Archived Questions and answers for Phys 1240
From CAPA #9
In question 6 of CAPA set 9, what is meant by "increasing/decreasing
the resonant frequencies"? Does this mean the number of frequencies
(f1, f2, f3. . .) or the frequency of each resonant frequency (Hz). Or
something else? Thanks!
Yeah, guess I worded it kind of awkwardly. What I meant was, "do the
particular numerical values of the frequencies of each of the harmonics
increase" (so if you think that you started with f1, 2f1, 3f1 = (say)
110 Hz, 220 Hz, 330 Hz, ... and then you slide out the trombone slide, will
the frequencies now all increase (to, like 120, 240, 360, ...) or does something
rather different happen - like they all decrase (to, say, 100, 200, 300...) or
maybe only the fundamental shifts, (like 100, 220, 330,...) or something
Does this makes sense? Please let me know if it's still ambiguous to you!
From a recent online participation
CAPA helps but sometimes I think on the long answers I get to
explaining things wrong and then never fully understand what the answer
should have been. I know we get feedback through E-mail but it would
be nice to have a general answer on the website
There IS a general answer to every CAPA AND long answer question,
posted as soon as the set is due! Just go to the "solutions" link,
you'll find them for the whole term. Sorry you didn't notice those (or
ask) earlier, I've mentioned it in class several times, guess it must
have slipped by you!
From the lecture on hair cells
a nice fun demo of a hair cell that always cracks me up .... enjoy !
From the participation questions
I dont understand what the relavance of the harmonic series is. Is it
just another term for a musical scale?
No... it's ALL the frequencies you hear when you play a SINGLE note (or
so you thought!) on an instrument. One tone (one fundamental) is always
really much more than a pure sin wave - many different resonant
frequencies, all "superposed" on top of each other. That's the harmonic
series. We'll talk about it more in class - it's MORE than just a
musical scale (and also LESS). Going from F1 to F2 skips ALL the notes
in the scale, it jumps up an octave. Then F3 skips a bunch (it's a
musical fifth), then F4 moves you up to the next octave. As you go
higher and higher, you fill in more and more notes. At some points you
catch all the notes in the (western) scale. As you keep going, you
start adding new tones that are NOT part of the western scale, you
won't find them on any piano! But we're getting just a little ahead of
ourselves, hang in there!
hi, i had a question about this week's capa. on number 4...the one
about 2 successive higher harmonics of a vibrating string, what is the
frequency of the fundamental. i thought it would be the formula
f=n*v/2L, but there aren't enough variables given in the problem to do
it that way. and i think L would be the same, but V changes for each
of them. i'm stuck...am i going in the right direction? thanks
Yes, you're on the right track. It DOES seem like there's not
enough info (but there is...) There's also one little thing wrong in
what you said: I would argue that for a given string, L is fixed (it's
whatever it is) and so is v (which depends only on the tension, mass,
and length, all of which are... whatever they are!)
So you have this given string with some fixed (unknown) v and L, or to
put it another way, some fixed(unkown) fundamental "f1", and so there's
this "chain" of frequencies, f1, 2f1, 3f1, 4f1, 5f1, and so on. I'm
only giving you TWO of those, that are "successive" (like, maybe 6f1
and 7f1, or perhaps it's 199f1 and 200 f1. I'm not telling where they
are in the chain, just that they're successive, i.e. one after the
other) So you don't know f1, and you don't know WHERE in the chain they
are, but you do know they're successive... Just puzzle over it a bit,
does this give you enough info to figure out f1?
Hope this helps.
The last participation question
Check out the answers your classmates gave to my question about "the
physics of sound and music" in your life!
Ode to the Ossicles
(Allison and Susan Sekuler)
Every creature, live or fossicles,
cannot hear without their ossicles.
Except, as hard as they may wish,
there are no ossicles in fish.
You might think this quite unfair,
but fish do not live in air,
so they simply do not care.
Air flows through the middle ear
'til a cochlear impasse does appear.
From air to liquid, sounds abate,
hence the ossicles must vibrate.
From the ear drum vibrations play,
setting the ossicles on their way.
Hammer moves anvil moves stirrup and so
sounds amplify at the oval window.
Now sound increased many-fold
enters the cochlea strong and bold
So that you may hear this story told.
>I feel like I do well in understanding the CAPAs but when it comes to
>the tests I feel completely lost (though I study) do you have any suggestions?
I don't have any simple "silver bullet", but can make a few generic
suggestions. If you want to come talk with me about the exam (bring
your copy if you have it) I'm happy to go over it with you, that might
help us get a little more specific. Also, my exams are hard - getting a
70.5 puts you in the top half of the class (!) But certainly no matter
what your score is, we should always be able to help anyone who wants
to, to do better...
Generic suggestions: do you study by *looking* over old things (CAPA?
lecture notes? powerpoints? concept tests? textbook?) or *working*
through them? Do you try to anticipate exam questions by "twisting"
CAPA or concept tests a little, trying to think of what the underlying
big idea was, and how I might vary it for the exam? Do you work through
CAPA on your own or are you getting help? Nothing wrong, at all, with
the latter, except that you do then have to be careful that you're able
to do fresh, similar problems entirely on your own. This stuff
generally makes a lot of sense when someone else, like me, is
explaining it, but to solve novel problems you have to go a step
further, figuring out what the key ideas and tools are that you need to
solve a problem that isn't quite the same as one you've done before.
When you look back over the exam, do you feel "dang, I knew that, what
a dumb mistake" or do you feel "huh, that's a hard question, I really
didn't know that". The strategies for studying would be kind of
different in those two cases, the first being more about test-taking
skills, the latter more about studying skills. Do you have a study
group that you ever work with? (*Explaining* the ideas from CAPA
or concept tests to someone else is huge in terms of your own
understanding and clarifying of it. If *they* don't get it, it might mean
that *you* don't actually get it yet, and can help both of you dig a
little deeper. )
Hope this helps a little - like I said, happy to meet with you during
office hours (or other times, if none of my hours works for you). See
you in class!
Decibels, so confusing at first!
I have a question about #2 on the cappa homework...I know we figured
out the intensity for the numbers in increments of 10 but what about
numbers inbetween...like 47??? Thanks!
This was the topic of the *very* end of the lecture on Tuesday, we'll
go over it more today in class. Basically, you need to use Table 5.1 of
your text, which helps you "interpolate" between increments of 10 for
dB (or pure powers of 10 for intensity ratios. ) Given dB, the trick
is to express it as "some number of tens + some number of ones", and
then figure out the intensity ratio for each of those separately, and
then multiply those!
23 dB = 20 + 3 dB, so that's two 10's, and 3 ones". The two 10's (i.e.
the 20 dB term) corresponds to a factor of 10^2 = 100 in intensity. The
3 ones corresponds (from Table 5.1!) to a factor of 2.0 in intensity,
so together we have 2.0* 100 = 200 times the intensity.
Hope this helps!
Check it out, a question that's NOT about CAPA 5!
A student wrote:
I was googling the internet and found a fun toy, or at least fun to me.
I like plugging in numbers and seeing what happens. Anyways it is a
Decibal-intensity converter at
I was actually really suprised that they would have something like this
online. And no need to fear, I know how to convert them w/o this
online source, i just think it's fun to play with.
See you on thursday,
I like it! So did you notice that multiplying *pressure* by 10
makes the dB go up by 20? (Does that make sense to you? I had to think
about it for a second, but sure enough, that's right)
And more CAPA 5 questions
Hi Professor Pollock, I'm having trouble with CAPA # 3, which is asking
us to convert 2E-2 intensity to dB. Here's my reasoning, which CAPA's
saying is wrong: 2E-2 equals 200. I divided 200 into powers of 10, to
get 2 X 10 X 10. Since there are 2 tens, the dB equivalent would be 20.
And an intensity of 2 would equal 3 on the dB. So the answer would seem
to be 23 dB. CAPA's not accepting this anwser, and I was wondering if
you could point out where my reasoning goes wrong? I'm also confused,
because this is the exact same problem as an example in your lecture
notes, and the answer was 23 dB. Thanks!
Couple things. First of all, 2E-2 is equal to .02, not 200. (That minus
sign after the E means "times ten to the MINUS 2 power")
Also, when you are given a sound intensity (in metric units, W/m^2),
and want to come up with an SIL, there's a convention (described in my
notes and in the book) that you're really talking about the decibels
above "I0", the "minimum (or threshold) intensity ", which is 1E-12
W/m^2. So before you start converting to dB, you really need to first
find the ratio of the given intensity to this minimum intensity. That
ratio (which is now a pure, unitless number) is the thing which you
convert to dB in the manner you so clearly described below.
If you look back at the lecture note example, you'll find that I had
given you a ratio there (the number 200 was already the ratio of two
intensities), not an "absolute intensity", which is what I've given you
in CAPA. (When given an absolute intensity, we almost always first find
its ratio to I0, and THEN go to dB.) dB really describe a DIFFERENCE
(or relative) loudness, and so if you aren't explicitly given two
different levels to compare, you're a little stuck, and by convention
just compare with I0, the human threshold...
Hope this makes sense! Please ask if it doesn't!
Yes, more CAPA 5 question(s)
Hi Professor Pollock, I'm pretty lost on CAPA 5 number 6. Although I
could get all the other questions this one has me at a lost. I
understand that if amplitude doubled then intensity is increasxed by a
factor of 4. Well if this is true then why doesn't the sound increase
to 40dB. Because if intensity is going up by a factor of 4, then would
the decibals increase at the same ratio? If you could help me out that
would be wonderful, Thank you.
Let's see. I'm with you that if amplitude doubles, then intensity goes
up by 4x.
But let's think more about the dB. dB and intensity are related, but
not in a linear way...
Remember, ADDING 10 dB means MULTIPLYING intensity by 10x. So adding
40 dB (which is what you thought the answer should be) would mean
multiplying intensity by 10x10x10x10x (that's 10^4) = 10,000x So
adding 40 dB makes the sound 10,000 times more intense (an awful lot
more than 4x!) So I think the answer will have to be a lot less than 40
In fact, 4x is LESS than 10x. So it seems to me that we must be going up by
LESS than 10 dB.
If you haven't looked over my lecture notes for Chapter 5, check 'em
out on the reading page. (Not the power point notes, the lecture notes,
though both might help). Around page 5-5 and 5-6 I talked about dB and
intensity, did an example a little like this one you're asking about
(for 2x, instead of 4x) and many more related examples after that. Take
a look, see if they help. If you're still confused, this kind of
problem is probably easier in person than by email, though if you're
really stuck and can't make any of our office hours, feel free to email
Good luck, hope this puts you on the right track.
CAPA 5 question(s)
Hi - I had two questions on the CAPA set for this week. First, for the
megaphone problem: would the opening near your mouth cause diffraction
because it is smaller than most frequencies of the human voice? But
then, would the megaphone body (and the other end) simply allow the
diffracted sound to act as a direct sound then, because it is so much
larger and won't diffract the sound again?
I think you're heading in the right direction, you just need to
sharpen it up a little! It's a subtle question, kind of a little open
ended. I think your ideas about diffraction are the key part of the
answer. You might be able to quantify just a little -your point seemed
to be (?) that the megaphone is effectively making the "opening to the
world" MUCH larger than your mouth. Think about how high pitched speech
is - can you make a guess, or look up, what frequency (and therefore,
wavelength) a typical "shouting voice" might have, and compare it with
the size of the megaphone opening? I think you're on the right
track, hope this helps.
My second question is on problem 8. If you put two violins together,
wouldn't you expect the sound to double? So why wouldn't the answer be
130 dB? Or would it be a square of the change, and therefore quadruple?
When you say "the sound would double", what does that mean,
exactly? What *about* the sound is it, that doubles? The frequency
doesn't double, right? but something does (what quantity, exactly,
will double?) Is it amplitude? Intensity? Decibel level? The answer
should make sense to you (in the end) - the text and my notes talk
about it (and we'll be covering this in class too) Once you've decided
this, then you need to think about what the relationship is between
decibels and other things (like intensity, or amplitude, or energy...)
Because for sure, doubling the energy transferred by a sound wave does
*not* double the decibels! That's a really key point (so no, two 65 dB
violins (65 dB is not all that loud) definitely don't sound like 130
dB, which would be painful to the point of rapid serious damage to your
ears! Otherwise, string quartets would kill people :-(
So... take a look at my online notes, especially the last couple of
pages, where I talk pretty explicitly about this kind of problem. A
word of caution - you might find an example that's TOO close to the
CAPA (which makes the CAPA kind of easy, but takes away a lot of the
need for you to think it through and make sense of it), so if you find
yourself "following what I'm doing" too much, try making up some
different questions and seeing if you can do them, you're welcome to
check with me or a TA. As always with CAPA, it's not about "getting
an answer" and then stopping thinking about it - CAPA is a guide to
help you figure out what topics you should be making quantitative sense
Hope this helps, see you in class.
hello, I am a student in your 1240 class. I am making a video for
another class and am trying to have the audio have a 50s archival sound
about it. I have the hissing and some of the artifacts running over
top of the sound I recorded which helps. I have the abilty to alter
about anything possible in the editing program i am using but do not
know what ot do. I was wondering if you or jeff knew what qualities
made the sound recordings in 50s educational video sound as they did?
In regards to frequency and reverb please be as specific as possible.
(From Jeff Merkel, our resident audio expert!):
We often do what your trying to do, "low-fi"ing stuff. We usually do it
to sound like an analog record, but old tv should be easy enough as
Basically older stuff had very limited bandwidth, so the first thing
you should do is put a band-pass filter on it or use an eq with a steep
slope and turn the treble and the bass down. You'll have to play with
the cutoff frequencies to get the sound you want. Basically it'll give
it something of an AM radio effect. Don't make it too narrow though as
you want it to be an effect, not make people cringe when listening to
it. There's actually plugins out there that do this effect, but I
don't know what kind of editing/mixing program you're using (Final
Cut?). All the FX plugins we use are PC VSTi, so they're pretty
specific and I can't really suggest one.
Adding some quiet static, or white noise across the whole thing will
add to it's "age" and effect as well. But then if you want the pops
and clicks to sync to the video you'll have to do it manually. You'll
have to either record your own pops or find something suitable online
then line them up with the film scratches in your editor.
If possible, recording them yourself will give you the best results. One
way to make some good sounding pops is to record the output of a
turntable and rub your finger and tap the needle. You can tweak the EQ
on these too to get the desired sounding effect.
That should get you started. If you need more suggestions feel free to
call or email me. Good luck!
More interesting related stuff
I ran across this article and found it quite interesting as well as
(somewhat) related to the ideas we're discussing in class so I thought I'd
share it with you.
Take a look at: http://www.newscientist.com/article.ns?id=mg18825205.800
I am having difficulty with the question about the dog whistle. Should I
be coming up with a negative answer? 22.0 kHz -17kHz= 5.0 kHz.
give whistle = 22.0 kHz
combined frequency = 5000 Hz
22000 Hz - 17000Hz = 5000 Hz = 5.0kHz
what am i missing here?
When you only know the beat frequency (5 kHz) and ONE of the two
primary frequencies (22 kHz in your case), there are always TWO
possibilities for the unknown.
Consider an example from class: if you have 440 Hz and 442 Hz, they
beat at 2 Hz. But if you have 440 Hz and 438 Hz, they ALSO beat at 2
Hz! So, if I told you that you have 440 Hz and "X", and they beat at 2
Hz, how would you decide between the two answers?
In the CAPA problem, there's an extra clue. The "X" is a dog whistle,
it is supposed to be inaudible to humans. (An audible whistle defeats
the purpose, I guess :-) Using your logic (which finds one of the two
possible answers), you concluded that X was 17 kHz, which is easily
audible to people. Can you find the other possibility?
Does this help/make sense?
I found this really cool article about the 'acoustics of sand.' The
thought it was a nifty 'real-world' example of what we've been doing!
I Recieved an email that gave me a grade for my clicker questions. I
have been to every class and clicked every question. I was under the
impression that the clicker questions were for participation not right
and wrong. And if that is the case then I should have a hundred
percent participation on the clicker question. As of now I do not have
a hundred on my clicker. Can you explain the grading to me please.
The scores I sent had two pieces of info. How many classes you
attended (based on clickers), which is just "one" for any lecture where
you clicked even once! That's the score used in your participation
grade. So it's 100% participation, and in fact, you still get 100% even
if you only participate on SOME questions, as long as you're there and
click at least once. I also sent you the "right-wrong" score (more
technically, 2 pts for wrong, 4 pts for right, 0 pts for no click. Some
questions that don't have a right or wrong answer become 4 pts for
everyone.). That's just for your own information, but also I give you
*extra credit* from this score (see the syllabus, I describe it there).
There's absolutely no penalty for getting clicker questions wrong. So,
even if you got them all wrong, if you got perfect scores elsewhere in
the class, your final grade could be 100%. But, they can *help* your
grade - the more correct you get, the less I "weigh" your exams. So
it's kind of a ratchet - clicker points can help, but can't hurt
(unless you don't do them at all)
Hope this helps explain it - take a look at the syllabus if you want
more details, and ask again if you're still not clear about it. Should
be designed as a supportive system, not a punitive one! That's the
I love these kinds of questions that I can't answer
why do finger nails on a chalk board sound so terrible?
Dunno, exactly! Certainly lots of high amplitude, high frequency
sound gets produced, but I don't know why that particular sound has
such a spectactularly strong negative emotional effect on us, though.
Maybe we should look at the signal on our oscilloscope and see if
there's anything about it that strikes us! I know that, in general,
"anharmonic" sounds are dissonant and unpleasant. (Like, two sounds of
nearly the same, but not quite the same, frequency. They get out of
"synch" with each and stay out of synch for a long time.) We tend to
like "harmonic" sounds, where you get multiple frequencies which are
related to each other by small integer ratios (like 2 to 1, or 3 to 2)
But why our brains should be like that I don't know! Can you think of
some evolutionary selection that would favor a strong emotional
response to screeching sounds? (So we run away from angry birds, or
creatures with large fingernails? :-) Do a little web research, maybe
somebody has come up with a good argument!
Two "random" questions
Hi! I had two kind of random questions...first, can sound really break
glass if it is at a high enough frequency? If so, how does that
happen? Second, how do they make sound-proof walls? I was just
wondering because you said in class on Tuesday that sound travels
through solids, so what do they do to make a solid soundproof?
Not random at all, and quite timely...
Yes indeed, sound can really break glass. I saw a demonstration of it
at a "Physics Demo Night" at the American Association of Physics
Teachers meeting just last month! It's hard to do, I don't think I'll
be able to get a reliable setup to do a demo for our class. It's not
that the frequency is high enough, it's that the frequency
is...resonant! Just like we were saying yesterday in class - if a
system has a natural vibrational frequency, then you can feed energy
into it slowly, bit by bit, just like tapping a kid on the swing. If
you tap at the resonant frequency, each tap ADDS energy, and the swings
get bigger and bigger. Same with a glass - if you can sing (or vibrate
it) at its natural frequency, you can get it to resonate. Like my
Tibetan bowl, which "sings" when we rub it! All resonant systems have
two properties - the natural frequency, and also some "damping" or
friction, some mechanism to LOSE the energy again. The more friction
there is, the smaller the resulting motion will be. (So, with a kid on
a swing, if you only tap, they will get so high but not higher - the
air resistance and drag will take OUT as much energy as your taps put
IN each swing, and you'll reach some balance point, where the kid does
NOT keep going higher and higher. The less internal friction, the
bigger the amplitude of the motion will get before it finally stops
growing. My Tibetan bell gets up to a certain loudness, but no louder -
at that point, the energy I feed in by rubbing it with the spoon is
equal to the energy it loses by radiating sound energy (and heating up
from the flexing). A *finely made*, delicate piece of crystal glass can
have SO LITTLE losses that you can get it vibrating wildly enough that
So, to break the glass you need: delicate glass, glass in a shape that
has a very well defined single resonant frequency, and so few
imperfections that there's no easy mechanisms for it to LOSE energy to
flexing, heating, etc. as it vibrates. And THEN you have to "stimulate"
it with an input that has exactly the resonant frequency. On occasion,
opera singers have been able to shatter glasses (at least, I think they
can, I hope it's not an urban legend!) In any case, I've seen videos of
a speaker next to a wine glass that does break it! The demo I saw used
a very large sheet of thin glass balanced right above a big speaker.
We talked in class briefly about the "sound proof wall" story
yesterday. I'd encourage you to look into it on your own if you're
curious, because I haven't studied this myself. All I can guess is that
you want to design a wall that, when it starts to compress, has
internal friction that takes the vibrational energy of the compressions
and turns them into random (thermal) energy. In other words, material
that *dissipates* the waves efficiently! Stiff materials tend to
transmit waves nicely, but floppy materials tend to convert more of the
sound energy. (I'm thinking of an analogy - car shock absorbers. They
have springs, but also "damping fluids". The idea is that if the car
starts to vibrate, the shocks quickly convert that vibration into
thermal energy in the shock, damping out the vibration quickly. ) Do a
little web hunting, I bet you can find some clear info about it - if
you find an explanation you like, let me know and I'll post it here!
CAPA #3, question 4
I'm completely stuck and baffled by question number 4 on our current CAPA
set. It's the one about percent of change in "speaking frequency" of a note
played by an organ when it was tuned in a 29 degree celsius room verses a 9
degree celsius room. I only have one more try, so I want to make sure to
get it correct. My understanding is that frequency isn't effected by
temperature...only speed and wavelength?
So the thing is, temperature affects speed of sound. That's what it
*directly* affects. Higher temperature means sound travels faster.
That's the only thing which is known for sure in any given problem
about the impact of temperature, is that it will change the speed of
sound. Well, the other thing we know for absolute sure is that the
produced sound wave will satisfy the equation lambda * frequency =
speed. So if speed is changed, SOMETHING has to change regarding
lambda and/or frequency? But what? Are they affected by temperature?
One of them? Both of them? That depends! It depends on the situation,
on the instrument.... ! There's not a general rule, you need to look at
For some instruments, the frequency is not affected by temperature
(e.g., most string instruments vibrate at a frequency determined by the
properties of the string, not the air, and so changing the temperature
has no noticeable affect on their frequency.) So for THOSE kinds of
instruments, your statement would be correct: frequency is unaltered,
but (always!) speed IS altered... Conclusion? wavelength must be
But like I said, it depends. For some instruments (e.g. most wind
instruments), the vibration is NOT of some material object like a
string, you are directly making pressure waves in air. For such an
instrument, it is the *wavelength* which is fixed, unaltered by
temperature. So the story is different! In this case, wavelength is
unaltered, speed IS altered... conclusion? frequency must be altered.
Is this making some sense?
The CAPA hint/help suggestion states:
"An organ pipe has a certain length, and that will NOT change noticeable
as the temperature changes. The length of a pipe is what determines the
wavelength of the sound produced. So, the wavelength (lambda) is NOT
changing as the temperature changes. But wavelength*frequency = speed,
and the speed of sound DOES change with temperature... So, what does
that tell you happens to the frequency you hear? "
Doesn't this mean that the frequency stays the same, while the other
two (speed+wavelength) change?
So, no, the hint is stating very explicitly that the wavelength is NOT
changing! (Like I said above, this is pretty common with wind
instruments, of which the organ is an example) The wavelength of the
produced sound is determined by the size of the organ pipe. So, just
to reiterate, as the temperature changes, the wavelength of sound from
an organ is NOT changing.
But no matter what, lambda*frequency = speed, always! So, if it warms
up, and lambda does NOT change, but the speed DOES change, then what
this formula is telling us is that the frequency will *have* to change.
The organ will "change its voice", the frequency will shift! (And
that's what you hear - you don't perceive wavelength, you perceive
frequency! So you will hear the organ getting a little bit
I can't figure out how to find the wavelength(lamda), or the
frequency. How can I find the percent difference if I'm not given the
wavelength or the frequency? Also, what does it mean to say, "Speaking
Frequency?" Hopefully I'm not "Over-thinking" this...
I like that you're "over-thinking", that's probably a good thing :-)
Speaking frequency is just frequency.
So it's a proportionality thing now. If lambda*freq = speed, and lambda
is FIXED, then if speed changes by some relative percent, frequency
will too. (Think about that, convince yourself exactly how it goes)
Thanks a bunch for your time,
No worries. Work it out before using your last try, maybe check with me
or a TA or some other students before using up that last one!
A student was concerned that my powerpoint lecture notes take up a lot of
paper, so I've added a new variation of "4-up", which should save some
trees and printing costs, if that's an issue for you! (See the "reading" web
page.) Most printers can also print double-sided, saves another factor of
two on paper!)
Some responses to Tuesday questions
>I do not understand the difference between steady and transient
>sounds. What is an example of a steady sound and an exampke of a
>transient sound? I actually did not understand this entire paragraph
>from your notes....i don' understand how a transient dies away?
Steady and transient are slightly ill-defined, but refer to how long the
note lasts. When you say a consonant ("P") it's transient, the sound
appears and quickly disappears. When you sing and hold a note, it's
steady. There's no sharp divide, but "you know it when you hear it". A
drum makes transient sounds, an organ makes steady ones (as long as you
hold the key down, the note continues unabated).
A piano makes something a little inbetween, but I guess you'd have to
call it transient. If you press and hold a key, the sound is clear and
loud, and then slowly (over a period of a couple of seconds) dies
away. It's just a vibrating string, you hammered it making it vibrate,
and then the vibrations natural die away. There's no mechanism to keep
feeding energy in, so friction and losses remove the vibrational
> I could not pull up the power point notes for today's lecture (sept. 6)
Thanks - fixed! When you notice broken links, send me a direct email, I
read email all the time!
> Since I joined the class late, I'm trying to do some major catch up work.
> I am still confused however, if the wave is moving through a medium,
> is it the wave itself moving? Or the medium through which it is flowing?
> It is just hard to grasp this concept.
> Is the water it self moving and then being displaced?
A wave is some sort of disturbance of a medium. Think first of "The wave"
we did in class (if you missed it, that's just the "Stadium wave" where
people stand and sit, and it sweeps across/around the stadium). The people
are the medium. People are disturbed, their arms or body move up and down.
But the PEOPLE do not really move anywhere, I mean, you remain in your
place as the wave sweeps past. You just "jostle" a little. The disturbance
is very visible, it's very real, but it's not a THING, it's a disturbance
of things (!)
Sound is pretty much the same. The room is filled with air. As a sound goes
through, it's not a THING going through the air. It's just a disturbance.
And, the air isn't traveling from one place to another, the air molecules
just jostle a bit. (In the case of sound, they jostle forwards and
backwards in the direction of the wave, rather than perpendicular. It's
"longitudinal") In some places, the molecules get temporarily bunched up.
It's "high pressure" there. They push their neighbors and "relax" right
back to where and how they were.( A bit like the stadium wave!) So there's
a region of high pressure that moves, but it's not the air (or medium)
that's actually moving.
Does this help at all? Let me know if not, it's kind of a basic thing
we should work on till you feel good about it! (And it's not easy, in
my opinion, none of this stuff is!)
> I really had a hard time with the CAPA on thursday, and I know a good
> chunk of kids who did also.
> We couldn't figure out if this was due to missing a class (I signed up
> late), or if we really missed something in class or what.
> Maybe we could talk about that in class.
Well, missing class can always make things harder, but one thing I also
discussed in the start that you may have missed is that CAPA is really
*supposed* to be hard - I make them hard enough that I believe people may
need to get together to work on them. It's one of my "meta goals" in the
class, since collaborative work is how science is done! I'm happy to have
you work with other students, friends, or of course any of our various
study sessions/help room hours. So... as long as you can make sense of the
homework in the end, you shouldn't feel like it's bad or a problem if you
can't do it all on your own, right away...
But do let me know if you just feel it's out of control hard, I don't
want people *suffering*, just learning! :-) (and let those "chunk of
kids" know this too, pass this email on to them if you like!)
> I read the vast majority of the reading and understood everything that
> I read, but I do have a question on the last CAPA hw.
> I remember number three several of us worked on together, and although I
> cannot remember the question, I was hoping to get it explained.
I post my solutions to CAPA problems, it's on the web page. Take a look
there (I'll post 'em over the weekend, generally), and see if that
helps. If it's still not clear, ask me in help room, or even class.
>I'm sure this will be addressed, but I'm wondering what the best way to
>study for the first exam would be
>and whether there will be a study guide provided.
yes, I'll try to talk about this soon in class. I don't have a study
guide, (yet) I'd say going over concept tests and class notes and
homework questions is the first order of business, and then letting the
book summaries point you to the "big ideas" we've been covering... What
I'm all about is having people make sense of what we're talking about -
be comfortable with the various "representations" of waves, work with
the relationships and think of them not as number crunching issues but
relationships. It's going to be a bit hard for everyone before the
first exam, and after that you'll have a better sense... (because I
haven't taught Phys 1240 before, so I've got no sample exams!) More
soon... (I'll post some info on the web page, there's a link for it)
>We know the speed of sound, but the book's discussion of high frequency
waves being used to broadcast radio led me to wonder how far a sound wave
>can travel while maintaining integrity of form.
Couple things - first of all, radio is carried by high frequency
electromagnetic waves (which "encode" the music in clever ways) I'm not
sure if I want to cover this in class, but maybe later in the term we
can, it's kind of cool, just brings in enough new/other ideas that it
requires some "overhead", it's not a quick lesson I'm afraid. Radio
waves can go a LONG way before losing integrity. (Indeed, short wave
radios can communicate huge distances, and we communicate with e.g. the
Cassini probe all the way from Saturn!)
Sound by itself does dissipate fairly quickly in air, and I don't
really know how to characterise this, it's complicated and depends on
the sound, the weather, and so on... I don't really have a clear answer
for this one! I can sometimes hear the announcer from the football
games at my house, maybe 2 miles away as the crow flies. It's a little
distorted. And those monthly warning sirens carry a good few miles too
- I can hear the voice and what it says, although it too sounds pretty
Hi! I had a question on #4 for the CAPA assignment this week. When it
is asking what is the longest and shortest wavelength that you could
produce by singing, are you just asking for a range of the human
singing voice? Where could we find this information?
Yes, indeed I was asking for a typical range of human singing. You
could do some web hunting, or perhaps just sing some high and low notes
and compare with a piano scale... (If you look at the back "flyleaf" of
your textbook, you'll see a picture of a piano keyboard and the
frequencies of each note!) Mostly just interested in you thinking about
the "orders of magnitude" of these numbers, not really worried about
getting it *exactly* right. Hope this helps, let me know if you're
Why the hole in string instruments
Many string instruments contain an opening under the strings that allows
the sound to be amplified. Would expanding or contracting the size of the
hole affect the amplitude, pitch, or tone.
you know, like with all good questions, I'm not 100% sure of the
answer! Instruments have a lot of "arcane" physics which developed over
years of trial and error. The odd shapes of the holes in violins, e.g.,
were surely not "calculated", but are the result of lots of
experiments! Still, we can make some educated guesses.
No hole at all would make the box very stiff, you wouldn't get much
movement, and therefore lousy volume. But a GIANT hole would take away
from surface area of the wooden surface which is actually driving the
air. So that would again make the volume go down. Presumably there's
some optimum compromise!
I don't think the pitch would be heavily influenced by the hole size,
because the pitch of a string instrument is determined by the vibrating
frequency of the string.
Tone - would surely be influenced, because the different hole sizes
would allow or dampen different "higher harmonics" of the vibrating air
in the "box".
We'll learn more about this as we go on (but not down to the level of
computing optimal hole size if you were inventing a new instrument,
that's way beyond the reach of this course!) Other readers of this - any
thoughts you'd like to add?
In space, no one can hear you scream
Does sound travel in a vacum? I know this question does not relate to
our text but I was just wondering.
I think this question TOTALLY relates to everything we're talking
about, so I'm glad you asked it!
Think about what we've said sound is - it's a pressure wave, which
results from molecules crowding and bumping into one other, a kind of
"chain reaction" of squeezing and expanding, like a compression running
along a slinky. If there are no molecules to wiggle and bump, could you
have a "wave"? (What would be waving? How would the energy be
transmitted?) . If there is no material (no medium), how could you have
a wave? (Could you do "the wave" in a stadium that had no people? Could
you send a wave down a rubber hose if the rubber hose wasn't there? Or
down a slinky if there was no slinky?!)
Sound isn't a *thing*, exactly - it's the result of nothing more or
less than alternating high and low pressure... It's not some thing
"traveling between the air molecules", it's a disturbance of the normal
pattern of those molecules!
Now, having said that, there are a couple of caveats. It doesn't have
to be AIR, per se. You can wiggle water molecules and send a pressure
wave through the water. (So, you can have sound traveling under water.
Your eardrums will wiggle due to being bumped by high pressure water
just like they wiggle due to being bumped by high pressure air. But
SOMETHING has to be there to bump them!)
The other caveat is that there IS a very special kind of wave that can
travel through a vacuum, namely electromagnetic waves. That's because
electric fields can exist in a region where there are no atoms or
matter. That's a whole conversation in itself, and quite different from
the sound story.
Hope this helps. Come visit office hours or problem sessions and ask
some more if you're still confused about this!
P.S. Remember the old movie Alien? Maybe it's ancient history now :-)
But when it came out, the ad campaign was "In space, no one can hear
you scream". They had it right - sound won't travel through empty
space! It annoys nerdy physicists like me to no end in sci-fi movies
where you can HEAR explosions of one space ship from another one!
Nonsense! Particles would of course fly out from an explosion (and
might bump into your ship, so I guess you could try to spin some story
of how you could "hear" the explosion?!) but that's really quite
totally different from pressure waves expanding outwards. In the end of
Alien, there's a giant nuclear explosion of a ship in space, and what
was cool was that they "filmed" it from a distance, and played it with
total silence... (2001 also got this right, but Star Wars and Star Trek
happily make noises when you see exploding stuff in space. Bah!)
CAPA long answer grades
Hi, I logged on to the CAPA system to view last week's problem set
solutions. I noticed that I received an "E" for my two long answers.
What does this mean?
It means "excused", which I did to everyone. I'm using CAPA to input your
answers, but we're going to record the grades separately from the CAPA
grading system (the hand-grading thing is just too cumbersome inside of
CAPA). I'm trying to figure out a way to deliver the information to
everyone, you'll probably get an email about it soon!
Credit for CAPA long answer
While typing out the essay part on CAPA, i completed both of them - but
for instance when i was completely done one said "A" for answered, and
the other one said "0" although i already submitted an answer. Anyhow,
I answered both of them, but one still had a "0" on it for no reason.
I also tried to log out by clicking the exit button but it didn't do
anything so i just x'ed out the window. Either way, I finished the
CAPA but i'm still semi-illiterate to the process and i appologize.
I'm almost positive everything was submitted correcly...but i couldn't
tell for sure. Sorry for bothering you, I was just didn't want to be
scored poorly when i did the assigned work. Thanks for your time
Yeah, this is a CAPA thing. It says "A" first, but then as soon as you refresh
the page, it shifts to 0 for "no points so far" (because it's hand graded,
and the grader can't get to it till after the set is done!)
So basically, no worries, you're fine!
A student wrote in with a very clear explanation of what he was doing,
which all looked correct, but CAPA said no. We finally realized that he
had taken his (perfectly correct!) answer and then ROUNDED it, and
typed in the rounded number... So e.g. if his calculator said
.85, he typed in .9 to CAPA (which is almost a 6% change, MUCH more than CAPA's
limit of 1%)! It's always prudent to keep 3 significant digits in your
calculator work (that
doesn't count leading zeros: 0.0002 has ONE "significant" digit, the
2. So if you compute .00016 as an answer, you should type that, and don't round to
The collapsing can
I am wondering why, if the pressure in the can in the demonstration was
decreasing steadily, there were several large pops and one huge bang.
Why these sudden phenomena? My guess would be this relates to the
shape of the can and its material. Perhaps as the can begins to
squeeze, and the integrity of the cylinder diminishes. . . I am not
sure where to go from here. Am I on the right track?
That can is very strong! There are reinforced circular steel rods
around the outside, so it really doesn't want to collapse! It's
designed to withstand a LOT of pressure. But when I got it, it was
dented. That already produced a weakness (circles are one of the
strongest shapes, but bends in the circle weaken it). The first pop was
the top bending inwards, I saw it... but it was just "flexing", no
structural collapse yet. Same with the second one... the reinforced
rods kept the main shape solid. But as we continued pulling air out,
the excess pressure on the outside just keep growing and growing. At
some point, the forces grew "catastrophic" and it collapsed. If we had
started with an undented barrel, it would have lasted LONGER, and then
when it final caved in, it would've been very flattened!
I figure the barrel is about 3 feet high and maybe 3 feet around, which
means surface area is about 1 m^2 (ok, I might be off by a factor of
two or three, but roughly...) So if we sucked all the air out, the
force = pressure*area = 100,000 N/m^2 * 1 m^2 = 100,000 N which is
about 20,000 pounds. So it's like there's a TRUCK sitting on that
thing! No surprise it collapses! (Air is heavier than you'd think,
especially since there's so darn much of it sitting on top of us!)
Getting credit for assignments?
Will we have any way of monitoring our Tuesday and Thursday homework
grades? For example, how do we learn whether or not we received full
credit for a Tuesday assignment?
When you submit the Tuesday homework, a page should come up that
serves as a "receipt". (Feel free to print or save that page if you're
concerned, but basically, if it appears that means we've already got
your data. The only issue might be if you accidentally mis-typed your
id, I guess) You'll get full credit for the Tuesday ones almost no
matter what you do, as long as it's even remotely reasonable. I'm only
going to take off points on the Tuesday assignment if someone really
isn't taking it remotely seriously.
With CAPA numerical/multiple choice, it's even simpler, because it
checks your ID when you log in. If CAPA says "Y" to a problem, that
means that it has been stored (it's dynamically reading the stored data
to decide if it's Y or N) So you don't need to worry about those. The
"long answer" questions in CAPA are new to me, I haven't used them
before, but when you see the box with your last input, that's also
being read from the data file so we have it.As for the score you get on
that - we'll hand grade those and enter the scores into the CAPA
system. So later (within a week), you can log in to CAPA for future
homeworks and, instead of clicking on "try current assignment", you can
look back at old ones, and see the grades you got.
I also plan to find a way to email everyone their "grades so far" at a
couple of intervals during the semester, I'll probably do it right
after each midterm, so if there are any potential "glitches" we'll have
lots of time to fix anything. (I may use webct for this, we'll see)
Hope this helps - let me know if you have any questions or concerns at
Wed and Thursday study sessions
I was wondering what is covered during the separate Weekly study and
review sessions, as in do they cover different things? Because my
schedule conflicts with the hours offered and I will only be able to
attend Jeff Merkels session (not including generic help room hours). Is
All of these are totally optional. The Thursday ones definitely
won't "cover" anything at all - it's really just a convenient time and
place to try to get students together who are likely working on the
CAPA homeworks. The TA's will not be lecturing at all - their job is
simply to facilitate group conversations, circulating and asking
guiding questions, steering people in the right directions. (Possibly
right before exams there might be a little bit more of a "review"
session, but even that is likely to be answering questions rather than
actually preparing anything.)
Jeff may do something a little different Wed night - I'm leaving that
up to him. I think he may like to talk a bit about how what we're doing
in class connects to his real world experiences in the audio recording
industry, possibly also give some class reviews/summaries... I'm not
sure, we'll find out, the Wed evening session is his voluntary
activity! I'm certain he will *also* provide a "study session"
environment there too, for much of his evening also, again letting
people do their own thing where he'll just be there as "learning coach"
on the side, just like the Thursday sessions do.
Hope this helps!
CAPA set #1
How do you counvert from Atm to Pa?
If you're looking for the conversion factor (how many Pa is 1 atm),
it's in your textbook, in the Appendix B which has many handy
conversions and information about metric units. It's also in section
1.5 on "pressure", they do a couple of examples there. (Section 1.5 is
the reading assignment for Tues, guess I wasn't expecting you to get
that far in CAPA a week in advance!
In general, lots of CAPA problems will be on material we won't
get to till Tuesday class...and on rare occasions, there might even be
one last CAPA problem (or two) on material we only get to on the
Thursday the CAPA is due! (That's one reason I give you till the end of
the day Thursday...)
If you're asking a more basic question (how do you do unit conversions
in general), I'll be talking about that briefly in class today (Thurs)
Hope this helps, if not, let me know, of course!
I had a quick question about the clickers. If we have the skinny ones
from last year, not the larger ones I saw people with in class, are we
still able to use them in your class or do we need to go buy the new
Skinny ones work fine if they have 2 little led's on the front end. If they
only have one, they're an older model and don't work. (I don't think those
have been sold in quite a while)
Hope it works, if you're not sure we can test it before or after class on
Homework the first week?
Hi, I am in your physics 1240 course and I couldn't understand if you meant
there to be homework this thursday or not. So is there anything due on
thursday? I have checked the webpage already and couldn't find any definate
information in regards to this. Thank you!
Nothing to turn in this week (just get starting on the readings)
Thanks for checking! I just changed the webpage to try to make it a
In the future, it's always the same: reading questions (and maybe a
short survey) on Tuesday, CAPA on Thursday. Let me know if anything is