In discussing the service on Yom Kippur, the holiest day of the Jewish Year, the Talmud notes that the voice of the Cohen Gadol, the High Priest, could be heard from a distance of ten parsangs. In case you were wondering, a parsang is an old Persian measure, and is about 3 miles or almost 5km. This means that the voice of the High Priest could be heard over 30 miles away! The Talmud notes that it could be heard at this distance even during the day, when sound does not travel as far as it does at night. Here it is, in the original:
שקלים כ,א
דְּאָמַר מָר: וּכְבָר אָמַר ״אָנָא הַשֵּׁם״ וְנִשְׁמַע קוֹלוֹ בִּירִיחוֹ. וְאָמַר רַבָּה בַּר בַּר חָנָה אָמַר רַבִּי יוֹחָנָן: מִירוּשָׁלַיִם לִירִיחוֹ עֲשַׂר פַּרְסֵי
וְאַף עַל גַּב דְּהָכָא אִיכָּא חוּלְשָׁא, וְהָכָא לֵיכָּא חוּלְשָׁא. וְהָכָא יְמָמָא, וְהָתָם לֵילְיָא
דְּאָמַר רַבִּי לֵוִי: מִפְּנֵי מָה אֵין קוֹלוֹ שֶׁל אָדָם נִשְׁמָע בַּיּוֹם כְּדֶרֶךְ שֶׁנִּשְׁמָע בַּלַּיְלָה? מִפְּנֵי גַּלְגַּל חַמָּה שֶׁמְּנַסֵּר בָּרָקִיעַ כְּחָרָשׁ הַמְנַסֵּר בַּאֲרָזִים
There already was an incident where the High Priest recited, in his confession that accompanied the placing of hands on his bull on Yom Kippur: Please God, and his voice was heard in Jericho. And Rabba bar bar Chana said that Rabbi Yochanan said: The distance from Jerusalem to Jericho is ten parasangs.
…here it was during the day, when sound does not travel as well, that the High Priest recited his confession… As Rabbi Levi said: Why is a person’s voice not heard during the day in the manner that it is during the night? It is due to the fact that the sound of the sphere of the sun traversing the sky generates noise like the noise generated by a carpenter sawing cedars, and that noise drowns out other sounds…
So today on Talmudology we ask: Is Rabbi Levi correct? Does sound really travel further at night? And if so, why?
Yes. It is true
Rabbi Levi is completely correct. Sound does indeed travel further at night, as you can see below in this helpful graphic. (For the man with the trumpet, think Cohen Gadol. For the dog, think Jericho.)
From here.
The first thing to know is that the speed of sound is dependent on the temperature of the air. Sound moves quicker in warm air and slower in cold air. During the day the sun heats up the earth’s surface, and in particular it heats up the air that is closest to the ground, which is where the sound travels the fastest. But a heat gradient bends the sound waves upwards, much in the same way that a lens bends light rays. (The gradient in which atmospheric temperature decreases with elevation by an amount known as the adiabatic lapse rate.) As a result, the sound waves travels up and away from the listener, and the sounds are quieter.
The reverse happens at night. At night the ground cools quickly. The higher air is warmer than the air close to the ground. The sound further from the ground travels faster at night causing the sound wave to refract back towards the earth. The listener now hears them as louder than they were during the day. It’s physics! (Though it should be noted that some physicists dispute this explanation.)
It’s not just sound waves
Why radio waves travel further at night. From here.
Here’s a fact that Rabbi Levi did not know. It’s not just sound that is heard better at night. Radio waves are heard better at night too, though for an entirely different reason. To be precise, this does not happen with all radio waves, but just those on the AM and short wave frequencies. Because radio waves only travel in straight lines, they do not follow the curvature of the earth’s surface, and so have a natural range of only 30-40 miles. But they can be transmitted up to the ionosphere, where they bounce off of it and down, back to earth. At night, that ionosphere is protected from the electromagnetic radiation that streams from the sun and tends to distort it. And so the radio waves are reflected back down with less interference, which means they travel further and are easier to detect. As a result, some distant AM radio stations (remember those?) can only be heard at night, though the whole thing becomes rather a moot point since nearly everything broadcast can now be found on the internet, for which the ionosphere is not needed.
Noise pollution
We have demonstrated that sound travels further at night, using our knowledge of the properties of sound waves and the phenomenon of refraction. Rabbi Levi knew none of this, but he had something that very few of us today have: a quiet natural environment. What many of us never experience thanks to noise pollution, he experienced each and every night. He, together with the other sages of the Talmud lived in a world that had no noise other than the sound of human voices around a crackling fire and the background music of the natural world. It was an utterly different experience. Our modern world has given us many things to be grateful for, but noise is not one of them.
