Common sonic decay test mistakes and how to avoid them

By Alex Shy, ISA Board Certified Master Arborist (MA-5832B)October 16, 2026

A sonic decay test is only as good as the readings that go into it. The method is simple, but small slips in the field can turn sound wood red on the map or, worse, hide decay that's really there. Most of these mistakes are easy to avoid once you know to look for them.

Here are the ones that come up most often, roughly in the order you'd make them on site.

1. Testing at the wrong height

A sonic map shows one slice of the trunk. If the fruiting body is at the root flare and you test at chest height because it's comfortable, the map can come back clean while the decay sits below it. Decide what question you're answering first, then set your sensor positions at the height that answers it. Record the height so a retest is comparable.

2. Positions not level

All the sensor positions should sit at the same height. If some are a few inches higher than others, the paths don't lie in one flat slice, and the distances the app works out won't match the real distances through the wood. Run a tape around the trunk and set every nail along it.

3. Poor contact at a position

With a two-sensor detector like the Tree Check, the sensors attach to nails or screws set at each position. If a nail sits in loose bark, in a callus ridge, or isn't driven firmly into sound wood, it adds time to every path that uses that position. The result looks like decay radiating from one spot.

How to catch it: if every path from one position is slow, be suspicious. Reset the nail, re-test a couple of paths from that position, and see if the times drop. Sound Wood's reading checks flag a position where every path is slow for exactly this reason.

4. Taking the first tap at face value

The first tap or two at a new pair can be inconsistent while everything settles. The Tree Check manual suggests discarding the first few taps and using a consistent cluster of readings. A single odd tap that you save without a second look can put one wrong path on the map.

5. Wrong circumference or positions

Every path's distance comes from the circumference and where each position sits around the trunk. If the circumference is off, or you measured positions from a different zero mark than you entered, every speed is off with it. A circumference that's too large makes every path look faster than it is, which can hide decay.

• Measure circumference at the same height as the positions, not at chest height out of habit.
• Mark zero clearly on the bark and measure every position clockwise from it.
• If the trunk is strongly out of round, note it. A circle is an approximation, and the distances for paths across the narrow side will be less accurate.

6. Typos

Readings are typed in by hand, so a dropped or doubled digit is easy: 285 becomes 2850, or 412 becomes 41. One bad number can create a red pocket that isn't there or erase one that is. Sound Wood checks each path as you type and flags readings that are faster than sound can travel through wood, or that don't fit with similar paths. Read the flags before you leave the tree, while you can still re-test.

7. The same pair entered twice

If you re-test a pair and enter both readings, that path counts twice. If they disagree a lot, which one is right? Keep the reading you trust and remove the other. The app warns when the same two positions appear under different path IDs.

8. Too few paths across the problem area

Decay is only confirmed red where slow paths cross. If you only take short paths between neighbouring positions, or skip pairs that cross the center, large areas of the trunk have no crossings at all. Those show as yellow, possible decay, and the range between minimum and "up to" gets wide.

How to fix it: take every pair if you can. Sound Wood's Fill all pairs adds a row for every pair of positions so you can see which ones are still missing. If time is short, prioritise paths that cross the area you're worried about from different angles.

9. Reading a crack as decay

A vertical crack, seam or included bark forces sound to travel around it, just like a cavity does. The map can't tell the difference. If you see a crack on the outside, expect slow paths that cross its line and factor that into how you describe the result. Resistance drilling or a visual check at the crack can help tell them apart.

10. Forgetting which way the map faces

A map is much more useful when you can say "the pocket is on the street side." Record which way S1 faces, or use the compass in the app, so the map has a north arrow. Without it, a client (or you, next year) has to guess how the map lines up with the tree.

11. Using the wrong wood group

The cutoff depends on the wood. Pick the species from the list rather than a nearby guess, so the app applies the right tier: 22 µs per inch for hardwoods, 25 for softwoods, 30 for very soft woods. A cottonwood tested against the hardwood cutoff will look riddled with decay.

A two-minute check before you pack up

• Any flags from the reading checks? Re-test those paths now.
• Any position where every path is slow? Reset the nail and re-test.
• Circumference and positions match what's on the tape?
• Every pair taken, or at least the ones that cross the problem area?
• Height, zero mark and S1 direction recorded?

Most mistakes are only expensive if you find them back at the office. Catch them at the tree and they cost a couple of extra taps.