Transmission Line Speakers for Artificial-Reality Headsets

1 . A sound-producing device, comprising:

a housing having:

(i) a head portion, which defines a chamber; and

(ii) a body portion, distinct from the head portion, which defines (1) a first audio passage to transmit a first sound wave from the chamber to a first audio outlet that outputs sound and (2) a second audio passage, distinct from the first audio passage, to transmit a second sound wave, distinct from the first sound wave, from the chamber to a second audio outlet that outputs sound; and

a driver, positioned in the chamber, for producing the first sound wave and the second sound wave simultaneously, wherein:

(a) the driver includes a forward-facing surface configured to produce the first sound wave in a first direction into the first audio passage;

(b) the driver includes a rearward-facing surface configured to produce the second sound wave in a second direction, distinct from the first direction, into the second audio passage, wherein the second sound wave is produced simultaneously with the first sound wave; and

(c) a length of the second audio passage is greater than a length of the first audio passage, such that the second sound wave constructively interferes with the first sound wave.

2 . The sound-producing device of claim 1 , wherein:

the driver has a first cross-sectional area, and

the second audio passage has a second cross-sectional area that is less than the first cross-sectional area.

3 . The sound-producing device of claim 2 , wherein the second cross-sectional area is less than half of the first cross-sectional area.

4 . The sound-producing device of claim 2 , wherein the second cross-sectional area is approximately one-tenth of the first cross-sectional area.

5 . The sound-producing device of claim 1 , wherein:

the head portion is sized to receive the driver.

6 . The sound-producing device of claim 1 , wherein:

the first and second audio passages are adjacent to each other in the body portion, and

the first and second audio passages both extend away from the head portion along a length of the body portion.

7 . The sound-producing device of claim 1 , wherein:

the head portion further comprises a lid; and

the lid seals the chamber to create a back volume between the rearward-facing surface of the driver and an interior of the head portion.

8 . The sound-producing device of claim 7 , wherein the lid is detachably coupled to the head portion.

9 . The sound-producing device of claim 7 , wherein:

the head portion comprises a surface and sidewalls extending from the surface; and

the surface and sidewalls collectively define the chamber.

10 . The sound-producing device of claim 9 , wherein:

the surface defines one or more first audio inlets joining the chamber and the first audio passage; and

the sidewalls define one or more second audio inlets joining the chamber and the second audio passage.

11 . The sound-producing device of claim 1 , wherein the second audio passage follows a serpentine path.

12 . The sound-producing device of claim 1 , wherein:

acoustic waves that pass through the second audio passage have a phase offset relative to acoustic waves that pass through the first audio passage; and

the phase offset corresponds to the length of the second audio passage.

13 . The sound-producing device of claim 12 , wherein acoustic waves output by the second audio outlet, due to the phase offset, constructively interfere with other acoustic waves output by the first audio outlet at a target location.

14 . The sound-producing device of claim 1 , wherein sound output by the first audio passage is directed in a predetermined direction according to a cross-sectional shape of the first audio passage and an arrangement of the first audio outlet.

15 . The sound-producing device of claim 14 , wherein the first audio outlet is composed of multiple openings defined along a length of the first audio passage.

16 . The sound-producing device of claim 1 , wherein the length of the first audio passage determines a minimum frequency of sound waves output by the first audio outlet.

17 . The sound-producing device of claim 1 , wherein:

the housing has opposing first and second end portions;

the driver is positioned toward the first end portion of the housing; and

the first and second audio outlets are defined toward the second end portion of the housing.

18 . The sound-producing device of claim 1 , wherein a non-zero distance separates the forward-facing surface of the driver from the first audio outlet.

19 . The sound-producing device of claim 1 , wherein the first and second audio passages are made from tubing.

20 . A head-mounted display, comprising:

a body; and

one or more strap arms securing the body to a user's head, each strap arm including:

a housing having:

(i) a head portion, which defines a chamber; and

(ii) a body portion, distinct from the head portion, which defines (1) a first audio passage to transmit a first sound wave from the chamber to a first audio outlet that outputs sound and (2) a second audio passage, distinct from the first audio passage, to transmit a second sound wave, distinct form the first sound wave, from the chamber to a second audio outlet that outputs sound; and

a driver, positioned in the chamber, for producing the first sound wave and the second sound wave simultaneously, wherein:

(a) the driver includes a forward-facing surface configured to produce the first sound wave in a first direction into the first audio passage;

(b) the driver includes a rearward-facing surface configured to produce the second sound wave in a second direction, distinct from the first direction, into the second audio passage, wherein the second sound wave is produced simultaneously with the first sound wave; and

(c) a length of the second audio passage is greater than a length of the first audio passage, such that the second sound wave constructively interferes with the first sound wave.