If you're looking for a material to build a subwoofer box out of, plywood is a popular choice. It's strong and relatively inexpensive, making it a good option for budget-conscious DIYers. However, there are some things to consider before building your box out of plywood.
One thing to keep in mind is that plywood is not an acoustically transparent material. This means that sound will be absorbed and dampened by the wood, which can affect the sound quality of your subwoofer. If you're looking for the best possible sound quality, you may want to consider using another material like MDF or fiberglass.
Another thing to keep in mind is that plywood is not as sturdy as some other materials. If you're planning on building a large or heavy subwoofer box, you may want to reinforce the plywood with metal brackets or braces. Otherwise, your box could end up collapsing under the weight of your subwoofer.
Overall, plywood is a decent option for building a subwoofer box. Just be sure to take into account its acoustic properties and lack of durability when making your decision.