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Sonic testing vs. resistance drilling: when to use each

By Alex Shy, ISA Board Certified Master Arborist (MA-5832B)

Sonic tomography and resistance drilling are the two tools most arborists reach for when they suspect internal decay. They're often presented as competitors. In practice they answer different questions, and the most useful assessments often use both.

How each one works

Sonic testing

You set sensor positions around the trunk and time a stress wave between pairs of them. Decay, cavities and cracks slow the wave, so slow paths point to a problem. With enough paths crossing the trunk, you can map where the slow wood is across the whole cross-section. A detector like the Tree Check uses nails or screws at each position, so the trunk takes only small holes in the outer wood.

Resistance drilling

A resistance drill pushes a very thin needle into the trunk and records how hard it is to drive, giving a profile of the wood along that one line. Sound wood shows steady resistance; decay and voids show drops. Growth rings often show up as peaks, and you can see how thick the sound shell is along the drill line very precisely.

What each one is good at

Sonic testing is good at

  • The whole cross-section. It shows where the decay is around the trunk, not just along one line.
  • Finding the problem side. You learn which direction the decay faces and whether it reaches the bark.
  • Low impact. Only small nail holes in the outer wood.
  • Monitoring. The same positions can be re-tested next year to see if a pocket is growing.
  • Client communication. A map is easy to show and explain.

Resistance drilling is good at

  • Precise depth. It tells you exactly how thick the sound wood is along the drill line.
  • Separating causes. A crack and a decay pocket can look the same on a sonic map. A drill line through the area can show whether it's a void, soft decay or a thin crack.
  • Small or awkward spots. Buttress roots, branch unions and stems too small for many positions.

Their limitations

Sonic testing

  • It finds slow wood, not the cause. Cracks, seams and included bark read like decay.
  • Accuracy depends on coverage. Areas without crossing paths are uncertain, which is why Sound Wood reports a range.
  • It depends on good contact and accurate measurements at every position.

Resistance drilling

  • It only tells you about the line you drilled. Miss the pocket by a few inches and the profile can look sound.
  • Covering a trunk takes several drill lines, each a wound through the outer wood and into the decay column.
  • Reading profiles takes practice, and results vary with species, moisture and drill settings.

In one line: sonic testing tells you where to look; resistance drilling tells you exactly what's there along one line.

Using them together

A common and sensible sequence:

  1. Visual assessment. Signs of decay, the target, the species.
  2. Sonic test at the height of concern to map the cross-section and find the problem side.
  3. Resistance drilling only where the answer matters: through a red pocket to measure the shell, through a yellow area to decide whether it's decay, or along a suspected crack.

That way the drill goes where it's most useful, and you make fewer holes. The map also makes it easier to explain why you drilled where you did.

When one tool is enough

  • Sonic alone is often enough for monitoring a known pocket, for a first look at a trunk with general signs of decay, or when the map comes back clearly sound with good coverage.
  • Drilling alone can be enough for a quick check of one spot, such as a small stem, a root, or a single suspect area you already know about.
  • Both make sense when the decision is close, the target is serious, or the sonic map shows slow areas you can't explain from the outside.

Recording what you did

Whatever you use, record it alongside the result. Note the drill lines in your report (direction, height, depth to sound wood) next to the sonic map. In Sound Wood, the notes and photos stay with the tree, and the field report puts the map, the decay range and your observations on one page, so the reader can see both pieces of evidence together.

Neither tool makes the decision for you. Together they give you a much clearer picture to make it with.

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