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Echoes in performance spaces can be very disturbing and hard to cure. They are among the prominent sound effects that are important to predict during design.

Echoes and flutter echoes are phenomena that occur when the energy-time distribution is uneven related to the temporal resolution of human hearing. It is therefore one of the acoustic opposites to diffusivity.

Anders Løvstad’s Master Thesis at NTNU from 2003 on Echo Criteria, introduces a new criterion: Evaluation of Objective Echo Criteria.

Flutter-echoes

Flutter-echoes appears to be wave phenomena similar to standing waves, only with periods long enough (>50ms) to be perceived as separate sound events. When occurring between parallel walls the axial modes normal to the parallel walls will constitute the harmonics of a flutter-echo with period T and harmonic frequencies 1/T, 2/T,…. If the walls are hard and smooth, the higher harmonics can be prominent so that discrete tones are being heard. Due to little absorption at normal incidence and long free paths, decays are slow and reverberation time long, often leaving a late double slope at mid-high frequencies.

In addition to the pair of parallel walls, there is usually one more basic condition for a flutter-echo to appear, namely that absorption is unevenly distributed so that there is less absorption from the parallel walls than from the other surfaces. In terms of modes, this means that tangential modes and oblique modes are more damped than the axial modes in the actual direction, making the latter more prominent. When this is not the case, the sound field is diffuse, the axial modes and the flutter-echo effect is masked. For this reason, added absorption to the gracing surfaces can make the flutter-echo more audible, and surprisingly make measured RT longer (EDT shorter, but T30 longer due to double slope), typically in mid-high frequencies.

The reason for the inherent mid-high frequency band pass filter of flutter echoes is due to a pair of conditions:

One is the fact that the parallel walls provides an aperture for the higher order image sources in the axis normal to the walls, and if the gracing surfaces provide more absorption than the parallel walls, this aperture will be to small compared to the Fresnel-zone for lower frequencies as image sources moves farther away from the receiver.

Secondly, higher frequencies are damped by air absorption and are also more sensitive to microscopic unevenness in the parallel walls. The result is a band pass filter. For this reason, flutter echoes can be prevented by surface treatment with rather shallow modulation, say 1” deep, having an effect above 1kHz.

Tilting or inclining the parallel walls somewhat is another way of preventing flutter echoes as well as normal modes. This measure will force the waves to hit the gracing surfaces since the subsequent image sources behind the parallel walls will “bend” and soon disappear under the acoustic horizon.

Prediction of echoes, and flutter-echoes

Echoes