>I am in the process of buying new speakers, I plan on buying the svs psd 12 sub to go along with them, would full range floor speakers be overkill with a sub,
>or would there be any benifit to using a full range speaker with a sub, thanks
1. The short answer is that speakers marketed as "full range" generally aren't. Speaker low frequency extension is specified at low and even potentially inaudible (for the fundamental - with harmonic distortion approaching 100% you can hear the 40Hz second harmonic and 60Hz third harmonic of a 20Hz tone at an inaudible 70dB SPL) levels.
Output at the maximum linear excursion into full space for various representative drivers at 3 feet is as follows at 120, 80, 40, and 20Hz. Many drivers have less excursion and lower output. Subtract 3-5dB for living room dimensions and more for a larger space for the SPL at your listening position
You can add 6dB for a floor mounted woofer (as in many 3-ways), 6dB if there are a pair of bass drivers, and 6dB at the cross-over point to a sub-woofer.
Size Driver Sd (cm^2) x xmax (mm) 120Hz 80Hz 40Hz 20Hz
4 1/2" Seas W12CY001 50 x 3 89dB 82dB 70dB 58dB
5 1/4" Peerless 830873 88 x 3.5 95dB 88dB 76dB 64dB
6 1/4" Seas L16RN-SL 104 x 6 101dB 94dB 82dB 70dB
7" Seas W18EX001 126 x 5 102dB 95dB 83dB 71dB
8.5" Seas W22EX001 220 x 5 106dB 99dB 87dB 75dB
10" Peerless 830452 352 x 12.5 118dB 111dB 99dB 87dB
I like to play nice jazz recordings around 85dB SPL average. With 20dB between average and peak levels the speakers are hitting 104-107dB measured at 3 feet. It's a logarithmic scale so that's over 30X the sound power you can get at 120Hz from the 4.5" driver in my chart.
Although you can get speakers with big drivers that only play to 80Hz, consumers want more bass extension so you often need to buy a speaker that plays deeper so you can have sufficient output in the range it'll be playing before it crosses to a sub-woofer.
When it comes to home theater you have the LFE channel which has an extra 10dB of headroom. At movie theater levels you're looking at 115dB SPL peaks at the seats (that could be 1000X the sound power you get from one 10" driver with moderate throw) and even when you keep dialog at a more domestically friendly 60-65dB you're looking at 100-105dB peaks that are 10-100X beyond what you can get. You also have less wiggle room because special effects put a lot of low frequency energy in the last octave compared to music where peaks there are generally at least 10dB down from the rest of the spectrum even in "bass heavy" music.
2. Your brain hears timbre as a weighted combination of what it identifies as a direct sound and its reflections with limited accomodation of high frequency roll-off in the delayed reflections.
When a driver becomes acoustically large with respect to the sound waves it's reproducing at higher frequencies there's less output into the reflections.
You hear changes in the timbre especially where the speaker crosses between acoustically large and small drivers and the high frequency roll-off in the reflections is not monotonic. An 8" driver is large compared to a 4" 3KHz wave and a 1" tweeter small compared to it.
With a "full range" speaker the need for larger bass drivers is more likely to combine with the larger acceptable price tag to net a 3-way with a midrange driver that can remain small acoustically at its cross-over point so the directivity change between midrange and tweeter and impact on timbre is far less significant.
3. People prefer speakers with deeper bass extension (Sean Olive has actually come up with a formula that accurately predicts relative speaker preference with this as a factor), like smaller cabinets, and want to spend less money.
Ported alignments allow designers to have lower bass extension at a given SPL or higher peak SPL for a given bass extension for the same driver displacement (area * stroke) instead of using larger more expensive drivers, 3dB more efficiency out of the same box size or another 1/3 octave extension at the same efficiency, and lower IM and harmonic distortion for the lowest frequencies. This makes ports common.
Unfortunately below the port's tune driver excursion increases far beyond what it would be in a box which nets problems with IM distortion and can even run the driver out to its mechanical limits where it fails after repeatedly banging its voice coil into the motor back plate.
Ported alignments also necessarily introduce more group delay as they approach their low frequency cut-off.
For those reasons ported enclosures tend to integrate better with sub-woofers when crossed over with an electrical high-pass filter an octave above their low frequency cut-off. If you're going to have a ported speaker and want an 80Hz cross-over to a sub-woofer you're better off when the speaker extends to 40Hz.
4. You can get bass which sounds more natural and has fewer measured notches when you augment speakers with sub-woofers (preferably multiples). Earl Geddes probably has the best approach here (commercialized by his company Gedlee and Duke LeJune at Audio Kinesis).