A Study of DMLs as a Full Range Speaker

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#7,665
chdsl said:
There's no pressure in the air in front of the little bobbin front. The bobbin is just trying to get out of the way from the magnetic field. It won't "come back" if it is not held tightly by the "spider." Search for the right hand 3-finger rule of a magnetic field. It was taught in the school those days.

There's no pistonic motion. The bobbin doesn't want to come back. There's no real word 'pistonic' in English. A piston needs a chamber to work in. You can't push air to a vast unlimited space in front of anything. Most of the people, who applied and got the patents were not scientists, or engineers. And, one of them coined a word and that stayed. Most of the mistakes in patents come from this "pushing air", "air pressure growing in", "pistonic motion" etc, etc.

Have a look at any exciter lying unfixed on a table face up with an amp connected to it. The coil tries to jump out of the magnetic field. It never tries to go into the table. You can change the polarity, and you'd still see it trying to jump away. Right hand 3-finger rule.
Click to expand...
Chdls,
Let me give a (last?) chance to this discussion.
Wording may be confusing.
A piston is a well known object in mechanic, in thermal or steam engine when it comes to transform the energy from the pressure of the vapor or the pressure from a combustion to a movement by a change of volume. This is not acoustics... even if engines are noisy.
The word "pistonic" is not English, neither French... It is a neologism used to describe a membrane for which all the points move the same direction, the same distance at a time (so not a DML); like a piston does. Thanks to that, some calculation in acoutics are possible.
Is there a pressure in front of the speaker in the meaning the pressure increases or decreases as detected by a barometer? No
Does the speaker change the pressure in the room by pumping air? No
Is there a variation of pressure in front of the membrane? Yes! This is what makes sound when this variation arrives in our ears.
Why the sound arrives to our ears? Because there is propagation (a wave). The molecules in front of the membrane are pushed, they pushed their neighbors and so on. No displacement of material from the source to the receiver (no "wind"), just a change of local pressure like little mass connected by springs will do.
Even a very low change... a 60dB sound is a variation of 0.01Pa when the atmospheric pressure is about 10000Pa.
The voice coil (bobbin) is a force generator working thanks to the Laplace force (no Lorentz Burnt! 😉 ) : a current, a magnetic field, a force. By it self it can't give a sound or if it does, it is a little one like a string will do.
Why a little sound? Because the change applied to the surrounding air is too little. To create audible sound, a larger surface with in consequence a small displacement is needed (otherwise additional energy is needed). We could speak here about impedance adaptation.
It is what happens in a piano, a violin, a guitar... The membrane being the soundboard.
Sound is pressure change which is displacement created by a force on a membrane itself being create by a current in a magnetic field (except for piezo where it is an electric field). Cone, DML... all the same. The differences are in the details of the vibrations : piston-like (pistonic, cone), thin plate modes (DML), vibration in a membrane, mix (canvas)...
Does the "bobbin" come back? Yes! Either by stopping or reversing the current into it. Think AC (not DC), think also about the springs (the spider, the air in a closed box).
Christian
 
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#7,666
homeswinghome said:
Chdls,
Let me give a (last?) chance to this discussion.
Wording may be confusing.
A piston is a well known object in mechanic, in thermal or steam engine when it comes to transform the energy from the pressure of the vapor or the pressure from a combustion to a movement by a change of volume. This is not acoustics... even if engines are noisy.
The word "pistonic" is not English, neither French... It is a neologism used to describe a membrane for which all the points move the same direction, the same distance at a time (so not a DML); like a piston does. Thanks to that, some calculation in acoutics are possible.
Is there a pressure in front of the speaker in the meaning the pressure increases or decreases as detected by a barometer? No
Does the speaker change the pressure in the room by pumping air? No
Is there a variation of pressure in front of the membrane? Yes! This is what makes sound when this variation arrives in our ears.
Why the sound arrives to our ears? Because there is propagation (a wave). The molecules in front of the membrane are pushed, they pushed their neighbors and so on. No displacement of material from the source to the receiver (no "wind"), just a change of local pressure like little mass connected by springs will do.
Even a very low change... a 60dB sound is a variation of 0.01Pa when the atmospheric pressure is about 10000Pa.
The voice coil (bobbin) is a force generator working thanks to the Laplace force (no Lorentz Burnt! 😉 ) : a current, a magnetic field, a force. By it self it can't give a sound or if it does, it is a little one like a string will do.
Why a little sound? Because the change applied to the surrounding air is too little. To create audible sound, a larger surface with in consequence a small displacement is needed (otherwise additional energy is needed). We could speak here about impedance adaptation.
It is what happens in a piano, a violin, a guitar... The membrane being the soundboard.
Sound is pressure change which is displacement created by a force on a membrane itself being create by a current in a magnetic field (except for piezo where it is an electric field). Cone, DML... all the same. The differences are in the details of the vibrations : piston-like (pistonic, cone), thin plate modes (DML), vibration in a membrane, mix (canvas)...
Does the "bobbin" come back? Yes! Either by stopping or reversing the current into it. Think AC (not DC), think also about the springs (the spider, the air in a closed box).
Christian
Click to expand...
Sound is an energy. It moves in sinusoidal waves, and in all planes.

When you are in one room, someone in another room two rooms away from your room, knocks on the horizontal table, and you hear it. Two parallel walls between you and that table. There's radio on that room, and you are sitting in the first room. You still hear the radio, behind two walls. How does that come to your ears? The door in your room is closed.

By the way, the radio has two pistonic speakers and the radio is facing away from you.
 
#7,667
homeswinghome said:
Aether is for the light in the vaccum of space! air is known to be the vector of sound 😉
By the way as I didn't remember the history of air as the medium for sound and the origin of the bell and vaccum experiment, I found this article (basics of acoustics).

@chdsl : please forget the model of "wind" which is like trying to transmit a signal with a DC voltage; analogy between AC voltage and sound seems better. Soud is a wave, an increase and decrease of local air pressure that thanks to the elastic property of the air can propagate. The molecules move locally for denser area (higher pressure) or rarefaction (lower pressure). A vibrating surface is able to create a sound by modifying locally the pressure.
Please think also to that :
  • is a string able to make sound? or in other words, is there a string instrument without a soundboard?
  • is there a string instrument without resonance box?
Click to expand...
homeswinghome,
Yes look at this video
 
#7,673
chdsl said:
Sound is an energy. It moves in sinusoidal waves, and in all planes.

When you are in one room, someone in another room two rooms away from your room, knocks on the horizontal table, and you hear it. Two parallel walls between you and that table. There's radio on that room, and you are sitting in the first room. You still hear the radio, behind two walls. How does that come to your ears? The door in your room is closed.

By the way, the radio has two pistonic speakers and the radio is facing away from you.
Click to expand...
Oh dear the journey will be long.
Sound is not an energy but a variation of pressure that propagates in the air.
The main sensor is our ear where the eardrum is deformed by the pressure difference on both side, which move the bones, that create a wave in the cochlea to fire the nerves.
Transmission of sound in well known in different type of construction like building, airplane, automotive. The transmission can be solidian (the table, the floor, the wall) or aerial (the air). This is a problem for flats having the same concrete slab where the noise of a falling marble, a drill machine, water pipe noise or shoe heel are transmitted far from their source by the solids and then the air of the room... and the doors are closed!
I am sure you can find many readings on all of that on the net.
To give a chance to this thread to come back to its main purpose, I will stop here on that.
Christian
 
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#7,675

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#7,676
@pixel1,
Have you sorted out your exciter issue yet?
I think Steve and Todd and others have suggested some of the same things I would suggest.
That is a heavy exciter, it it looks like the wires and connectors you have hanging it too are also a bit heavy.
I too think it's most likely that the support of a spine will likely solve your problem.
In post linked below there are some images showing how I make the spines I use to support my exciters. In the images, the black block between the exciter and spine is a piece of soft foam with adhesive on both sides. I like the soft foam there as it allows you to a little more room for error in positioning the spine the correct distance from the panel to avoid pressing the exciter too hard against the panel. I think attaching the exciter to the spine with a screw instead of the foam block is fine also as long as you are able to precisely position the spine.

Eric

https://www.diyaudio.com/community/...s-as-a-full-range-speaker.272576/post-7121486
 
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#7,677
pixel1 said:
Sorry I'll add more I'm not sure if you see the first one
Click to expand...
@pixel1
Can you share more details of your build here?
I recall you were looking at CF/nomex panels, and I assume that is what you are showing, here right? I seem to recall you were looking at two different thicknesses, which did you get? Can you reshare the link describing the panels? Either one seemed like it would be very high efficiency, equaling or perhaps even exceeding PS foam.
What size is the panel and frame, and how is the panel attached to the frame? I think I see some screws in the panel at the perimeter but it's not clear how they are being used. Can you explain?
Thanks,
Eric
 
#7,680
Veleric said:
@pixel1
Can you share more details of your build here?
I recall you were looking at CF/nomex panels, and I assume that is what you are showing, here right? I seem to recall you were looking at two different thicknesses, which did you get? Can you reshare the link describing the panels? Either one seemed like it would be very high efficiency, equaling or perhaps even exceeding PS foam.
What size is the panel and frame, and how is the panel attached to the frame? I think I see some screws in the panel at the perimeter but it's not clear how they are being used. Can you explain?
Thanks,
Eric
Click to expand...
thank you
I think I will follow the advice of the structure to support everything
even if I have to see the photos to understand how to adjust the pressure, I imagine that the squeeze must stop at a slight push
 
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