OFF TOPIC; Resonance of open end tube and node position...

[John Walliker]

On Saturday, 18 December 2021 at 13:27:15 UTC, amdx wrote:

My question why is my testing seeing results backwards from what I see
in the literature.
It depends upon whether your microphone is sensitive to pressure changes or velocity changes.
There will be maximum pressure changes (with zero velocity) at the centre and maximum velocity changes at the ends.
The open tube can be considered as being two closed-end tubes connected at the centre then removing the ends.
kw
I didn\'t know that existed, here\'s the mic, can you tell what type it is?

https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf
Mikek

That microphone responds to pressure and not velocity. Its a nice microphone
I have used a lot of them.
John

 

[amdx]

On 12/17/2021 11:51 PM, Jasen Betts wrote:

On 2021-12-18, amdx <amdx@knology.net> wrote:
On 12/17/2021 7:51 PM, Rich S wrote:
Your answer leaves me in the dark, I\'m looking for some light.

Mikek
--
Hi Mikek,
Let\'s all try to be light producing ;-)
Could you re-post your exact question, again,
if you don\'t mind, BUT without the formatting?
(On my computer, the post was horribly littered with a
characters, /, * My brain shuts down if I have to read
something that non-decipherable.)
regards, RS
Sure, I copied and pasted from a pdf and it went bad.

I have a 22\" long tube with open ends. ( ID is 1-3/16\")

 I find a resonance at 300Hz. My understanding is a tube with both ends
open will have null (node)

in the center and antinode at both ends. I would think at a node, a mic
would pick up little sound and at the
Because of the counter-intuitive way that open and closed are defined
in electonics, in pipes open and closed are the opposite, so an open in
a pipe (gas-accoustic transmission line) is like a short in an
electrical transmission line, so it gets you a node. (pressure
minimum, velocity maximum)

There will be an antinode (pressure maximum,velocity minumum) in the middle if it\'s driven at
its fundamental frequency (or some odd harmonic).

 I went to pull up a couple of diagrams showing the opposite, showing
antinodes at the open ends.

The first hit was Khan academy, showing what I was looking for, BUT,
reading further, it said,

\"Another way to think about standing waves in open tubes is by instead
considering how the air pressure varies

along the length of the tube. For open tubes, the air pressure at the
ends equalizes with the atmosphere.

Thus, the pressure stays constant at the open ends and they are pressure
nodes.\"

It goes on to show an air pressure diagram, with nulls at the open ends
and an antinode in the center.

> https://www.khanacademy.org/science/in-in-class11th-physics/in-in-11th-physics-waves/in-in-11th-physics-sound-topic/a/intro-to-sound-ap1
A couple other posts brought up the pressure/velocity idea and what type
mic I had. I hope to glean more from their responses.

                                                      Mikek



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[amdx]

On 12/18/2021 7:45 AM, John Walliker wrote:

On Saturday, 18 December 2021 at 13:27:15 UTC, amdx wrote:

My question why is my testing seeing results backwards from what I see
in the literature.
It depends upon whether your microphone is sensitive to pressure changes or velocity changes.
There will be maximum pressure changes (with zero velocity) at the centre and maximum velocity changes at the ends.
The open tube can be considered as being two closed-end tubes connected at the centre then removing the ends.
kw
I didn\'t know that existed, here\'s the mic, can you tell what type it is?

https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf
Mikek

That microphone responds to pressure and not velocity. Its a nice microphone
I have used a lot of them.
John

Great, that seems to be making sense with what I\'m measuring.

I do have a question about the mic the author used, a 2-1/8\" Lafeyette
PA-27 Crystal Cartridge.

Because the author mounted it at the end of a 1/2 wave open end tube, my
thinking is the crystal

cartridge is a velocity mic. What do you think?

                                    Mikek


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[Phil Allison]

amdx wrote:
=========

With a mic setting at the open end
** What mic are you using???

.... Phil

This is what I had purchased a few years ago and will probably use
if/when I do this.

https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf

** Ok - it\'s a *small* pressure mic.

Not a cardiod ( aka pressure gradient ) type.
That could have invalidated all your tests.

( Nor a velocity mic either )


..... Phil

 

[amdx]

On 12/18/2021 8:21 AM, Phil Allison wrote:

amdx wrote:
=========
With a mic setting at the open end
** What mic are you using???

.... Phil

This is what I had purchased a few years ago and will probably use
if/when I do this.

https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf
** Ok - it\'s a *small* pressure mic.

Not a cardiod ( aka pressure gradient ) type.
That could have invalidated all your tests.

( Nor a velocity mic either )


.... Phil

Oh, this is all good, it tells me why I found the measurements numbers I
did.

 It also tells me I could use a closed end 1/4 wavelength tube and
mount the mic in the closed end.

I\'m about to cut a 1/4 wavelength tube and verify the same amplitude as
on the open end 1/2 wavelength tube with center mic.

 Btw, do you know what type, Pressure or Velocity a  Lafeyette PA-27,
2-1/8\" Crystal Cartridge would be.

Because of the way the author used it, I\'m guessing Velocity.

                                           Thanks for the input, Mikek


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[Phil Hobbs]

amdx wrote:

On 12/17/2021 7:51 PM, Rich S wrote:
Your answer leaves me in the dark, I\'m looking for some light.

                                                   Mikek
--
Hi Mikek,
Let\'s all try to be light producing ;-)
Could you re-post your exact question, again,
if you don\'t mind, BUT without the formatting?
(On my computer, the post was horribly littered with a
characters, /, *    My brain shuts down if I have to read
something that non-decipherable.)
regards, RS

Sure, I copied and pasted from a pdf and it went bad.

I have a 22\" long tube with open ends. ( ID is 1-3/16\")

 I find a resonance at 300Hz. My understanding is a tube with both ends
open will have null (node)

in the center and antinode at both ends. I would think at a node, a mic
would pick up little sound and at the

antinode the mic would pick up maximum sound. What I find is minimum
signal amplitude from the mic at the

end (opening) and maximum signal in the center of the tube.

The centre is a _velocity_ node but a _pressure_ antinode.

The ends are not exactly at the nodal points because there\'s a
mass-spring loading at the ends due to having to move ambient air out of
the way. (This is the same physics as the Helmholtz resonance.)

The Helmholtz resonance is always the lowest mode of any cavity with one
opening. (There\'s obviously a DC mode when both ends are open.)

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[amdx]

On 12/18/2021 10:38 AM, Phil Hobbs wrote:

amdx wrote:
On 12/17/2021 7:51 PM, Rich S wrote:
Your answer leaves me in the dark, I\'m looking for some light.

                                                   Mikek
--
Hi Mikek,
Let\'s all try to be light producing ;-)
Could you re-post your exact question, again,
if you don\'t mind, BUT without the formatting?
(On my computer, the post was horribly littered with a
characters, /, *    My brain shuts down if I have to read
something that non-decipherable.)
regards, RS

Sure, I copied and pasted from a pdf and it went bad.

I have a 22\" long tube with open ends. ( ID is 1-3/16\")

  I find a resonance at 300Hz. My understanding is a tube with both
ends open will have null (node)

in the center and antinode at both ends. I would think at a node, a
mic would pick up little sound and at the

antinode the mic would pick up maximum sound. What I find is minimum
signal amplitude from the mic at the

end (opening) and maximum signal in the center of the tube.

The centre is a _velocity_ node but a _pressure_ antinode.

The ends are not exactly at the nodal points because there\'s a
mass-spring loading at the ends due to having to move ambient air out
of the way.  (This is the same physics as the Helmholtz resonance.)

The Helmholtz resonance is always the lowest mode of any cavity with
one opening.  (There\'s obviously a DC mode when both ends are open.)

Cheers

Phil Hobbs


Thanks Phil,

    I found answer that answer on a Khan Accademy website. So, with
that info I can mic the closed end of a tube and use 1/4 wavelength

tube, cutting my longest (300 Hz resonance) tube down to 11\".

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

Thanks, Mikek


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[Phil Hobbs]

amdx wrote:

On 12/18/2021 10:38 AM, Phil Hobbs wrote:
amdx wrote:
On 12/17/2021 7:51 PM, Rich S wrote:
Your answer leaves me in the dark, I\'m looking for some light.

                                                   Mikek
--
Hi Mikek,
Let\'s all try to be light producing ;-)
Could you re-post your exact question, again,
if you don\'t mind, BUT without the formatting?
(On my computer, the post was horribly littered with a
characters, /, *    My brain shuts down if I have to read
something that non-decipherable.)
regards, RS

Sure, I copied and pasted from a pdf and it went bad.

I have a 22\" long tube with open ends. ( ID is 1-3/16\")

  I find a resonance at 300Hz. My understanding is a tube with both
ends open will have null (node)

in the center and antinode at both ends. I would think at a node, a
mic would pick up little sound and at the

antinode the mic would pick up maximum sound. What I find is minimum
signal amplitude from the mic at the

end (opening) and maximum signal in the center of the tube.

The centre is a _velocity_ node but a _pressure_ antinode.

The ends are not exactly at the nodal points because there\'s a
mass-spring loading at the ends due to having to move ambient air out
of the way.  (This is the same physics as the Helmholtz resonance.)

The Helmholtz resonance is always the lowest mode of any cavity with
one opening.  (There\'s obviously a DC mode when both ends are open.)

Cheers

Phil Hobbs


Thanks Phil,

    I found answer that answer on a Khan Accademy website. So, with
that info I can mic the closed end of a tube and use 1/4 wavelength

tube, cutting my longest (300 Hz resonance) tube down to 11\".

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

Thanks, Mikek

You have to worry about the phases, of course, but I tend to be partial
to wiring that sort of stuff in series. Phil A will of course trash
that idea.



Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Walliker]

On Saturday, 18 December 2021 at 17:14:58 UTC, amdx wrote:

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

The simplest way is not the best. Each mic will draw a current of about
0.5mA from the pullup resistor. That current is modulated with the
audio signal. The mic needs around 1.5V across it, but would survive
with considerably more - up to about 10V.
So if you have four mics you could connect them all in parallel and use
a single lower value resistor calculated to give a standing voltage of
about 1.5V at 2mA. If you use a 3.3V power supply, the resistor will be
(3.3-1.5)/2mA = 900 ohm.
The problem with this method is that the output audio current from
each mic is being applied to a lower value resistor than normal, so the
output is lower in proportion to the number of mics which will
worsen the noise performance of the system.
With four mics the output will be reduced by a factor of four, or 12dB.
If you can cope with this gain reduction, the parallel approach should
work well. Don\'t try to connect them in series. You will not like the result.
A better approach would be to have a separate preamp for each mic
and mix the outputs of the preamps, but this is a lot more complex.

John

 

[amdx]

On 12/18/2021 12:11 PM, Phil Hobbs wrote:

amdx wrote:
On 12/18/2021 10:38 AM, Phil Hobbs wrote:
amdx wrote:
On 12/17/2021 7:51 PM, Rich S wrote:
Your answer leaves me in the dark, I\'m looking for some light.

                                                   Mikek
--
Hi Mikek,
Let\'s all try to be light producing ;-)
Could you re-post your exact question, again,
if you don\'t mind, BUT without the formatting?
(On my computer, the post was horribly littered with a
characters, /, *    My brain shuts down if I have to read
something that non-decipherable.)
regards, RS

Sure, I copied and pasted from a pdf and it went bad.

I have a 22\" long tube with open ends. ( ID is 1-3/16\")

  I find a resonance at 300Hz. My understanding is a tube with both
ends open will have null (node)

in the center and antinode at both ends. I would think at a node, a
mic would pick up little sound and at the

antinode the mic would pick up maximum sound. What I find is
minimum signal amplitude from the mic at the

end (opening) and maximum signal in the center of the tube.

The centre is a _velocity_ node but a _pressure_ antinode.

The ends are not exactly at the nodal points because there\'s a
mass-spring loading at the ends due to having to move ambient air
out of the way.  (This is the same physics as the Helmholtz resonance.)

The Helmholtz resonance is always the lowest mode of any cavity with
one opening.  (There\'s obviously a DC mode when both ends are open.)

Cheers

Phil Hobbs


Thanks Phil,

     I found answer that answer on a Khan Accademy website. So, with
that info I can mic the closed end of a tube and use 1/4 wavelength

tube, cutting my longest (300 Hz resonance) tube down to 11\".

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

Thanks, Mikek



You have to worry about the phases, of course, but I tend to be
partial to wiring that sort of stuff in series.  Phil A will of course
trash that idea.



Cheers

Phil Hobbs

 I don\'t see how to wire electret mics that have internal fets in series.

Am I missing something simple?

                              Mikek



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[piglet]

On 18/12/2021 5:14 pm, amdx wrote:

On 12/18/2021 10:38 AM, Phil Hobbs wrote:
amdx wrote:
On 12/17/2021 7:51 PM, Rich S wrote:
Your answer leaves me in the dark, I\'m looking for some light.

                                                   Mikek
--
Hi Mikek,
Let\'s all try to be light producing ;-)
Could you re-post your exact question, again,
if you don\'t mind, BUT without the formatting?
(On my computer, the post was horribly littered with a
characters, /, *    My brain shuts down if I have to read
something that non-decipherable.)
regards, RS

Sure, I copied and pasted from a pdf and it went bad.

I have a 22\" long tube with open ends. ( ID is 1-3/16\")

  I find a resonance at 300Hz. My understanding is a tube with both
ends open will have null (node)

in the center and antinode at both ends. I would think at a node, a
mic would pick up little sound and at the

antinode the mic would pick up maximum sound. What I find is minimum
signal amplitude from the mic at the

end (opening) and maximum signal in the center of the tube.

The centre is a _velocity_ node but a _pressure_ antinode.

The ends are not exactly at the nodal points because there\'s a
mass-spring loading at the ends due to having to move ambient air out
of the way.  (This is the same physics as the Helmholtz resonance.)

The Helmholtz resonance is always the lowest mode of any cavity with
one opening.  (There\'s obviously a DC mode when both ends are open.)

Cheers

Phil Hobbs


Thanks Phil,

    I found answer that answer on a Khan Accademy website. So, with
that info I can mic the closed end of a tube and use 1/4 wavelength

tube, cutting my longest (300 Hz resonance) tube down to 11\".

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

Thanks, Mikek

Seem to recall some years ago Adrian Tuddenham posted a design with a
large array of (16?) electret capsules to make a low noise synthethic
large diagraphm. I think they were paralleled in groups of four with
each group in series.

piglet

 

[Jasen Betts]

On 2021-12-18, amdx <amdx@knology.net> wrote:

I do have a question about the mic the author used, a 2-1/8\" Lafeyette
PA-27 Crystal Cartridge.

Because the author mounted it at the end of a 1/2 wave open end tube, my
thinking is the crystal

cartridge is a velocity mic. What do you think?

If it has holes in the front and back it\'s probably a velocity
microphone



--
Jasen.

 

[Jasen Betts]

On 2021-12-18, amdx <amdx@knology.net> wrote:

On 12/18/2021 10:38 AM, Phil Hobbs wrote:

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

current, voiltage, or power amplitude?

why not just use more amplifier gain?

--
Jasen.

 

[amdx]

On 12/18/2021 4:51 PM, Jasen Betts wrote:

On 2021-12-18, amdx <amdx@knology.net> wrote:

I do have a question about the mic the author used, a 2-1/8\" Lafeyette
PA-27 Crystal Cartridge.

Because the author mounted it at the end of a 1/2 wave open end tube, my
thinking is the crystal

cartridge is a velocity mic. What do you think?

If it has holes in the front and back it\'s probably a velocity
microphone

The PA-27 is list as a part number from a 1964 article, I have no idea
if it has holes front and back.
Mikek


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[Phil Allison]

amdx wrote:
=========

I do have a question about the mic the author used, a 2-1/8\" Lafeyette
PA-27 Crystal Cartridge.

** Only Crystal mics I ever came across in the 60s were all omnis.

But in truth they can be any type.


...... Phil

 

[Phil Allison]

amdx wrote:
============

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

** Connect each mic to a small, step down transformer and wire the secondaries in series.

The signals from you mics will not be in-phase so will not add in a simple way.



........ Phil

 

[Joe Gwinn]

On Sat, 18 Dec 2021 10:11:42 -0800 (PST), John Walliker
<jrwalliker@gmail.com> wrote:

On Saturday, 18 December 2021 at 17:14:58 UTC, amdx wrote:

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

The simplest way is not the best. Each mic will draw a current of about
0.5mA from the pullup resistor. That current is modulated with the
audio signal. The mic needs around 1.5V across it, but would survive
with considerably more - up to about 10V.
So if you have four mics you could connect them all in parallel and use
a single lower value resistor calculated to give a standing voltage of
about 1.5V at 2mA. If you use a 3.3V power supply, the resistor will be
(3.3-1.5)/2mA = 900 ohm.
The problem with this method is that the output audio current from
each mic is being applied to a lower value resistor than normal, so the
output is lower in proportion to the number of mics which will
worsen the noise performance of the system.
With four mics the output will be reduced by a factor of four, or 12dB.
If you can cope with this gain reduction, the parallel approach should
work well. Don\'t try to connect them in series. You will not like the result.
A better approach would be to have a separate preamp for each mic
and mix the outputs of the preamps, but this is a lot more complex.

What I\'ve done is to use the Mic\'s FET as the bottom of a cascode
transistor circuit. The base of the upper cascode pair is a NPN
transistor whose base is held at a convenient voltage by a zener and
bypass capacitor.

This results is essentially constant voltage on the long cable between
Mic assembly and the cascode unit, so cable capacitance has little
effect on bandwidth. The audio is carried by the varying current.

Joe Gwinn

 

[amdx]

On 12/18/2021 5:00 PM, Jasen Betts wrote:

On 2021-12-18, amdx <amdx@knology.net> wrote:
On 12/18/2021 10:38 AM, Phil Hobbs wrote:

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?
current, voltage, or power amplitude?

why not just use more amplifier gain?

Each mic will be connected to resonant tube, each tube is resonant at a
different frequency.

 I want to keep the noise low and not allow the mics to load each other.

 I want the final amp to drive a headphone or speaker.



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[amdx]

On 12/18/2021 5:43 PM, Phil Allison wrote:

amdx wrote:
============
Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

** Connect each mic to a small, step down transformer and wire the secondaries in series.

The signals from you mics will not be in-phase so will not add in a simple way.



....... Phil

 I have thought about signals of different frequencies adding and
subtracting, but don\'t they do that in air also?

Mikek


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[amdx]

On 12/18/2021 5:52 PM, Joe Gwinn wrote:

On Sat, 18 Dec 2021 10:11:42 -0800 (PST), John Walliker
jrwalliker@gmail.com> wrote:

On Saturday, 18 December 2021 at 17:14:58 UTC, amdx wrote:

Another question, what is the best approach to paralleling multiple
electret mics together for maximum amplitude from each?

The simplest way is not the best. Each mic will draw a current of about
0.5mA from the pullup resistor. That current is modulated with the
audio signal. The mic needs around 1.5V across it, but would survive
with considerably more - up to about 10V.
So if you have four mics you could connect them all in parallel and use
a single lower value resistor calculated to give a standing voltage of
about 1.5V at 2mA. If you use a 3.3V power supply, the resistor will be
(3.3-1.5)/2mA = 900 ohm.
The problem with this method is that the output audio current from
each mic is being applied to a lower value resistor than normal, so the
output is lower in proportion to the number of mics which will
worsen the noise performance of the system.
With four mics the output will be reduced by a factor of four, or 12dB.
If you can cope with this gain reduction, the parallel approach should
work well. Don\'t try to connect them in series. You will not like the result.
A better approach would be to have a separate preamp for each mic
and mix the outputs of the preamps, but this is a lot more complex.
What I\'ve done is to use the Mic\'s FET as the bottom of a cascode
transistor circuit. The base of the upper cascode pair is a NPN
transistor whose base is held at a convenient voltage by a zener and
bypass capacitor.

This results is essentially constant voltage on the long cable between
Mic assembly and the cascode unit, so cable capacitance has little
effect on bandwidth. The audio is carried by the varying current.

Joe Gwinn

In my case I don\'t expect any cable between the mic and the amplifier to
be any longer than 10\".

Mikek


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