It may well be that my inability to understand the phenomenon points
only to a massive gap in my knowledge of physics. I can live with that, for no
man's education is ever complete (Thank God).
On the other hand there are
strange phenomena associated with resonant cavities that have never been fully
explained. (Sonoluminescence comes to mind.)
I would very much like to hear
your opinion on the subject.
To which Professor Greenslade replied:
The usual technique, when you don't understand
something, is to quote the source! Let me try to examine the situation...
the cavity were somehow completely closed on both ends, yet filled with acoustic
radiation of the proper (resonant frequency) the forces on both ends would be
Now drill a hole in one end that is smaller than the diameter of
the cavity. The standing wave can still be set up, but some acoustic radiation
would come out of the hole, and the unbalanced force would drive the resonator
This begs the questions of how the radiation gets into the cavity.
Clearly it comes in through the hole. Let me suppose that the radiation comes in
almost parallel to the length of the cavity. You can split the incoming
radiation into two components -- parallel to the length of the cavity and
perpendicular to this direction. The latter component will set up circular
standing waves -- I suspect that a Bessel function might be involved here
because of the circular boundary conditions. The parallel component will
certainly be refracted at the entrance hole -- standard problem of a plane wave
interacting with an aperture. This will provide more transverse components --
but there will also be longitudinal components. Having written this, I am
conscious of the fact that you just can keep stuffing radiation into the cavity
-- eventually an equilibrium situation mmust be reached.
Depending on the
entrance angle, some of the incoming radiation has a longitudinal component that
must come out of the hole -- and this drives the system forward.
What do you
In my next letter I wrote:
….. As to our little problem, I must confess that early on I
entertained thoughts similar to yours and would have probably left it at that if
it weren't for one awkward fact.
The device is said to move in the opposite
This was over two months ago, I did not get a reply. Perhaps I was a
little too forthright and offended the good Professor. This was never my
intention. Perhaps he felt that since he had already admitted he didn't know,
there was no need for further correspondence.
If you have an explanation for the phenomenon please let me know, I will
publish all answers here. Thank you.
Christopher Koveleski writes:
The Acoustic Turbine seems simple enough to build.
I find it hard
to believe that it spins "backward" ... and I think that the limited knowledge
of sound waves / english jargon when the article was published has defined
"backwards" as the opposite direction of the open cavity .. which could be
Such as this device:
Nice to see
someone else interested in Keely ...
Thanks for the link Chris, interesting stuff. Hans.Graham Ormiston
I think this may be connected with non-linearity in the air movement in and
out of the Helmholtz resonators. At the sort of frequency at which the resonance
will occur, the wavelength will be much greater than the size of the resonator,
so it is best to think of a resonance between the enclosed air compliance and
the 'inertance' of air in the port, causing an alternating stream of air through
the port. A lot of thinking has been done on this sort of system by the
designers of reflex loudspeaker cabinets. One of the problems that concerns
them, of course, is distortion. If the air moves too fast through the port,
there will be significant non-linearity. Acting on a sine wave, this will
produce both second harmonic and a 'DC' component. I have seen suggestions to
the effect that, in a loudspeaker, this may produce a DC pressure shift in the
cabinet, causing the drive unit to be displaced from its optimum rest position.
Looking at the picture of the turbine, I think the design of the ports, with
their sharp edges, may cooperate with the non-linearity to entrain a stream of
air with a non-alternating, unidirectional component.Good Comments Graham. As to one resonator facing in the wrong
direction you will see if you look closely that the resonators are rivetted or screwed to
the cross , not too tightly I would suspect, to enable the resonators to
be adjusted to their optimum position. Since this device is a museumspiece
and in remarkably clean condition after 130 years I would guess that whoever
cleaned the device turned one resonator the wrong way before putting the rotor
back on the pivot. Hans.
I'm not sure why one
resonator is facing in the 'wrong' direction. They look as if they're
spot-welded, so this may be intentional
Another physics demo that uses
non-linearity in a Helmholtz resonator is the smoke-ring generator.
Dear Mr. von Lieven:
I read your posting about the acoustic turbine with interest. I had not
heard of the device before, and it certainly is intriguing.
First, I assume the turbine rotates "nozzles-first", or
counterclockwise as seen from above.
Here are my thoughts on a possible operative mechanism:
First, I considered the device as an "air-cannon", ejecting small
quantities of air in order to rotate in a jet-like manner.
(Sticking to basics--action and reaction.)
I then considered one container...
Resonance in the cavity may (should) cause the entire container to
vibrate. The blunt end (closed end) may flutter as a result. Like a
drumhead, so to speak.
If the fluttering of the closed end is more effective at pushing air
away from the device than in sucking air back towards it, a net linear
force and resulting motion might be created.
On the outward flutter of the container endwall., the air would be
displaced linearly--thus causing motion in the correct direction.
On the inward flutter, replacement air would be drawn in from the
sides--radially--as the departing pulse of air blocks direct reflux
by its inertia. This would not affect the previously induced linear
On the inward side of the container base, a similar pulse of air would
be formed, but the small diameter and conical shape of the nozzle would
act to cancel the motion of the air somewhat in the manner of Tesla's
one way pipe--as if the container were closed at the nozzle end.
Pressurization rather than ejection of the contained air would be the
As a test of this solution, a rigid tube, sealed at one end with a
wall, could be fitted with an internal sound generator. The open end of
the tube would then be fitted with a flexible membrane of thin metal.
A turbine structure made with four such tubes, upon actuation, should
then rotate with the rigid ends "forward".
Good line of reasoning. The experiment suggested is not difficult to do. I shall get to it as soon as time permits. Hans.
I would look for something similar. It's very interesting. Thank you for bringing it to our attention..
Although it's not a microwave cavity, it being a cavity resonance with a mystery, it reminds us somewhat of the unidirectional thrust of EMdrive.
What are the similarities (if any) between the Acoustic Turbine and an EMdrive?
Other than cavity resonance, (one of sound, the other of microwaves), and end reflectors that are not of equal area, I don't know. Could it be they both operate due to the difference in the energy gradient between the standing wave reflector surfaces?
The mystery remains.. :)
steve edward george writes:
I am not an expert but I believe that sound is a pressure wave that usually involves the rapid back-and-forth movement of air.
My suspicion is that the chambers take the back-and-forth movement of the air (sound pitch) and convert it into a directional pulsed movement much like a whetstone bridge would take AC electricity and convert it into pulsing DC.
The sound (pulsed air) would then exit from the chambers much like the old German V1 Buzz Bomb to cause propulsion.
Steve Edward George
Largo, Florida, USA
nicolaie vlad writes:
mr.Hans von Lieven ,
very interesting 'news' on 'old' Acustic Turbine !
Hans von Lieven,