I don't know about backpack style vacuums (the few I've seen seem plenty loud), but the idea that manufacturers deliberately create loud vacuums to make them seem more powerful is ridiculous. There is an entire market segment of vacuums that are marketed as 'quiet', so there is plenty of demand for quiet vacuums. There is simply a trade off between suction / loudness / price, and various models make different trade offs (corresponding to various points in the market).