Multiway off-axis irregularity - causing midrange distortion (Long post)

[jesper]

Hi,

Recently I've locked into the lobing issues for a normal multiway speaker. I've written a Matlab program that can simulate the vertical polar pattern and frequency response at any off-axis angle for a TM, MTM or TMM configuration. Crossover point, slope and phase difference between driver can be arbitrary chosen.

All configurations exhibit serious off-axis irregularities, where MTM and TMM are worst. Even with a 24dB crossover slope the off-axis response of a TM have a dip of at almost 60dB (at 30deg). The depth is mathematical and in real life it will be less, but some 10dB at least is to be expected I guess. At 20 and 40 deg off-axis the dip is some 7dB. The case I simulated had a crossover point at 2khz and the width of the dip is about a half octave, with 12dB slope it becomes an octave wide.

MTM configuration is best when following d'appolitos rules by having one wavelength between adjacent drivers and third order filter. This way I get a quasi spherical vertical dispersion pattern at crossover frequency, but the lobing at lower frequency gets worse due to the 2 M's. In my case, with crossover at 2khz, I get a 15dB deep dip centered at about 1khz which is about one octave wide. At large off-axis angles (60deg) irregularities at crossover point is starting to appear aswell.

Now you think, who is listening so much off axis? Well, at least in my room the ceiling bounce will be at approximately 60deg vertical off-axis. This bounce will be almost perfect since my ceiling is of concrete. With a softer ceiling you might get some loss but it can't be much. I also have a floor bounce at about 30deg but there I have a large table...

For a TM config the center frequency of the dip is the same regardless of off-axis degree, only the depth change. For an MTM config the frequency change somewhat but not to the extent where one can expect smoothing of the dips.

No matter how I look at the problem there HAVE to be some serious midrange distortion due to this for all normal multiway systems.

For obvious reasons a fullrange driver will not have this problem. Dispersion at higher frequencies may be worse than a tweeter, but the midrange is intact, all reflections are (more or less) undistorted in the midrange.
A very common comment of fullrange driver is "lovely midrange"... I strongly believe it has mostly has to do with lack of off-axis irregularities and not so much with crossover phase shift, doppler distortions or similar theories.

I’m on the verve of putting together a active filtered MTM speaker using two Seas L15 and a Scan-speak tweeter crossed at 2khz. I’m seriously considering exchanging the tweeter for a Fostex FE103E fullrange and lowering the crossover point to 500hz. This way I get almost no off-axis irregularities…

Please comment on this!

Regards Jesper

[tiroth]

Your math seems good, but are the conclusions correct? The lobing characteristics of certain configurations (MTM, dipole come to mind) are often touted as benefits. The ultimate FR (_at the listening position_) of ambient (off axis) radiation is also going to be heavily dependent on room treatment and geometry.

I would posit that given a suitably wide sweet spot it is more important to have a smooth transition to the off-axis FR than to meet some artificial ideal.

[jesper]

The dips in the off-axis response is too narrow to be a good thing, only an octave or so. It is pritty much omnidirictional up to the point where the tweeter start to beam, except for the dip(s).

/Jesper

[Svante]

Originally posted by jesper
The dips in the off-axis response is too narrow to be a good thing, only an octave or so. It is pritty much omnidirictional up to the point where the tweeter start to beam, except for the dip(s).

/Jesper

This is interesting! A collegue of mine has done similar matlab measurements and simulations and also came up with narrow dips off axis near the crossover, as I recall it. His system is a two-way with a crossover near 2kHz I think.

I also agree with you that the comparison with dipoles is not really valid, since the dipole has a large frequency range in which it is directed (correct me on this if I am wrong. Is there maybe a range some 0.3-2kHz when it isn't directed?).

[angel]

As I recall, the recommendation with regards to placement of the drivers in an MTM configuration was less than 1/2 or 1/4 (can't remember) wavelength. The idea is that this causes line source behaviour.

Have you listened to an electrostatic panel before? I think you will find that many such panels have narrow frequency variations of as much as 20dB in the ON-AXIS response (not to mention off-axis )..

The matter of reflections is dependent on the distance to the ceiling, walls and floor. Depending on the timing of a reflection, it may be perceived as (a) part of the sound, (b) part of the ambience, or (c) an echo.

[jesper]

I've listen to electrostatic panels, and they sure do beam. The dips at low angles, say 10-20 degrees are not my concern, it only gives a narrow sweet spot, reflections are mainly from the back wall and bounce away from the listner. It's the 30-60 degrees which bounce off the ceiling and floor that are bugging me.
Electrostatic panels beam over a wide frequency since they are panels and dipoles, they have a "figure eight" radiation pattern. They should have little ceiling bounce. Line source behave similar, beam over a wide frequency range.
TM and MTM are mostly omnipolar in the midrange.

High frequency floor bounce can probably be perceived as ambience. The wavelength of a 2khz wave is 0.17m. The length difference between direct wave and ceiling bounce in my room is maybe 2-3m, some 12-17 wavelength, it might be enough to be perceived as ambience, I don't know. Does anyone have a good figure? For the 1khz dips in the MTM though it's only 6-9 wavelength....

/Jesper

[Bill Fitzpatrick]

Originally posted by jesper


High frequency floor bounce can probably be perceived as ambience. The wavelength of a 2khz wave is 0.17m. The length difference between direct wave and ceiling bounce in my room is maybe 2-3m, some 12-17 wavelength, it might be enough to be perceived as ambience, I don't know. Does anyone have a good figure? For the 1khz dips in the MTM though it's only 6-9 wavelength....

/Jesper

Floor and ceiling bounce occur to early to be perceived as ambience although the ceiling bounce probably lends as sense of height to the presentation. Many believe that reflections occuring within the first 15 to 25 milliseconds are destructive to the first arrival signal and should be absorbed by placing acoustic panels at the appropriate positions.

[jesper]

It all comes down to how we perceive sound and when we "hear" the combfilter effect caused by superimposed waves.
No matter where we are in a room we all sound pretty much the same when talking, there's not much perceived distortion even though there is considerably measurable destructive interference, due to bounce off nearby walls.
I think two sound sources have to be pretty much in minimum phase difference in order to be perceived as one, distorted source. In the bassarea most reflections are minimum phase. Offcasue in the bassarea we also have standing waves to a large extent.
The 2khz ceiling bounce in my room is some 12-17 wavelength delayed from the on-axis one. I think this is perceived as another sound source, at least from lower midrange and up. This source is attenuated, colored in the midrange and has less treble.

/Jesper

[jesper]

Just made some measurements on a friends 2-way bookshelf (TM) and my two Seas L15 in a M-M config, and the result correlate very well with my simulated result. There are a 10-12dB dip at approx 3khz in the TM at 30 degrees off axis. At other degrees the dip is noticeable but not so pronounced (some 5dB). Treble start to rolloff at approx 10khz.
The M-M conig have a very pronounced dip at about 1khz of some 10-15dB. The dip doesn't change much from 30-60 degrees off-axis.
So, it's true that a MTM config gives less floor and ceiling reflections, but only so in a very limited frequency range... Anything else is just marketing.

I'd like to have no ceiling reflection, but since it can't be avoided (unless you go for line source or dipole), I prefer undistorted midrange ceiling reflection. This means a fullrange driver instead of a tweeter, crossed over at 500hz.

/Jesper

[Salas]

I had in mind to combat those problems by building an experimental small line array using 2x5.5'' + Ribbons in an 8+8 small cabinet L&R ceiling suspended stack. 10'' cardioid subs could be providing the lows. It remained an idea due to practical considerations. Maybe I will try it if some brave celibate friend with a big livingroom is going to volunteer at a point...Chinese ribbons are about $ 90 these days retail. A discount is very probable if to buy 16 + 32 midbasses....anybody tickled?