Hugh Hetherington Hearing Aid Museum
Hugh Hetherington Hearing Aid Museum

The Hearing Aid Museum

Hearing Aids of all types—Ear Trumpets, Carbon Hearing Aids, Vacuum Tube Hearing Aids, Transistor Hearing Aids, Body Hearing Aids, Eyeglass Hearing Aids and much more!

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Click on the "General Information" button (top button above) for an overview and general information on this category of hearing aid.

 

Carbon Hearing Aids: 1900-1939

Sonotone Model 400 (Binaural Bone Conduction) Carbon Hearing Aid

The Sonotone Model 400 bone conduction carbon hearing aid was manufactured in 1932 by Sonotone International, Inc. of New York.

The Sonotone Model 400 came with either a single bone conduction transducer or in a binaural configuration. Some Model 400s may have come with an earphone or a receiver instead of a bone conduction transducer.

The picture at the right shows the rare  binaural bone conduction Sonotone Model 400 carbon hearing aid. It had two bone conduction transducers, two volume controls, bilateral wiring and a heavy duty carbon amplifier.

Without the batteries, this hearing aid weighed 1 lb. 1.4 oz. (494 g).

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View of the Sonotone Model 400 monaural bone conduction carbon hearing aid. The monaural version did not have an external volume control. It also used a smaller carbon amplifier. It may not have come with a resonant carrying case. Otherwise, it was basically the same as the binaural model.


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The Sonotone Model 400 carbon hearing aid could be used with or without the box-like carrying case.

In the top picture, it is set up for use without the carrying case. In the picture at the right, it is set up for binaural bone conduction using the resonant carrying case to house the battery case, microphone and carbon amplifier.

The total weight of this hearing aid including the case, but without the batteries was 1 lb. 11.7 oz. (786 g).

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Front view of the Sonotone Model 400 carbon hearing aid double microphone showing the pattern of each of the microphone grills—12 holes in the outer ring, 6 holes in the middle ring and one hole in the center.

The double microphone measured 5 3/16" wide x 2 9/16" high x ⅝" thick (13.2 x 6.5 x 1.7 cm), and weighed 4.9 oz. (130 g).

Double microphones were used when more power was needed than that produced by single microphone hearing aids.

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Close-up view of the front center between the double microphones of the Sonotone Model 400 carbon hearing aid showing the logo. The large stylized "S" in the triangle is one of the identifying characteristics of the Model 400. (Other models with double microphones used a small logo and small "S".)



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Rear view of the Sonotone Model 400 carbon hearing aid double microphone showing the pocket clip (upper center).

The back was black.
 

 

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Close-up view of the pocket clip on the upper back of the Sonotone Model 400 carbon hearing aid double microphone.

Note the hole in the top of the pocket clip for threading a lanyard through if you wanted to wear the microphone around your neck.
 

 

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Below the pocket clip and in the center of the back are the words "Sonotone Corp, New York" and about half an inch  below these words is the serial number S-32111.

At the very bottom of the microphone (not shown) are the words "U.S. Pat. No." with such faint numbers following so as to be unreadable. Below that is, "Other Pat's Pend." and "Made in U.S.A."

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Top view of the Sonotone Model 400 carbon hearing aid double microphone showing the three-pin body plug and socket.

Note that the socket is recessed into the body, not flush with the surface as was the case with later Sonotone carbon hearing aids.

Also note the larger left hole and the larger pin (bottom) so you couldn't plug this cord in the wrong way.

If you look closely, you'll see the letter "T" on the end of the plug. There is a "T" on the other end too.

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Top view of the Sonotone Model 400 carbon hearing aid double microphone showing how the plug snuggly fitted into the socket.


 

 

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Side view of the above plug showing the name "Sonotone" and below that, "New York".

 

 

 

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The opposite side view of the above plug showing the words "Pat. Pend."

Note the slots on the pins. There was a wire spring in each slot that stuck out a bit (see left pin). This did two things—first it ensured a good contact with the socket for maximum current flow, and second, it held the pins tighter in the socket so they wouldn't work loose.

 

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Bottom view of the Sonotone Model 400 carbon hearing aid double microphone showing the sliding rheostat and to the right of the rheostat, the folding "foot" shown in the folded position.

 

 

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Close-up view of the bottom of the Sonotone Model 400 carbon hearing aid double microphone showing rheostat in the off position. To the right are the words "Soft" and "Loud". You got maximum volume when the slider was all the way to the right.

Note the "foot" shown swung out at right angles so the microphone could stand by itself on a table.

 

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One of the characteristics of carbon microphones is that they don't work at all when laying flat. They had to be tipped up at an angle, or better yet, held vertically.

This view of the Sonotone Model 400 carbon hearing aid double microphone shows the foot turned out, thus holding the microphone in the vertical position when it was not used in its carrying case or hung around a person's neck.

The front was made of faux tortoiseshell of mottled red and yellow When backlit, this hearing aid looked beautiful.

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Bottom front view of the carrying case for the Sonotone Model 400 carbon hearing showing the cutout in the bottom so you could adjust the volume or turn the hearing aid off without having to take it out of its case.

 

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Outside view of the metal battery case of the Sonotone Model 400 carbon hearing aid

The battery case measured 4 11/16" x 2 15/16" x 1⅝" (12.0 x 7.5 x 4.1 cm).

It contained 3 standard "D" cells giving an output of 4½ volts.

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Inside view of the battery case of the Sonotone Model 400 carbon hearing aid showing the three battery holders.

There is no indication which way to install the batteries. In fact, each of the three battery holders can accept a battery facing either way.

The secret is that you have to have the first and third batteries facing the same way and the the middle battery facing the opposite way.

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Close-up view of the left side of the battery case of the Sonotone Model 400 carbon hearing aid showing the battery terminals.

Note the outside ring for matching to the negative terminal of the battery, and the center button for matching the positive terminal. Thus the battery would work no matter which way it was installed.
 

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Close-up view of the right side of the battery case of the Sonotone Model 400 carbon hearing aid showing the battery terminals.

Note that they are identical to the left side terminals. As a result, a battery would work no matter which way it was installed.

 

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Front view of the heavy-duty carbon amplifier for use with binaural bone conduction Sonotone Model 400 carbon hearing aids. It shows the name "Sonotone", "New York" and "Pat. Pend."

Note the plugs on each end—one goes to each of the bone conduction transducers.

 

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Rear and bottom view of the heavy-duty carbon amplifier of the Sonotone Model 400 carbon hearing aid showing the two pins on the bottom for plugging into the battery case.

The left plug is slightly smaller than the right one so you can't accidentally plug them in backwards.

This carbon amplifier measured 2⅜" x 1 9/16" x 15/16" deep (6.0 x 3.9 x 2.3 cm). It was quite heavy for its size and weighed 2.7 oz. (78 g).

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Close-up view of the end of the heavy-duty carbon amplifier of the  Sonotone Model 400 carbon hearing showing the bone conduction cord unplugged. Both pins are the same size, so it didn't matter which way you plugged them in.

 

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Close-up view of the end of the heavy-duty carbon amplifier of the  Sonotone Model 400 carbon hearing showing the bone conduction cord plugged into the carbon amplifier.

 

 

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The heavy-duty carbon amplifier of the Sonotone Model 400 carbon hearing aid plugged into the socket on the front of the battery case.

 

 

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View of the Sonotone Model 400 carbon hearing aid carbon amplifier plugged into the end of the battery case. You used this configuration if you were not using the carrying case.

 

 

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Inside view of the empty carrying case of the Sonotone Model 400 carbon hearing aid showing the mounting hardware for the carbon amplifier (top left corner) and the battery case cord and plug (bottom center).

This hearing aid could be used plugged into the carrying case, or taken out and used separately.

 

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If you use the carrying case with the Sonotone Model 400 carbon hearing aid then you plugged the carbon amplifier into the two jacks in the back left corner of the carrying case.

Shown here is the bottom of the carbon amplifier (bottom) showing the amplifier pins and the corresponding jacks in the carrying case (top) into which they were pinned.

 

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View of the back left corner of the carrying case of the Sonotone Model 400 carbon hearing aid showing the carbon amplifier plugged into the case hardware.

 

 

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When using the carrying case of the  Sonotone Model 400 carbon hearing aid, the battery case also needed to be plugged into the wire coming from the bottom of the carbon amplifier as shown.

 

 

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Close-up view of the plug attached to the carbon amplifier mounting hardware that plugged into the battery case (shown here unplugged).

 

 

 

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Side view of the lighter-duty carbon amplifier for use with single bone conduction Sonotone Model 400 carbon hearing aids. Embossed on the side are the words "Sonotone", "New York" and "US. Patent 1,939,627".

The two pins on the bottom of the carbon amplifier plugged directly into the front of the battery case shown above.
 

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End view of the light-duty version of a carbon amplifier for use with the Sonotone Model 400 carbon hearing aid showing the 3-pin plug and socket.

This plug is slightly smaller than the socket on the carbon microphone so you can't accidentally interchange them.

 

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This Sonotone Model 400 carbon hearing aid with binaural bone conduction transducers actually had three volume controls. The master volume control was on the bottom of the carbon microphone.

In addition, there were two sliding rheostats, one on each cord leading to the transducers. This way you set the overall volume via the master volume control and balanced the sound between both ears using the individual rheostats on each transducer cord.

"Off" is at the bottom and "Loud" in at the top. There are 10 numbers between the bottom and the top so you could remember your preferred settings.

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End view of a Sonotone Model 400 carbon hearing aid bone conductor transducer showing the end of the transducer with the cable plugs and transducer jacks. The plugs are different sizes so you can only plug them in the "right" way.
 

 

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View of a Sonotone Model 400 carbon hearing aid bone conductor transducer showing the pins plugged into the jacks.


 

 

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Rear view of a Sonotone Model 400 carbon hearing aid bone conductor transducer showing the pattern on its back.

 

 

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End view of a Sonotone Model 400 carbon hearing aid bone conductor transducer showing the Sonotone stylized "S" in a circle. (It looks almost like an "8" if you didn't know better.)

Notice the slot in the top into which the end of the headband slides.

 

 

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Close-up view of a Sonotone Model 400 carbon hearing aid bone conductor transducer showing the end of the headband about to be pushed into the slot.

 

 

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View of a Sonotone Model 400 carbon hearing aid bone conductor transducer with the tip of the headband properly inserted.

 

 

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View of the swivel-bracket on the end of the headband so the bone conductor transducer could be adjusted to sit flat on the mastoid bone.

 

 

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Front view of a bone conductor transducer. The tan-colored surface fits against your head on the mastoid bone just behind your ear.

Each transducer measured 1½" x ¾" x ⅝" (3.7 x 1.8 x 1.5 cm) and weighed ¾ oz. (22g), although they feel much heavier.

 

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View of a single-sided headband with a bone conduction transducer mounted on the swivel bracket.

The headband held the transducer tightly to the mastoid bone behind the ear for efficient transfer of the sound vibrations to the skull.

 

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View of a headband with bone conduction transducers mounted on both sides.

Note that this is really a single-sided headband. It does not have the swivel bracket on the left side so the transducer on that side will not fit as well as it otherwise would.

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Close-up view of the carrying case of the Sonotone Model 400 carbon hearing aid showing the notch in the middle of the left end for the bone conduction transducer cord.

 

 

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Close-up view of the carrying case of the Sonotone Model 400 carbon hearing aid showing the notch in the middle of the left end with the bone conduction transducer cord sticking through.

There was corresponding notch on the other end for the right transducer cord.

 

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The Sonotone Model 400 carbon hearing in its resonant case. The hearing aid could be used mounted in the carrying case, or taken out and used separately.

 

 

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Inside view of the lid of the Sonotone Model 400 carbon hearing aid's carrying case showing the Sonotone name and crest (upper center) and its address (19 W. 44th St. New York) on the left and right sides. The words "Patent Pending" are at the bottom.

 

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The resonant (carrying) case for the Sonotone Model 400 carbon hearing aid.

The case measured 5 13/16" x 4 1/16" x 3¾" (14.8 x 10.3 x 9.5 cm) and empty weighed 10.3 oz. (292 g).

 

 



 

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