The frequency response depends on a number of environmental, mechanical and materials factors. The piezo sounder 110, comprises its own resonant frequency which may depend on factors (e.g., including the attachment method of the piezo disc 116 to the diaphragm 114, the attachment of the diaphragm o the outer ring 112, the dimensions of the diaphragm, the dimensions of the piezo disc, the piezo disc and diaphragm material type). The sound chamber geometry also has its own resonant frequency. These factors may be controlled during manufacture of the device 100.

Further, tight control of the manufacturing process, materials and geometries of the components can yield repeatable frequency responses generated by each piezo manufactured. If these controls were not in place, otherwise identical piezo sounders may vary from one another, such that the resonant frequencies of multiple piezo sounders 110 may differ. It may otherwise be necessary to characterise each piezo sounder 110 in order to determine its frequency response characteristics and resonant frequency which may be time consuming and costly.

In addition to the above factors affecting the frequency response of the device 100 is the effect of temperature. Temperature can change the frequency response of the both the piezo sounder and sound chamber. Higher temperatures typically shift the frequency response peak to a higher frequency. Lower temperatures typically shift the frequency response peak to a lower frequency (FIG. 1C). By examining one of the curves in FIG. 1C, it can be observed that a selected frequency of 100 Hz or greater away from the resonant peak, can result in a significant drop in SPL (loudness). As the temperature depends on the environment of the device 100 at the instant of activation, this factor cannot be controlled.

Accordingly, when a piezo sounder device 100 is provided as part of a firefighter's safety alarm, the peak frequency which provides the best response and so the most audible alarm, will depend on the ambient temperature of the device at the instant of activation. Firefighters are exposed to such a wide range of temperatures that the frequency giving the peak frequency response at one operating temperature may not be readily audible or may be significantly reduced at another operating temperature (FIG. 1C).

Method of Manufacture

With reference to FIG. 2, a method 200 of manufacturing a piezo sounder device 100 which better addresses the above variability is described. The method 200 comprises characterising 202 the piezo sounder 110 at a first operational temperature to determine a first frequency corresponding to a resonant frequency of the piezo sounder 110 at the first operational temperature. For example, the piezo sounder 110 may be characterised immediately post-manufacturing at room temperature in order to determine a resonant frequency of the sounder 110 at room temperature. In the example of FIG. 4, the resonant frequency of the sounder 110 at 23° C. may be 3350 Hz.

The method 200 further comprises configuring 204 the controller 120 to drive the piezo sounder 110 at the first frequency when the device 100 is activated.

The method 200 further comprises configuring 206 the controller 120 to drive the piezo sounder 110 at a second frequency when the device 100 is activated, the second frequency corresponding to a resonant frequency of the piezo sounder 110 at a second operational temperature. For example, the piezo sounder 110 may be characterised at a second operational temperature, e.g., by heating or cooling the device 100 to the second operational temperature, in order to determine a resonant frequency of the piezo sounder at the second operational temperature. Alternatively, in order to determine the second frequency corresponding to a resonant frequency of the sounder 110 at a second operational temperature, the second frequency may be calculated based on the first resonant frequency. In the example of FIG. 4, the resonant frequency of the sounder 110 at 85° C. may be 3550 Hz.

The method 200 may further comprise configuring 208 the controller 120 to drive the piezo sounder 110 at a third resonant frequency when the device 100 is activated, the third resonant frequency corresponding to a resonant frequency of the sounder 110 at a third operational temperature. For example, the piezo sounder 110 may be characterised at the third operational temperature, e.g., by heating or cooling the device 100 to the third operational temperature, to determine a third frequency corresponding to a resonant frequency of the piezo sounder 110 at the third operational temperature. Alternatively, in order to determine the third frequency corresponding to a resonant frequency of the sounder 110 at the third operational temperature, the third frequency may be calculated based on the first and/or the second resonant frequency. In the example of FIG. 4, the resonant frequency of the sounder 110 at 260° C. may be 3800 Hz.

The method 200 may further comprise configuring the controller 120 to drive the piezo sounder 110 at a fourth resonant frequency when the device 100 is activated, the fourth resonant frequency corresponding to a resonant frequency of the sounder 110 at a fourth operational temperature. For example, the piezo sounder 110 may be characterised at the fourth operational temperature, e.g., by heating or cooling the device 100 to the fourth operational temperature, to determine a fourth frequency corresponding to a resonant frequency of the piezo sounder 110 at the fourth operational temperature. Alternatively, in order to determine the fourth frequency, the fourth frequency may be calculated based on the first, the second and/or the third resonant frequency. In the example of FIG. 4, the resonant frequency of the sounder 110 at −30° C. may be 3200 Hz.

The resonant frequency at other temperatures of interest can be determined also. For example, depending on the location in which the device 100 will be used, it may be common for a firefighter to operate at a minimum temperature of −50° C. rather than −30° C.

An audible sound pattern can then be carefully constructed, the sound pattern having frequency components comprising at least two resonant frequencies, which correspond to different operating temperatures (determined by the previous characterisation), spanning all or part of the operating temperature range of the device 100. The controller 120 then uses this carefully constructed sound pattern to drive the sounder 110 at multiple resonant frequencies upon every activation, regardless of an operational temperature of the device 100 at the instant of activation. In this way, the sounder 110 will be driven at a frequency which is at or very close to a peak frequency response of the sounder 110 at the instant of activation. This may obviate the requirement to: measure a temperature of the device 100 at the instant of activation; adjust the driving frequency of the controller 120 at the instant of activation based on the measured temperature; and/or adjust the dimensions of the sound chamber at the instant of activation based on the measured temperature. The device 100 may not therefore require a temperature sensor to operate, such that the manufacture and construction of the device 110 may thereby be simplified when compared with previously proposed devices.

By employing this method, a degradation in sounder loudness due to external temperature effects can be minimised. This benefits general sounder performance and also helps to ensure a firefighter's safety, regardless of temperature at the instant of activation.

Method of Emitting a Sound

With reference to FIG. 3, a method 300 of emitting a sound by a piezo sounder device 100 according to the present disclosure is described.

The method 300 comprises driving 302, by the controller 120, the piezo sounder 110 at a first frequency substantially corresponding to a resonant frequency of the piezo sounder 110 at a first operational temperature. For example, upon every activation, the piezo sounder 110 may be driven at a frequency which corresponds to a resonant frequency of the device at −30° C. A frequency may correspond to a resonant frequency at an operational temperature when it is within 100 Hz of the resonant frequency, or more specifically within 50 Hz of the resonant frequency.

The method 300 further comprises driving 304, by the controller 120, the piezo sounder 110 at a second frequency substantially corresponding to a resonant frequency of the piezo sounder 110 at a second operational temperature. For example, upon every activation, the piezo sounder 110 may be driven at a frequency which corresponds to a resonant frequency of the device at 260° C.

The method 300 may further comprise driving the piezo sounder 110 at a third frequency substantially corresponding to a resonant frequency of the piezo sounder 110 at a third operational temperature and/or driving the piezo sounder 110 at a fourth frequency corresponding substantially to a resonant frequency of the piezo sounder 110 at a fourth operational temperature.

The first and second operational temperatures may be any temperatures at which the device may be intended to operate, and thus any temperatures at which the device is expected to emit a sound according to specified requirements. The first and second operational temperatures may be sufficiently spaced apart that the peak frequency response of the device 100 at the first operational temperature is substantially (e.g., significantly) different from the peak frequency response at the second operational temperature (e.g., such that a driving frequency corresponding to the resonant frequency at the first operational temperature would not satisfy specified requirements at the second operational temperature). Further, an operational temperature may not include a temperature which is so high as to cause the device to disintegrate or otherwise malfunction.

In the context of a safety device for a firefighter, a piezo sounder device 100 may be expected to provide a readily audible alarm at a range of temperatures spanning −30° C. through 260° C. In one example, the first and second operational temperatures may be opposite ends of a range of temperatures through which the device is required to function according to specified requirements. Accordingly, the first operational temperature may comprise one of −30° C. and 260° C., and the second operational temperature may comprise the other of −30° C. and 260° C. The third and fourth operational temperatures may comprise 23° C. and 85° C., which may be at or near other modal operating temperatures for the device.

The device 100 may thereby be operated in the same manner upon every activation. For example, the method 300 may not comprise determining a temperature of the device 100 at the instant of activation. Accordingly, the method 300 may not comprise adjusting the dimensions of the sound chamber depending on a determined temperature or adjusting a driving frequency of the controller 120 depending on a determined temperature.

The method 300 thereby represents a simplified method of operating a piezo sounder device 100.

Sound Pattern

FIG. 4 is an example driving pattern 400 of the controller 120 when driving the disc 116. The pattern 400 has been determined for a device 100 which is expected to function at a range of operational temperatures spanning −30 to 260° C. The sound pattern 400 may be particularly beneficial for use in a DSU in the UK or Europe.

The pattern 400 may be emitted by a device 100 which has been manufactured according to the method 200 above, and thus a device 100 having a controller 120 configured to perform the method 300 above.

Accordingly, it may have been determined by method 400 that a peak frequency response: at −30° C. is 3200 Hz; at 23° C. is 3300 Hz; at 85° C. is 3500 Hz; and at 260° C. is 3800 Hz. The pattern 400 therefore comprises of a tone at each of the above frequencies.

The pattern 400 comprises a first portion 410, a second portion 420, and a third portion 430 provided consecutively according to Table 1.

The pattern 400 generally comprises two types of tones-high tones and low tones. The high tones target resonant frequencies of the device, whilst the low tones target non-resonant frequencies of the device (e.g., tones which are not resonant frequencies of the device at any temperature). The low tones are below the range of resonant frequencies of the device.

The first portion 410 comprises a continuous high tone at 3806 Hz for a duration of 21450 microseconds. The first portion 210 thus corresponds to 3800 Hz which is where the resonant peak moves at extremely high temperatures (e.g., 260° C.). Portion 410 may be repeated multiple times in a “Full Alarm” alert.

The second portion 420 comprises a series of steps including high tones and low tones. In particular, the second portion 420 comprises pairs of descending high tone steps, consecutive pairs being interleaved by a step at a low tone. The duration of each step varies according to Table 1. Portion 420 may be repeated multiple times in a “Full Alarm” alert. In the example sound pattern 400, portion 420 is repeated 12 times.

The higher tones span 3800 to 3100 Hz in 100 Hz increments, such that the resonant frequency of the device 100 at any temperature between the maximum and minimum expected operating temperatures (260° C. to −30° C.) is covered, within a tolerance of 50 Hz. The range of frequencies covered by the high tones may therefore be defined according to the maximum and minimum expected operating temperatures, the steps then being defined by dividing this range into sufficiently small (e.g., 100 Hz) increments. Accordingly, when the sound pattern 400 is emitted by the device 100, a sound at or near the resonant frequency of the device will be emitted regardless of the temperature at the time of operation. For example, the resonant frequencies of the sounder 110 at −30° C., 23° C., 85° C. and 260° C. are all provided in the sound pattern 400 (within a tolerance of 100 Hz or, more specifically, 50 Hz). In this manner, the sound pattern