I understand that the bass below 80Hz would be non-directional (although, I read somewhere that it is non-directional below 60Hz).
As others have said the point at which bass seems non-directional (in a room) varies. The larger the room the higher the frequency that bass is perceived as non-directional. The closer the source the more easily bass can be perceived as directional, worst case is when you can actually feel the air from the driver movement on you body. Differing placement of the “satellite” speakers and subwoofer can increase the frequency at which bass coming from a subwoofer can feel diferent from the direction intended by the mixing of the audio. Since most bass not directed to the LFE channel is likely to come from the mains placing the subwoofer behind the seating rather than up front with the mains can cause some people to perceive the base to be coming from behind down to a lower frequency rather than from up front where the other audio cues should be placing it.
As Alan explains, bass instruments may produce a combination of directional and non-directional frequencies, in that case the ideal cross-over should be at 60Hz/80Hz.
Cross-over at frequencies higher than this should start distorting the bass imaging.
Not necessarily. For the reasons listed above and some others the only way to know the “ideal” crossover for a given setup is to experiment. There are just to many variables involved to come up with an “ideal” generalization for all or even most situations. 80Hz is a compromise and generally good starting place but far from ideal IMO.
I also read that cross-over for any speaker should generally be set on 10Hz higher than its lowest frequency. This would essentially mean that while selecting the speakers for 5.1/7.1 setup, the lowest frequencies for the speakers should be around 50Hz/70Hz. Do you agree?
While I agree that the 10Hz rule is a generally good idea to account for any early roll-off from the satellite speaker to the subwoofer not all speakers are normally going to be required to produce base down below 70Hz. The front 3 speakers especially the mains are most likely to be mastered for music and moves to with audio information going below 70 or 50 Hz but since sound engineers know that most people don’t have full range speakers all around they don’t even bother sending much low frequency information to the surrounds and rears. Besides most low frequency information is integrated to “onscreen” action which means it coming from the front anyway.
The matching of tonal quality of the speakers and sub-woofer also becomes important so as to have an unnoticeable transition from speakers to subs. Does that mean that the speakers and subs should always be from the same manufacturer?
I’ve never noticed a tonal difference between speakers and subwoofers across manufactures up to 100Hz which is the highest I’ve ever run a cross-over.
I would be more worried about a tonal match between the front three speakers L/C/R than any others. Next I would worry about wide speakers if using them. Then surrounds. Then rears and finally height speakers. I’ve found two main things drive tonal differences (something I’m notoriously sensitive to on this forum). The amount of audio information shared between the speakers and the proximity of those speakers to each other. Almost by default this means the front three are the most important to have matched. Personally I will never use anything but three identical speakers up front again.
Some people suggest crossing-over main, center and rear at different frequencies, based on the handing capacity of the speakers. What are your views on it?
I have done this with good results but it’s a very situational thing depending on the speakers and distance from the speakers to the seating for me.
The best speaker set and settings for any given situation are so variable that generalities often fall apart if one is trying to obtain an “ideal” solution. I’ve give you some of my examples.
In my apartment 16x12x8 listening area seated about 6-8 feet away from the mains/center surrounds in the back corners and subwoofer directly behind the seats. My mains and center were crossed over at 40Hz because sitting that close to them I could feel the mid-bass punch (which is best produced near-field) and gave a much more realistic feel to the true directionality of bass outside of a room (been knocked on my butt or felt the slam in my chest on more than one occasion from explosions and they are very directional). Plus even crossing them over at 60Hz I could feel the subwoofer behind me. However, I have my surrounds crossed at 100Hz because being corner loaded reenforced the bass so much that they felt “boomy” playing anything lower. With the subwoofer behind me, my surrounds were also pretty far back, caused them to blend well at 100Hz.
In my house 21x13x8-12(vaulted) listening area seated 12 feet from the mains/center and 5-8 feet from the surrounds/rears w/o any corner loading this time. At first I crossed my mains/center over at 40Hz because I have a room mode that dips down 10dB around 50Hz but goes up about 10dB around 20Hz and 90Hz and I couldn't get a good balance out of a subwoofer alone. At a 40Hz cross-over I could run my mains a little hot to get some more mid-bass w/o the low bass from the subwoofer becoming overpowering if I turned it up. All the rest of my speakers were crossed at 80Hz just because the Auto EQ detected them that low or lower in which case I bumped them up to 80Hz to help take some load of my receiver.
Then Charles (CV) loaned and later sold me his SVS AS-EQ1 which flattened out the bass in my “sweet spot” to +/- 2dB and making the bass from the subwoofer sound better than from the mains. So I now run all my speakers including my front 3 M80s at 80Hz.
If there’s a moral in my long winded post it’s that now “ideal” is ever going to survive the reality of the room/seating and listener preferences. Only experimentation will yield optimal results. Generalizations will get you in the ballpark but “ideal” IMO only comes with tweaking.
