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: $1,400 plus shipping and applicable taxes. This does not include the associated equipment (listed below), which can add up to $3000 or more to the price. As the rigs are individually assembled at Joseph Curtin Studios, a lead time is required. Delivery is currently limited to the USA. Contact Joseph Curtin to place an order.

Software: ObieApp is a free software developed by Chris Rogers and Joseph Curtin for impact hammer measurements. Built on the LabView platform, versions for Mac and Windows can be downloaded here: Oberlin Acoustics App – Downloading Software (google.com). The software is based on George Stoppani’s pioneering Acquisition and Overlay applications, available at 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.

Associated Equipment: the Impulse Measurement Rig is built around (but does not include) an impact hammer and microphone with associated power supplies, a soundcard, and a computer.  The equipment shown below is a top-end, highly portable suite of components from PCB totaling about $3,400 without an accelerometer, and about $4,200 with.  All prices are approximate (updated 2024) and may be higher outside the USA. While other components, such as audio mics and consumer soundcards can work very well, they involve more discrete components and connecting cables, and may not be as reliable.

Impulse Hammer: PCB model 086C80. ~ $1,220

Power Supply/DA converter: The hammer needs a power supply. We recommend Model 485B39 | PCB Piezotronics (~$1,000 in USA). This two-channel unit can supply a hammer and microphone or 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. Note that this will not power audio mics that require phantom power.

Microphone: omnidirectional condenser.  The PCB precision microphone Model 378B02 | PCB Piezotronics   ($1,150) goes with the above-listed power supply/DA converter.

Also required is a mic cable with BNC connectors, such as the 10′  Model 012A10 | PCB Piezotronics (~$45). (Similar cables are readily available on Amazon for less. A 3 – 4 ‘ cable works well in the rig.

Accelerometer: An accelerometer is needed for bridge admittance measurements or modal analysis. A lightweight (<0.5 g) model is essential. PCB offers a couple in this range, but model 352A73 (~$770) is the only one to consider. No additional power supply is needed.

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.

USB Cables: a female to male USB extender cable is needed to connect the power supply to your computer. About 10′ suggested. Standard microphone cable: about 10 feet (3 meters).