Low Frequency Room Modeling –an Acoustician’s Perspective

An acoustic engineers’ approach to medium and large room computer modeling has a slightly different perspective than the audio system designers. While the audio system designer is primarily concerned with loudspeaker interaction and pattern control, acousticians must make the complete audio system work within the confines of the physical room.This includes all the challenges of the reverberant field, background noise and sound transfer in and out of the room. Therefor close collaboration between the system designer and acoustic engineer is crucial for a successful project.

System design tools available to the audio designer allows modeling of the broadband and sub-bass loudspeaker dispersion patterns but only look at the direct field without any room interaction.Of course, very helpful to the design process but must be used carefully when the project is indoors. At present, these tools are the only viable option for the acoustician to use as well. Here’s why.

Common room modeling tools such as Odeon, EASE/AURA, CATT and OTL share one thing in common, inaccuracy at low frequencies and no low frequency estimations below 100-125Hz.

Allthe programs mentioned use some form of geometrical-acoustic calculation technique (GA). Whether beam, ray, cone or hybrid tracing methods are used, predictions below 125 Hz are very inaccurate.Below this transition region (the Schroder frequency) wave behavior dominates in a room. In medium to large rooms, this transition frequency can be anywhere from below 50 Hz to over 100 Hz. This of course is where sub-bass room modeling is needed.

Another limiting factor is, per ASTM C423, construction and acoustic treatment materials are tested for absorption characteristics from 100 –5000 Hz using random incidence, diffuse noise. In areal room, sound is anything but random or diffuse and, depending on the room geometry,can strike room surfaces at shallow angles. This can literally turn absorptive materials into reflectors. For those materials specific to low frequency absorption that have been tested in very large reverberation chambers or impedance tubes, current GA modeling programs can’t be used to accurately model their impacts.Finite element modeling (FEM) or boundary element method (BEM) could be used but are very computer and time intensive. Currently, not very realistic for real-world project budgets or design time lines.

In future room modeling software development, with more powerful computing now available, hybrid modeling engines (GA+FEM) are on the horizon which will give the acoustician greater insight into the low frequency room interaction phenomenon. This would also require the construction material industry to retest some of their products with yet to be implemented standards and few testing facilities able to perform them.

Thomas Ryan AES/ASA/INCE
Principal Consultant
Technological Design Studios, LLC