We (GR Research and Skiing Ninja) recently concluded our upgrade viability tests for these two models and below I will detail exactly what we found with the Axiom M80Ti tower and VP150 center speaker. The VP150 is the top of the line center and the M80Ti is the predecessor of the current top of the line tower speaker, the M80v2.

When we decided to look at these two speakers, I had a large question in my mind – how did they solve the issue with using multiple dome tweeters without any dispersion restrictions? Read on to find out ;)

A quick note on the M80Ti…

Between the Ti and v2, the driver complement has remained unchanged. The speaker’s dimensions are the same as is the weight (57lbs). The crossover between the two HAS changed so you cannot take the information presented here on the Ti and transfer over to the v2. The M80v2 is claimed to be more linear than the M80Ti, the rest of the specs remain the same. The nominal impedance is listed at 4 ohms. (info in the preceding paragraph from soundstage.com review)

OK, here we go with the M80Ti:

The box was lightly braced with only two braces used in the whole box.

There was a light amount of poly fill in the box. Just enough in there to say that it had it but really not enough to be effective.





The dual mids were separated from the lower woofers with cardboard tubes located behind each mid.

The woofers had no venting of the voice coil through the frame or through the pole piece. We find this curious to be sure…





No shielding cup was used on the motor structure; just a smaller bucking magnet.





The crossover, as you can see here is bottom of the barrel when it comes to parts quality.





We are going to give you the facts here and back everything up with measurements.

All measurements were made by Danny Richie at GR Research. Danny has been contracted as an outside party for measurements before and has an established reputation in the community for high quality work that is beyond reproach.


Let’s start with the impedance issues:

The one place that you don't want to see low impedance is in the low frequency range where it will cause a high current demand on the source. Unfortunately this is exactly where the impedance is lowest in the M80Ti. It drops to 2.4 ohms between 10Hz and 20Hz. At the tuning frequency of the port it drops to 3 ohms. I can't recommend using this speaker with any typical home theater receiver though I am sure many do.





As this speaker allows for bi-amplification, we were able to run an impedance sweep on the mids and tweeters separate from the lower woofers.

The Blue line shows the two lower woofers to be a pair of 8 ohm nominal woofers in parallel. The real alarming part though is that the mids have no high pass filter at all. They are only crossed over to the tweeter and allowed to play as low as the lower end roll off allows. Not only does this mean that they will easily reach exertion limits with low frequency information but that they also contribute to making the impedance in the lower range even lower. There is no reason we can see for doing this.





Moving on to the frequency response:

The tough part was finding an absolute middle between the two tweeters to take a measurement so as not to get cancellation between the two of them. Since the binding posts allowed for bi-amplification we also shot the mids and tweeters separate from the lower woofers.

Obviously the Green line is the mids and tweeters and the Blue line is the lower woofers. The Red line is with all drivers playing. The response falls just outside the +/-6db range (pretty rough). There is also cancellation from an out of phase issue between the woofers and mids in the 650Hz to 1,800Hz range. There is also a slight cancellation in the 2,500Hz to 3,500Hz range.





Horizontally the response stays fairly consistent with just the top end rolling off and only a minor peak popping up between 2 and 3kHz.





The vertical off axis is…not so good. I even had to move the graph up to try and keep most of it on the 25db range of the graph. Keep in mind that this is 1 meter away and each additional measurement going from Red, Orange, Yellow, and Green are made by only moving the mic up 4 inches.

From the on axis (Red line) to moving up only the first 4 inches (Orange line) caused a 16db swing from one measurement to the next as the tweeters became out of phase in the top octave.

Moving up 4 more inches (Yellow line) we now have the tweeters response coupling in the top octave and being out of phase just one octave below it causing a 20db swing in output at that distance.

Finally moving up the last 4 inches (Green line) shows the tweeter to tweeter cancellation to have shifted down in range with a maximum dip at 5kHz and a peak at about 12kHz with a deviation of about 22db.





The spectral decay shows some minor ringing in the tweeters range with only one fairly significant one at 3kHz. There is also some resonance issues in the upper vocal region coming from the lower woofers in the 600Hz to 1khz range.





Recommendations:

The easy fix is to disconnect the top tweeter. It could be left in place or replaced with a black painted MDF insert and covered with felt to minimize the surface reflections that the added baffle space adds. The speaker would then get a redesign of the crossover to correct the other issues. Upgrading the cabinet with NoRez is highly recommended as well due to the light bracing.

The more ambitious fix that will yield the best results would be to cut a new front baffle for this speaker and overlay the old baffle. Some of the driver mounts from the mids up would have to be cut away from the old baffle. The upper section can then consist of an MTM section and the lower woofers can be left in the same location. An all new crossover would have to be designed as well.

A front baffle could be made that will allow the old grill to snap right on just like it was before.

Additional bracing and No Rez can also be added to stiffen up the box and dampen out the resonances.



The VP150…

The tough part about explaining the measurements of this speaker is trying to stay objective. We must say though, that this is not the way one would want to go about designing a center channel speaker.

We can start with just the impedance. It is a typical 4 ohm sealed box speaker:






Where it really gets messy is in the response:

It was a challenge to try and hit the absolute dead center so as to minimize the cancellation effects of the tweeters. The curve seen in Red shows a range that is just within +/-6db. This is not realistic though as one person will never be able to get both ears in this same spot. What the listener will really hear can vary greatly even from one ear to the next.





As an example: The graph with the Red line and Green line show the effect of moving the mic (1 meter away) over by 1 inch. Let me state that one more time. The mic was moved over by 1 inch! Over only 1 inch creates a deviation in the response that is 20db.





What one would want in a center channel is a speaker that will hold a consistent output over a wide coverage range. This speaker does not do that.

Looking at the horizontal off axis the measurements: These are taken at 0 (on axis) and at 10, 20, 30, and 40 degrees off axis. This is represented in Red, Orange, Yellow, Green, then Blue.

At 10 degrees off axis (Orange) we see various comb filtering effects causing peaks and dips all across the upper range.

At 20 degrees off axis (Yellow) we not only see cancellation in the upper ranges but the woofers are now also out of phase and causing cancellation in the range just above 1kHz.

At 30 degrees off axis (Green) the cancellation effects are off the charts.

Likewise at 40 degrees off axis (Blue) the cancellation effects are again off the chart.






The vertical off axis remains consistent so long as you stay directly in line with the two tweeters. Any movement slightly to the left or right of center causes huge cancellations again.





The spectral decay shows some minor resonance issues too but that is really overshadowed by the response problems.





There are two possible ways to correct these issues.

One would be to disconnect the two tweeters or remove them and replace them with MDF inserts and cover the areas on the ends with felt. Then mount one of the tweeters to the top of the box right above the middle woofer. The new crossover would then be designed crossing the tweeter only to the middle woofer and allowing the outer woofers to only join in at the lower frequency ranges.

The second possibility would be to cut a new front baffle for it allowing the mounting of a center tweeter and a single woofer on each side of it. This would make it just like the VP100 model. A new crossover would also have to be designed.

In Conclusion...

I'm sure this report will draw a lot of comment and we welcome them here. We certainly didn't like having to write something negative about a loudspeaker; it is a hard thing to do.

We know designers take many variables into consideration when designing a loudspeaker and no speaker is perfect so those choices can be very hard to make. Some make better choices than others. In the case of the M80Ti and VP150, we can't agree with many of those choices. The M80Ti has been replaced by the M80v2 which sports a new crossover. As the two tweeters are still not dispersion controlled, we don't see the possibility for much improvement in that area over the Ti model. We will gladly test a v2 crossover if one is made available.

Our final word is: what would you like seen done to correct these models for all the customers out there?

Remove one tweeter and cover with felt? New baffles cut? Custom tweeter pod for the 150?

Let us know by your comments below.

We also welcome comments from anyone about this report. Do it in a civil and adult manner and we will all get along great.