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).

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).

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.

The back was black.
 

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.
 

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."

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.

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.

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.

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.

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.

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.

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.
 

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

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

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).

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

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.

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

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

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.

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

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.

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

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.

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

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).