Listening-Room Bass Problems: Why Average Reverberation Time Is Not Enough

When a listening room exhibits prolonged bass decay or an unusually prominent pitch, an acceptable average reverberation time does not by itself establish that the room is free of problems. An average summarizes a specified frequency range. Low-frequency diagnosis also requires frequency-specific response and decay information, with the loudspeaker configuration, measurement positions and room arrangement recorded. This article concerns listening rooms for evaluating loudspeaker reproduction, not anechoic rooms or building sound-insulation acceptance tests.

Why an acceptable average can miss a problem

ITU-R BS.1116-3 addresses subjective assessment of small impairments in audio systems. Section 8.2.3.1 uses an average reverberation time over 200 Hz to 4 kHz, together with frequency-dependent tolerances, and notes difficulties in measuring short reverberation times at low frequencies. It does not replace all sound-field checks with one average. A development listening room that does not fully adopt this method should not treat one of its metrics as proof of overall conformity.

The first question about an engineering report is what has been averaged: which frequency bands, which positions and which room condition? Without retained band-by-band results, an anomaly in a narrow frequency region may become less apparent in the summary. Recalculating the average alone cannot identify which part of the low-frequency sound persists longer.

Separate frequency response from decay

At low frequencies, a small room can be dominated by relatively isolated modes; its behaviour cannot automatically be interpreted as that of a sufficiently diffuse sound field. REW's official documentation therefore recommends examining low-frequency decay, waterfall or spectrogram plots rather than relying only on the reverberation-time graph. This is an analytical approach, not a requirement to purchase particular software or to apply one low-frequency boundary to every room.

Record two questions separately: at which frequency is the energy prominent, and at which frequency does it decay slowly after excitation stops? A frequency-response curve mainly helps locate the first issue. A joint time-frequency display helps examine the second. A relatively flat frequency-response curve is not evidence that decay is appropriate at every frequency.

A modal model provides clues, not an acceptance result

An ideal rectangular room with rigid boundaries permits calculation of natural frequencies and mode shapes. Those results alone cannot establish low-frequency decay in the actual room. In their 2018 study, Magalotti and Cardinali used a rectangular-room test model and explicitly distinguished natural-frequency calculations with sound-hard boundaries from decay analysis after introducing finite wall impedances.

When a proposal includes modal plots, ask about geometric simplifications, boundary losses and the verification method. Actual walls, doors, absorptive constructions and furniture should not appear only in renderings while disappearing from the explanation of the model. Modelling can help select frequency regions for closer examination; it does not replace measurement in the completed room.

Build evidence that can be checked again

As an engineering recommendation, retain the original impulse responses for each loudspeaker configuration and document measurement coordinates, equalizer settings, frequency resolution, time range and noise floor. Keep these conditions comparable when assessing a modification. Examine adjacent frequency regions as well as the target region, rather than presenting only the plot with the largest apparent improvement.

The contract or acceptance plan can separately define applicable reverberation metrics, low-frequency response and decay checks, and conditions for retesting anomalies. Limits should follow the room's intended use and the selected method; this article supplies no universal numerical limits. This makes the remedial discussion more specific: which response or decay feature needs to change, and what evidence will demonstrate that change, rather than simply whether to add more absorptive material.

References

ITU-R BS.1116-3 (2015), Methods for the subjective assessment of small impairments in audio systems, Section 8.2.3.1. Official current-version entry:
https://www.itu.int/rec/R-REC-BS.1116-3-201502-I/en

Room EQ Wizard official help, RT60 Graph. Cited for the limitations of low-frequency analysis in small rooms, not as a universal acceptance standard:
https://www.roomeqwizard.com/help/help_en-GB/html/graph_rt60.html

R. Magalotti and V. Cardinali, A simulation test bench for decay times in room acoustics, 2018 COMSOL Conference, page 1. A modelling study, not an engineering acceptance standard:
https://www.comsol.com/paper/download/568711/magalotti_paper.pdf