Two subs are better than one! At least that’s what a sub-woofer manufacturer wants you to believe; what better way to instantly double their profits. The thing is, they’re right. Two subs really are better than one, and we’ll explain why.
The story all starts with how we perceive sound, and especially how loud certain frequencies sound. There are some graphs call the Fletcher-Munson curves that show how loud relative frequencies sound to humans. They are called equal loudness curves; which shows what real in room SPL levels are required for us to believe that all the frequencies are the same loudness. The immediate take away is that we need to produce much higher SPL levels in the sub-bass range in order for them to sound as loud as the vocal range. This makes sense, as humans have evolved with increased sensitivity to frequencies in the range of human speech, and lower sensitivity to frequencies outside that range. So what does this mean for your sub-woofer? It means it has to work significantly harder than your main speakers in order to keep up with them.
This helps explains why sub-woofers need to have large sound radiating surfaces (10″ +) and high input power. It also is one of the reasons there is a LFE (low frequency effects) channel in 5.1 and 7.1 recordings, as it is a way to include more sub-bass information in a recording. These large transducer sizes and high power requirements have another downside: enclosure size. Sub-woofers tend to be large. There is a general rule for sub-woofer design that states of the three main constraints, size, output, and power, you can only pick two. So if you want a small sub-woofer with high output, you will need to feed it a lot of power. Likewise if you want to use a small amplifier and have high output, your sub-woofer will need to be very large.
So you might be asking “if one sub-woofer needs to be large, how could I fit two of them into the same room?” That’s because you can use two smaller and/or lower powered sub-woofers to achieve the same sound level as a single larger, higher powered sub-woofer. Two smaller boxes tend to have an easier time fitting into a room then one large sub; your wife will appreciate this. The two subs also don’t need to work as hard as a single sub to get to the same SPL levels, and this reduces distortion and directly increases sound quality. Distortion is highest in the low frequency range, and anything you can do to lower distortion will have immediate audible benefits. On the other side, if you do have the space to fit larger sub-woofers in your space, two of them will be able to play louder and deeper than a single; 6db louder.
Two sub-woofers also allow greater flexibility in positioning them for optimal in room response. Multiple sound points around the room help to fill in dips and suck-outs caused by Standing Waves and Speaker Boundary Interference based issues. Standing waves are caused by the size of a room and a speaker’s relative position inside that room. Boundary issues are caused by early reflections causing destructive and constructive interference. In the frequency region less the 250 Hz, these peaks and dips can be much greater than 25db swings in very tight bands. You may have noticed before when moving around a room that the bass sounds stronger, weaker, or just ‘better’ in certain spots in the room. These points correlate to the peaks and suck-outs in the frequency response of the system. Multiple sound sources will help to fill in these dips and suck-outs across the entire listening space creating a much smoother response.
Dual subwoofers in your home theater or two channel system is a great way to lower distortion, increase headroom, smooth out room modes and frequency response, and will also provide for better installation and integration options then a single, larger subwoofer might be able to. If you have the budget, stepping up to 3 or 4 sub-woofers continues to add value to your system; but after 4 the return on investment starts to be minimal. Contact us for room modeling simulation and recommendations for the right sub-woofer set up and positioning for your set up.