by Pat Brown
GratisVolver Pro™ is a major upgrade to a powerful, proven open-loop measurement process.
Early measurement systems such as TEF, MLSSA, and others did not allow the investigator to directly listen to room measurements. We depended solely on measures such as the reverberation time, clarity score, %Alcons, and STI to assess the performance of the system.
The original GratisVolver™ was conceived by CATT-Acoustic author Bengt-Inge Dalenback as a simple, powerful, and free convolution tool for room acoustics work. GV allowed the RIR to be evaluated by listening as a complement to the use of various acoustic measures. In addition, listening may also reveal potential measurement problems.
Open vs. Closed-Loop
Soon after its introduction, GV’s capabilities were expanded to allow “open-loop” room measurements. An overview of open vs. closed-loop measurements can be found on the Audio Precision website. In short, a traditional closed-loop measurement requires both the analyzer output and input be connected to the Device (or room) Under Test (DUT). A sweep is generated and collected, and the IR or transfer function is displayed. An open-loop measurement allows a disconnect between the data collection step and the determination of the system response. This process is necessary to measure “smart” devices that cannot be measured using a closed-loop process, which is why Audio Precision is adding it to their analyzers. These include media players, cell phones, and other modern gadgets.
An open-loop process allows collection of the RIR from sweep recordings made in the room. This requires that the recorded sweep be convolved with an inverse version of the dry sweep played into the room. We have demonstrated the process using GV in both our in-person and online training courses for many years. It allows anyone armed with a recorder and a wave editor app to collect superb RIRs in any space, without the requirement of a computer on site. The wave editor app is necessary for some housekeeping steps such as trimming the raw recordings and resultant RIRs. The “GV method” has been my “go to” for room acoustics studies for many years, and I have published many Room Surveys in our SynAudCon Library. The possibility to deconvolve four channels opened the door to make B-Format RIR measurements in a similarly simple way.
Onward and Upward
One of the most notable new features of GVPro is the ability to trim wave files within the app itself rather than requiring a separate WAV editor and also to extract trimmed IRs or recorded sweeps from a single file with recorded sweeps from several receiver positions. The short video below shows how it is done. In it I process one of the sweep recordings made during the Hinkle Fieldhouse site visit described in Room Survey #35.
GratisVolver Pro™ is a major step forward for the open-loop measurement process. A list of the many new features can be found here along with download and pricing information. It is complemented by a new version of ReflPhinder™, a powerful app used to analyze the RIRs made with GVPro. ReflPhinder now supports stereo files along with mono and B-Format. I’ll overview it in a future article. pb