A log drum is a section of seasoned hardwood hollowed to a wall of at least 3 cm, and different parts of the wall ring at different pitches according to their thickness and length. The tone of a vibrating wooden bar rises in proportion to its thickness and falls with the square of its length, so thinning a tongue lowers its note and shortening it raises it. Season the log before and after hollowing, seal the end grain, and burn out only with fire controls in place.
Part of Music & Simple Instrument Making
A hollowed trunk used as a struck instrument, how wall thickness sets its tones, and how to stop it cracking.
Wall Thickness Sets The Notes
The parent article recommends a log 30-60 cm in diameter and 40-60 cm tall, hollowed to walls at least 3 cm thick, and says walls of varying thickness give 2-3 distinct tones. That is consistent with how a wooden bar behaves. For a bar of uniform cross-section that is free to vibrate, standard beam theory gives a frequency that rises in proportion to its thickness and falls with the square of its length (the relation is f proportional to t divided by L squared, times a constant set by the wood’s stiffness and density). It is an idealisation for a straight bar and only a guide for a curved tongue cut in a log, but it points the right way for tuning.
Worked example, thinning a tongue. Suppose a tongue 30 mm thick and the same tongue after being worked down to 20 mm, with the length unchanged. The frequency ratio is 20 mm ÷ 30 mm = 0.667. In pitch terms that is 1200 × log₂(0.667) = −702 cents, about a fifth lower (a fifth is 702 cents, a ratio of 3:2). So a thinner tongue gives a lower note, and removing one-third of the thickness drops the note about a fifth. Doubling the length of an otherwise identical tongue divides the frequency by 2² = 4, which is two octaves lower.
Use this to plan: leave the walls thicker than needed, test, and thin only the tongue you want lower. You can lower a note by removing wood but you cannot put it back.
What You Need
- A seasoned hardwood log 30-60 cm in diameter and 40-60 cm tall, the dimensions the parent article gives; for seasoning see Wood Seasoning
- An axe, adze or gouge; see Hand Tool Making
- Fire controls if burning out: water, sand, a watcher
- Sealer for the end grain — beeswax or pitch; see Beeswax and Pitch and Waterproofing
Method: Hollow Without Splitting
Step 1: Choose a log without cracks or rot, and cut it square at both ends. A log with radial splits already in the end grain will open further.
Step 2: Seal both end faces at once with beeswax or pitch. Water leaves end grain much faster than it leaves the sides, and uneven drying is what splits logs.
Step 3: Mark the opening (a slit or a round mouth) on the top face and the wall you want to leave. Keep a margin: do not cut below 3 cm anywhere on the first pass.
Step 4: Hollow in stages. Use an adze or gouge, or char the interior and scrape the char off, the same logic used for a dugout; see Dugout Canoe, which sets a 3-5 cm wall for a boat. Take a layer, rest, check the wall by feel.
Step 5: Dry slowly in shade, out of wind and sun, for months, turning it occasionally. A hollowed trunk that dries too fast checks on the surface.
Step 6: Tune last, by thinning the tongues you want lower, as in the worked example.
Common Mistakes
- Hollowing a green log and drying it fast. The wall shrinks across the grain and splits.
- Making every wall the same thickness. You get one dull tone; the parent article’s 2-3 tones come from thickness variation.
- Cutting the wall under 3 cm. The wall is the instrument and the structure; too thin and it cracks when struck.
Safety
Burning out is a fire hazard and heavy logs injure
Never burn out a log indoors or near stored wood; keep water and sand at hand and never leave the fire. See Fire Management. Logs of this size weigh tens of kilograms; roll or lever them, do not lift them alone. Sharp tools slip on round surfaces, so clamp the log.
What’s Next
- Learn next: Skin Drums (Frame & Goblet) — a hollow trunk with a hide head
- Combine with: Woodworking — the cutting skills this relies on