239A Stress Wave Timer


This portable instrument measures sonic propagation time in wood structures such as living trees, waterfront structures, panel products, and poles.

  • Detects decay in live trees
  • Non-destructive testing to assess the structural integrity of wood structures
  • Functions as a crosscheck for other wood testing equipment
  • Features multiple capabilities for handling a wide variety of laboratory and field testing needs
  • Accurately measures and displays mechanical stress wave propagation time or speed of sound through a specimen
  • Stress wave propagation time is an excellent indicator of the mechanical properties of wood and woodbased products
  • Ideal for testing standing timber, oriented fiberboard and other panels, waterfront pilings, or other structural timbers in place
  • The Stress Wave Timer uses two accelerometers along the propagation path to detect the signal. The first accelerometer may be built into the impact device while the second is secured to the specimen.
  • In operation, the electronic timer starts when the stress wave front arrives at the first (“start”) accelerometer and stops when the wave arrives at the second (“stop”) accelerometer.
  • Propagation time displayed in microseconds
  • Timer automatically filters out noise and other signals, providing a consistent, accurate measurement.
  • Accelerometers and impact devices are sold separately to allow customization of stress wave measurement systems.
  • Powered by a single 9-volt battery; designed to be very portable, lightweight, durable, and reliable.
  • System is configured in kits for the various testing situations.


Digital readout indicates stress wave propagation time from 1 to 9999 microseconds with a resolution of 1 microsecond. Controls: Dual gain control switches and internal sensitivity controls for the start and stop channels.



9 volt

Battery Life

in excess of 40 hours



9 x 6 x 8 in (230 x 150 x 200 mm) (L x W x H)


5 lb (2.3 kg)

Shipping Dimensions

L x W x H

33 x 12 x 12 in (838 x 305 x 305 mm)


35 lb (15.9 kg)

Approximate shipping container size and weight are given for Model 239A with Laboratory Clamp Set. Sizes and weights will vary with other accessory options.

The Model 239A is available in kits configured for specific uses. The “Everything Kit” includes all the special tools and parts needed to fully realize the potential of this testing instrument. Individual components may be ordered to build your own customized system.

Description Part Number

Kit 1

Kit 2 Kit 3 Kit 4
KIT 1: Laboratory system 239-KIT-1001 Kit 1
KIT 2: Field survey system 239-KIT-1002 Kit 2
KIT 3: Panel testing system 239-KIT-1003 Kit 3
KIT 4: Everything 239-KIT-1004 Kit 4
Model 239A electronic unit 239-BAS-1001 1 1 1 1
Model 239A manual LIT-MAN-1001 1 1 1 1
Laboratory clamp set with carrying case 239-ASM-1003 1 1
Trombone clamp 239-ASM-1001 1 1
BNC/BNC cable WIR-MUL-1001 2 2
10-32 Micro/BNC cable WIR-MUL-1006 2 2 2
Stress wave impact hammer SEN-PIE-1005 1 1
Field test accelerometer SEN-PIE-1002 1 1
Laboratory test accelerometer SEN-PIE-1001 2 2 2
Lag screw clips with screws (set of four) 239-ASM-1002 1 1

Laboratory Clamp Set


Ideal for laboratory testing. Easily fastened to bench top or other work surface for fixed-gauge-length testing. Set includes separate start and stop clamps. The start clamp includes a pendulum ballhammer device for initiating stress waves; start accelerometer mount; and toggle clamp for convenient attachment to specimen. The stop clamp includes a stop accelerometer mount and toggle clamp. Two Laboratory Application Accelerometers and two 10-32 Micro-to-BNC connector cables are required (sold separately). A portable carrying case is included.

Trombone Clamp


Ideal for testing panel material; panels may be tested without cutting a specimen. The Trombone Clamp includes a fixture with a pendulum ball-hammer device for initiating a stress wave in a specimen. The fixture is pressed against the surface of a specimen and held in place by hand while the hammer device is actuated and readings are taken on the 239A Stress Wave Timer. Two Laboratory Application Accelerometers and two 10-32 Microto-BNC Connector Cables are required (sold separately).

Stress Wave Impact Hammer


Ideal for field testing, this self-contained, stainless steel impact device has a built-in start accelerometer for field testing, such as in structural decay surveys. Striking a test specimen with the Impact Hammer activates the start accelerometer and sends a stress wave through the specimen. The stress wave is then picked up by a stop accelerometer.

A BNC/BNC Cable is required. Either a Field Test or Laboratory Application Accelerometer can be used as the stop accelerometer. (WIR-MUL-1001)

Laboratory Application Accelerometer


This Lab Application Accelerometer is ideal for research of stress wave propagation in wood and wood-based products. It includes a 10-32 micro connector, and requires a 10-32 Micro/BNC Cable.

Cable, 10-32 Micro/BNC


Use with Laboratory Application Accelerometer (SEN-PIE-1001)

Field Test Accelerometer


The Field Test Accelerometer is used in field testing to pick up stress waves initiated by the Impact Hammer. It includes a BNC output connector. A BNC/BNC cable is required. The Field Test Accelerometer is shown with a lag screw clip (sold separately).

Lag Screw Clip


A lag screw clip is ideal for field tests where the test specimen will be examined repeatedly over a length of time. This clip provides a stable mounting point for the field test accelerometer. The kit includes one lag screw and one clip. Made of stainless steel.