Piezoelectric sounder device

1 . A piezo sounder device comprising a piezo sounder and a controller, wherein the controller is configured, upon activation of the piezo sounder device, to drive the piezo sounder at:

a first frequency for a first duration, the first frequency substantially corresponding to a resonant frequency of the piezo sounder at a first operational temperature; and

a second frequency for a second duration, the second frequency substantially corresponding to a resonant frequency of the piezo sounder at a second operational temperature, wherein the first duration is different from the second duration.

2 . The piezo sounder device of claim 1 , wherein the controller is configured to drive the piezo sounder in a frequency sweep including at least the first and the second frequencies, optionally wherein one of the first and the second operational temperatures is an uppermost expected operating temperature and the other of the first and the second operational temperatures is a lowermost expected operating temperature.

3 . The piezo sounder device of claim 1 , wherein the controller is configured to drive the piezo sounder in the same manner upon every activation, regardless of an operational temperature of the piezo sounder at the instant of activation.

4 . The piezo sounder device of claim 1 , wherein the controller is configured to drive the piezo sounder at a third frequency corresponding to a resonant frequency of the piezo sounder at a third operational temperature.

5 . The piezo sounder device of claim 4 , wherein the controller is configured to drive the piezo sounder in a stepped pattern, such that the piezo sounder is driven at the first frequency for the first duration, at the second frequency for the second duration, and at the third frequency for a third duration.

6 . The piezo sounder device of claim 1 , wherein the controller is configured to drive the piezo sounder such that each frequency corresponding to a resonant frequency of the piezo sounder is preceded and/or succeeded by a frequency which is not a resonant frequency of the piezo sounder.

7 . The piezo sounder device of claim 6 , wherein the controller is configured to drive the piezo sounder such that:

each frequency corresponding to a resonant frequency is provided temporally adjacent a further frequency corresponding to a resonant frequency; or

each frequency corresponding to a resonant frequency is preceded and succeeded by a frequency which does not correspond to a resonant frequency.

8 . The piezo sounder device of claim 1 , wherein the first frequency is greater in value than the second frequency, the controller being configured to drive the piezo sounder at the first frequency prior to the second frequency, optionally wherein the second frequency is greater in value than a third frequency and the controller is configured to drive the piezo sounder at the second frequency prior to the third frequency.

9 . The piezo sounder device of claim 1 , wherein the second frequency is greater in value than the first frequency, the controller being configured to drive the piezo sounder at the first frequency prior to the second frequency, optionally wherein a third frequency is greater in value than the second frequency and the controller is configured to drive the piezo sounder at the second frequency prior to the third frequency.

10 . A breathing apparatus comprising a piezo sounder device, the piezo sounder device comprising a piezo sounder and a controller, wherein the controller is configured, upon activation of the piezo sounder device, to drive the piezo sounder at:

a first frequency for a first duration, the first frequency substantially corresponding to a resonant frequency of the piezo sounder at a first operation temperature; and

a second frequency for a second duration, the second frequency substantially corresponding to a resonant frequency of the piezo sounder at a second operational temperature, wherein the first duration is different from the second duration.

11 . A method of manufacturing a piezo sounder device comprising a piezo sounder and a controller, the method comprising:

characterising the piezo sounder at a first operational temperature to determine a first frequency substantially corresponding to a resonant frequency of the piezo sounder at the first operational temperature;

configuring the controller to drive the piezo sounder at the first frequency for a first duration when the piezo sounder device is activated; and

configuring the controller to drive the piezo sounder at a second frequency for a second duration when the piezo sounder device is activated, the second frequency substantially corresponding to a resonant frequency of the piezo sounder at a second operational temperature,

wherein the first duration is different from the second duration.

12 . The method of claim 11 , further comprising configuring the controller to drive the piezo sounder at a third frequency when the piezo sounder device is activated, the third frequency substantially corresponding to a resonant frequency of the sounder at a third operational temperature.

13 . The method of claim 11 , further comprising:

characterising the piezo sounder at a second operational temperature to determine the second frequency corresponding to the resonant frequency of the piezo sounder at the second operational temperature; and optionally

characterising the piezo sounder at a third operational temperature to determine a third frequency corresponding to a resonant frequency of the piezo sounder at the third operational temperature.

14 . A method of emitting a sound by a piezo sounder device, the piezo sounder device comprising a piezo sounder and a controller, the method comprising:

driving, by the controller, the piezo sounder at a first frequency for a first duration, the first frequency substantially corresponding to a resonant frequency of the piezo sounder at a first operational temperature; and

driving, by the controller, the piezo sounder at a second frequency for a second duration, the second frequency substantially corresponding to a resonant frequency of the piezo sounder at a second operational temperature, wherein the first duration is different from the second duration.

15 . The method of claim 14 , wherein the method does not comprise:

determining an operational temperature of the piezo sounder at the instant of activation;

selecting the first frequency or the second frequency based on an operational temperature of the piezo sounder at the instant of activation; and/or

mechanically adjusting a dimension of a sound chamber of the piezo sounder device.