Autotuba measurements, plus shiva

[tiroth]

I've built two versions now of Bill Fitzmaurice's Autotuba design. This is a horn sub with a 7' path, using the MCM 55-2421 8" driver.

The first is the textbook one, with 14.5" outer dimensions. The second is slightly larger, with 16" outer dimensions. Although it doesn't sound like much, the difference in mouth area is significant.

I've attached nearfield measurements taken with Speaker workshop, smoothed 1/10 octave. Behringer ECM-8000 mike, with Claudio's calibration file. These measurements have been adjusted so that the SPL of the tuba and shiva match at 40Hz--this is not showing the absolute sensitivity.

The black and red lines are the 16" tuba--the only difference is the black has 2" legs, the red 5.5" legs.

The brown line is the 14" tuba. For comparison, the yellow line is an Adire Shiva in a sealed 75L box.

[tiroth]

I'm interested in any critique of my measurement setup. I was unable to measure any loading differences, whether I located the sub hard in the corner, farther out, or in the middle of the room. Does the nearfield measurement remove the contribution such loading would normally impart?

The Shiva F3 appears to be textbook at ~38Hz, so I am inclined to think that these measurements are spot on.

It looks like the 16" version picks up a few dB over the 14" below 32Hz, and the Shiva definitely is a big winner below 30Hz.

This is only part of the story of course. I will try to post measurements tomorrow indicating relative efficiency, and max SPL at 40, 30, and 20Hz. I'll also include some subjective comments after I've had more time to listen...I don't want to overstate my initial impressions.

[tiroth]

Anyone out there have a summed response nearfield measurement of an EBS (142L) vented Shiva? I'm interested in trying this out, but I'd love to see a graph before building the box.

[simon5]

I'll try to get a graph from the girl I built the subwoofer for.

[tiroth]

Thanks a lot Simon!

[morbo]

Frankly, from all the talk about the Autotuba being optimized for typical room gain, I expected to see a gradual shallow rolloff rather than a sharp dropoff with a hump at 40 hz.

[tiroth]

Sub measurements always seem to be hopelessly optimistic. Grey has been hammering on this for years...the TS equations work, but there is generally a 1st order highpass starting fairly high up that causes problems.

A suggestion was made to measure farfield near the ground plane. This measurement shows the nearfield, on-axis gated farfield, and ungated farfield measurements for Shiva. If room loading is being measured at all with this technique, it isn't helping much.

[tiroth]

This graph shows the tuba at various distances from the wall, with 2" and 5.5" legs. There are some variations, but they are subtle for the most part.

I also include the reference Shiva. There were no significant response differences, no matter how far from the wall I moved it. So, it does show that the tuba has some sensitivity to loading.

None of these curves are scaled; that is, each curve is printed as measured, with equal voltage gain. Because the Shiva has much lower impedance, it is drawing anywhere from 2-4 times as much power for the same voltage. (3-6dB)

[tiroth]

I also did some SPL tests for 20-40Hz band limited (4th order) pink noise. I turned the output on my sound card up as high as possible, and ended up maxing the controls on the amp as well for this test. (250W MCM plate amp)

Measurements were done with a Radio Shack analog meter, set to C weighting and fast response. I applied a correction factor of +4.5dB.

Shiva was peaking at 109.5dB/1m. Excursion was high, and I was hearing some small louder pops at intervals. I do not think this was bottoming, just loud transients. The lights were dimming noticebly from the current draw.

Autotuba was peaking at 114.5dB/1m. There were some scary loud pops, which were either bottoming or the effect of 115dB LF audio. No dimming of the lights. Autotuba may be cheating due to extra ~80Hz range response.

I still plan to do more power testing, but not tonight.

[johninCR]

Not bad for an 8" driver in a horn designed for use in a car. I would like to see a smoother response though. I guess there's no getting around the laws of physics.