Acoustic metamaterials

A kit of parts comprising:

• a grid structure comprising one or more grid elements, each grid elements being configured to receive a respective acoustic metamaterial cell;
• and
• a set of one or more acoustic metamaterial cells that can be inserted into the respective grid elements to provide a device for controlling noise,
• wherein the device for controlling noise comprises:
• a flow passage that is open to allow air or another fluid to flow through the flow passage in a primary flow direction;
• and
• one or more acoustic metamaterial cells, wherein each of the one or more acoustic metamaterial cells is provided with an inlet opening on a first side of the acoustic metamaterial cell, an outlet opening on a second, different side of the acoustic metamaterial cell, and an internal channel extending between the inlet and outlet openings and arranged to allow air or another fluid to flow between the inlet and outlet openings of the acoustic metamaterial cell through the internal channel of the acoustic metamaterial cell, and wherein each of the one or more acoustic metamaterial cells is structured such that the respective internal channel for the acoustic metamaterial cell provides a tortuous flow path through the acoustic metamaterial cell between the inlet and outlet openings, each acoustic metamaterial cell being configured to introduce a respective time delay to a portion of an acoustic wave incident at the inlet opening of the acoustic metamaterial cell based on the structure of the internal channel;
• the one or more acoustic metamaterial cells being arranged relative to the flow passage such that when an acoustic wave propagating through air or another fluid is incident on the device, a first portion of the acoustic wave propagates through the flow passage and a second portion of the acoustic wave propagates through the one or more acoustic metamaterial cells, the second portion of the acoustic wave thereby being delayed relative to the first portion of the wave, and the first and second portions of the acoustic wave destructively interfere to provide noise reduction at one or more frequencies based on the respective time delays associated with the one or more acoustic metamaterial cells.

A method of designing a device for controlling noise, wherein the device comprises:

• a flow passage that is open to allow air or another fluid to flow through the flow passage in a primary flow direction;
• and
• one or more acoustic metamaterial cells, wherein each of the one or more acoustic metamaterial cells is provided with an inlet opening on a first side of the acoustic metamaterial cell, an outlet opening on a second, different side of the acoustic metamaterial cell, and an internal channel extending between the inlet and outlet openings and arranged to allow air or another fluid to flow between the inlet and outlet openings of the acoustic metamaterial cell through the internal channel of the acoustic metamaterial cell, and wherein each of the one or more acoustic metamaterial cells is structured such that the respective internal channel for the acoustic metamaterial cell provides a tortuous flow path through the acoustic metamaterial cell between the inlet and outlet openings, each acoustic metamaterial cell being configured to introduce a respective time delay to a portion of an acoustic wave incident at the inlet opening of the acoustic metamaterial cell based on the structure of the internal channel;
• the one or more acoustic metamaterial cells being arranged relative to the flow passage such that when an acoustic wave propagating through air or another fluid is incident on the device, a first portion of the acoustic wave propagates through the flow passage and a second portion of the acoustic wave propagates through the one or more acoustic metamaterial cells, the second portion of the acoustic wave thereby being delayed relative to the first portion of the wave, and the first and second portions of the acoustic wave destructively interfere to provide noise reduction at one or more frequencies based on the respective time delays associated with the one or more acoustic metamaterial cells, the method comprising:
• determining one or more frequencies of noise to be reduced;
• and
• selecting a desired arrangement of acoustic metamaterial cells to provide noise reduction at the determined one or more frequencies.