Echoes and flutter-echoes

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Echoes

An echo is an auditory effect associated with hearing a repetition of a previous sound, having delay and level proper to be detected as a separate sound event. Echoes are commonly caused by sound reflected from hard, plane surfaces that form sound paths that makes the sound arrive at the receiver at least 50ms after the arrival of direct sound. This delay corresponds to a reflection path that is 17m longer than the direct sound path. Indoors, there are commonly many reflections, one masking the other, forming a smooth reverberant sound decay. A single reflection can still be prominent enough to be recognized as an echo, either because it arrives separate in time or because it is stronger than the reflections arriving before and after. Transient sounds are more often detected as echoes than continuous sounds, e.g. echoes from percussive instruments may be heard when echoes from winds are not being heard. For the same reason, acousticians often tests performance spaces for echoes by clapping their hands.

While echoes are often disturbing to perception of speech and rhythmical music, they can provide acoustic support to singers themselves. An echo from the back of the hall may return to stage some 200-300ms delayed, making the singers able to judge the audibility of her/his voice, giving the feeling of ‘filling the hall’, thus reaching out to the rearmost seats.

Prediction of echoes

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.

Periodic echoes

Periodic echoes may occur when a close-to-plane wave is reflected back and forth inside certain geometries, e.g. between a pair of hard parallel surfaces. A classical example is the repeated fading echo heard in so-called echo-valleys, typically found in valleys with steep sides. Periodic echoes with short periods are often referred to as flutter-echoes, see below. By Fourier Transformation, periodic echoes and frequency modes are transformation pairs. (more). As a consequence, two fading harmonic components with frequency spacing Df are sufficient to create the perception of a periodic echo with period T=1/Df. However, the higher the number of audible harmonics, the more distinct will the temporal components of the echo be perceived.

When the period is less than 1/20 second (<50ms), our hearing system will tend to merge together the subsequent reflections, making the temporal feature perceived as echo or flutter-echo less prominent.

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, thus the axial modes and the flutter-echo effect is masked. For this reason, added absorption to the gracing surfaces can surprisingly make the flutter-echo more audible, and 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 several reasons:

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.

Thirdly, air absorption cuts off high frequencies.

Flutter echoes can be abated by applying proper surface treatment to one of the parallel surfaces, or to both. Any hard objects of at least 1” thickness that does not leave large plane areas exposed, would work. In some rooms, this is provided naturally by paintings or ornamentation on the walls. Designed diffusers can be larger convex panel elements or modulation by a series of smaller elements. However, the latter should not have rhythmical or periodic patterns

If porous absorbers, e.g. mineral wool or polyurethane foam, are being used as surface treatment, a thickness of ~2cm will in most cases be sufficient to suppress flutter echoes. Textile and a slight air-gap in front of a wall or a thick carpet on the floor can sometimes do the job.

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 apparent sequence of image sources behind the parallel walls will “bend” and soon disappear under the acoustic horizon.

There are cases where flutter-echoes may appear even if no surfaces are parallel. The sound may take loops that are not obvious at first sight, such as: a) between a flat wall and a pair of walls that meets in a right angle corner, b) between to sides of a room that is strictly symmetrical, or c) between a staircase with right-angled steps and a ceiling with the same inclination as the staircase.

Cuboids

In buildings, rooms are often cuboids. Not only does cuboids allow periodic echoes between parallel walls, it can be shown that all standing waves (modes), even oblique ones, are frequency domain features of periodic reflections travelling in opposite directions. E.g., the beating effect of two prominent neighboring modes separated by Df can during their decay sound like a soft flutter echo of period T=1/Df (see presentation p 11).