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.
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.
Attachments
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.
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.
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.
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.
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.
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.
Attachments
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)
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)
Attachments
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. 🙂
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. 🙂
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.
If the AT rolls off at 2nd order below 80hz id be glad,since in the car this will yield an approximately flat response - otherwise youd be complaining about how bloated and slow it sounds 😀
I expected more change with distance from corner- but remember that you have to consider how acoustically close things are - considering 40hz waves that are 8m long youd have to near 2m from the corner to affect things down here
where as 80hz waves much easier to affect things with a 1m distance from the boundary.
I expected more change with distance from corner- but remember that you have to consider how acoustically close things are - considering 40hz waves that are 8m long youd have to near 2m from the corner to affect things down here
where as 80hz waves much easier to affect things with a 1m distance from the boundary.
Shiva EBS measurements :
This is not a max SPL chart and this is in the center of a small room, not in the corner.
I don't know if she used the RatShack SPL correction chart on these numbers, I'll ask her.
16 107,5
18 107
20 107,5
22 108,5
25 109
28 111
31,5 113
36 114,5
40 114,5
45 114
50 114,5
56 113,5
63 111,5
71 112,5
80 113,5
89 111,5
100 110
125 109
This is not a max SPL chart and this is in the center of a small room, not in the corner.
I don't know if she used the RatShack SPL correction chart on these numbers, I'll ask her.
16 107,5
18 107
20 107,5
22 108,5
25 109
28 111
31,5 113
36 114,5
40 114,5
45 114
50 114,5
56 113,5
63 111,5
71 112,5
80 113,5
89 111,5
100 110
125 109
Attachments
I know it's not a really fair comparison, but it would be fun if you can include my measurements in your graph. Drop the curve to match the sealed Shiva with a reference point at 70 Hz or so maybe?
simon5
Could you show you box on this post and tell me the specs of that box, eg how many litres, box dimension, port tuning watt driving the woofer and what shivs model.
because those numbers are unbeleivable
16HZ at 107db that better then any subwoofer on the planet and its not even corner loaded
your box must be HUGE or somthing
juding from your success if i bought a tempest 15" could i beat those numbers (DB)
Could you show you box on this post and tell me the specs of that box, eg how many litres, box dimension, port tuning watt driving the woofer and what shivs model.
because those numbers are unbeleivable
16HZ at 107db that better then any subwoofer on the planet and its not even corner loadedyour box must be HUGE or somthing
juding from your success if i bought a tempest 15" could i beat those numbers (DB)
Greets!
It appears you've led a rather sheltered life WRT infrasonic BW output capability from a single driver. 😉 107 dB/16 Hz is rather ho-hum by today's high Xmax/power handling, low Fs driver standards.
GM
It appears you've led a rather sheltered life WRT infrasonic BW output capability from a single driver. 😉 107 dB/16 Hz is rather ho-hum by today's high Xmax/power handling, low Fs driver standards.
GM
It must be me because i have a bigger xmax than a shiva and I dont get 16hz at 100Db! 
even corner loaded
even corner loadedre subs
Some years ago an Australian hi-fi magazine did some tests on typical Australian houses for room gain. They concluded that for all the typical room gain you get you may as well set your sound system up in the middle of a field.
Typical Australian houses are brick veneer, plasterboard wall and ceiling, picture windowed creations, that are open plan and leaky. The fact is you are flat out setting up a decent standing wave in them let alone pressurising them.
In short if you live in a typical Australian house your subwoofer has to be flat down to its lower cut off. and if that is 20Hz.it needs to play at least at 103dbm. otherwise you can't hear it.
Some years ago an Australian hi-fi magazine did some tests on typical Australian houses for room gain. They concluded that for all the typical room gain you get you may as well set your sound system up in the middle of a field.
Typical Australian houses are brick veneer, plasterboard wall and ceiling, picture windowed creations, that are open plan and leaky. The fact is you are flat out setting up a decent standing wave in them let alone pressurising them.
In short if you live in a typical Australian house your subwoofer has to be flat down to its lower cut off. and if that is 20Hz.it needs to play at least at 103dbm. otherwise you can't hear it.
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