Air-transparent selective sound silencer using ultra-open metamaterial
A bilayler metamaterial silencer allows substantial fluid through the apparatus, while mitigating the propagation of sound through the apparatus, and while providing a form factor that is significantly more compact than previously-known devices. Moreover, illustrative embodiments allow a designer to specify one or both of the frequency or frequencies at which the apparatus mitigates sound propagation, and/or the bandwidth around the frequency or frequencies at which the apparatus mitigates sound propagation.
1. An apparatus comprising two or more metamaterial silencers, each such metamaterial silencer comprising a first channel open to the propagation of a target frequency therethrough and configured to remain in a continuum state during propagation of the target frequency therethrough, and a second channel open to propagation of the target frequency therethrough and configured to resonate at the target frequency, wherein said second channel is disposed, relative to said first channel, such that the target frequency of said second channel is capable of destructively interfering with the target frequency of said first channel;
wherein a first metamaterial silencer comprises a first channel configured to remain in a continuum state during propagation of a first target frequency therethrough and a second channel configured to resonate at the first target frequency, and wherein
a second metamaterial silencer comprises a first channel configured to remain in a continuum state during propagation of a second target frequency therethrough and a second channel configured to resonate at the second target frequency;
wherein said second metamaterial silencer is disposed in series with said first metamaterial silencer; and
wherein the first target frequency is different from the second target frequency.
2. The apparatus of claim 1 , wherein each of the first metamaterial silencer and the second metamaterial silencer further comprises an acoustically rigid spacer disposed between the first channel and the second channel and capable of reducing transmission of acoustic energy between the first channel and the second channel.
3. The apparatus of claim 1 , wherein the first channel is open to a flow of fluid therethrough.
4. The apparatus of claim 1 , wherein the first channel defines an axis of fluid flow therethrough.
5. The apparatus of claim 1 , wherein each second channel comprises a second outlet, and each such second outlet is an un-ducted outlet.
6. The apparatus of claim 1 , further comprising:
a third metamaterial silencer, having a second channel configured to resonate at a third target frequency, the third metamaterial silencer disposed in series with said first metamaterial silencer and said second metamaterial silencer, to receive a wave comprising the third target frequency,
wherein said third target frequency is different from said first target frequency and said second target frequency.
7. The apparatus of claim 1 wherein, for each of the first metamaterial silencer and the second metamaterial silencer:
the first channel has a first area in cross-section; and
the second channel defines a second area in cross-section, and first area in cross-section is larger than the second area in cross-section such that the apparatus has an openness ratio of at least 0.8.
8. The apparatus of claim 1 wherein, for each of the first metamaterial silencer and the second metamaterial silencer:
the second channel is a helical channel disposed around the first channel.
9. The apparatus of claim 1 wherein for each of the first metamaterial silencer and the second metamaterial silencer:
the first channel is disposed radially outward of the each second channel.
10. The apparatus of claim 1 wherein:
each of the first metamaterial silencer and the second metamaterial silencer has a cylindrical shape having an upstream face on an upstream side and a downstream face on a side opposite the upstream side; and
each of first metamaterial silencer and the second metamaterial silencer has a thickness corresponding to a cylinder height between the upstream face on an upstream side and a downstream face, the thickness being less than one quarter of a wavelength of the first target frequency and the silencer target frequency.