[[email protected]]

On Jul 10, 7:31 pm, ChrisCoaster wrote:
If you have a sound with a frequency of 20kHz, that means that the
wave has to go through one complete cycle 20.000 times a second.
That is: up *and* down 20.000 times a second. To record that you
need to record both the ups and the downs, so that's 20.000 ups plus
20.000 downs, makes 40.000 samples per second.
Why they went up from 40kHz to 44.1kHz is some weird
technical reason,
explained in another part of this thread
You are, I hope, joking. Cos that is total crap.

Alrighty geoff, then YOU explain it. And remember,
easy on the math!

Any explanation that's CORRECT and ACCURATE
involves math beyond addition and subtraction:
assuredly multiplication and division for one.

However, perhaps an explanation by analogy might
help.

Imagine one of them ol' western movies, where the
indians on horses are chasing the settlers in their
covered wagons. Did you ever sometimes notice
that the wheels seem to be turning BACKWARDS?
Even backwards slowly?

That's due to a phenomenon known as "aliasing."
It's a direct result of having the wheel spin too fast
for the movie camera. What's actually happening
is quite simple. Pretend our movie camera is running
at 25 frames per second. And let's also preten that
ONE of the wagon wheel spokes is painted white and
all the rest are dark wood colored.

If the wagon wheel rotates slowly, sy, one revolution
per second, in one second, we will have captrued 25
images of the wheel, each one with the white spoke
rotated about 14 dgrees further than the previous frame.

When I play this movie back, and measure the wheel,
I will measure its rotation at 1 per second

If I speed it up to 2, 3, 4, 5 revolutions per second,
it's ALWAYS the case that on playback, the image
of the wheel is spinning at the right rate in the right
direction.

But now let's see what happens if it's spinning
EXACTLY 12.5 revolutions per second (gee,
that's HALF the frame rate. At that point, the wheel
spins just fast enough to make it halfway around.
So in the first frame it's pointing up. in the second
it;'s pointing down, the third up, and so on.

On playback, how fast and in what direct is the wheel
turing? Well, it's impossible to tell, because all you
see is two white spokes pointing up and down, not
moving at all.

And that happend at 1/2 the frame rate of the camera.

But slow it down JUST a little, say 10 revolutions per
second. in the first frame, say, it's pointing up. By
the second frame, it's moved 144 degrees, so it's
NOT quite poiting down. Third frame, it's moved a
total of 288 degrees and has NOT quite returned to
it's start.

Play this back, and you will see the wheel turning
at the right speed and in the right direction.

Now speed it up a little, say to 15 revolutions per
second. Now the wheel has moved just past the
bottom, 216 degrees, then 72 dgrees beyond the
start point, and so on.

And what you'll find is that NOW the wheel, instead
of spinning dorwards at 15 RPM, looks like it's
spinning BACKWARDS at 10 RPM.

As an extreme case, consider the wheel spinning
forwards at 24 RPM: because your movie camera
is not sampling fast enough, the wheel will actually
look like it's moving BACKWARDS at a measily
1 revolution per second.

What's the FAsTEST the wheel can turn before the
playback does NOT accurately depict it? It HAS to
be spinning LESS THAN 1/1 the frame rate of the
camera.

Conversely, how fast does you camera have to
run if it want's to accurately capture a wheel spinning,
say 20 revolutions per second? Well, it HAS to be
MORE THAN twice that, or FASTER than 60 frames
per second.

Now, the interesting thins about aliasing like this
is that ALL possible aliases get folded down. With
your frame rate of 25 per second you;'' get the same
picture back again with the wheel spinning at 1 rpm,
26 rpm, 51 rpm, 76 rpm, 101 rpm and so on on,
and at 24 RPM, 49 RPM, 74 RPM 99 RPM, the wheel
will look like it's spinning BACKWARDS at 1 RPM.
Notice also that aliases are occuring at MULTIPLES
of the original sample (or frame) rate +- 1 RPS, e.g.,
25*2 = 50+-1, 25*3+-1, and so on, up to infinity.

Do the experiment yourself in your head: What's the
fastest the wheel can spin without seeming to stop,
spin backwards, or spin at a rate different on playback?

The answer is ALWAYS, less than 1.2 the sample rate.

ALWAYS (assuming it's the baseband you want).

And, thank you for your concern, but your ability to
"feel my heart racing through your high-speed
connection" needs some serious reevaluation.
Your ability to sense my stree, kind sir, is quite
defective.

It has been said, however, that I'm not first in line
for the Nobel prize in suffering fools gladly. Especially
when this is easily a topic that's been discussed
HUNDREDS of times before.

[Geoff]

ChrisCoaster wrote:
If you have a sound with a frequency of 20kHz, that means that the
wave has to go through one complete cycle 20.000 times a second.
That is: up *and* down 20.000 times a second. To record that you
need to record both the ups and the downs, so that's 20.000 ups plus
20.000 downs, makes 40.000 samples per second.

Why they went up from 40kHz to 44.1kHz is some weird technical
reason, explained in another part of this thread.

You are, I hope, joking. Cos that is total crap.

geoff- Hide quoted text -

- Show quoted text -
_______________
Alrighty geoff, then YOU explain it. And remember, easy on the math!

'Cause at this point I'm about hit "Wiki" in my favorites.
-CC

Simple. To accurately recontruct a waveform, you need two samples of the
highest frequency specificied. Look up Nyquist Theory on you beloved Wiki -
even Wiki has that right !

Nothing to do with 20KHz worth or 'up' and 20KHz worth of 'down'.

geoff

[Arny Krueger]

"ChrisCoaster" wrote in message


What I meant with my Cayman wall analogy was the actual
analog frequency response curve of the CD medium. That is, if
you started to roll sine wave from 20Hz all the way up to 22kHz, and
record it to a CD, the drop off at the high end of that would be
instantaneous, not a soft gradual roll off as on a record or cassette.

Sort of. So what?

To me the roll-off between 20 KHz and 22.05 KHz is not *instantaneous*.
There is a discernable, measureable slope.

For example, on a Behringer UCA 202, response is less than 1 dB down at 20
KHz, but its only about 4 dB down at 21 kHz, and 8 dB down at 21.8 kHz.
Measuring response between 21.8 and 22.05 kHz gets a little hairy, but if
you want to do the work, the slope is there.

High frequency response is a little like clipping. The debate over soft
roll-offs and sharp roll-offs is like the debate over soft clipping versus
sharp clipping.

Would you rather have a power amp that rises smoothly to 10% THD at 50 watts
and clips at 100 watts, or would you prefer a power amp that is almost
perfectly clean up to 99 watts, and clips at 100 watts? Well, as long as
you stay below 100 watts, you'd probably want the amp that is clean up to 99
watts. With a reasonable application, avoiding going over even 50 watts is
completely doable.

So, would you rather have a recorder that drops off smoothly to 1 dB down at
10 kHz, and is 3 dB down at 15 kHz, or would you rather have a recorder that
is nearly perfectly flat to 20 kHz, and then the response drops off pretty
sharply. I can tell you that if you did an ABX test, the recorder that is 3
dB down at 15 kHz will be pretty easy to pick out, and the one that is
almost perfectly flat to 20 Hz will be very, very tough.

As far as the So what part goes, here's what I mean. Does the shape of a
system response curve actually matter that much above even 12 kHz? In fact,
all we know for sure is what we hear, and what we hear even as low as 10 KHz
is actually pretty corrupt. I'm speaking acoustically, of course.

It is well-known in the perceptual coder community that with almost all real
world music, you can throw away *everything* above 16 kHz and nobody is the
wiser. Well they won't be the wiser until they stop listening to the music
and start looking at frequency response curves. They they may have anxiety
attacks. But, its not what they hear that is bugging them. Its what they
think they hear.

[[email protected]]

On Jul 10, 9:32 pm, "geoff" wrote:
Simple. To accurately recontruct a waveform, you need two samples of the
highest frequency specificied. Look up Nyquist Theory on you beloved Wiki -
even Wiki has that right !

Omigod, this is painful!

Look, TWO samples per cycle WILL NOT DO IT.
You MUST have MORE than two samples.

Let me try it a different way

TWO samples per cycle WILL NOT DO IT.
You MUST have MORE than two samples.

[[email protected]]

On Jul 10, 2:17 pm, ChrisCoaster wrote:
What I meant with my Cayman wall analogy was the actual analog
frequency response curve of the CD medium. That is, if you started to
roll sine wave from 20Hz all the way up to 22kHz, and record it to a
CD, the drop off at the high end of that would be instantaneous,
not a soft gradual roll off as on a record or cassette.

No, the rolloff is NOT instantaneous, nothing like it.
While maybe steeper than a cassette, it is far from
the hypothetical "brick wall." A theoretically perfect
brick wall filter is not possible to implement in a
practical world.

Rather it merely has to be "good enough," which means
that across the significant audio bandwidth, it has to
affect the audible response in the least possible way,
and as you approach 1/2 the sampling rate, it has to
be far enough down so that any expected images or
aliases are sufficiently attenuated so has to have no
impact.

That means that if you're criteria is to be flat at 20 kHz,
you have a 2.05 kHz transition band available for your
filter, and that provides sufficient margin for many
competent filter implementations.

[GregS[_3_]]

In article , "geoff" wrote:
ChrisCoaster wrote:
If you have a sound with a frequency of 20kHz, that means that the
wave has to go through one complete cycle 20.000 times a second.
That is: up *and* down 20.000 times a second. To record that you
need to record both the ups and the downs, so that's 20.000 ups plus
20.000 downs, makes 40.000 samples per second.

Why they went up from 40kHz to 44.1kHz is some weird technical
reason, explained in another part of this thread.

You are, I hope, joking. Cos that is total crap.

geoff- Hide quoted text -

- Show quoted text -
_______________
Alrighty geoff, then YOU explain it. And remember, easy on the math!

'Cause at this point I'm about hit "Wiki" in my favorites.
-CC

Simple. To accurately recontruct a waveform, you need two samples of the
highest frequency specificied. Look up Nyquist Theory on you beloved Wiki -
even Wiki has that right !

Nothing to do with 20KHz worth or 'up' and 20KHz worth of 'down'.

Not any waveform. Just a sinewave.

Its interesting aliasing can be seen with nothing to do with time,
but its still a sample, such as with viewing lines through various
filters.

greg

[[email protected]]

On Jul 11, 8:24 am, (GregS) wrote:
Not any waveform. Just a sinewave.

No, it's ANY waveform whose components are within
the Nyquist bandwidth (i.e., 1/2 sample rate) ANY
waveform whatsoever.

[Willem]

wrote:
) On Jul 11, 8:24 am, (GregS) wrote:
) Not any waveform. Just a sinewave.
)
) No, it's ANY waveform whose components are within
) the Nyquist bandwidth (i.e., 1/2 sample rate) ANY
) waveform whatsoever.

And what are those components ? Sine waves, right ?


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

[Chronic Philharmonic]

"geoff" wrote in message
...
Chronic Philharmonic wrote:
"geoff" wrote in message
...
Earl Kiosterud wrote:
I notice that Audacity displays sample points connected with
straight lines. I don't think that's meaningful. Other editors
show what a post-filter, presumably at 20 KHz, would show, along
with the sample points.


That would be very presumptive of editing software. SOundForge shows
straight lines between samples - I would be most peeved if SF made an
arbitrary decision about a filter.

If they're drawing lines between samples, they're making arbitrary
decisions about filters. They should either be properly smoothing the
waveform (they know the sample rate don't they?), or showing a
histogram, in my opinion.

Naa , a line is important in case you miss a sample dot out of field of
veiw. Histogram just plain messy, and how thick would you make the columns
?

The columns on the editor I use are as thick as the dots that represent the
samples. There are a multitude of ways to represent off-screen data in the
UI that does not imply something that is misleading and mathematically
incorrect. I still maintain, if they are going to connect the samples with
lines, the line should reflect the smoothing filter that the sample rate
demands.

[Industrial One]

On Jul 10, 6:56 pm, wrote:
[SNIP]

Very eloquent explanation. However, anything spinning over 24 fps will
appear spinning backwards to the human eye anyway, cuz we don't
perceive movement any more precise than that, just like no
neurotypical dude can hear over 20 KHz.

[Mark Nelson]

On Jul 11, 3:20*pm, Willem wrote:

And what are those components ? *Sine waves, right ?


You're stretching it a bit there. A waveform is a waveform. It happens
that we can take an arbitrary waveform and decompose it into a set of
sine waves. But we can also decompose it into other types of
components. It's not like sine waves are some atomic part of any
waveform. It just means that we have built up a body of analysis that
works very well on sine waves, and as a result, decomposing it that
way is convenient.

You can also decompose an arbitrary waveform into wavelets, and then
use different analysis tools. But if this leads to arguments about
whether waveforms are made up of wavelets or sine waves, well, it
shows that we're missing the point.

- Mark

[[email protected]]

On Jul 12, 10:58 am, Industrial One
wrote:
On Jul 10, 6:56 pm, wrote:
[SNIP]

Very eloquent explanation. However, anything spinning over 24 fps will
appear spinning backwards to the human eye anyway, cuz we don't
perceive movement any more precise than that, just like no
neurotypical dude can hear over 20 KHz.

No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

There's another phenomenon called "aperture error" or
"aperture filtering" which, in the case of the eye, is due to
the so-called "persistance of vision." It, in effect, "smears"
the continuous input of images.

Now, interestingly enough, as a complete analog
of my wagon-wheel gadanken, how might we prevent
the aliasing of the wagon wheel motion, given that we're
limited to, oh, 25 frames per second?

Well, you "low-pass filter" the wagon wheel, i.e.,
blur its motion, before sampling, so that nothing
makes it through to the "sampler (the shutter"
any faster than 1/2 the shutter or frame rate
That's a little tough to actually accomplish in the
real world. "Aperture filtering" is one way to get
part of the way the you make your shutter speed
as long as possible. IN the simplest case, it can't
be any longer than 1/25 of a second, which would
have the effect of blurring things enough such that
things goign faster that 1/25 of a second are blurred
wnough that there's nothuing left to alias, but doesn't
solve the problem of what happens between 12.5
and 25/second.

One might imaging a somewhat more sophisticated'
movie camera that exposes each frame for, say 1/12
second, but interleaves frames such that halway
through frame 1, it starts exposing frames to, and
frame 1 continues unitl halway through frame 2, at
which point frame 3 starts getting esposed, etc.

[UnsteadyKen]

Industrial One said:

Very eloquent explanation. However, anything spinning over 24 fps will
appear spinning backwards to the human eye anyway, cuz we don't
perceive movement any more precise than that, just like no
neurotypical dude can hear over 20 KHz.

We may not "hear" it but the brain registers the presence of
frequencies considerably higher than 20Khz:
http://jn.physiology.org/cgi/content/full/83/6/3548

Similarly we are aware of visual frequencies over 24FPS. Most of us
have come across a faulty fluorescent tube or a computer screen with
a low 50-60hz refresh rate which we sense as an almost subliminal
flicker.

--
Ken

[Richard Crowley]

wrote ...
No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

Then you've never done the experiment youself.
Actually try it and get back to us.

[Industrial One]

On Jul 13, 3:05 pm, wrote:
On Jul 12, 10:58 am, Industrial One
wrote:

On Jul 10, 6:56 pm, wrote:
[SNIP]

Very eloquent explanation. However, anything spinning over 24 fps will
appear spinning backwards to the human eye anyway, cuz we don't
perceive movement any more precise than that, just like no
neurotypical dude can hear over 20 KHz.

No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

Depends how fast they spin. If enough to capture part of the frame
when you focus your eyes on the oscillation rather than on a fixed
spot at the wheel then it may appear to spin backwards, at least with
me. I believe the limit is... yeah, 'bout 25 fps. If spinning faster,
'course it'll appeared blurred.

There's another phenomenon called "aperture error" or
"aperture filtering" which, in the case of the eye, is due to
the so-called "persistance of vision." It, in effect, "smears"
the continuous input of images.

Now, interestingly enough, as a complete analog
of my wagon-wheel gadanken, how might we prevent
the aliasing of the wagon wheel motion, given that we're
limited to, oh, 25 frames per second?

Well, you "low-pass filter" the wagon wheel, i.e.,
blur its motion, before sampling, so that nothing
makes it through to the "sampler (the shutter"
any faster than 1/2 the shutter or frame rate
That's a little tough to actually accomplish in the
real world. "Aperture filtering" is one way to get
part of the way the you make your shutter speed
as long as possible. IN the simplest case, it can't
be any longer than 1/25 of a second, which would
have the effect of blurring things enough such that
things goign faster that 1/25 of a second are blurred
wnough that there's nothuing left to alias, but doesn't
solve the problem of what happens between 12.5
and 25/second.

Isn't it already blurred before sampling? No camera captures a
perfectly still image. All got expose times of at least 1/30th a
second, 1/40th 1/50th whatever so if you took a picture of a wheel
spinning faster than that, it'll already be blurred.

One might imaging a somewhat more sophisticated'
movie camera that exposes each frame for, say 1/12
second, but interleaves frames such that halway
through frame 1, it starts exposing frames to, and
frame 1 continues unitl halway through frame 2, at
which point frame 3 starts getting esposed, etc.

You got it.

On Jul 13, 4:24 pm, UnsteadyKen wrote:
Industrial One said:

Very eloquent explanation. However, anything spinning over 24 fps will
appear spinning backwards to the human eye anyway, cuz we don't
perceive movement any more precise than that, just like no
neurotypical dude can hear over 20 KHz.

We may not "hear" it but the brain registers the presence of
frequencies considerably higher than 20Khz:http://jn.physiology.org/cgi/content/full/83/6/3548

Yeah, it's called binaural beats, you don't hear it, you feel it.. I
once had the priviledge of getting a friend's high-class electronic
playground all to myself for a while, I had this bigass subwoofer
output about 5 Hz and amplified the volume to hardcore maximum. I
didn't hear **** but I swear to god I FELT like ****, like a complete
drop to an emo kid's state -- suicide (I swear to god.) It's real hurl-
inducing too I should note. I wonder what happens at say 500 dB, 1000.
Can some mind-control weapon be built on this idea? Drop an emo bomb
on Iran, say?

I got a ****load of binaural beat files that advertise mind-altering
effects, I only tried S-Angel2 so far (valium) which actually worked.
I gotta try the heroin and microdots ones sometime to see if they
work.

Similarly we are aware of visual frequencies over 24FPS. Most of us
have come across a faulty fluorescent tube or a computer screen with
a low 50-60hz refresh rate which we sense as an almost subliminal
flicker.

--
Ken

You're imagining it, take your pills boi.

[UnsteadyKen]

Industrial One said:

Yeah, it's called binaural beats, you don't hear it, you feel it..

It's called infra sound. The US, UK and Russian military investigated
its use as a weapon years ago. It works but can't be projected so it
wasn't practical. The same effect is can cause problems in large
buildings with lift shafts which act as Helmholtz resonators.
It appears to be a very old technique:
http://www.orkneyjar.com/history/tom...bacoustics.htm

once had the priviledge of getting a friend's high-class electronic
playground all to myself for a while, I had this bigass subwoofer
output about 5 Hz and amplified the volume to hardcore maximum. I
didn't hear **** but I swear to god I FELT like ****, like a complete
drop to an emo kid's state -- suicide (I swear to god.) It's real hurl-
inducing too I should note. I wonder what happens at say 500 dB, 1000.
Can some mind-control weapon be built on this idea?
Yes it'scalled shell shock.

Drop an emo bomb
on Iran, say?

No I don't say. You can practise on Calgary if you like.

--
Ken

[isw]

In article ,
"Richard Crowley" wrote:

wrote ...
No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

Then you've never done the experiment youself.
Actually try it and get back to us.

Are you confusing what happens in the movies ( a sampled data situation
if there ever was one), with what happens in "real life"?

Isaac

[Mr.T]

"Industrial One" wrote in message
...
I wonder what happens at say 500 dB, 1000.

If you mean dB SPL, that is a physical impossibility at normal atmospheric
pressures. From memory a swing to complete vacuum, gives around 180dB SPL,
but of course an asymmetrical swing to greater than twice air pressure can
increase things a bit.

MrT.

[[email protected]]

On Jul 14, 1:04 am, "Mr.T" MrT@home wrote:
"Industrial One" wrote in message

...

I wonder what happens at say 500 dB, 1000.

If you mean dB SPL, that is a physical impossibility at normal
atmospheric pressures. From memory a swing to complete
vacuum, gives around 180dB SPL,

Actually, this is a common misconception. The low
pressure swing has no need to go to a vacuum for
symmetry. It merely has to match, factor-wise, the
positive swing.

For example, consider a symmetrical pressure swing
where it swings a factor of 2 about atmospheric pressu
that means at its highest, the pressue is twice that of
atmosphere, and at its loweest,. it's hald that of
atmosphere.

New, where the problem comes is the fact that such
large swings cause nonlinearities because the ideal
gas equation likely no longer holds. Consider:

PV = n R T

which, for small swings in atmospheric pressure
typical with "survivable" (!) sound pressure levels
holds true. THere are some underlyingh assumpitons,
e.g., that based as it is on the kinetic theory of gases,
that the medium consist of essentially point sized
particles that interact through perfectly elastic collisions,
and that the properties of these particles are independent
of temperature, and that, as far as sound is concerned.

These are all true fro small deltas of P, V, and, especially T,
but is increasingly less true as the magnitude of things
start to increase. For example, air molecules behave like
point paritcles at low sound levels, but no so at high
levels. You encounter abrupt discontinuities as energies
start to reach those necessary to effect diassociation of
air molecules, changes in the linearity as more energy is
put into non-translational kinietic ebergy modes, i.e.,
expanding of the molecule, and so on.

But as far as "hitting" the vacuum at the bottom of the
swing: nope. That's not an issue.

Consider one more gedanken: imagine a speaker set in
the wall of a seled room. One could determine that for
all frequecnies whose wavelength is large compared to
the dimensions of the room, the sound pressure level
in the room is a function of the excursion of that speaker.
In other words, assuming we adopt the convention
that excursion of the speaker's diaphragm into the room
is "positive, then since the volume in the room is an
inverse function of the excursion of the diaphragm, and
that pressure is an inverse function volume, then pressure
is a function of diaphragmn excursion.

(for the nit-pickers, whether we are ssuming adiabatic
or isothermal conditions is relatively unimportant at
this point since the end conclusion remains essentially
the same in principle, differing only in magnitude)

Now, let's take you assertion:

"a swing to complete vacuum, gives around
"180dB SPL,

Ignoring whether it's 180 dB or 210 dB or a bazillion
dB, how far sdoes that diaphragm have to move in the
nnegative direction to create a pure vacuum in that room?
(hint, it has to move an infinitie distance).

More importantly, if you want to skip a WHOLE bunch
of physics, how much energy would it take to create that
vacuum? (hint: inifinite).

All this because:

PV = nRt

or,

P = nRT/V

Thus, the relation is a reciprocal one, and when
you plot the relation, you end up with a function whose
graph is asymtotic to the X and Y axes and can never
equal 0.

[[email protected]]

On Jul 13, 7:11 pm, "Richard Crowley" wrote:
wrote ...

No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

Then you've never done the experiment youself.
Actually try it and get back to us.

Actually, I have. Any number of times.

Could you elcidtae the conditions under which you
saw the phenomenon?

[[email protected]]

On Jul 13, 7:25 pm, Industrial One wrote:
On Jul 13, 3:05 pm, wrote:
Well, you "low-pass filter" the wagon wheel, i.e.,
blur its motion, before sampling, so that nothing
makes it through to the "sampler (the shutter"
any faster than 1/2 the shutter or frame rate
That's a little tough to actually accomplish in the
real world. "Aperture filtering" is one way to get
part of the way the you make your shutter speed
as long as possible. IN the simplest case, it can't
be any longer than 1/25 of a second, which would
have the effect of blurring things enough such that
things goign faster that 1/25 of a second are blurred
wnough that there's nothuing left to alias, but doesn't
solve the problem of what happens between 12.5
and 25/second.

Isn't it already blurred before sampling? No camera captures a
perfectly still image. All got expose times of at least 1/30th a
second, 1/40th 1/50th whatever so if you took a picture of a wheel
spinning faster than that, it'll already be blurred.

Professional movia and video cameras allow you
to adjust shutter speed independently of frame
rate, up to a limit. For example, with a rate of 25 FPS,
you could set the shutter speed to anything from, say,
1/1000 second to about 1/30 second in the normal
1-stop increments. This was to allow you to be able
to say, have a shallow depth of field (wide aperture)
under birght light without having to switch to a slower
film.

[[email protected]]

On Jul 11, 4:20 pm, Willem wrote:
wrote:

) On Jul 11, 8:24 am, (GregS) wrote:
) Not any waveform. Just a sinewave.
)
) No, it's ANY waveform whose components are within
) the Nyquist bandwidth (i.e., 1/2 sample rate) ANY
) waveform whatsoever.

And what are those components ? Sine waves, right ?

Read it again: he said:

"Not any waveform, Just a sinewave"

"Just a sine wave," which seems to definitely say
"one" sinewave.

[[email protected]]

On Jul 11, 8:24 am, (GregS) wrote:
Its interesting aliasing can be seen with nothing to do with time,
but its still a sample, such as with viewing lines through various
filters.

It has to do with "sampling," and as such has the
property whether one is sampling in the time domain
or one is sampling in the spatial domain. It could happen
in ANY data domain: take a bunch of social security
number gathered at random. Sort them numerically.
Pick out every 5, and try to tell us that most social security
numbers end in 2 or 7. or 4 and 9, or some such. Aliasing.

[ChrisCoaster]

On Jul 14, 1:59*pm, wrote:
On Jul 13, 7:11 pm, "Richard Crowley" wrote:

wrote ...

No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

Then you've never done the experiment youself.
Actually try it and get back to us.

Actually, I have. Any number of times.

Could you elcidtae the conditions under which you
saw the phenomenon?
___________________________
Sure! I'll elcidtae one instance - actually many - when I've seen the
spinning backward phenonemon:

Whenever a pilot throttles up a prop-driven plane or turns the engines
off. For a brief few seconds the props appear to change directions as
the props wind down to a stand still.

Ever elcidtae that? If you haven't then you are either blind or from
a planet where propellers are not used for air transport.

-CC

[Mr.T]

wrote in message
...
On Jul 14, 1:04 am, "Mr.T" MrT@home wrote:
"Industrial One" wrote in message
...

I wonder what happens at say 500 dB, 1000.

If you mean dB SPL, that is a physical impossibility at normal
atmospheric pressures. From memory a swing to complete
vacuum, gives around 180dB SPL,

Actually, this is a common misconception. The low
pressure swing has no need to go to a vacuum for
symmetry. It merely has to match, factor-wise, the
positive swing.

For example, consider a symmetrical pressure swing
where it swings a factor of 2 about atmospheric pressu
that means at its highest, the pressue is twice that of
atmosphere, and at its loweest,. it's hald that of
atmosphere.

New, where the problem comes is the fact that such
large swings cause nonlinearities because the ideal
gas equation likely no longer holds. Consider:

PV = n R T

which, for small swings in atmospheric pressure
typical with "survivable" (!) sound pressure levels
holds true. THere are some underlyingh assumpitons,
e.g., that based as it is on the kinetic theory of gases,
that the medium consist of essentially point sized
particles that interact through perfectly elastic collisions,
and that the properties of these particles are independent
of temperature, and that, as far as sound is concerned.

These are all true fro small deltas of P, V, and, especially T,
but is increasingly less true as the magnitude of things
start to increase. For example, air molecules behave like
point paritcles at low sound levels, but no so at high
levels. You encounter abrupt discontinuities as energies
start to reach those necessary to effect diassociation of
air molecules, changes in the linearity as more energy is
put into non-translational kinietic ebergy modes, i.e.,
expanding of the molecule, and so on.

But as far as "hitting" the vacuum at the bottom of the
swing: nope. That's not an issue.

Consider one more gedanken: imagine a speaker set in
the wall of a seled room. One could determine that for
all frequecnies whose wavelength is large compared to
the dimensions of the room, the sound pressure level
in the room is a function of the excursion of that speaker.
In other words, assuming we adopt the convention
that excursion of the speaker's diaphragm into the room
is "positive, then since the volume in the room is an
inverse function of the excursion of the diaphragm, and
that pressure is an inverse function volume, then pressure
is a function of diaphragmn excursion.

(for the nit-pickers, whether we are ssuming adiabatic
or isothermal conditions is relatively unimportant at
this point since the end conclusion remains essentially
the same in principle, differing only in magnitude)

Now, let's take you assertion:

"a swing to complete vacuum, gives around
"180dB SPL,

Ignoring whether it's 180 dB or 210 dB or a bazillion
dB, how far sdoes that diaphragm have to move in the
nnegative direction to create a pure vacuum in that room?
(hint, it has to move an infinitie distance).

More importantly, if you want to skip a WHOLE bunch
of physics, how much energy would it take to create that
vacuum? (hint: inifinite).

All this because:

PV = nRt

or,

P = nRT/V

Thus, the relation is a reciprocal one, and when
you plot the relation, you end up with a function whose
graph is asymtotic to the X and Y axes and can never
equal 0.


Thanks for the comprehensive analysis Dick, I am certainly no expert on this
topic. (yes that's obvious :-)
However I wasn't quite so stupid as to think a vacuum was possible from any
sound driver, or indeed a "perfect vacuum" even in space. My only point was
that *I think* 500dB or 1000dB SPL is still *theoretically* impossible in
air.

So is there in fact a *theoretical limit* (ignoring physical implementation,
and non linearities), and if so do you know what it is?
Or am I completely off base here?

MrT.

[ChrisCoaster]

On Jul 15, 4:22*am, "Mr.T" MrT@home wrote:
. My only point was
that *I think* 500dB or 1000dB SPL is still *theoretically* impossible in
air.

MrT.- Hide quoted text -

- Show quoted text -
____________________
Ask anyone who has been within 5-miles of a 1950s atomic bomb test.

-CC

[Mr.T]

"ChrisCoaster" wrote in message
...
My only point was
that *I think* 500dB or 1000dB SPL is still *theoretically* impossible in
air.
____________________
Ask anyone who has been within 5-miles of a 1950s atomic bomb test.

Ask them what exactly? Whether they have a better clue what 500dB-1000dB SPL
means than you?
No one could have first hand experience and still be alive, assuming it is
even possible.

MrT.

[Richard Crowley]

"Mr.T" wrote ...
"ChrisCoaster" wrote ...
My only point was
that *I think* 500dB or 1000dB SPL is still *theoretically* impossible
in
air.
____________________
Ask anyone who has been within 5-miles of a 1950s atomic bomb test.

Ask them what exactly? Whether they have a better clue what 500dB-
1000dB SPL means than you?
No one could have first hand experience and still be alive, assuming it is
even possible.

"Around 260 personnel were present, none closer than 5.6 miles (9 km)."
http://en.wikipedia.org/wiki/Trinity_test

[Phil Carmody]

"Richard Crowley" writes:
"Mr.T" wrote ...
"ChrisCoaster" wrote ...
My only point was
that *I think* 500dB or 1000dB SPL is still *theoretically* impossible
in
air.
____________________
Ask anyone who has been within 5-miles of a 1950s atomic bomb test.

Ask them what exactly? Whether they have a better clue what 500dB-
1000dB SPL means than you?
No one could have first hand experience and still be alive, assuming it is
even possible.

"Around 260 personnel were present, none closer than 5.6 miles (9 km)."
http://en.wikipedia.org/wiki/Trinity_test

And as this is cross-posted to comp.compression, I can
only assume that you audio dudes want to know how to compress
5.6 miles to less than 5 miles?

Phil
--
Dear aunt, let's set so double the killer delete select all.
-- Microsoft voice recognition live demonstration

[Industrial One]

On Jul 13, 8:03 pm, UnsteadyKen wrote:
Industrial One said:

Yeah, it's called binaural beats, you don't hear it, you feel it..

It's called infra sound. The US, UK and Russian military investigated
its use as a weapon years ago. It works but can't be projected so it
wasn't practical. The same effect is can cause problems in large
buildings with lift shafts which act as Helmholtz resonators.
It appears to be a very old technique:http://www.orkneyjar.com/history/tom...bacoustics.htm

I know what infrasound is, binaural beats consist not only of
infrasound but audible frequencies as well as ultrasound. After
reading an article with astute, detailed information on infrasound, I
think my induced emotional drop and nausea was realistic after all.
The sound makes surrounding particles vibrate -- more times per second
with higher frequencies and shake harder with additional dBs. So, if
the volume was cranked to something like 1000 dB, your particles would
probably vibrate so violently they'd fall outta place and you'd
crumble.

Yes it'scalled shell shock.

Drop an emo bomb
on Iran, say?

No I don't say. You can practise on Calgary if you like.

--
Ken

It's not too late to move outta that ******** you know. I once moved
there thinking the prices would be low since the city is essentially
an isolated Bum****ville with practically nowhere worth going and
plenty of oil-pumping jobs available. I was surprised to find it
jewfiltrated to the ****ing max.

[Industrial One]

On Jul 14, 7:39 pm, ChrisCoaster wrote:
On Jul 14, 1:59 pm, wrote: On Jul 13, 7:11 pm, "Richard Crowley" wrote:

wrote ...

No, we do NOT perceive them as "spinning backwards,"
we perceive them as blurred.

Then you've never done the experiment youself.
Actually try it and get back to us.

Actually, I have. Any number of times.

Could you elcidtae the conditions under which you
saw the phenomenon?

___________________________
Sure! I'll elcidtae one instance - actually many - when I've seen the
spinning backward phenonemon:

Whenever a pilot throttles up a prop-driven plane or turns the engines
off. For a brief few seconds the props appear to change directions as
the props wind down to a stand still.

Ever elcidtae that? If you haven't then you are either blind or from
a planet where propellers are not used for air transport.

-CC

lol, true.

[Mr.T]

"Richard Crowley" wrote in message
. ..
My only point was
that *I think* 500dB or 1000dB SPL is still *theoretically* impossible
in air.
____________________
Ask anyone who has been within 5-miles of a 1950s atomic bomb test.

Ask them what exactly? Whether they have a better clue what 500dB-
1000dB SPL means than you?
No one could have first hand experience and still be alive, assuming it
is
even possible.

"Around 260 personnel were present, none closer than 5.6 miles (9 km)."
http://en.wikipedia.org/wiki/Trinity_test



Doesn't say they experienced 500dB-1000dB SPL though does
it!!!!!!!!!!!!!!!!!!!!!!!
(And IF it did it would be total bull**** anyway)

If they can still hear at all, it was probably less than 120-140dB SPL at
5.6 miles, and an unknown level at ground zero

MrT.