Continued from part 3.
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In between GenRad’s strange photo-film oscillograph in the ’20s and its own pinfeed chart recorder in the ’50s, GR collaborated with Esterline. The Esterline pinfeed chart recorder was the standard in many industries. It was tough, portable, and simple to use. GenRad made a signal conditioner and preamp calibrated to feed the Esterline from various transducers or sources.
Around the same time in the mid-30s, GR also made its first sound level meter, the 759, establishing several traditions and setting the STANDARD for such instruments.
Here’s the GenRad 759 sound level meter measuring a noisy NYC street, nicely illustrating the time and place. Horse carts, the Elevated, humpback car. (I think it’s a ’36 Buick.)
This was the first SLM to use the ABC weightings. It also used a durable crystal mic, a consistent and superior feature of all later GenRad SLMs. The meter itself had a linear scale for db, which was formed by the physical shape of the magnets, not by electronic ckts. Incidentally, the price in the 1936 catalog was $200, which translates after official inflation to $4500. The new price for the newest SLM is $3500, actually cheaper than the 1936.
In our setup, Martian is singing into the mic. Note that the mic is on a pivot, and folds down into its own little nest when not in use.
GR repeated the folding mic in its nest on the 1933 sound-level meter with octave analyzer. The hidden compartment also held some of the controls and several extra mics.
The calibrated audio output from the SLM goes through a solid-state fullwave rectifier module (ICs were nothing new!) into the signal conditioner and preamp.
The rectified audio goes into the signal conditioner, which has all the features you’d want on such a device. Input impedance is variable, and degree of amplification is variable. The recorder output, and the meter, are rigged as a bridge or difference between the amplified signal and a fixed standard voltage. You can offset the fixed standard up and down to focus the difference on one part of the range.
The unrectified signal is also going into a GenRad oscilloscope, which was poorly described. Its configuration differs from modern scopes, with the controls on one side and only the screen on the front. This was clearly inconvenient in the usual ‘row-house’ setup of instruments as seen here, and wouldn’t have worked at all in a rack mount.
The Esterline itself had no controls; it simply picked up an input in the required range and moved a pen back and forth on paper while a motor pulled the paper forward with pinwheels.
Finally, here’s the whole setup running in sync with the intensity of Martian’s aria.
Polistra's Mill 