Axiom Home Page Forums General Discussion Home Theater Multi-channel Bass

An ideal HT setup would consist of full range speakers for all 5/7 channels. The advantage being that you get the low frequencies from the channel intended. Also, different low frequency effects can be played simultaneously from different channels.
My question being, how does this ideal setup compares to a 5.1/7.1 channel setup crossed-over at 80Hz/100Hz? Can you get bass imaging similar to the ideal setup? How does a sub-woofer produce 2 different low frequency effects that could be simultaneously produced through multiple channels in an ideal setup?
Its my first post on this forum and in the time to come hope to gain a lot from the experienced members.

IMO “ideal” is a relative term here. AFAIK most sound engineers mix their channels so that most of the LFE is coming from the LFE channel and Mains. I even have some Blu-ray 7.1 audio that has all the front channel information mixed into the Mains with only some reverb and ambient stuff coming from the center channel.

Also when using DSP like Dolby PLIIx the information being sent to the surrounds and rear is the ambient (out of phase) information which most people find is better reproduced using di-pole or bi-pole speakers.

I did a lot of research and experimentation into getting what you term the “ideal” setup and found the only place using another full range speaker improved the imaging was the center channel. I found that using bi-pole speakers as surround and rear is far superior for my uses than using either full range or direct radiating bookshelf speakers.

Also having identical full range speakers all around is usually considered the ideal for multi-channel music and not HT. like I mentioned AFAIK most movies are mixed to account for the types of speakers people typically have in various positions. Even most of my music SACDs DVD-As and concert DVDs just have ambient sounds of the venue mixed into the surround channels.

If I can find it, on a different computer, I have a link to an article that makes a very good argument for why using either identical bookshelf speakers crossed over to a subwoofer is probably the best bet for most people for multi-channel music. And that for a combined music HT system that using di/bi-pole speakers as surrounds and rears may even be a better option.

In short even if money were no issue I still wouldn’t upgrade my QS8 surround/rears to M80s since the QS8s are better designed for those duties.

One member here, Onn, does have M80s surrounds and might be able to add his opinion of their value.

I think Grunt hit the nail on the head, also for most humans freq. under 80Hz can not be located. That means even if there is sound comming from a any speaker in your room, if it is below 80HZ you will hear it, you just can't tell where it is comming from.

A good rear or side and center, speaker that can get to around 80hz and a good quality sub should work for any multi channel program, music or movies.

That is the theory at least.

paul

Sarang,

It's a tricky concept to understand, but by using one or two subwoofers for the very low non-directional deep bass energy and relatively smaller speakers (center, surrounds and even mains), you really can achieve highly realistic bass imaging.

As others have pointed out, we do NOT hear very low frequencies in a directional sense. It is the upper bass and midrange sounds that provide the directional cues that tell your ears and brain where the bass sounds are coming from.

For example, if a mallet hits a big bass drum (about 30 Hz or lower depending on the diameter of the drum), the deep 30 Hz sound issues from the subwoofer, but the "crack" of the mallet hitting the skin of the drum (likely around 800 Hz or higher)is what tells your ears where the drum is located. The deep bass reverberates through the room (or the concert hall) but your ears/brain instantly locate the "thwack" of the mallet which, for example, comes from the front right or left side of a typical orchestra.

Regards,
Alan

Thanks Grunt, Paul and Alan for the replies.

I understand that the bass below 80Hz would be non-directional (although, I read somewhere that it is non-directional below 60Hz). The basis of my question was explosions in war movies and the thumping sounds in movies like Jurassic Park which originates in the rear channels. Since some or the rear channels are crossed over at frequencies over 80Hz, in such cases, how do they maintain accurate bass imaging?

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. 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? 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?

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?

Hope I'm not confusing with my questions.

Sarang

Good questions.

I think that the 80hz is a rule of thumb and the actual frequency for humans for non-directional sound is differnt for everyone (probably within +/-10Hz max). 60hz is pretty low, i bet it is closer to 70 / 75 Hz. I bet this is why most recievers are in 20 Hz incrments under 100hz for crossover freq.

This is why I always recommend the QS8 over the of the QS4.

I have done some extensive testing and really don't notice that much, if any difference between setting my QS8 at 80 Hz or 100hz.

Some people like to have all their speakers in an Epic 80 st to 80Hz. I think that actually works great for movies, but for 2ch music, I like my M80's at 40hz.

I suspect the recommendation of keeping the crossover at 10Hz over the lowest tested freq. in theory can makes sense. However I really would be suprised if you could hear the difference between the QS8 set at 80Hz and 90Hz with a good sub like the EP500.

I don't think the timbre of a speaker is nearly as critical at lower frequences say below 80Hz. So, no subs don't have to be from the same manufacturer. It isn't like matching your mains with a center channel.

That is not to say there arn't very bad sounding subs out there, but it usually isn't because they don't match the rest of the speakers.

paul

Sarang, welcome. The studies I've read(most notably by THX)indicate that 80Hz is the lowest frequency which is localizable. It varies with individuals and rooms and in some cases even 150Hz was non-directional. I use an 80Hz crossover for all speakers, regardless of a lower bass extension capability.

Arguments of some that separate bass reproduction should be carried down to very low frequencies, at least for two channels, and even better, for all, haven't been proven by experimental evidence. For example, Dr. Toole, briefly discussing this in "Sound Reproduction" at pp. 238-239, titles the section "Stereo Bass: Little Ado about Even Less"(with apologies to Shakespeare).

I really don't think we should let you get away with this characterization of "ideal HT setup". Ideal how? Why? What sense has "the low frequencies from the channel intended" for frequencies that humans can't tell what channel the frequencies come from?

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.


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.



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.



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.



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.

I will just say that there is no ideal set up that every one will like as listener/room/system interactions are all different. You must tweak till it sounds the best to you. To me direct radiators near field are not optimal as the sound is too directional, to have M80s as surrounds that are only 2' away makes no sense to me. I like a more enveloping surround experience and QS8's fill that role at only 2-4' away.