How do I know if I need a sub woofer?

@elliottbnewcombjr....buy a stereo pair of front facing subs, no ports, and locate them adjacent to your mains, to you gain more lows and get the directionality contributing to imaging that the overtones give.

Your comment may work very well with -6dB sub-bass woofers as their dramatic mid 30hz roll off does not excite a rooms bass modes despite the main speakers often being located in a middle of the rooms extra low frequency draining null.

-3dB subwoofers require very precise standing wave bass mode room positioning to enjoy the subwoofers delicate extra low frequency potential which is present on many recordings.

Unfortunately there are no requirements to qualify using the word subwoofer. Fortunately manufacturers such as REL and Perlisten publish their low frequency limitations. Buyer beware.   

I have large speakers with 15" woofers that are spec'd to 30 hertz.  When I measure my in-room response I have strong output to 20 hertz, the lower limit of my analyzer.  I would still like to have subs but space and monetary concerns have kept me back.

A long time ago my speakers were Dahlquist 30i's which were considered full range but I felt the low end was too polite and I picked up a 15" Velodyne sub just to see what a sub would do in my system.  As expected, there was no increase in slam but the low end reached much lower, which I did like.  I picked up a second sub mostly because I could tell where the sub was when listening and I wanted to even out the output.

What I didn't expect was the change in soundstage on live recordings.  I attended a lot of large venue rock shows back then and with the subs I had a lot more of a feeling of the venue space.  I reasoned that in a large hall or stadium there was a whole lot of subliminal sound that I could now hear-- noise produced by the shuffling of thousands of feet, people talking and eating, and more stuff that was producing low frequency sound that I just hadn't been aware of in the recording until I added the subs.  It added a whole new dimension in my room, and made those live shows much more realistic.

The setup I have now plumbs the depths pretty well but not like those subs did, and I miss the environment they provided.  But to get what I think I would like would require large subs that reach real low with high output, and a pair of such would be physically imposing and lots of $ so I get by with what I have....

Bill

Room size and acoustics factor into bass levels which is why you need to measure to truly know what is missing or not. 

Maybe you haven’t heard a truly good system that delivers excellent imaging and soundstage without the aid of subwoofers.
 

@lanx0003  Well yes I’ve heard plenty of those systems, and it’s clear you’ve never heard good subs properly dialed in or you’d understand what I and several others here are saying, but clearly you do not.  Saying something ignorant like subs aren’t needed if your speakers go down to 30Hz just underscores you’ve no idea what you’re talking about here and what subs done right can actually do.  There’s a reason companies like Magico, Wilson, and others make subs costing into five figures, and it’s not just to pair with monitors.  Case in point, here’s a Stereophile review of the Wilson LōKē sub paired with the Alexia V that go down to 19Hz where the subs made a substantial improvement not only in the bass but in the overall soundstage.  Pretty much says it all and completely debunks your position. 

https://www.stereophile.com/content/wilson-loke-subwoofer

From the conclusion of the review…

I expected bass to be bigger, deeper, and more fleshed out with the LōKē. That it certainly was. What I did not expect was the ability of Wilson's smallest subwoofer to enhance every aspect of the listening experience—especially the sense of space. As the soundstage grew deeper, wider, and more coherent, as soundstage boundaries became more evident, as air between and around instruments and voices increased and the quality of music reproduction rose to mesmerizing levels, my appreciation for the transformative potential of the LōKē grew exponentially.

I am happy with the sound....enjoy listening to music...have an incredible fleshed out sound with delightful micro dynamics that make the music accessible. So why would I need a subwoofer?