First, the speed of sound is commonly measured at 968 feet per second. The fundamental calls for f = v / 2L, where f is frequency, v is velocity, and L is length. For a 20 Hz note, this equation becomes 20 = 968 / 2L. After simple algebra, you get 24.2 feet for L. In other words, a 20 Hz note will require 24.2 feet to complete an oscillation. A typical mid-size sedan has an interior length of 8 feet (front-to-back windshields). That's enough for 60 Hz note at best. Now, I understand that most cheap car speakers can't reproduce 60 Hz note properly, and so-called subwoofers for cars certainly can't reproduce 25-30 Hz note that most mid-end home theater subwoofers can, but isn't it still an overkill to get a subwoofer? Or am I missing something (e.g., bad math)? It's been awhile since I took physics, so please be kind.