This conclusion is absolutely true. It is one of the energy sources with one of the lowest energy density we can harvest. So, is it worthless? No, we don't think so; with all the recent developments in micro energy harvesting new opportunities arise to harvest the energy more efficiently and better, as more effective transducers are coming to market.  Next to technology developments directly involving harvesting sound energy, there have been a lot of developments in other fields as well. Just to name a few examples:

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• The number of noisy wind turbines mentioned on our homepage is growing rapidly. So is their noise pollution, which has an increasing social and environmental impact. By using sound wave harvesting equipment, the turbines could be generating more power, while making less noise.

• When MIT was studying the subject in the 1990's, there were not as many EV's (electric vehicles) around and today they are causing grid constraints. If energy from sound waves could increase their range, this would be a big relief for the national and regional grid operators. 

• The number of smaller devices that consume just a little power, like all the recent IoT (Internet of Things) applications, that could benefit from micro power harvesting is growing rapidly; no one could have seen that coming twenty years ago.

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So, even if harvesting sound energy is not as effective as wind or solar power are today, we still see a lot of use for it. Like marketing and real-estate; "location, location, location" is what makes it valuable! It's all about harvesting at the right places. And, we must not forget that solar power in the 1960's wasn't nearly as efficient as it is today and it's even still evolving.

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Some interesting pages to fill your day:

• Science Direct on sound energy harvesting

• MIT: Can sound be converted to useful energy?

• Just Energy: Sound energy: Everything you need to know

• Manilla: Sound energy, a noise pollution based power bank

• Medium.com: Harvesting energy from urban noise pollution

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