Room Acoustics

An important feature of every room is its acoustical properties. These properties not only influence the speech intelligibility but can also determine how we perceive a musical performance.

For many rooms the acoustic qualities are an afterthought and corrective measures have to be implemented, often at great cost or with suboptimal results. Our Sabin prediction software tool can serve as a fast and easy to use aid in finding the appropriate treatment for a room. A more time-consuming and expensive option is the use of ray tracing based simulation software. Better yet of course is to consider the acoustical requirements during the initial design of the room. In projects where the acoustics is a defining feature, such as concert halls, the use of scale models to verify designs is indispensable.

Whether it is in a space or room that has problematic acoustics, or in rooms designed to have great acoustics for a specific purpose, measuring the acoustic properties of the room is essential for an objective assessment. The reverberation time is probably the best known room acoustical parameter used to quantify the acoustics, but there are many more that are regularly used by acousticians and researchers. The most important of these are formalized in the ISO 3382 series of standards. All parameters described in ISO 3382 (and many more) can be measured using Dirac software.

Acoustic Measurements:

The measurement of acoustical parameters is usually based on impulse responses. To perform these measurements in compliance with ISO 3382 you need an Omni-directional sound source and a power amplifier such as the Type 2734 to emit the MLS or sweep test signals. The room response is then picked up using a microphone or a handheld meter with an analogue output or an audio analyzer. The signal is captured by Dirac through a standard sound device, and de-convolved to arrive at the impulse response.

For spaciousness parameters such as the lateral energy fractions (LF and LFC), Dirac supports dual channel measurements using a microphone probe or a combination of Omni- and bi-directional microphones. To measure the interaural cross correlation (IACC) an artificial head needs to be used. For measurements of the Strength (G) and related parameters such as the early and late strength (G80 and LG), and the early and late lateral sound levels (GEL and LG), the sound source needs to be calibrated. This calibration can be performed either in a reverberation chamber or in a free field environment.

Open loop measurements:

Traditional impulse response measurements based on the de-convolution of MLS or sweep signals, require the sound source and the microphone to be connected to the same sound device. This is because the slightest difference in clock rate between playback and recording could result in a very noisy impulse response or even no recognizable response at all.1) The MLS in particular is very sensitive to timing variations. Dirac is able to compensate for any clock rate difference between playback and recording. This makes it possible to play an MLS stimulus from a CD or MP3 player (asynchronous to the recording) and still get almost the same results as with synchronous (or closed loop) measurements. This unique feature of Dirac is also used with the Echo Sound Source, which doesn't require any connection to the PC that records the response.

Automated Measurements:

In large rooms many measurements are needed to capture the acoustic properties. In house of worships, concert halls and similar venues many hundreds of measurements may be required. The measurement tools that are used should support these scenarios effectively. This not only means automated measurements, but also, perhaps even more importantly, the efficient and flexible handling of a large number of impulse responses.

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