Which of the following is NOT a form of acoustical systems?

The correct answer is the rectangular form, which is not typically classified as an acoustical system design. Acoustical systems are primarily concerned with how sound interacts with surfaces, and their configurations are designed to manage sound waves in a space effectively.

Concave, convex, and angular forms are known to create specific sound reflections and diffusions. A concave shape, for instance, can focus sound waves to a point, which is beneficial for amplifying sound. A convex shape, on the other hand, tends to scatter sound waves, which can help in reducing echoes and creating a more even sound distribution in a space. Angular geometries can also be effective in manipulating sound waves, as they can help in redirecting sound towards desired areas while minimizing unwanted sound buildup.

In contrast, a rectangular shape does not inherently provide any sound management characteristics. While rooms or surfaces can be rectangular, this form does not contribute to the acoustical behavior in the same way that the other geometries do. Rectangular systems may require additional acoustic treatments or adjustments to achieve desired sound quality, which is why it stands out as not fitting into the category of acoustical systems naturally.