Perception of modal distribution in critical listening spaces

Critical listening rooms, like recording studio control rooms or listening rooms, are carefully designed
to provide the listener with the most accurate listening environment. It is well known that in small
rooms resonant modes may occur, well separated in the lower frequency range, and cause some
amplification or attenuation of sound at certain frequencies. This may distract from the optimum
perception and the correct judgement of the reproduced sound. The modal distribution is directly
associated with the dimensional aspect ratios of the room. There are in literature various design metrics
that assess the distribution of these modes in frequency and help the designer to decide on room
dimensions that avoid worst situations and aim for an “optimum” distribution of the eigenfrequencies.
The purpose of this study is to identify the subjective perception of two general types of metrics, one
based on modal spacing and the other based on pressure response, and to attempt to classify their
perceptual importance compared to other factors associated with the perception of room modes.
Tests were carried out to evaluate differences between three “virtual” rooms that score extreme
classifications in each of the metrics. The results of the tests indicate that room aspect ratios do have
some effect on the perception of the modal distribution, but the effect is very much dependent on the
frequency content of the original signal. This indicates that a room that scores well using a certain
metric may still suffer from problems if the frequency content of the driving signal matches one
particularly strong modal artefact.