I recently suggested to someone here, that thermoacoustics would be a better way of making a loud noise. But had to admit that I did not know enough about the subject.
Here are 3 of ideas that seem obvious from my reading up on the subject.
1 / A modified stack for research proposes The ends of
the stack would be of a fine copper mesh, and its mass would be made of long thin wires or whiskers of a thermoelectric material such as Bismuth Telluride. This would turn the stack into a thermocouple, a device that turned a temperature difference into electricity, or electricity into a temperature gradient. An obvious combination of technologies to my mind, witch as well as providing a chance at getting a better match between theory and measurement, it also lends its self to power generation.
The simple pulse tube refrigerator ( not the more complex orifice pulse tube refrigerator ) is a the thermoacoustic device that will fit well with an IC engine. The pulses of exhaust gas taking the place of the piston or valve derived pulses in the classic device. The question of witch end of the stack gets cold / hot is set by the location of the stack, relative to the nearest pressure node.
2 / A pulse tube refrigerator with the stack before the 3/4 pressure node, so that the end of it that is away from the hot exhaust is being thermoacoustically cooled. If the stack is of the the thermocouple type the end of it that is being kept cold thermoacoustically, giving a large temperature difference to be turned into electricity.
3 / A pulse tube refrigerator with a regenerator after the 3/4 pressure node, trying to refrigerate the exhaust but really getting air behind the regenerator hotter than the exhaust. A suitably placed stack will, if the other end is kept cool, generate a significant amount of acoustical energy. This cooling can be achieved by allowing ambient temperature Nitrogen to penetrate into the end of the stack before leaving the device through the side. The acoustical energy passes into a long tube that has air being pumped in to it half way along. This tube is a Thermoacoustic separator, Nitrogen moving towards the stack, and the other end becoming enriched with Oxygen.