Sound Waves - What Trees Can Tell Us

Nov 14, 2017

In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses sound waves.

If we listen carefully, trees have a great deal to tell us.  You may have an image in your mind of someone bear hugging a tree with their ear pressed tightly to the trunk.  This strategy may work if you need a hug, but you won’t likely hear anything of consequence.  What I am referring to is the process of using sound waves to “see” inside the tree.  The process is called Stress Wave Timing.  In a rudimentary way, this process is similar to medical imaging, such as echocardiography, which uses sound waves to generate an internal picture of the patient’s heart. What is great about this type of technology is that it is fast and nondestructive.  In the past, a patient had to undergo exploratory surgery to be able to seen inside.  For a tree, it would have to be cut down to see inside.

So why do we need to see inside of a tree?  This is extremely useful for determining hazard trees in campgrounds, parks, your back yard, or other high traffic areas.  The goal is to safely remove these trees before they fall and harm a person or damage property.  This technology can also be used to determining the value of standing timber.  For example, as trees age, they reach a point where fungal decay will eventually become established, which reduces the economic value of the tree.

How does stress wave timing work?  The concept is simple.  Sound waves travel from particle to particle.  You are hearing me now because the vibrations from your radio speakers are causing the air to vibrate.  These vibrations or sound waves eventually get to your ears causing your eardrums to vibrate.  Sound waves can also travel through a liquid or solid.  In fact, the more dense the material, the faster sound will travel.  This is important and we will come back to density and speed in a minute.

To apply stress wave timing on a tree, you need a hammer, two nails, two accelerometers, and a timer.  One nail is driven into the tree trunk so that it is firmly embedded through the bark into the wood.  The second nail is positioned directly opposite the first across the diameter of the tree. When you strike one of the nails with the hammer, a stress wave is sent out from that nail through the tree to the second nail.  The accelerometers and timer measure the time it takes for the sound wave to travel from one nail across the tree to the other.  By measuring the diameter of the tree, we know the distance the sound had to travel between nails, so we can calculate speed in feet per second.  Since sound is so fast, we measure the time in microseconds, so speed is recorded in feet per microsecond.

I’m guessing that you don’t have a couple of accelerometers lying around the house.  Not to worry since compact versions of stress wave timers have been developed.  A friend and colleague, Professor Bruce Allison, worked closely with the USDA Forest Products Laboratory and the University of Wisconsin to develop a stress wave tool called Tree Check.  He saw a need for an affordable handheld system for the tree care and forestry professionals.

Back to seeing inside of the tree.  When a tree has fungal decay within its main stem, the decayed wood is much less dense than normal wood.  That makes sense because the fungus is consuming the wood tissue.  I mentioned earlier that sound travels faster through more dense materials, so sound travels faster through normal wood and slower through decayed wood. Average stress wave travel speeds have been established for common tree species. These values are available to tree professionals when they are using stress wave tools. Consider red oak for example, sound will travel at 188 microseconds per foot through normal wood.  While testing a large red oak on Kemp Station, it took more than double the time at 400 microseconds per foot. What was going on?

Recall the two nails on opposite each other on the tree trunk.  In a healthy tree, when the nail is struck with the hammer, the leading edge of the sound wave travels the shortest distance, a straight line, across to the second nail.  In a tree with decay at the center, the leading edge of the sound wave can still travel in a straight line across to the other nail, but it slows down as it moves through the less dense decayed wood.  In a more extreme case with a tree that is hollow in the middle, the sound wave can not travel in a straight line, but it must follow the circumference of the tree traveling in a semicircular path just under the bark, again a longer distance to travel than a straight line, which takes more time.  This was the case with the oak at Kemp Station, it was hollow in the middle.

Stress wave timing is a fantastic technology that provides useful and sometimes lifesaving information to arborists and foresters.  If we listen carefully, with the help of a stress wave tool like Tree Check, trees really do have something to tell us.

For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.