How sonic tomography finds decay inside a tree

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

Most of the decay that brings a tree down is decay you can't see. A trunk can look solid from the outside while the xylem behind the bark, the wood that holds the tree up, has been breaking down for years. You can sound it with a mallet, drill it, or probe a cavity, but each of those only tells you about one spot. Sonic tomography is a way to look across the whole cross-section of the trunk without cutting into it.

This guide explains how it works in field terms: what the detector is actually measuring, why a slow reading means trouble, and how a set of readings becomes a map you can put in front of a client.

Sound travels through wood, and decay slows it down

Tap one side of a trunk and a stress wave travels through the wood to the other side. In sound wood the fibers are stiff and continuous, so the wave moves quickly and arrives in a short time. Decay breaks down the cell walls that carry that stiffness. Cracks, cavities and hollows do something similar: the wave can't jump across an air gap, so it has to travel around it, which takes longer.

That's the whole idea in one sentence: sound that takes too long to cross the trunk has met something other than solid wood. Everything else is about measuring that time carefully and working out where the slowdown is.

What a transit time measures

A sonic decay detector, such as the Nu Arbor Tree Check, has two sensors you set into the trunk. You tap the start sensor with a small hammer, the stress wave travels through the wood to the other sensor, and the detector shows the time in microseconds (millionths of a second) it took to get there. That number is the transit time for one path through the tree.

A transit time on its own doesn't tell you much, because a big trunk naturally takes longer to cross than a small one. So the time is divided by the distance between the two sensors, giving a figure in microseconds per inch. That turns every path, long or short, into the same unit you can compare against a cutoff.

In Sound Wood you don't measure that distance with a tape. You enter the circumference (or diameter) and where each sensor sits around the trunk, measured in inches from a zero mark. The app works out the straight-line distance through the trunk for every pair of sensors.

Cutoffs depend on the wood

Not all sound wood is equally fast. Dense hardwoods carry sound quickly, conifers are generally slower, and very light woods such as poplars, cottonwoods and willows are slower still. A reading that would mean decay in an oak can be perfectly normal in a cottonwood. That's why the cutoff has to match the species.

Sound Wood uses three tiers, based on the reference values Allison Tree publishes for the Tree Check and on the USDA Forest Products Laboratory's guide to stress wave decay testing (see Sources below):

• 22 µs per inch for hardwoods.
• 25 µs per inch for softwoods, such as most conifers, boxelder and silver maple.
• 30 µs per inch for very soft woods: poplars, cottonwoods, aspens, willows and tulip poplar.

Pick the species and the app picks the tier. A path slower than its cutoff is flagged as a slow path. The tiers were chosen to lean strict: when a species sits near the line, it goes in the stricter group, because calling decay sound is a far worse mistake than asking for a second look.

One path tells you something is there, not where

Here's the catch with a single reading. If the path from sensor 1 to sensor 4 is slow, you know there's a problem somewhere along that line, but not whether it's near the bark on one side, in the center, or near the bark on the other side. A single slow path is a lead, not a location.

That's why a sonic test uses several sensor positions spread around the trunk, marked with nails or screws, and takes readings between many pairs of them. With a two-sensor detector you simply move the sensors from pair to pair. The paths form a web across the cross-section. Each path you add narrows down where the slow wood can be.

Crossing paths pin decay down

The key is where paths cross. If two slow paths cross each other, the slow wood that explains both of them is most likely near the crossing. If a slow path crosses a fast (sound) path, that crossing point is probably solid, so the decay on the slow path must be somewhere else along it.

Sound Wood colors the map using those rules:

• Green paths are sound: faster than the cutoff.
• Red marks decay that's confirmed, where slow paths cross with no sound path between them.
• Yellow marks possible decay: a slow path with no other slow path crossing it, or a slow stretch that sits between two sound crossings.

The red areas are joined into pockets on a grid, the pocket map, so you see a shape rather than a tangle of lines. When several slow paths meet at the same sensor with no sound path near it, the app treats that as decay reaching the bark at that sensor, which is often what you find at a basal cavity or an old wound.

Why the result is a range

Because yellow means "possibly," Sound Wood reports the decay as a range: Decay (minimum X%) from the confirmed red area, and Possible decay (up to Y%) if the yellow areas turn out to be decay too. The verdict uses only the confirmed red, but the "up to" figure is there so you never present a best-case number as if it were the whole story.

If the two numbers are far apart, that's useful information in itself. It usually means more sensor positions or more paths would tighten the picture.

What can fool a sonic test

Sound slows down for reasons other than decay, and it's worth knowing them before you write anything up:

• Cracks and seams. A crack forces sound around it just like a cavity does, so it can read as slow wood even when the wood beside it is sound.
• Poor sensor contact. A nail or screw that's loose, or set in loose bark or a callus ridge instead of sound wood, can add time to every path that uses that position. If all the paths from one position are slow, check it before you call it decay.
• Measuring mistakes. A circumference that's off, or a sensor position typed wrong, changes every distance the app works out. Sound Wood's reading checks look for numbers that don't fit with the rest, such as likely typos or a sensor that reads slow on every path.
• Species and condition. Wood properties vary with species, moisture and season. A cutoff is a guide line, not a law of nature.

Where it fits in an assessment

A decay map doesn't replace your judgment. It's one more piece of evidence alongside what you see on the outside, the species' known decay patterns, the target, and the rest of the tree. What it does well is turn a hunch about a hollow-sounding trunk into something you can measure, show a client, and repeat next year to see if the decay is spreading.

The readings take a few minutes once the sensor positions are marked. Turning them into a map, a decay estimate and a report used to be the slow part. That's the part Sound Wood is built to do for you, on your phone, before you leave the site.