Headphone

1. A headphone comprising:

an electro-acoustic transducer configured to reproduce sound from electrical signals;

a housing to which the electro-acoustic transducer is attached;

a noise cancelling circuit configured to attenuate a noise sound by adding an antiphase sound to the noise sound;

a resonant frequency converter configured to change a resonant frequency of a Helmholtz resonator configured to include a cavity in the housing and a tubular cavity communicating with the cavity; and

a switch configured to perform alteration of the resonant frequency and switching of the noise cancelling circuit, in conjunction with each other.

2. The headphone of claim 1 wherein the resonant frequency converter is configured to change any one of a cross-sectional area of the tubular cavity, a length of the tubular cavity, and a volume of the cavity.

3. The headphone of claim 1 wherein the resonant frequency is higher when the noise cancelling circuit is turned on than when the noise cancelling circuit is turned off.

4. The headphone of claim 1 wherein the resonant frequency is lower when the noise cancelling circuit is turned off than when the noise cancelling circuit is turned on.

5. The headphone according to claim 1 , wherein

the housing includes an outer case and an inner case that partitions an inner space of the outer case into a front space having the electro-acoustic transducer and a rear space,

the inner case has a through-hole penetrating between the front space and the rear space, and

the through-hole is at least part of the tubular cavity of the Helmholtz resonator.

6. The headphone of claim 5 , further comprising an adapter having an auxiliary through-hole, wherein

the switch is adapted to switch a position of the adapter between a first position and a second position,

at the first position, the adapter is disposed so that the auxiliary through-hole avoids communication with the through-hole to form an entirety of the tubular cavity from the through-hole, and

at the second position, the adapter is disposed so that the auxiliary through-hole communicates with the through-hole to form the tubular cavity from the through-hole and the auxiliary through-hole.

7. The headphone of claim 6 , wherein the Helmholtz resonator includes:

a first Helmholtz resonator in which the cavity is in the front space; and

a second Helmholtz resonator in which the cavity is in the rear space.

8. The headphone of claim 5 , further comprising an adapter having a depressed surface, wherein

the switch is configured to switch a position of the adapter between a first position and a second position,

at the first position, the adapter is disposed so that a portion around a depressed surface is in close contact with an inner surface of the housing, to exclude a space sealed with the depressed surface and the housing from the cavity, and

at the second position, the adapter is disposed so that a portion around the depressed surface is away from the inner surface of the housing.

9. The headphone of claim 8 , wherein the cavity of the Helmholtz resonator is in one of the front space and the rear space, where the adapter is located.