I just had to comment on a couple of things here.
Vertical imaging...
In a sense both of you guys are right about this. Denny is right in that it is not something that is an effect picked up during recording. Meaning that a mic placed higher up or something will not place an image higher up in play back. No speaker will change this either.
There is the effect though and it is a result of a time arrival of the signal from the two speakers. I easily hear effects come directly from spots high up on the ceiling verses effects way down low in the sound stage. Q sound recordings are especially good about this effect. Typically down low images are forward and up front while higher up sounds are deeper and further back in the sound stage.
If you unplug one speaker you will quickly note that the image height is limited to the speaker and driver placement. It will not extend above or below the speaker in any way. Just give it a shot and you'll see.
Now a line source will not in any way diminish this effect in fact the varying degrees of height in image is typically better with the line sources than point sources.
The next question I have is that I seems to notice that most line arrays are built with small drivers (less than 8"), so aren't these drivers limited by their inherent limitation to produce frequency below say 50Hz??
Not at all. Low end extension has little to do with driver size. I'll elaborate more on that below in another response.
There seams to be some misconceptions that I will address directly as some of what was posted was really directed to me.
The bottom line is that the bass extension is not going to be any different whether you have one woofer (like a compact 2-way) or several woofers in an array.
This is not at all true. The longer the line of woofers the lower they will play due to the collective gain effects in the lower region. For instance: I designed a group of line sources for Epiphany Audio. Most of you have seen or read about them. The Absolute Sound reviewed two of them and gave them an Editors Choice award and a Golden Ear award.
They used a 4" woofer that I had custom built for that application. A single woofer ported low and in a bit of an oversized box could hit a -3db of 57Hz. The 6/6 model used 6 of them per speaker and would hit a -3db of nearly 50Hz (at least low 50's). The 12/12 model used 12 woofers and easily hit -3db figures in the mid to upper 40's (about 45Hz). The larger 20/21 (20 woofers) would hit a -3db range in the upper 30's.
The only accurate way to measure the true low frequency extension of a system is to do a near field test where you place the microphone a 1/4" from the woofer (sealed box system) or take a sum of the woofer's output and the port's output in a ported design.
This really doesn't work at all for line source speakers. One reason is mentioned above and involves the collective low frequency gain due to the length of the array.
Secondly, a line source will always couple with the room differently than most point source speakers. Lower woofers for example will couple with the floor and the floor reflections must be included in the measured output as it will always be there regardless of the rest of the room acoustics. The ceiling can cause gain too depending on the height of the ceiling and length of the array. All of this can effect the low end extension.
Summing a rear port and a woofer rarely gives a true resemblance of a room response either. It all depends on the room. I used to measure speakers in my anechoic chamber. From these measurements one can easily note that the rear firing port output gets absorbed into the foam behind the speaker and no port output is measured.
Most people are not concerned with a simulated low end response. It really means nothing. Most people want to know how low it will play in their room. Unfortunately all rooms are different but an average sized room and room response is very telling. Measurements will follow below, but first one more thing to note.
The array will move more air (increased cone area = more displacement); however, it you want true low octave extension you'll need a subwoofer to do that. Without active equalization the best you can expect from a ported 7" driver is a -3db point of 40-45hz. Some drivers I've seen (and measured) in other commercial arrays won't even go that low.
Again the collective gain from the length of the array yields more low end extension. Aside from that though if you are limited to a single woofer and off the shelf units then a 40 to 45Hz range is about the limit.
However it is no problem to get a 6.5" or 7" woofer to play down to a -3db point in the mid 20's. It's pretty easily actually. You just have to give up something somewhere else and in this case it is sensitivity.
The low sensitivity 6.5" woofers used in the LS series have a Fs of 34Hz and a single woofer is capable of hitting a -3db of 32Hz in a ported enclosure. So a group of 8 or 12 of them have no problem hitting -3db down points in the upper teens. XBL^ motors don't hurt either. This keeps the woofer linear even with long exertions and maintains a constant BL and driver control.
Most line sources utilizing ribbon tweeters can't be paired with woofers like these. The fragile ribbons are not capable of playing down very low or handling much power at all in a lower range. Crossover points must be kept in the 2kHz to 2.5khz range. So a woofer with a lighter weight diaphragm is needed to reach them well.
The custom planar magnetic tweeters used in the LS series can be crossed much lower. The LS-9's cross at 850Hz. This means the woofer need not play as high. So lighter weight cones are not needed.
The low crossover point also maintains a more even off axis response as horizontal lobbing effects are considerable less when crossing in the lower frequency ranges. This gives an even off axis response in both directions.
Another thing regarding the low end extension. The speed and accuracy of 12 or even 8 small woofers like those used in the LS series are way beyond any sub. I have been working on the fastest and cleanest subs currently available (more on this announcement soon). Still there is no comparison to a group of smaller drivers all sharing a small percentage of the load. Moving mass is much less than any single sub and the settling time is much faster. Plus the multiple small woofers can hit the same output levels as a single large sub and never be forced to move very far. This further reduces settling time and gives a cleaner tighter bass response.
Oh yea, here are a few measurements for you guys.
Here is a LS-6 in the middle of my listening room. These are an RTA responses.
There is a slight top end roll off in my well treated room when measuring at this distance. For comparison here is an A/V-1 brought up to the same level and in the same position. It is within +/-1.2db throughout its operating range. Obviously the blue line is the A/V-1. These were also moved over to the listening position on the right side of my room.
Here are a couple of the LS-9's. One is in the right listening position and the other is in the left listening position. 30hz and down actually kick up a little in my room.
Here are the same speakers measured with an un-gated MLS response.
Sorry, my Clio only measures down to 20Hz. Art measured his prototype LS-6's in his room with a DB meter to be -3db down in the upper teens.
Just guessing it sounds like the LS-9's go a few Hz lower than the 6's as they should.
There is plenty of room gain in any room with these but fortunately the bass management system allows some equalization for the room gain. So one could get a pretty flat response in just about any room of reasonable size.
I hope this helps.
