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High Sensitivity HeadPhones, new direction

A

amdx

Jan 1, 1970
0
Hi guys,
I ran across this electromechanicoustic device (speaker) called a Gallows
headphone.
This is used as the speaker for a crystal radio.
It is pictured about 2/3 of the way down the page.
You will understand it's operation by its picture.
http://www.hpfriedrichs.com/

I have cross posted looking at this as having three disciplines, acoustics,
electronics
and magnetics.
I would like to see ideas to design one with these objectives in mind.
Objectives:
Maximum audio from minimal (crystal radio) signal (high sensitivity)

High impedance, if we can get enough wire on the electromagnet, the matching
transformer could be eliminated. (100k to 1 meg ohms, tapped) That's a wish
:)

Questions;
What should the diaphragm look like? low mass? with ridges? needs to be
magnetic at
some point.

Can we put an electromagnet on both sides in push pull? Can needs to be
sealed.

Earphone is connected with hollow tubing, what type of tube would be low
loss?
does length matter? I'm thinking stethoscope earphones till something better
is found.

Do we want the center of the diaphragm to move or the whole circumference to
breath?
How do we focus the magnetic field, if we do.

Thanks for your input, MikeK
 
S

Sjouke Burry

Jan 1, 1970
0
amdx said:
Hi guys,
I ran across this electromechanicoustic device (speaker) called a Gallows
headphone.
This is used as the speaker for a crystal radio.
It is pictured about 2/3 of the way down the page.
You will understand it's operation by its picture.
http://www.hpfriedrichs.com/

I have cross posted looking at this as having three disciplines, acoustics,
electronics
and magnetics.
I would like to see ideas to design one with these objectives in mind.
Objectives:
Maximum audio from minimal (crystal radio) signal (high sensitivity)

High impedance, if we can get enough wire on the electromagnet, the matching
transformer could be eliminated. (100k to 1 meg ohms, tapped) That's a wish
:)

Questions;
What should the diaphragm look like? low mass? with ridges? needs to be
magnetic at
some point.

Can we put an electromagnet on both sides in push pull? Can needs to be
sealed.

Earphone is connected with hollow tubing, what type of tube would be low
loss?
does length matter? I'm thinking stethoscope earphones till something better
is found.

Do we want the center of the diaphragm to move or the whole circumference to
breath?
How do we focus the magnetic field, if we do.

Thanks for your input, MikeK
Electromagnetic phones will cause to much load on a crystal radio.
The output and selectivity will become to low to be useful.
Their impedance is only about between 10 ohms and ~2kohms(with
extremely thin wire, and a few thousand windings).
The crystal earphones are a much better choice.
 
A

amdx

Jan 1, 1970
0
Sjouke Burry said:
Electromagnetic phones will cause to much load on a crystal radio.
The output and selectivity will become to low to be useful.
Their impedance is only about between 10 ohms and ~2kohms(with
extremely thin wire, and a few thousand windings).
The crystal earphones are a much better choice.

I would point out that Electromagnetic phones (sound powered headphones)
are very commonly used with crystal radios and yes, that's why I want
this to be very high impedance.
So maybe we need 10.000 turns and high (AsubL?) core.

Thanks, MikeK
 
A

amdx

Jan 1, 1970
0
Salmon Egg said:
I could not make head or tail out of these pictures. A well thought out
diagram would be much more helpful.
Ok, first page down about 2/3 until you see a caption that says Gallows
headphone.
It is a sealed container with a diapragm mounted on top. The sealed contain
has
a tube sticking out, rubber hose is connected and this is connected to an
air piece.
Above the diaphragm is a a coil with an iron core (electromagnet). When an
audio
electrical signal drives the coil it causes the diaphragm to move and sound
is emitted
from the tubing to the ear.
There are two pictured one has a large container the other is much smaller.

In general, you have a number of impedance matching problems. The
crystal set provides the source impedance. That power has to be matched
to a mechanical device like the stirrup in your ear. That in turn is
attached to a diaphragm which must be matched to the air. Everywhere in
this path, a mismatch will reduce the amount of power that can be
transferred. That is the trick.

Bill
Yes matching is the trick, that's why I ask the experts.
MikeK
PS. a perfect match is never going to be made over the useful audio range,
hopefully just better than what's available now.
 
A

answerman

Jan 1, 1970
0
Maximum power across the band is what you want.

No, maximum pressure at the eardrum across the band is what you want.

Exceptions would be if
you are listening to a a narrow audio band signal like tone modulation
of an rf signal detected by the crystal. The amplification will only
occur if the acoustic network is the equivalent of a lumped circuit.
Resonance at 3kHz is not going to help listen to an ordinary voice
signal.

I never said that it would. It's the combined effect of system resonances
at 1kHz (middle ear) and 3kH (ear canal) that together contribute to the
sensitivity of the human ear in the frequency range between 500Hz and 6kHz.
The point, which you apparently either missed or ignored, is that when it
comes to hearing sensitivity, an impedance mismatch can be and is an
advantage, and not, as you claim, something to be avoided.
 
A

amdx

Jan 1, 1970
0
Answerman said:
For whatever reason, you are still not listening. You can not talk about
building a high-sensitivity headphone when the headphone is to be used in
a passive circuit such as a crystal radio. This is so because the load
impedance of the headphone affects the terminal voltage of the circuit to
which it is connected and because the source impedance of the circuit to
which the headphone is connected affects the acoustic output of the
headphone. It is because of this interaction that the sensitivity of a
headphone is specified in terms of the pressure that it produces in
specified aoustic load (eg IEC-318 coupler) per applied volt when driven
from a low-impedance voltage source.
Taken to the extreme, you're saying no headphone is any better than any
other when
connected to a crystal radio. Bullshit! I understand impedance matching, I
understand
loading the tank to an unusable Q. Go look at this 4 Megohm to 8 ohm audio
transformer.
http://www.crystal-radio.eu/entrafounit2.htm
If you don't like my semantics get over it, everyone else understands I
want maximum
audio volume for the input power available.
You are welcome to ignore this thread.
MikeK
 
A

amdx

Jan 1, 1970
0
Answerman said:
I agree with your conclusion. The problem is that what you say I said
isn't what I said or even implied.



That's a good start.


Interesting but irrelevant.
Not irrelevant if you want to connect a low impedance headphone to a
detector with a high output impedance.
At this point I don't know the output impedance of high Q tank driving
a diode that matches the tank, but I do think 200Kohm to 2.5megohm
are not unreasonable numbers*.
MikeK
I like factors greater than 10, almost easier the horseshoes to get close.
;-)
(it does give a range and tank Qs vary a lot in crystal radio building)
 
A

Angelo Campanella

Jan 1, 1970
0
Answerman said:
For whatever reason, you are still not listening. You can not talk about
building a high-sensitivity headphone when the headphone is to be used in
a passive circuit such as a crystal radio. This is so because the load
impedance of the headphone affects the terminal voltage of the circuit to
which it is connected and because the source impedance of the circuit to
which the headphone is connected affects the acoustic output of the
headphone.

Agreed. The main difficulty in our understanding of the CR system is
that at this time no model including the transformation of the electrical
power derived from the radio wave into acoustical power radiated from a
speaker transducer has been advanced. The "Gallows" phone is an example of
effort to improve the conversion of electrical audio power (demodulated RF
power, no amplification) into sound one can hear. When speaking of
reproduced sound pressure level, the distance at which that is heard is not
yet in the model as far as I can see.
It is because of this interaction that the sensitivity of a
headphone is specified in terms of the pressure that it produces in
specified acoustic load (eg IEC-318 coupler) per applied volt when driven
from a low-impedance voltage source.

..... which is a human ear simulator... It's a start. The "distance" at
which the sound is heard is about an inch.

(My mnemonic for conversion of sound power data to SPL in dB is that the
sound pressure found one foot from a point source (small speaker) is
numerically equally to the sound power expressed in picowatts (10^-12 W)).
So the sound dB sound level created in that coupler's geometry is
numerically about 20 dB more than its sound power value... A +10dB sound
power level produced by the transducer under test would then produce a +30
dB SPL in that coupler.... (ad-hoc WAGs by me).

(FYI, a vacuum cleaner emits about +85 to +90 dBA sound power).

Ange
 
A

amdx

Jan 1, 1970
0
Angelo Campanella said:
Agreed. The main difficulty in our understanding of the CR system is
that at this time no model including the transformation of the electrical
power derived from the radio wave into acoustical power radiated from a
speaker transducer has been advanced. The "Gallows" phone is an example of
effort to improve the conversion of electrical audio power (demodulated RF
power, no amplification) into sound one can hear. When speaking of
reproduced sound pressure level, the distance at which that is heard is
not yet in the model as far as I can see.


..... which is a human ear simulator... It's a start. The "distance" at
which the sound is heard is about an inch.

(My mnemonic for conversion of sound power data to SPL in dB is that
the sound pressure found one foot from a point source (small speaker) is
numerically equally to the sound power expressed in picowatts (10^-12 W)).
So the sound dB sound level created in that coupler's geometry is
numerically about 20 dB more than its sound power value... A +10dB sound
power level produced by the transducer under test would then produce a +30
dB SPL in that coupler.... (ad-hoc WAGs by me).

(FYI, a vacuum cleaner emits about +85 to +90 dBA sound power).

Ange
Here are some numbers thrown out on one of the crystal radio groups.

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

"The big question here is: Do phones that produce 125 dB SPL when 1 mW is
applied to them . . . produce 35 dB SPL when 1 pW is applied ?

1 pW is 90 dB below 1 mW. I believe 35 dB SPL should be very comfortable
listening. 1 pW audio might be what a crystal radio would deliver from a
weak DX signal.

Much debate has transpired in the past over this issue, without any definite
conclusions. Anecdotally speaking: some Sennheiser "120 dB/mw" earbud-type
phones failed miserably on weak DX when compared to a good set of
sound-powereds."
MikeK
 
A

Angelo Campanella

Jan 1, 1970
0
Answerman said:
For either a circumaural headphone or an insert earphone, which is what
is required for this application, it's the enclosed air volume, not the
distance between the transducer and the ear that matters. This is so
because the acoustic impedance looking into the ear canal is stiffness
controlled (approximately 2cc equivalent air volume) in the frequency
range below about 1kHz. Consequently, up to about 1kHz, pressure at the
eardrum is proportional to the volume displacement of the diaphragm of
the transducer.

Classical room acoustics (the earcup on head is a room for this
discussion) has it that regardless of the volume, the sound level in a room
is calculated directly from the sound power inserted and the absorption,
volume does not matter (!!). In decibels,

SPL = PWL-1*log(A) + 6 (A in square meters)

SPL = PWL-10*log(A) + 16.3 (A in square feet)

One picowatt is 0dB PWL

Assume the cup and ear to have a surface area of about 30 square
inches (0.2 square feet), with an absorption coefficient 50%, A = 0.1,
so the 10*log(A) is -10 dB.

This leaves us with 6 dB of SPL. At 1 or 2 kHz, this is ( for a good
ear, fellas) about at threshold...

Now, that's 1 picowatt of Sound Power...
Considering audio transducer conversion efficiency of 10%, the
electrical-audio-frequency power needs to be 10 picowatts of audio ac.
Figure a factor of ten for demodulation (in)efficiency of 10% ups
this level to 100 picowatts..
The RF power that engendered that, let's say ten times more, or 1,000
picowatts (one microwatt).

Therefore AM RF absorbed or captured by the crystal set and its antenna
must be more than one microwatt of RF energy.
Therefore, the best transducer for this application is
one that provides a the greatest volume displacement per applied volt,
that minimizes the amount of added air volume (relative to 2 cc) and that
has a critically-damped, lumped-element diaphragm resonance at about 3
kHz.

I think we can assume that he earcup volume SPL is uniform throughout
for frequencies up to about 2 kHz.
The ear canal resonance will be as it is for a open ear.

Ange
 
A

amdx

Jan 1, 1970
0
Considering audio transducer conversion efficiency of 10%, the

And there is the where I started! How do I increase that 10% to 20% or 30%?
The easy 1 to 3db is to eliminate the impedance transformer, so make the
speaker impedance high, with a tapped coil (for matching) on the speaker.
(I don't know if it is feasible to make a speaker coil with 500kohm
impedance)
Or a multi peizo speaker with series/parallel switching arrangement to alter
the impedance.
All this used in the gallows style diaphragm.
Thanks, MikeK
 
R

Rich Grise

Jan 1, 1970
0
amdx said:
And there is the where I started! How do I increase that 10% to 20% or
30%?

Proper enclosure design.

Good Luck!
Rich
 
A

Angelo Campanella

Jan 1, 1970
0
Answerman said:
If I understand your analysis correctly, your bottom line is that 10pW of
electrical power is required to produce 6dB SPL at 1kHz at the eardrum.
If so, there is no reason to go any further, because your numbers thus
far don't comport with reality.

Remember, guys, these are all WAG's, just to get an analysis framework
up.

We can tweak as we go along.
One of the more efficient headphones, the Sennheiser HDA-280, produces
117dB SPL at the eardrum at 1kHz for 1mW of electrical input, or

From one milliwatt to one picowatt is -90 dB . OK. = 27dB for 1 pw.
equivalently, 37dB SPL at the eardrum at 1kHz for an electrical input of
10pW. So, without going into why, the fact of the matter is that your
calculation thus far is in error at least 30dB.

Not too bad for a beginning system analysis, I say... This relates to
the audio sensitivity that can be achieved.

37 dB @ 1 kHz is nicely audible in a quiet room, so that the HDA-280 should
be useful to use with a crystal receiver. But what is the input impedance of
that headset? if it is 8 ohms or even 50 ohms, there will be a impedance
mismatch of as much as 30 dB....


Ange
 
A

amdx

Jan 1, 1970
0
Salmon Egg said:
I think you are missing the fundamental quandary,The major cause of
inefficiency of a speaker is poor impedance matching between the speaker
cone and the wave you are trying to propagate in the air. It is NOT the
mismatch between an amplifier and putting power into the voice coil or
diaphragm.

Bill
It's not that I'm missing it, you'll note I said;
"The >easy< 1 to 3db is to eliminate the impedance transformer".
The quandary is that I don't know how to do the mechanical matching.
As far as the piezo, I think that is just a matter of picking the right
piezo?
(assuming a completed bender)
Thanks, MikeK
 
A

amdx

Jan 1, 1970
0
One of the more efficient headphones, the Senheiser HDA-280,
produces 117dB SPL at the eardrum at 1kHz for 1mW of electrical input, or
equivalently, 27dB SPL at the eardrum at 1kHz for 1pW electrical input.


Is this a good assumption? Will the audio output SPL be linear with the
power input?
Is the answer the same for all transducer types?
Thanks, MikeK

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

THIs from my previous respoNSE

"The big question here is: Do phones that produce 125 dB SPL when 1 mW is
applied to them . . . produce 35 dB SPL when 1 pW is applied ?

1 pW is 90 dB below 1 mW. I believe 35 dB SPL should be very comfortable
listening. 1 pW audio might be what a crystal radio would deliver from a
weak DX signal.

Much debate has transpired in the past over this issue, without any definite
conclusions. Anecdotally speaking: some Sennheiser "120 dB/mw" earbud-type
phones failed miserably on weak DX when compared to a good set of
sound-powereds."
---------------------------------------------------------------------------------------------------
 
A

amdx

Jan 1, 1970
0
Salmon Egg said:
I think you are missing the fundamental quandary,The major cause of
inefficiency of a speaker is poor impedance matching between the speaker
cone and the wave you are trying to propagate in the air. It is NOT the
mismatch between an amplifier and putting power into the voice coil or
diaphragm.

Bill
Hey Bill,
In an effort to make this match between cone and air, would I do better to
get an efficient
speaker* and remove the magnet and voice coil and build my own driver.
Here's a simple concept sketch.
http://i395.photobucket.com/albums/pp37/Qmavam/ModifiedSpeaker.jpg
Now where getting into an enclosure to tune the speaker.
MikeK
*someone else already did the design work .
 
A

Angelo Campanella

Jan 1, 1970
0
amdx said:
Hey Bill,
In an effort to make this match between cone and air, would I do better to
get an efficient
speaker* and remove the magnet and voice coil and build my own driver.
Here's a simple concept sketch.
http://i395.photobucket.com/albums/pp37/Qmavam/ModifiedSpeaker.jpg
Now where getting into an enclosure to tune the speaker.

WOW! The first speaker working like that I ever saw was while cleaning
out the finished attic of an old house circa 1950. That speaker was built
exactly like that. I recall it to be a large cone like you show, maybe a
foot or more in diameter. The coil driver was just a big old relay
coil-looking sort of thing. The lever and its fulcrum looked just like a
relay armature; the cone driver just an extension rod on the other side of
the armature bearing-shaft. This is an easy way to get more leverage, a
mechanical impedance transformer if you will. That unit was obviously a
refugee from an ancient AM radio from some where in 1920's. The former owner
was a past Mayor of Wilkes-Barre, PA, Dan Hart... He was sai dto have had
some wild parties up in that finished garret during Prohibition days.

Anyway, the important factors are the steps made to mechanically
"impedance match" the electrical force available; a strong armature force
which can only go a short distance over coil - to the cone wagging in the
air presenting little reaction force, but should travel over a much longer
distance.

Go for it... build one..

You could use an old high impedance relay, mounted with coil axis
horizontal, on an inertial base (e.g. a brick) .

The mechanical transformer ratio is adjusted as the length of the
extension rod.

It was about an inch or so long as I recall...

The cone drive point has an axial pin glued into the apex tip of the
cone; use super-glue, I think.

Manila folder stock, painted with clear dope to stiffen it, may work as
the cone. I'm not sure of the cone base support, but I suspect that any
hanging arrangement will do; the axis of the cone is horizontal.

This could be a "Big-Gallows" speaker; completely impedance matched to
air.

Ange
 
A

amdx

Jan 1, 1970
0
Angelo Campanella said:
WOW! The first speaker working like that I ever saw was while cleaning
out the finished attic of an old house circa 1950. That speaker was built
exactly like that. I recall it to be a large cone like you show, maybe a
foot or more in diameter. The coil driver was just a big old relay
coil-looking sort of thing. The lever and its fulcrum looked just like a
relay armature; the cone driver just an extension rod on the other side of
the armature bearing-shaft. This is an easy way to get more leverage, a
mechanical impedance transformer if you will. That unit was obviously a
refugee from an ancient AM radio from some where in 1920's. The former
owner was a past Mayor of Wilkes-Barre, PA, Dan Hart... He was sai dto
have had some wild parties up in that finished garret during Prohibition
days.

Anyway, the important factors are the steps made to mechanically
"impedance match" the electrical force available; a strong armature force
which can only go a short distance over coil - to the cone wagging in the
air presenting little reaction force, but should travel over a much longer
distance.

Go for it... build one..

You could use an old high impedance relay, mounted with coil axis
horizontal, on an inertial base (e.g. a brick) .

The mechanical transformer ratio is adjusted as the length of the
extension rod.

It was about an inch or so long as I recall...

The cone drive point has an axial pin glued into the apex tip of the
cone; use super-glue, I think.

Manila folder stock, painted with clear dope to stiffen it, may work as
the cone. I'm not sure of the cone base support, but I suspect that any
hanging arrangement will do; the axis of the cone is horizontal.

This could be a "Big-Gallows" speaker; completely impedance matched to
air.

Ange

Hi Ange,
This is basically the same arrangement found in some of the older headsets.
Although being the Gallows style it can be built big, so fat fingers can do
it,
also the size will allow many turns for impedance matching. The reason I
went with a manufactured speaker assembly is because maybe the solved
some of the matching problems that I'm clueless about. Although by sealing
the speaker and adding the tube, I've probably messed up all the good design
that the speaker had. I don't really like the fulcrum idea as drawn, but I
have
another idea (probably not new either) that I'll draw up later.
Re:
This is an easy way to get more leverage, a mechanical impedance
transformer if you will.
It would be neat to have an adjustable fulcrum to fine tune the
leverage, I don't know where the trade off is between cone
displacement and the power available to overcome the leverage.

Now to find/build an iron core with a gap as drawn.
Are the laminations thinner on audio transformers than on power
transformers?
Any tape wound cores used for audio? (might be easy to mill the gap)
Mike
 
A

amdx

Jan 1, 1970
0
Robert Baer said:
Well, an air gap would decrease coupling a lot and the effect i think
would be the equivalent of a loss.
Yes, audio (and modem) transformers have a lot thinner laminatins - so
there is a lower loss at the higher frequencies.
Never heard of tape wound cores for audio, but if you could get such
with thin "laminations" that should work quite well.
Forget gaps!
I need the gap to put the driver in.
MikeK
 
A

amdx

Jan 1, 1970
0
Answerman said:
According to the diagram, the acoustic load seen by the speaker cone is a
confined volume of air, the impedance of which is a purely reactive
compliance. In order to match that load impedance, the source impedance
would need to be a purely mass reactive, and the match would only exist
at a single frequency. For a broadband (300Hz-3000Hz) impedance match,
both the source and load impedances would need to be predominantly real
and approximately equal in magnitude. I don't see the latter requirment
being satisfied in the proposed design. What am I misssing?
Answerman, I have realized you are very intelligent and well educated
in the subject of acoustics. I'm reassigning you to a higher position, you
my friend are now the Chief Design Engineer of the High Sensitivity Crystal
Radio Headphone Project. Now known world wide as the HSCRHP.
The task at hand is to design a very efficient audio transducer with a
minimum
frequency response of 300Hz-3000Hz, a wider range would be ok.
The transducer will be used with crystal radios in contest situations.
There are two options for electrical impedance matching;
1) taps can be made on the transducer for matching. (Taps at 1 Megohm and
less)
2) A transformer can be used, realizing losses of 1 to 3 db.
Electromagnetic or piezo driver are acceptable.
The transducer assembly must be produced from easily available parts
and be able to be built by a skilled homebrewer without any special tools.
Putting together a winder or a few jigs is acceptable.
Congratulations on your promotion and thanks in advance for
your dedicated work on the HSCRHP.
MikeK
 
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