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Re: [Phys-L] Suggestions for audio speakers



Thanks everyone for all of the information, this was very helpful in clarifying my thoughts about what I want to accomplish (and subsequent choice of speakers).

kyle

On Mar 18, 2013, at 12:00 PM, <phys-l-request@phys-l.org<mailto:phys-l-request@phys-l.org>>
<phys-l-request@phys-l.org<mailto:phys-l-request@phys-l.org>> wrote:

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Today's Topics:

1. Re: this stuff is easy ... or not (Paul Lulai)
2. Re: axes versus contours (John Denker)
3. Re: Suggestions for audio speakers (John Denker)
4. Re: Suggestions for audio speakers (John Clement)
5. Re: Suggestions for audio speakers (Bernard Cleyet)
6. <jsd@av8n.com> is a new physics wikipedia. Was: Re: mass,
energy, and spacetime (Bernard Cleyet)
7. Re: Suggestions for audio speakers (Chuck Britton)


----------------------------------------------------------------------

Message: 1
Date: Sun, 17 Mar 2013 21:09:01 +0000
From: Paul Lulai <plulai@stanthony.k12.mn.us>
To: "phys-l@phys-l.org" <phys-l@phys-l.org>
Subject: Re: [Phys-L] this stuff is easy ... or not
Message-ID: <6E5B0A1D-7FB5-4C7C-9745-D39A7182D944@stanthony.k12.mn.us>
Content-Type: text/plain; charset="Windows-1252"

So...
What is the difference between an x axis, x plane, and an x contour?

...::. Sent from a touchscreen.::...
Paul Lulai


----- Reply message -----
From: "John Denker" <jsd@av8n.com>
Date: Sat, Mar 16, 2013 7:28 pm
Subject: [Phys-L] this stuff is easy ... or not
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>

In the context of
http://www.av8n.com/physics/spacetime-welcome.htm

On 03/16/2013 05:58 AM, Brian Blais wrote:
Sometimes I think the guide assumes something is simpler (for the
reader) than it really is.

That's unintentional.

Given the choice, I would prefer to err on the side of exaggerating
how easy stuff is, rather than exaggerating how hard it is ... but
really I'd rather not exaggerate either way. IMHO the motto should
be: "This stuff is easy if you know how; let me show you the trick."

Of course if you say that but don't sufficiently explain the trick,
that's a problem, but that's usually a fixable problem. I would
much rather face that problem ... as opposed to saying "this stuff
is hard" and giving up without even trying to explain it.

So .... If you or your students find something that is significantly
overstated and/or underexplained, please let me know. I'll fix it
if I can.

==================

I make a big fuss about "hard" versus "easy" in the context of
special relativity because I see a lot of bad horrible no-good
pedagogy in this area. Specifically, I'm talking about the so-
called "paradoxes". As far as I can tell, there are no paradoxes
in the correctly-stated laws of special relativity. The only way
to set up a paradox is to mis-state the laws of physics. I want
to tear my hair out every time I see that. There are some teachers
out there that positively revel in making relativity seem weird,
paradoxical, unnecessarily complicated, and disconnected from
everyday life -- which is almost 100% wrong.

I'm not saying SR is easy like falling off a log, but I insist it
is a whole lot easier than many texts and many teachers make it
out to be.

Also: Generally speaking, it's easier to explain SR to high school
students than to high school teachers. That's because the latter
have learned the wrong way of doing things, based on the pre-1908
way of looking at things, and unlearning all that is incomparably
more difficult than just doing things the easy way from the get-go.

If you look at my screed
http://www.av8n.com/physics/spacetime-welcome.htm
you may be amused to note that:
-- It does not even mention FiztGerald-Lorentz contraction.
-- It does not even mention time dilation.
-- It does not even mention velocity-dependent mass.
-- It does not even mention rest mass.

It's not like a I found an easy way to explain those things; the
point is that you don't need those things. Not ever. Not at all.
A na?ve high-school student knows that when you rotate a ruler in
the xy plane, its length does not change. So if you tell him that
when you rotate the same ruler in the xt plane, its length does
not change, there is no explaining necessary. He looks at you and
says "Well, duh." It is only the poor guy who learned the pre-1908
way of doing things that needs an explanation.

Please, folks, let's stop teaching stuff that is more than 100
years out of date.

If you want to read about the hard way of doing things, I have a
separate document for that ... but please keep in mind that these
are ideas you should not share with the students. It would just
pollute their brains.
http://www.av8n.com/physics/spacetime-dirty-laundry.htm

This makes it hard to publish a book that contains what students
need. That's because the students don't choose the books. The
explanation that makes sense to the student doesn't make sense to
the teacher.

That's one nice thing about publishing on the web. Students have
ways of finding what they need. Sometimes the teacher finds out
about it from the students.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l


------------------------------

Message: 2
Date: Sun, 17 Mar 2013 16:35:07 -0700
From: John Denker <jsd@av8n.com>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] axes versus contours
Message-ID: <5146532B.3070800@av8n.com>
Content-Type: text/plain; charset=windows-1252

On 03/17/2013 02:09 PM, Paul Lulai asked:
What is the difference between an x axis, x plane, and an x contour?

I assume "x contour" is shorthand for "contour of constant x".

In three _Cartesian_ dimensions, contours of constant x are
planar, so I assume that is what "x plane" refers to. However,
if there any number of dimensions other than 2, the contours
of constant x are not planar, so "x plane" is not a viable
concept AFAICT. Also, there are lots of situations where the
geometry is not Cartesian -- or where there is no geometry
at all -- in which case the concept of "plane" goes out the
window, but the concept of contour survives.

In particular, in thermodynamic state-space, there is no notion
of distance or angle. There is topology, but no geometry.
There is no natural way to define a "flat" plane. Even if you
arbitrarily impose the idea that the contours of constant P
and constant T and constant V are somehow flat, then the
contour of constant S will not be flat. To make progress,
you need a formalism that can handle "some" non-flat contours,
and at that point you might as well use the same formalism
for all contours, not even pretending that any of them are
flat.

In three Cartesian dimensions, the X axis is perpendicular
to the contours of constant X, so these are easily seen to
be different concepts. In two dimensions, there is a slight
pedagogical problem, because the contours of constant X look
like lines and the X axis looks like a line, so students may
wonder why one kind of line is better than another kind of
line. That answer, of course, is that as soon as there are
more than two variables in two dimensions, *or* there are
more than two dimensions, then axes get you into trouble
and contours get you out of trouble.

For the last N years, whenever I make a plot using a computer,
I habitually turn on the "grid" feature so as to plot the
contours. I haven't fully imposed this policy on all my
older plots, but I am gradually upgrading any plots I get
my hands on. I do this even when it's not important,
because I want people to get in the habit of seeing the
contours.

This is example #754 of the fact that somebody who knows enough
to /pass/ the course doesn't know nearly enough to teach the
course. That's because there are always N different ways of
doing things, and it's hard to know which way will get you
into trouble later, and which way lays the foundation for
further progress. Drawing the grid -- even when it is not
important -- lays a good foundation.



------------------------------

Message: 3
Date: Sun, 17 Mar 2013 16:38:50 -0700
From: John Denker <jsd@av8n.com>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Suggestions for audio speakers
Message-ID: <5146540A.4010004@av8n.com>
Content-Type: text/plain; charset=UTF-8

Let me address a couple of points that are relevant to the body of
the original message. Some of this is relevant to the subject line,
and some not.

On 03/11/2013 07:20 AM, Forinash III, Kyle wrote:

I will be teaching a physics of sound class in the fall

OK.

I have a CD that plays a sine wave starting at about 20Hz and sweeps
up to 20,000 Hz

For those who don't already have the CD and/or want to do things
that aren't provided by that CD ... you can make simple audio files
from scratch using the "mkwav" utility. The C++ source is here:
http://www.av8n.com/sound/

It will make sine waves or frequency sweeps.
It can do mono if you like.
++ It can also do stereo, with the right channel offset from the
left channel by a fixed musical interval (i.e. frequency ratio).
++ It can also do N channels if you like, for any N.
You can specify the frequencies.
You can specify the amplitude.
You can specify the duration of the recording.
You can specify the sample rate.

In the same directory you can find pre-fabricated frequency sweeps
(as .wav files) sweeping from 20 Hz to 20 kHz at the rate of
0.5 octaves per second.

I ask students to raise their hands when they can't hear it anymore

You'll get better psychophysics data if you play a short beep-beep-beep
sound at a selected frequency ... and selected amplitude ... and ask them
to raise their hands if/when they hear something.

====

There are "virtual piano keyboard" apps that run on your computer, so
you can play any sound you want, whenever you want. There are midi
patches that put out a sine wave.

Midi codes span more than 10 octaves, which would be more than you need
for the given application, *except* that the midi low-end and high-end
are both lower than is ideal for this application. You can fix this by
finding a "transposing instrument" that plays all midi codes an octave
higher than written. I assume a transposing sine-wave patch exists or
could easily be constructed, although I can't point to one at the moment.
See e.g. http://freepats.zenvoid.org/

============

I would like to get a good set of speakers/monitors and amp system (has to be portable).

To serve the narrow purpose of the raise-your-hands demo, and a lot
of other purposes besides, your best option might be a consumer-grade
"home theater" system, perhaps a 7.1 system, perhaps something like
this:
http://www.pyleaudio.com/sku/PT798SBA
I have no experience with that particular item; I cite it only as an
existence proof, not a brand-specific or model-specific recommendation.

Within the 7.1 category, it would be ideal IMHO to have 7 identical main
speakers and one subwoofer. Usually the best you can do is 6 identical
main speakers, plus one differently-shaped "front center" speaker, plus
the subwoofer. That should be good enough for the application.

Having a large number of smallish speakers works waaaay better than having
a couple of larger speakers ... especially in a room as big as an ordinary
classroom. The smallish speakers are definitely portable. You can port them
one at a time ... or you can pile them into a box and port them all at once,
and they will /still/ weigh less than two big speakers with comparable overall
output quality.

Also, having a multi-channel system allows flexibility to do lots of other
stuff in the future, including sending different signals to each of the N
channels.

Note that for a few bucks you can get a dongle that takes USB in and puts
7.1 S/Pdif out. It looks to the operating system like a sound card. I
mention this so that you don't need to worry about being limited by the
capabilities of your existing sound card.

Along the same lines, there are good reasons why you do not want an analog
connection from your computer to your fancy audio stuff. S/Pdif is a fine
way to make this problem go away once and for all.


------------------------------

Message: 4
Date: Sun, 17 Mar 2013 22:07:00 -0500
From: "John Clement" <clement@hal-pc.org>
To: <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Suggestions for audio speakers
Message-ID: <750221DCF3054EEC8F9059D8CE0FC529@ClementPC>
Content-Type: text/plain; charset="us-ascii"

But it all depends on the application and the size of the room.
A home 7.1 system may not have enough volume if you are in a large lecture
hall.

A 7.1 system will not be suitable for demonstrating interference patterns.
For that you need 2 dedicated speakers or 2 stacked arrays of R/L with no
sub-woofer. Any stacked speakers would have to be identical to prevent
phasing problems.

A 7.1 system works by sensing the phase of the 2 channels and then channels
the sound appropriately to the specified speaker. 7.1 messes up any
interference or phase demos. Also the signals are added together at the low
frequencies for the sub-woofer so you can not demo superposition at low
frequencies.

So in the end the simplest is to have 2 speakers that fit into a small
duffel for portability. And of course you will not achieve usable response
anywhere near to 20Hz with such a system. There are any number of speakers
which claim to be monitors that would satisfy this. But they will not have
very wide dispersion so the high frequency response will be severly
attenuated at off angles. You will pay dearly if you want good dispersion,
and broad frequency response. And adding response below 50Hz at decent
volume will also generally cost a lot as well as adding mass.

When setting up a speaker system it is desirable to have the speakers either
above or below the audience. You need to aim them at the back row, with the
front row off axis so as to prevent the front from from getting very loud
sound. The people in the front row will have incresed volume due to
distance, but decreased due to being off axis. While monitors should be
placed aimed at the listener for a single person or a small group, this
should not be done for a larger group. Of course if you keep the speakers
at a modest volume level the only problem will be that the back row will not
hear them as well. But if you are demonstrating how the sound falls off
with distance this may be desirable.

7.1 systems may be very pleasing for reproducing audio designed to take
advantage of them, but they do have some severe limitations, and are
probably not optimal for physics demos. The subwoofer will probably not
output anything if the 2 channels are out of phase. I don't know exactly
how the active circuits handle the phasing. Sometimes this is done
deliberately for special effects. Newer recordings are probably designed
with the bass in phase so as to prevent subwoofer problems. Older
recordings or poorly mastered recordings may not have proper phasing. At
one time recordings were all auditioned to sound good both in mono and
stereo, but I don't know if this is still done.

In the end the best option is to try various speakers until a satisfactory
one is found. Since the actual purpose of the demo was not specified it is
very difficult to recommend specific equipment. If it is a one shot demo,
then expensive equipment would not be desirable.

John M. Clement
Houston, TX


Within the 7.1 category, it would be ideal IMHO to have 7
identical main speakers and one subwoofer. Usually the best
you can do is 6 identical main speakers, plus one
differently-shaped "front center" speaker, plus the
subwoofer. That should be good enough for the application.





------------------------------

Message: 5
Date: Sun, 17 Mar 2013 21:38:58 -0700
From: Bernard Cleyet <bernardcleyet@redshift.com>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Suggestions for audio speakers
Message-ID: <83E55034-F8DD-4BDF-8BAE-B992C5EAA791@redshift.com>
Content-Type: text/plain; charset=us-ascii


On 2013, Mar 17, , at 20:07, "John Clement" <clement@hal-pc.org> wrote:

But it all depends on the application and the size of the room.
A home 7.1 system may not have enough volume if you are in a large lecture
hall.

A 7.1 system will not be suitable for demonstrating interference patterns.
For that you need 2 dedicated speakers or 2 stacked arrays of R/L with no
sub-woofer. Any stacked speakers would have to be identical to prevent
phasing problems.


they all must be the same distance from the hearer. Having speakers different distance from the hearer is only a partial solution to the phase prob. created by cross overs.


A 7.1 system works by sensing the phase of the 2 channels and then channels
the sound appropriately to the specified speaker. 7.1 messes up any
interference or phase demos. Also the signals are added together at the low
frequencies for the sub-woofer so you can not demo superposition at low
frequencies.

So in the end the simplest is to have 2 speakers that fit into a small
duffel for portability. And of course you will not achieve usable response
anywhere near to 20Hz with such a system. There are any number of speakers
which claim to be monitors that would satisfy this. But they will not have
very wide dispersion so the high frequency response will be severly
attenuated at off angles. You will pay dearly if you want good dispersion,
and broad frequency response. And adding response below 50Hz at decent
volume will also generally cost a lot as well as adding mass.

When setting up a speaker system it is desirable to have the speakers either
above or below the audience. You need to aim them at the back row, with the
front row off axis so as to prevent the front from from getting very loud
sound. The people in the front row will have incresed volume due to
distance, but decreased due to being off axis. While monitors should be
placed aimed at the listener for a single person or a small group, this
should not be done for a larger group. Of course if you keep the speakers
at a modest volume level the only problem will be that the back row will not
hear them as well. But if you are demonstrating how the sound falls off
with distance this may be desirable.

7.1 systems may be very pleasing for reproducing audio designed to take
advantage of them, but they do have some severe limitations, and are
probably not optimal for physics demos. The subwoofer will probably not
output anything if the 2 channels are out of phase. I don't know exactly
how the active circuits handle the phasing. Sometimes this is done
deliberately for special effects. Newer recordings are probably designed
with the bass in phase so as to prevent subwoofer problems. Older
recordings or poorly mastered recordings may not have proper phasing. At
one time recordings were all auditioned to sound good both in mono and
stereo, but I don't know if this is still done.


Blumlein or crossed pair -- I started out using spaced pair onmis until clued in by a pro.

I've used both omni and cardioid, but true Blumlein uses bi-cardioid.

In the end the best option is to try various speakers until a satisfactory
one is found.

I suggest the student make the system(s) -- won't this instruct much?


Since the actual purpose of the demo was not specified it is
very difficult to recommend specific equipment. If it is a one shot demo,
then expensive equipment would not be desirable.


I wrote:


For what is your intended use?

Rich physics in the design of speakers and their enclosures - the best Hi-Fi system I know of included feed back with microphone integral to the speakers; feedback from the microphone to the input of the power amp. in addition to "regular" amp. feedback. Used 6BL7s as is a very linear tube (vertical deflection amp. and oscillator use is a double triode).

bc notes six days and ten posts.


John M. Clement
Houston, TX


Within the 7.1 category, it would be ideal IMHO to have 7
identical main speakers and one subwoofer. Usually the best
you can do is 6 identical main speakers, plus one
differently-shaped "front center" speaker, plus the
subwoofer. That should be good enough for the application.



_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




------------------------------

Message: 6
Date: Sun, 17 Mar 2013 21:39:46 -0700
From: Bernard Cleyet <bernardcleyet@redshift.com>
To: Phys-L@Phys-L.org
Subject: [Phys-L] <jsd@av8n.com> is a new physics wikipedia. Was: Re:
mass, energy, and spacetime
Message-ID: <ABCDA8AC-0A91-49B8-9E9E-E36D30D3ADDD@redshift.com>
Content-Type: text/plain; charset=windows-1252


On 2013, Mar 16, , at 07:08, John Denker <jsd@av8n.com> wrote:


I just now reworded the paragraph in question at
http://www.av8n.com/physics/spacetime-welcome.htm#eq-e-mc2
_______________________________________________


bc is thankful.




------------------------------

Message: 7
Date: Mon, 18 Mar 2013 01:27:54 -0400
From: Chuck Britton <britton@ncssm.edu>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Suggestions for audio speakers
Message-ID: <AB7517A3-3FA5-4F50-84C0-362D90BCC920@ncssm.edu>
Content-Type: text/plain; charset=us-ascii

A coupla notes for those with less money to devote to this -


On 03/11/2013 07:20 AM, Forinash III, Kyle wrote:
I ask students to raise their hands when they can't hear it anymore

I got better student data when I would start 'High' (20 kHz) and come down,
asking students to raise their hand when they DO begin to hear the whine.
(Have then put their heads down - not looking - so they are not influenced by their peers.


On another 'Note' - use a single tiny (tinny) speaker to play music and then cup your hands around the speaker.
The increase in bass respond is VERY dramatic.
This is keeping the bass waves generated from the BACK of the cone from destructively interfering with those from the front.

A very simple demo that gets GREAT response from the crowd.

Or just hold a sheet of paper with a speaker-sized hole directly in front of the speaker.
(Amazing)

------------------------------

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Forum for Physics Educators
Phys-l@phys-l.org
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------------------------------

End of Phys-l Digest, Vol 99, Issue 15
**************************************

--------------
'Those who can make you believe absurdities and make you commit atrocities.'
- Voltaire

Kyle
kforinas@ius.edu<mailto:kforinas@ius.edu>