Originally Posted by aarons915
I'm not talking about people who have no choice but rather people who's speakers extend plenty low to crossover at 80 but prefer the sound of a 100hz crossover for their fronts and center?
For most practical purposes quoted low frequency extension is only a marketing number since it may not apply to the output levels you're interested in. A speaker that's "flat" to 40Hz may lack the displacement (product of driver size and how far they move in and out) to play comfortably down to 120Hz.
Maximum excursion limited SPL from a monopole operating into free space at 1 meter is
102.4dB + 20log(displacement) + 40 log(f) with displacement in meters cubed and frequency f in Hz.
102.4dB + 20log(travel) + 20 log(area) + 40 log(f) with travel in meters an area in meters^2 if you prefer.
Output at the maximum linear excursion into full space for various representative drivers one meter away is as follows at 120, 80, 40, and 20Hz. Subtract 3-6dB getting to your listening position in a typical living and more for a larger space.
You can add 6dB for a floor mounted woofer (as in many 3-ways), 6dB if there are a pair of bass drivers, 10dB for three, and 6dB at the cross-over point to a sub-woofer. A few drivers have more excursion although the things you do to increase excursion (like a longer voice coil) reduce high frequency output (more inductance) and can't be applied freely to mid-bass drivers. Many mid-bass units have less. A port gives you about 1/3 octave of extension where excursion is less but introduces other issues.
Size is the nominal marketing size (cone speakers are usually sold according to their mounting flange dimensions, and manufacturers often embed this in the part number with the Seas W22EX001 being a 22cm driver), Sd effective radiating area of the cone, and xmax about how far it can move one way before the motor looses strength which makes distortion go up.
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
12" Peerless 830500 483 x 12.5 121dB 114dB 102dB 90dB
Where a pleasant -10dB from Dolby reference level with dialog at 64dB SPL implies 91-95dB peaks at your listening position and perhaps 97-101dB at the speaker many consumer market 2-ways are likely to have problems with an 80 Hz cross-over.
The same holds for music. Where jazz sounds great at 85dBC average and good recordings have 20dB of dynamic range peaks are hitting 105-108dB a meter from each speaker. Feeding _Take Five_ through 60Hz second order Butterworth IIR low-pass filters I noted right channel low frequency peaks 10dB down from that; although that's still 30 times the acoustic power you can squeeze out of a 6" driver at 40Hz.
My bedroom system used for lower level listening has the Peerless 830873 midrange drivers crossed to stereo sub-woofers at 100Hz for this reason although the -3dB point from their mechanical roll-off is about 60Hz.
The other side of this is amplitude variations and phase shift caused caused by the speaker's low-frequency roll-off.
The low-frequency cut-off isn't a brick-wall. A sealed speaker may be -6dB (1/4 power) at 40 Hz and -3dB (1/2 power) at 60Hz. Where that's close to your cross-over frequency you get an additional dip.
A high-pass response like that caused by a speaker's electrical and mechanical parameters (All speakers have the driver's moving mass and the spring formed by its suspension; conventional speakers add a spring in the form of the air compressed by the driver in the box; and ported speakers have a moving mass of air in the port) implies a phase lead which varies from 90 degrees per pole at very low frequencies to 0 degrees at very high frequencies. A sealed enclosure has two poles (180 to 0 degrees) and vented one four (360 to 0 degrees).
Where the speaker's low frequency cut-off is too close to your electrical cross-over frequency and this isn't taken into account (one octave is the rule of thumb, which is to say a speaker which plays to 40Hz is good for an 80Hz cross-over and one that manages 60Hz just 120 Hz) you get peaks and nulls in the summed response (180 degrees of phase shift from a ported speaker could really hurt - if you reverse the leads on one of your front speakers, stand it beside the other, and feed them bass test tones or music at safe levels you'll notice that the bass is gone compared to a normal wiring configuration).
I think because of the 80hz THX "standard" I want to crossover my stuff at 80hz
The THX 80Hz cross-over was specified as a part of a total system standard.
THX speakers were sealed with a two-pole high-pass function -3dB at 80Hz with 90 degrees of phase lead.
They got paired with a matching two-pole electrical high-pass function yielding 4 poles -6dB at 80Hz with 180 degrees of phase lead and the sub-woofer got a low-pass filter with 4 poles -6dB at 80Hz with 180 degrees of phase lag so they're 360 degrees out of phase at all frequencies which is in-phase.
Add them together and they sum flat at all frequencies (disregarding the effects of distance which is compensated for electronically and phase lead from the sub-woofer's inherent high-pass function which is not).
Other permutations of speaker and electrical high-pass functions at 40-80Hz won't work as well.
but the more I listen to each I think 100hz just sounds more crisp especially the center channel, I find voices to be too "bassy" from my center when I cross over at 80 and it's -3db point is at 55hz so it can definitely handle it.
1. You have speaker interactions with the environment. Low frequency output increases significantly as you near a boundary (6dB or 4X the acoustic power in theory; although in real rooms which leak and absorb sound you get more like 3-5dB and may only have double the bass). A center channel speaker in an entertainment center is going to have a lot more bass than one on a stand and one near the floor or ceiling will have more output than one at ear level behind an acoustically transparent screen.
You also get a dip in output when the low frequencies wrap around the speaker and bounce off the wall behind them and the resulting total delay is about 1/2 a wavelength. With a speaker's front 4' from the wall this occurs at about 70Hz and at just 2' it's 140Hz.
2. Many small (they don't take up a few cubic feet of space. A 30" tall x 15" wide x 12" deep speaker can be only medium sized depending on how low it plays) speakers have a small bass boost which causes your brain to believe lower frequencies they can't reproduce are present (it fills in the fundamental note it's expecting from the harmonics). Where those lower frequencies are actually present because you have a sub-woofer it'll be too much.
And then there's music which I'm much more picky about and same story here. I find with a 100hz crossover I am able to crank my music much louder without it fatiguing my ears and the midrange seems improved as well.
I know this is all subjective and if it sounds better to me I should run with it but has anyone else noticed the same thing?
Actually that's what the science suggests.
I really think this THX 80hz standard makes us want to crossover at that frequency even if our ears tell us otherwise cause I know it pains me to set mine at 100 lol..
Although people like simple rules of thumb they're not always accurate.
You're balancing lower distortion and flatter response against the risk of localizing the sub-woofer which is more likely with shallower cross-over slopes, mismatched levels, port noises and rattles from bad design, etc. Without those problems and a sub-woofer near the front wall you don't have down-sides through higher frequencies than with one or more of those issues.Edited by Drew Eckhardt - 11/4/12 at 10:42am