Why softwoods and poplars need different decay cutoffs

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

If you test an oak and a cottonwood with the same sonic detector, the cottonwood will almost always read slower, even if both trees are completely sound. Use one cutoff for both and you'll either call the cottonwood rotten or miss decay in the oak. That's why sonic decay cutoffs have to depend on the kind of wood.

What makes sound faster or slower in wood

A stress wave moves faster through material that's stiff for its weight. In wood, that comes down mostly to the structure of the cell walls, the density, and how the fibers are arranged. Dense hardwoods with stiff fibers carry sound quickly. Lighter woods are generally less stiff, and sound moves through them more slowly.

Other things move the number too: moisture content, temperature (frozen wood behaves differently), and the direction you're measuring relative to the grain. Across a trunk, a sonic test measures sound moving across the grain, which is slower than along it. That's normal and built into the cutoffs.

Healthy ranges differ by wood type

The Tree Check manual gives typical readings for sound wood of roughly 18 to 22 microseconds per inch for hardwoods and roughly 25 to 30 for softwoods. Readings above the healthy range suggest decay, and higher readings make decay more likely. The manual is clear that there's no single fixed decay number and that judgment and a reference reading on sound wood of the same tree both help.

That's the core idea: a reading only means something compared to what's normal for that wood.

Sound Wood's three tiers

The tiers follow the reference table Allison Tree, the maker of the Tree Check, publishes by wood type: 22 µs per inch for hardwoods, 25 for softwoods and 30 for the softest woods. The USDA Forest Products Laboratory's guide to stress wave decay testing gives the same picture: softwoods are slower than hardwoods even when sound, and readings should be judged against what's normal for the wood.

• Hardwoods: 22 µs per inch. Oaks, maples (other than silver maple), hickories, ashes, beech, elms, blackgum and most other broadleaf trees.
• Softwoods: 25 µs per inch. Most conifers, plus boxelder and silver maple, which are lighter than most hardwoods.
• Very soft woods: 30 µs per inch. Poplars, cottonwoods, aspens, willows and tulip poplar.

A path slower than its tier's cutoff is flagged as slow. Pick the species from the list (over 1,200 are included) and the app picks the tier.

Why the tiers lean strict

Any cutoff is a line drawn through natural variation. Some sound trees will read a little slow; some decayed wood will read only a little slow. You have to decide which mistake you'd rather make.

Sound Wood leans toward calling borderline wood slow. Calling decay sound can leave a dangerous tree standing. Calling sound wood slow costs a second look. So:

• The hardwood cutoff sits at the top of the healthy hardwood range, not above it.
• The softwood tier sits at the low end of the softwood range, so conifers, boxelder and silver maple are judged more strictly than a single 30 µs cutoff would.
• Only the genuinely light woods, poplars, cottonwoods, aspens, willows and tulip poplar, get the 30 µs cutoff.
• Where a species sits near the boundary between two groups, it goes in the stricter one. For example, yews and alligator juniper are dense for conifers, so they use the hardwood cutoff.

What goes wrong with the wrong tier

Too lenient

Test a silver maple against a 30 µs cutoff and paths that are well above its normal sound-wood speed can still show green. Early decay gets missed, and the decay estimate comes out too low.

Too strict

Test a cottonwood against the hardwood cutoff and much of the sound wood shows red. The map looks alarming, and the result won't hold up if anyone checks.

Both are why choosing the right species matters more than it might seem.

Reference readings: tuning to one tree

Group cutoffs cover the typical tree. Individual trees vary. A reference reading lets you measure what sound wood looks like in the tree in front of you: one reading between two positions on wood you're confident is sound, away from the problem area.

In Sound Wood, a reference reading can only make the cutoff stricter, never looser. Its speed plus a 10% allowance, in line with the USDA guide's advice to suspect decay when readings run more than about 10% slower than sound wood of the same tree, becomes that tree's cutoff when it's stricter than the group's. If the reference comes out slower than the group cutoff, the app keeps the group cutoff and suggests you re-check, because a slow "sound" reading may not be sound at all.

The short version

Sound speed depends on the wood, so cutoffs have to as well. Hardwoods at 22, softwoods at 25, very soft woods at 30 microseconds per inch, with borderline species placed in the stricter group. Pick the species carefully, take a reference reading when you can, and treat the cutoff as a careful line, not a law of nature.