Studies in modal density – its effect at low frequencies

The ability to objectively measure the reproduction quality of a small room at low frequencies has long been desired. Over many years, there have been attempts to produce recommendations, metrics, and criteria by which to define a particular room. These have often concentrated on some aspect of the modal distribution, such as spacing or density. Other attempts have focused upon the deviation from a desired frequency response. Whilst the subjective validity of objective measures such as these has often been questioned, the notion that a transitional region between a modal and diffuse sound fields exists, dependant on the room volume and reverberation time continues to permeate much thinking. The calculation of this transitional frequency relies on the calculation of a desired modal density. In the case of the most well known definition, the Schroeder Frequency1, the transitional frequency is that point where the density becomes sufficient that three modes lie within one bandwidth. Although this idea may well be useful in some instances, such as defining points for the use of statistical sound field analysis, recent thought has cast some doubt over its relevance as a subjective frequency above which we may ignore modal issues2. This paper highlights a number of studies along with a new listening test, which help us to better understand the role of modal density upon subjective perception of modal soundfields

The assessment of low-frequency room acoustic parameters using descriptive analysis

In small rooms, low-frequency modes have a degrading influence on the quality of the bass components of music. Using objective measures to correct these modes often fails because they do not correspond to the subjective experience of listeners. This research begins with a procedure that elicits a compact set of four verbal descriptors from subjects: articulation, resonance, strength, and depth. These are then mapped to three objective parameters: modal decay time, room volume, and source/receiver position. Results show the importance of reducing the decay time, which then provides an increase in articulation. Discussions suggest ways of extending the results

Subjective preference of modal control methods in listening rooms

Methods to control the unwanted effects of low-frequency modes in critical listening rooms range from complex signal processing to the positioning of loudspeakers and listeners. A rigorous scientific experiment has been conducted to evaluate the perceived quality of eight low-frequency reproduction systems used to control the unwanted effects of room modes in a standard listening room. A strong correlation has been demonstrated between perceived improvements in quality and the decay times of low-frequency energy. For critical listening conditions, those systems ensuring a faster decay of low-frequency energy are preferred over those attempting to “flatten” the magnitude frequency response