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Updated: May 21, 2025
So it is only necessary to twist the phonometer about on its pivot until the sound is received most loudly in the horns and the band of light is greatest. I know then that the horns are at right angles to the direction from which the sound proceeds, and that, as I lift my head, I am looking straight toward the source of the sound. I can tell its direction to a few degrees."
The light of the phonometer had flashed up. A car was following us. "There's just one chance!" cried Kennedy, springing to the wheel. "We might make it on the rim." Banging and pounding, we forged ahead, straining our eyes to watch the road, the distance, the time, and the phonometer all at once. It was no use. A big gray roadster was overtaking us.
I turned and shot up the detour, stopping in the shadow of some trees, where we switched off every light and shut down the engine. Kennedy continued to watch the instrument before him. "What is it?" I whispered. "A phonometer," he replied. "It was invented to measure the intensity of sound. But it is much more valuable as an instrument that tells with precision from what direction a sound comes.
I looked through it myself to see how sound was visualized by light. "Hush!" cautioned Kennedy. Down on the main road we could see a car pass along slowly in the direction of Montrose, from which we had come. Without the phonometer to warn us, it must inevitably have met us and blocked our escape over the road ahead. That danger passed, on we sped.
It needs only a small dry battery and can be carried around easily. The sound enters the two horns of the phonometer, is focused at the neck, and strikes on a delicate diaphragm, behind which is a needle. The diaphragm vibrates and the needle moves. The louder the sound the greater the movement of this needle.
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