You shall make the altar of acacia wood, five cubits long and five cubits wide—the altar is to be square—and three cubits high.
Make it hollow, of boards. As you were shown on the mountain, so shall they be made.
נבוב לחת HOLLOW WITH TABLETS — Render it as the Targum does, חליל לוחין, hollow as regards boards (לוחין is what is termed an accusative of respect): boards of shittim-wood on all sides (cf. v. 1) and a hollow space in the middle; it shall not be in its entirety a single block of wood, the thickness of which in two directions should be five cubits by five cubits — like the trunk of a tree, which has been hollowed out.
נבוב לוחות, “make it hollow with boards;” it was to be hollow underneath in order for it to be lighter when carrying it. These hollowed out boards were inside of the mesh on which blood would be sprinkled, occupying the lower half of the altar. When the Israelites were encamped they would fill the hollow space with earth and offer the sacrifices on top of that earth. When they would move again, the accumulated earth would fall between the boards enclosing that hollowed space and the mesh, which looked like a sieve. Each time the came to rest they would fill that hollow space up with earth again.
Author:Chizkuni
Commentary on the Torah of Rabbi Hezekiah ben Manoah. Chizkuni, composed in mid-13th century, is actually a compilation of insights culled from the Midrashim, as well as the writings of twenty other Rishonim, including Rashi, Rashbam and Ibn Ezra. However, Chizkuni does not name any of his sources (other than Rashi), in order to encourage objective study, as he felt that one should focus on the message rather than the messenger.
Composed: Middle-Age France (c.1220 - c.1260 CE)
Author:Sforno
Commentary on the Torah by Rabbi Ovadiah ben Jacob Sforno, a 16th-century Italian rabbi and physician.
Composed: Bologna (c.1500 - c.1550 CE)
ומלת נבוב לוחות. כמתורגם ארמית. וכמוהו ואיש נבוב ילבב. שהוא ריק בלי לבב. על דרך האומרים ראש בלי מוח.
[HOLLOW WITH PLANKS.] Nevuv luchot (hollow with planks) is to be explained in accordance with the Aramaic translation. The phrase ve-ish nevuv yillavev (But an empty man will get understanding) (Job 11:12) is similar, for ish nevuv means an empty person, that is, a person lacking intelligence. The verse is similar to the aphorism, a head without a brain.