Dear Kerry,

From your description, your cycle is between 1,6 to 4 seconds as per your 25% duty cycle and pulse length (how many cycles ie how long do you intend to transmit?). It looks like you are discarding batteries as an option (why? charging time? efficiency? weight? dimensions? safety?) but you could potentially have other options based on super or ultracapacitors (these have wide working temperature ranges, high efficiency both charging and discharing - unlike conventional capacitors -, low weight, long lifetime, usually support cycling in the range of 1 million cycles ... though, they don't like fast cycling at large currents but I suppose audio pulsing should not be too power hungry and this should be no concern); cons, depending on the specific application and your operating voltage might be dimensions or form-factors, safety... there might be more. If the use is not continous but sparse, as you don't seem to have a reliable constant power source (you talk about solar or kinectic charging), you could charge the ultracapacitors as a battery substitute, but of course, always based on the premise that your average power consumption is lower than your average power generation. I suggest you take a look at Maxwell as a starting point for acquiring more information on the pros and cons of Ultracaps, design guidelines, etc. in order to make up your mind on whether they can fit or not any of your use cases.

Best of lucks.

what i mean by 25% duty cycle is 1 second transmit and 3 seconds off to recharg. So yes Ultracap is a possibility to hold the recharge. The pulse would repeat indefinitely and the duty cycle could depend on the duration to charge the cap.

It may also be a single pulse from a radio charge like is done from a tollway car pass when exposed to a microwave. For that use case, the audio circuit is passive and the audio pulse only occurs when external power is transmitted to it (like when a car passes under a toll booth transmiitter/receiver). The down side to that is (I assume) a smart phone can not transmit enough microwave energy in a burst to power an audio circuit even for a second. But I don’t know what all I don’t know thus the reason for this query.

should be cheap, flexible and durable solution, and of course safe to humans( like a toll tag). Maybe even kinetic energy that builds up to power the pulse??? Obviously a stretch:)