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first sub build reality check
first sub build reality check
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Old 20th February 2015, 11:26 PM   #1
kcducttaper is offline kcducttaper  United States
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Default first sub build reality check

I'm building my first subwoofer and would like to toss in my plans here for a reality check before I tear into it too much.

My goal for this sub is to keep it under $100, but still hit reasonably well and deep enough to cover most musical situations. It will be about 90% music (mostly rock/metal type stuff, but some classical, jazz, dubstep, rap, etc thrown in occasionally) and 10% movies.

The driver I have have selected is a Dayton Audio SD215A-88 chosen for its price, QTS, sensitivity, power, and response range. I plan to power it using a Dayton Audio SA70 subwoofer plate amplifier. I know the amp is just slightly under powering the speaker, but I think it will work out alright for what I'm using it for.

The box is planned to have 1.25 ft^3 of volume empty. Once I throw in the ports, bracing, and driver, I figure I'm looking at 0.9 - 1 ft^3 final volume. 2 x 2" diameter by 10" long ports will give it a tuning of 38Hz, which is represented on the graph below.
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Here's the general layout. It's going to be down firing with 2" legs although I wonder if I should make the legs a little longer. A lot of guys run 3-4" minimum, but that's also with bigger, much more powerful drivers.
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Underneath view:
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Plate amp on the back:
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SPL graph
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Am I missing anything? Look reasonable?

Last edited by kcducttaper; 20th February 2015 at 11:28 PM.
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Old 21st February 2015, 11:43 AM   #2
oculi is offline oculi  Australia
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any reason not to just have it on legs? what's the board the legs attach to for?
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Old 21st February 2015, 03:18 PM   #3
kcducttaper is offline kcducttaper  United States
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Partially because I thought it looked better that way, but I also figured since it will be sitting on carpet, the hard wood will radiate the sound out a bit better than just firing it into the carpet.
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Old 22nd February 2015, 04:18 PM   #4
truecolour is offline truecolour  United Kingdom
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I think the amp is under powered for your project, you will want 200w i would say. Think twice the driver power handling...
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Old 23rd February 2015, 01:05 PM   #5
TBTL is offline TBTL  Germany
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Nice project. 70W of amplifier power is sufficient, it drives to woofer to it's maximum cone excursion at 54Hz. Air velocity is quite high below 55Hz, it peaks at 21 m/s which will cause chuffing at high volume levels. A single, bigger vent has less problems. Flaring the exits of the port also helps.

It would be nice to have a subsonic filter (highpass) at around 35Hz, to protect the woofer from frequencies below the tuning frequency.
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Old 23rd February 2015, 01:34 PM   #6
kcducttaper is offline kcducttaper  United States
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After this post, I boosted the volume of the box by .125 feet, so it's now 1.375 empty. I also figured out where to plug in the system wattage in WinISD and TBTL hit it right on the head.

At 70 watts, the max cone excursion just slightly maxes out around 55Hz, then drops back down. I doubt that will be a huge problem, but we'll see how it turns out. I also noticed the port velocity is up there, but there seems to be no definite "keep the velocity under XX". Some people seem to get away with 30+ just fine.

I would like to throw in a highpass too. In my sims, a 4th order at 32Hz should do the trick nicely. I'm not entirely sure where to put said highpass though. Is that something I can rig up electrically, or perhaps an audio DSP running on my computer (which would be pretty sweet anyways)?
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Old 23rd February 2015, 02:49 PM   #7
kouiky is offline kouiky  United States
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Depending on how you listen, you may have enough power to cover all the dynamics in the media you listen to. My general rule is to have 10dB of headroom over the average level I plan to listen at, and since I don't listen loud this is quite easy to acheive. You may be in the same camp, but be sure to double check your numbers.

I would probably say, take little notice of the port's secondary resonance, group delay and port air velocity. If velocity exceeds 12m/s, it may become audible as chuffing at the port's second resonance. Group delay is largely what distinguishes the sound between sealed and ported woofers. As group delay increases at audio frequencies, it impacts dynamic response.

The easiest way to get around these three behaviors is to use a passive radiator and tune low in an oversized enclosure, although there is slighly higher loss than with a port tuning. I used an 18" passive radiator (and a slot port would work just as well with some time and care in design) witha native resonance of just under 4Hz. The woofer was 10", loaded into 130L, total system Fs was around 15Hz, placing the bulk of group delay and phase shift below the important band. Adding mass, I could shift the Fs dpwn to 7Hz, but that wasn't nessecary, nor audible.

That was a few years ago before DSP caught on. We can do quite a bit with a signal processor now to coax more out of the lower response, and a highpass and low-pass response equalization to integrate with the main speakers is a great example. Some software will allow phase correction and impulse shaping, although it's not clear to me which open source programs work in this way.

I like your project, and I think that building a subwoofer is the best way to meet your goals. I'm just not sure that 38Hz or so will be low enough for long term enjoyment. I don't know!
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Old 23rd February 2015, 07:08 PM   #8
kcducttaper is offline kcducttaper  United States
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I'd thought about a PR, but that would put me over budget.

I'm still trying to figure out the best way for a HPF, but I haven't done any research on it really.

For normal music listening, about 35Hz seems to be about as low as most anything goes (except for some rap songs, dubstep, and orchestral type stuff), but I built the box a bit larger than planned, so now, I should hit around 35Hz before a significant roll off. I may have to get some larger PVC for the ports too and I definitely will flare them a bit. I might make the legs a bit longer too - we'll see.

If all else fails, I can drop in an 8" Dayton Audio HO driver that does quite well in a smaller box with a bigger amp.Dayton Audio RSS210HO-4 8" Reference HO Subwoofer 4 Ohm
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Old 23rd February 2015, 07:32 PM   #9
kouiky is offline kouiky  United States
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I have music that extends into the 20Hz regions. My tastes range quite wide from dance tracks to large orchestrations, as you say. No rap, however. Of course other media can go below that, such as video games, films, and other media. When you hear a subwoofer that's carefully crafted and tuned below 20Hz, its difficult to go back because you notice larger impactful sound that articulates and doesn't boom at one note. The thud-rumble is pretty cool.

But, as you say that may not be what you need for your own requirements. You can make a slot port that is wide and thin and even fold it with different second dimensions. That will spread port resonance out, so that it doesn't align all at one tone. As for the filter, check out miniDSP's DRC-AN module. They use the 2+2 plugin that gives you 6 parameters of your choice at any frequency from 10Hz to 20kHz. There are more advanced plugins, too. There is another:

dspCrossover - High Performance Audio - danvillesignal.com
dspMusik 2/8 - High Performance Audio - danvillesignal.com
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Old 25th February 2015, 11:40 PM   #10
kcducttaper is offline kcducttaper  United States
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Dang it guys! Now you're making me want to drop in a Dayton Audio RSS265HO!

In a 1 cubic foot box tuned to 28, it gives me an F3 of 27Hz (instead of 35), max SPL at 110+ until the roll off (instead of 106), and no excursion constraints (except way down low below a HPF). That would be more along the lines of $300 though with the amp to push it. I might as well go with an iNuke DSP if I use that driver. Maybe I should just shut up and build it.
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