this point. So basically, the middle ear has taken pressure waves from the outside, used these levers and the different areas on the two ends to make a lot MORE pressure variation on the "oval window". Since intensity grows like (pressure)^2 (remember?!) we have REALLY ratcheted up the intensity by this point, it's like we have a little "amplifier". Very cool - it's all mechanical, and it doesn't violate conservation of energy, we're just "concentrating" the energy... The text argues that all together, this middle ear business amplifies intensity by about 100 to 1000 times , which is 20-30 dB improvement in sensitivity. There's lots of other nifty things going on - e.g. muscles which are designed to prevent damage from overly loud sounds. 3) The INNER EAR: The cochlea is a coiled up region that contains mechanisms to convert pressure variations into electrical signals. That's the big idea here - converting the physical signal into something your brain can interpret! Part of the key is the 20,000 "hair cells" which get disturbed by motion, and initiate nerve signals. At low frequencies, the fluid in the inner ear can transmit motion all the way towards the back, but higher frequency sounds tend to cause more response closer to the front (window) So, this provides a simple physical mechanism to distinguish frequencies. The text goes into more details, but I think this is what I pulled out as the key physical ingredients. Quite a remarkable little device. Damaging these little "hair cells" is part of what causes hearing loss as you grow older, or get exposed to too much (or too loud) sound, by the way. The box on page 95 of the text has a little more info about this, but some web hunting might be worthwhile if you're interested!Phys 1240 Fa 05, SJP 5-2 The next section of the text talks first about the range of hearing. We've discussed this in round numbers (20-20,000 Hz for normal hearing), but in reality, it's more complicated. For one thing, you are more sensitive to certain pitches than others (we'll get back to this soon), and esp. at the low end, you can "detect" low frequencies but it's not clear if you're "hearing" or "feeling" them! The text goes on to discuss "JND" or "Just noticeable difference". It's a fun psychology question - what small differences can normal humans detect in frequency or amplitude? Think a little bit about how you might set up such an experiment, it might not be the same as what the book talks about! They discuss a scheme involving "two-alternative forced-choice" where you hear two sounds and choose which is (e.g.) louder or softer. They give random pairs, and figure out where most people can reliably determine which is louder (or higher pitch) Fig 6.6 makes the case that a SIL (that's "Sound Intensity Level", or decibel level) difference of about a couple of dB is pretty much the JND for normal people at 1000 Hz at a normal (40 dB) level. Changing the pitch will change that graph (you won't be so sensitive at very high pitches, for example) and changing the "base level" (dB of one or the other tone) will also change the results. That's what fig 6.7 shows. Pick the "1000 Hz" curve: if your "base tone" is 40 Hz, your JND is a bit over 1 dB, that's how much DIFFERENT it has to be to notice a difference. Up at 80 dB, the JND has gone DOWN, you are able to tell a difference of only 0.5 dB. This surprises me, I would have thought that as the base gets really loud, it would become MORE difficult to detect a small change. Huh! Of course, the graph doesn't go up to painful levels, so maybe it's not crazy. (At SOME point I claim those curves would have to head back up again? Do you agree with me?) Fig 6.8 shows how changing the pitch impacts your sensitivity to pitch. E.g., at 80 dB (solid curve) as the frequency goes UP, you steadily become less sensitive to pitch (your JND in pitch goes up, which means you need MORE of a change to reliably notice it) Do you see how the curve TELLS you this? (This says if you play a bit "off key" at really high pitches, apparently people are less likely to notice!) Do you see why JND in fig 6.8 is given in Hz, but it's in dB in fig 6.9? They measure the "just noticeable difference" of two DIFFERENT quantities: loudness in Fig 6.7 (dB), and pitch in Fig 6.8 (Hz) You could imagine all sorts of variations on this - what happens to your JND of loudness as the pitch goes up? (That's what the various curves in 6.7 are trying to show, do you understand how?) Are you making sense of what
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