Introducing the Impulse Measurement Rig, Model 2.0

The Impulse Measurement Rig allows violin-makers and researchers to measure the sound radiation of violins and violas. The original version was introduced by Joseph Curtin in 2010. Since then, the design has been refined through several generations of prototypes, and then field-tested in workshops, concert halls, and laboratories. Model 2.0 is more portable, more versatile, and significantly less expensive than the original rig. A violin or viola can be taken through a cycle of 24 individual measurements in less than ten minutes. In another ten minutes, the rig can be disassembled and packed into a carry-on suitcase.

How it works

The Impulse Measurement Rig relies on a miniature impulse hammer, a precision measurement tool with a head weighing just a few grams. When the hammer taps an instrument’s bridge, the instrument body is set into vibration. A sensor in the hammer head records the force of the blow, while a microphone records the resulting sound radiation. This data is digitally processed to give the instrument’s frequency response – a record of how much sound is produced at each frequency for a given force at the bridge. For an in-depth discussion of this process, see “Measuring Violin Sound Radiation Using an Impulse Hammer”.

Features

• Accommodates violins and violas of any size
•  3-axis positioning stage allows precise alignment of hammer and bridge
• Meaningful data can be gathered to 7 kHz and beyond
• Instruments can be rotated 360 degrees with respect to microphone
• Distance between microphone and instrument can be readily changed
• Mounts on standard photo tripod legs (not included)

While the Impulse Measurement Rig is fully capable of producing calibrated, laboratory-grade measurements, doing so requires an understanding of the basics of violin acoustics and a working familiarity with the relevant hardware and software. Prospective users should consider getting hands-on experience at the VSA Oberlin Acoustics Workshop. Joseph Curtin can provide only very limited after-sales consulting.

Price: $1,200 plus shipping and applicable taxes. This does not include the associated equipment (listed below), which can add $1,800 or more to the price. As the rigs are individually assembled at Joseph Curtin Studios, a lead time of up to six weeks is required. Delivery is currently limited to the USA. Contact Joseph Curtin to place an order.

Software: Acquisition and Overlay applications are available from George Stoppani (www.stoppani.co.uk). They run on Windows, but can be used on a Mac using Boot Camp or Parallels. There is no charge for the software, but a donation is appreciated. Note that there is currently no manual for the software, so hands-on training at the VSA Oberlin Acoustics Workshop is recommended.

Professor Chris Roberts of Tufts University is currently developing a free, easy-to-use acquisition and analysis application built on the LabVIEW platform. Versions should be available for Windows and Mac computers by September, 2017.

Associated Equipment: The Impulse Measurement Rig is built around (but does not include) the equipment listed below. Depending on the models chosen, this can add an additional $1,800 or more to the total cost. (Note: a computer is also required.)

Impulse Hammer: PCB model 086C80. About $850.

Power Supply: The hammer needs a power supply. We recommend PCB model 485B362 (~$500). This two-channel unit can supply a hammer and accelerometer simultaneously, and so can be used for modal analysis and bridge admittance measurements. The unit itself can be powered by a USB port on a computer, or by any other USB power supply, such as an iPhone charger.

Microphone: omnidirectional condenser. Many models are available in all price ranges. The most important considerations are a flat frequency response (~100 Hz –10 kHz), low noise, and stability over time. Microphones with small cartridges have the flattest response at higher frequencies, but tend to be noisier. An inexpensive and popular example is the Behringer ECM 8000 at about $60 – a good mic to get started with. Curtin uses the Josephson C617, a top-end mic with a ½” cartridge, a very flat response, and extremely low noise. It costs about $2000 – $1000 for body, $1000 for the cartridge. Josephson also makes a less expensive model 550H at about $450. The Earthworks M23 looks promising (~$700). Measurement microphones typically come with a calibration certificate. If the goal is to make calibrated measurements, look for a mic with a ½” cartridge that will fit a standard microphone calibrator.

USB Audio Interface: The Octa-Capture by Rolland is recommended (~$500). It is an 8 channel USB interface with digital gain control (great for calibration) and plenty of extra inputs for expansion or for multitrack recording. It is also very well designed and easy to use. That said, any stereo USB sound card with phantom power for a microphone will work. If the preamps have analog gain, it is wise to make some kind of mechanical lock for the gain knobs, to ensure they remain at the intended level.

Accelerometer: An accelerometer is needed if you want to do bridge admittance measurements or modal analysis. A lightweight (<0.5 g) model is essential. PCB offers a couple in this range, but model 352A73 (~$600) is the only one to consider. No additional power supply is needed if you have USB power supply listed above.

Camera Tripod Legs: The rig attaches directly to any standard camera tripod with a ¼” stud. No tripod head is needed. For maximum portability, consider lightweight tripods with 4-section legs.

Cables: the following are required

• Standard microphone cable: about 10 feet (3 meters)
• USB cable (typically comes with the audio interface)
• To get from the audio output on the USB power supply to a sound card such as the Octa-Capture, you need a Y-Cable Splitter Cord that goes from an 1/8″ TRS stereo jack to a pair of 1/4″ TS jacks. (The two ¼” jacks are used simultaneously only when doing admittance rather than radiation measurements.) About ten feet should be long enough.