In §4.1, I distinguished two major versions of medial theory. The first one is wave theory, which identifies sounds with concrete entities in the medium. My view falls into this category. As for the second one which I called wave pattern theory, it could also be treated as a variant of wave theory. It identifies sounds with patterns or structures of frequency components instantiated by compression waves.
In this section, I argue against wave pattern theory.
First, in identifying sounds with abstract entities, wave pattern theory makes the causal role of sounds problematic. Sounds themselves would have no causal power. It is rather the compression waves instantiating them which can produce in us auditory experiences. In contrast, wave theory does not have this consequence.
Compression waves and their constituent fluctuations are all concrete events which have the right ontological nature to enter the causal structure of auditory perception.
I can see no compelling reason to identify sounds with abstract patterns or structures of frequency components rather than the compression waves which con-tain those components. Nudds motivates his wave pattern theory by appealing to the way our auditory system serves its function of perceiving sound sources. He says:
… the sounds we experience are the result of the way the auditory system groups the frequency components that it detects in order to extract information about the sources that produced them. (Nudds, 2009, p. 75)
This claim would be blatantly objectionable if it means that what sounds objectively are out there in the environment results from the way the auditory sys-tem functions. It would be plausible if it instead means that the way we experience sounds to be results from how the auditory system works. However, this would have no direct implication for what sounds are objectively. Moreover, even if this alleged relation between sounds and our experiences of them is granted, it still does not follow that sounds are abstract patterns of frequency components. The very same observation could also be explained if sounds are concrete, patterned combinations of frequency components, in which case sounds can unproblematically be the causes of our auditory experiences. Wave pattern theory is therefore unmotivated.
Second, in Nudds’s view, the identity of a sound is fixed by the causal origin of the frequency components instantiating it. Sounds cannot be instances of patterns of frequency components, because people hearing the same sound can be hearing different instances at different locations (ibid., p. 76). Nor can sounds be types of patterns of frequency components, because numerically distinct sounds can be qual-itatively identical (ibid.). Different instances of patterns are counted as the same sound only if they are instantiated by frequency components produced by the same event source (Nudds, 2010b, p. 293).
I agree with Nudds regarding the role of sound sources in fixing the identi-ties of sounds. However, my view has a more elegant explanation for the three ob-servations given by Nudds: the identities of compression waves is determined en-tirely by the identities of their event sources, such that qualitative similarity is simply irrelevant. In my view, constituent fluctuations belong to the same compres-sion wave because of their common causal origin. This is explained by the causal nature of compression waves as events in which pressure fluctuations propagate in the medium. Constituent fluctuations of the same compression waves are not con-nected by qualitative similarity but by causal chains linking them back to their event sources as the common causes.
As for Nudds’s view, it remains unexplained why the identity of a sound, considered as an abstract pattern, is fixed by another property of its instantiator, viz.
the causal origin of the frequency components instantiating it. While a propagation event necessarily contains a causal link leading back to its event source, there simply does not appear to be any necessary connection between an abstract pattern and the causal origin of its instantiator.
Third and relatedly. If Nudds is right that a sound is just a pattern, it is nat-ural to wonder how dissimilar two constituent fluctuations of the same sound can be. Nudds does not offer any answer to this question, so it is not clear if he thinks there is any limit to this dissimilarity. In contrast, I simply deny the relevance of qualitative similarity to the identities of sounds.
It is implausible to limit how dissimilar a sound could be at two different locations. There are many ways in which the waveform of a compression wave can change dramatically. For example, if the amplitude of a compression wave is high enough, the compressions and the rarefactions are themselves significant changes to the medium which respectively increase and decrease the speed of sound. The compressions and rarefactions of the same wave would then propagate at different speeds, and hence the waveform would change in the process of propagation. Also, the faster rate of attenuation for frequency components at higher frequency means that only the lower frequency ones will remain in the later career of a compression wave. These are examples in which very different patterns of frequency compo-nents are instantiated while still being counted as the same compression wave, and hence should be counted as the same sound.
We might even imagine a world in which the medium obeys an entirely different set of physical laws. A compression wave propagating in this medium would change its waveform in whatever imaginable way. Constituent fluctuations at different locations would appear to us like randomly generated, although they are indeed causally connected by the physical laws in that world. Nonetheless, insofar as they are caused by the same event source, they still constitute the same compres-sion wave in my view. However, the patterns of frequency components would be as many as the constituent fluctuations there are.
It is unclear how Nudds would conceive of this imaginary world. If he thought that there is only one sound, then it is an unnecessary complication to iden-tify a sound with infinitely many patterns of frequency components rather than a compression wave constituted by infinitely many patterned local parts. If he instead thought that there are as many sounds as there are patterns of frequency components, then the same judgement should be made regarding sounds in our world. This is because the only difference between the two worlds is just the degree of qualitative similarity between constituent fluctuations, and this is accepted by Nudds as neither
sufficient nor necessary for the identity of sound. As a result, he would need to give up the plausible idea that people at different locations can hear the same sound.
Overall, wave pattern theory is unmotivated, has inferior explanatory power, and leads to implausible judgement regarding the identity of sound. My view, in contrast, does not have these problems and hence should be preferred.