Reflection: Echo, Reverberation, Echolocation

Echo has been discussed in relation to our echo experiences a few times in previous chapters. The main idea is that locating sounds at or near their sources and treating echo experiences as distorted or illusory experiences of primary sounds fails to do justice to the objective content of our echo experiences. This shortcoming of distal theory can be avoided by wave theory.

Wave theory, in general, allows that our echo experiences are veridical ex-periences of the reflected waves, as the common agreement that sounds are auditory objects just means that compression waves are auditory objects. I can hear the pri-mary wave and the reflected wave, both of which are correctly experienced when they arrive at my location. Indeed, the two experiences are experiences of the same compression wave at different stages. For those who accept also that sound sources are experienced auditorily, both my experiences of the primary wave and that of the reflected wave represent the sound source as well.

The echo experience might be illusory insofar as it may mislead me about the temporal and spatial location of the sound source. However, echo experiences, at least the distinctive ones which represent the echoes as causally related to the primary sounds, have their distinctive phenomenal characters, such that normal adults would not be misled by them. The case is like our visual experiences of

mirror images. Such experiences can be misleading, but only occasionally would we be deceived. As soon as we look around, the different ways in which the worlds

“inside” and “outside” the mirror change their appearances in response to our move-ment immediately lead us to correctly interpret our experiences.

My view is not an ordinary wave theory. I hold that our auditory experiences do not represent the entire compression wave but its constituent fluctuations. An experience of the primary sound thus represents the first constituent fluctuation at my location which is caused by the event source. An echo experience then repre-sents another constituent fluctuation at my location which is also caused by that event source. If this is a distinctive echo experience, it further represents this con-stituent fluctuation as causally related to the concon-stituent fluctuation represented in the corresponding experience of primary sound.

It is not clear how this causal relation is represented. The later constituent fluctuation may be represented as caused by the earlier one, or simply as being caused by the same source. I cannot determine which option better characterises the phenomenology of distinctive echo experiences. Anyway, both options allow dis-tinctive echo experiences to be veridical, as constituent fluctuations of the same compression wave do share the same causal origin, and the later ones are caused by earlier ones.

Notice that not every echo leads to echo experiences. As I mentioned in

§4.2.1, there is an effect called “echo suppression” which happens when the delay between the arrival of a primary sound and the arrival of its echo is shorter than a certain threshold. When it happens, our auditory system fuses the echo with the primary sound, such that the resulting experience represents the primary sound (and the event source) but with slightly different qualities. The qualitative difference be-tween hearing your singing in the bathroom and on an open field is a result of this process. Ordinarily, we talk about such a difference in terms of the reverberation of a venue. To the extent that we can distinguish the reverberation of a venue from the event source, our experiences of primary sound still represent the echoes but only obscurely.

One question arises here: do we auditorily experience the venue? If we can hear the difference between singing in the bathroom and singing on an open field, it seems implausible to say that the venue is entirely absent in our experience. How-ever, Young (2017) takes our experiences of reverberation as showing that we can

hear spaces, but he also holds that they fall short of representing the venues. It is unclear how theories other than wave theory would consider this possibility.

My view need not and cannot give a determinate judgement on this issue, but it should be compatible with the possibility that both venues and spaces are also represented in our auditory experiences. At least, a compression wave in my view can carry information about the venue and space in the surrounding in virtue of being modified by the enclosing surfaces. However, it is not explicit in our auditory experiences that venues and spaces are represented alongside event sources. We may nonetheless learn through reflection that some of the qualities of the experi-ences can only be explained by the contribution from the venues and the spaces.

Therefore, the case is like how we learn that constituent fluctuations are represented in our auditory experiences, though it seems it is even more difficult to distinguish features of venues and spaces from features of event sources.

We should not generalise the case of echo and reverberation to all objects which reflect compression waves. We discussed human echolocation in §3.3 and

§4.2.4.2. The impressive performance of human echolocators in locating reflective objects strongly suggests that such objects are represented in their experiences. In-deed, following the instructions of Schwitzgebel and Gordon (2011, p. 61), I tried to echolocate my hand and can attest to the auditory phenomenal difference be-tween hearing my silent hand held at different positions. I did not merely hear the clicks I made with my tongue as being modified by my hand in different ways.

There was also a clear impression of some silent object located fairly determinately at a certain nearby location, a location which is, in fact, the location of my hand.

I do not, however, pretend to have settled the question concerning whether reflective objects are represented in auditory experiences via echolocation. Echo-locatory experiences are far from being familiar to most people. It would not be surprising to find out that I am somehow abnormal in this respect. I should, there-fore, end this subsection merely by a remark on how my theory of sound could explain echolocation.

Compression waves are, in my view, propagation events in which constitu-ent fluctuations are causally connected. This claim leaves room for objects in the environment to be involved in the causal process. Right next to a reflective wall, after the compression wave is reflected, the constituent fluctuation there is both caused by its predecessor at the same place and the wall’s response. As a result, the

constituent fluctuation after reflection contains information about both the event source and the wall.

In general, reflective objects do not all reflect compression waves in the same way. They absorb energy. The smoothness of their surfaces in comparison to the wavelengths of the incoming wave affect the reflection in a way similar to how light is reflected by matte or glossy surfaces. Since the smoothness of a surface is relative to wavelength, this means the same surface can be smooth for lower fre-quency but rough for higher frefre-quency. As a result, the spectrum of the constituent fluctuation after reflection is altered by the reflective wall, and therefore the char-acters of reflective objects are best revealed by broad-spectrum noise. This is simi-lar to the case of light, where reflective objects have different reflectance properties.

For the same reason as the colours of objects are determined under full-spectrum illumination (e.g. sunlight), blind people also make use of broad-spectrum noises (e.g. white noise, clicks) for echolocation.

There is, however, a dissimilarity between visual and auditory experiences of reflective objects. Very often, light sources and reflective objects are not within the same view. Our visual system, therefore, needs to extract information about the character of light sources from the invariances among multiple reflective objects in the same view. In contrast, echolocators always hear the sound sources as well.

Therefore, at least at the level of subconscious auditory processes, it is plausible that the primary sound and the reflected sound can be compared. It would not be surprising if reflective objects are processed somewhere along the auditory pathway and figure in some way in our auditory experiences. If this is the case, my view would then need to say that our auditory experiences represent reflective objects in virtue of representing two constituent fluctuations, although phenomenologically speaking they are not represented as two distinct individuals.

It is still an open question whether the phenomenology of an echolocatory experience is more like a modified experience of the event source or an experience of two explicitly distinct individuals, i.e. the event source and the reflective object.

Both options are compatible with my view, as they are consistent with the fact that the stimulus is a local part of a modified compression wave, the modification of which could be identified by comparing the modified and the original features of the same compression wave.