The Complete Design and Build of The Sonorous 10S Subwoofer – Hits 26 Hz!

This subwoofer project was 6 months in making but after a couple of weekends of really focusing on it, I’m happy to say that it is finally done! This will be the complete (and exhaustive) detailed design and build review of The Sonorous 10S Subwoofer. This subwoofer has changed my perspective of how much deep bass can be achieved in small packages. And you don’t need to spend a fortune to get good, detailed, deep bass for your home theater or stereo system. So read on to find out more about this sweet little subwoofer project and maybe it will fit the bill for a similar project you’re working on.

Here’s what would be the marketing brochure for The Sonorous 10S Subwoofer:

Dayton Audio RSS265Hf-4 10″ Reference Series Subwoofer
Dayton Audio SPA300-D 300W Plate Amplifier with Selectable Bass Boost
RCA Inputs, Auto On/Off, 10 mV detect threshold, 30 minute on-time, Adjustable Gain (volume)
Adjustable Low-pass Filter 45 Hz to 150 Hz (24 dB/octave)
0/180 Phase Shift Switch
Switchable +5 dB bass Boost, Q 1.5, at 30 Hz
20 Hz Subsonic Filter (-6 dB at 20 Hz) 12 dB/Octave
Sealed Design, 25L Enclosure (0.88 cuft.)
System Qtc = 0.785 (slightly under-damped)
f3 = ~40 Hz (Bass Boost OFF)
f3 = ~26 Hz (Bass Boost ON, +5 dB at 30 Hz)
Max SPL = ?????? (measurements to come….) mathematical/physical limit is 109 dB
13.5″ x 13.5″ x 14.5″ (H x W x D)
3/4″ + 1/4″ MDF Construction
Internal Cross Bracing + Corner Cleats
Fiberglass Insulation Damping, Walls Lined, 2″ thick
Semi-gloss Brushed White Finish
3/4″ Radius Cosmetic Roundovers
(4) Large Rubber Feet
5 Year Warranty on amplifier and driver (from Dayton Audio)

I don’t normally give my speakers names, but I decided to try something new with this one. Sonorous means “capable of producing a deep or ringing sound.” I thought it fitting for this sub project. Now on to the real fun!

Enclosure Design – My Thought Process

First and foremost, my main reason for selecting a 10″ driver was two parts: size and cost. First a 10″ driver doesn’t require a huge enclosure so a small footprint can be easily be achieved. In most cases this will come at a compromise in terms of total output and low-end extension, but depending on your needs, that may be acceptable trade to make. Although with this sub you’ll see that we hardly give up that much and still are able to deliver a very compelling bass experience. And second, this driver as of this writing costs about $209 with free shipping from Parts Express. Compared to larger comparable drivers, this is definitely cheaper than most options and very reasonably priced for what you get. And since this is only a 10″ driver, the amplifier needs are reasonable as well. You can get away with a simple 300-watt plate amp and you’ll be able to push this driver to its limits. The SPA300-D amp is brand new from Dayton Audio and runs $149 and is the cheapest of the bunch that PE offer in the 300W range. Other options you might consider would be the SPA250, Bash 300S, SPA250DSP, or the Yung SD300 which will run you from $180 – $225. I’ll go ahead and recommend the SPA300-D as it measures well, has the bass boost which can be turned on and off, plus all the other features you’d expect from a basic plate amp. In my opinion it’s the better value of these 5 units, unless you want the DSP options for additional in-room tailoring.

Since we’re going for a cute little sub here, I’m going to rule out a ported design from the start. Not that the RSS265HF-4 doesn’t model great in a port box, because it absolutely does, it just requires more than twice the volume to get there. So I modeled up 6 different sealed enclosure volumes, that were based on going from an 11″ cube and working my way up to a 15″ cube in 1″ increments to see if there was sort of a sweet spot between output, extension, excursion and size. Ultimately what you see in the plots is that all of these enclosure volumes can work. The smallest sub sacrifices some low-end extension, but excursion is best kept in check. The largest box does produce the lowest f3, but excursion limits will be exceeded at max power, so you gotta be careful with the volume knob. Somewhere in the middle you get what I would call the Goldilocks sealed design: acceptable f3, Qtc < 0.8 and driver excursion is no more than 15% above the specified Xmax.

With Xmax being the physical design constraint and all other parameters being design nice-to-haves, if we fix the input power at 300 watts (based on the amplifier selection) and we target 14.1 mm excursion (12.3 + 1.8) we end up with a volume of 23 liters. After playing around with some physical box dimensions, this ends up being an odd 13.25″ inch cube. What I ended up settling on was the dimensions shown above which result in a total enclosure volume of 25 liters, give or take. In this case we are at about +20% of xmax instead of the general rule of +15% but since PE doesn’t spec Xmech, the 15% is only a rule of thumb, so I will allow it. Plus, as you will see, once we model the excursion taking into consideration the high-pass cutoff of the SPA300-D, our peak excursion drops to 14.4mm at 300W which reduced the excursion to only +17% of xmax which is even better. Fundamentally you can play around in this region of about 20 to 30 liters pretty safely with this driver without a high-pass filter, but with the inclusion of a high-pass filter around 20-25Hz, you can get away with a slightly larger volume, which results in a slightly better f3 and lower Qtc.

So with the enclosure size decided upon and the basic dimensions roughed out, I drew up an cut-sheet and hopped on over to Home Depot some 3/4″ MDF. Man oh man has the cost of lumber gone up! I paid over $60 for a 4×8 sheet of 3/4″ MDF. I’ve never paid that much for MDF in my life. I’m hoping costs comes down because with lumber costs this high, it really starts to eat into the savings that make DIY speaker building so appealing. I bought a 2×4 foot piece of 1/4″ MDF as well as some other odds for projects around the house (gotta make the trip worthwhile since gas prices are also insane). I had one of the employees cut the board into three ~32″x49″ pieces so I could fit them into the back of my Durango. I’m hoping to actually get either another subwoofer out of this sheet or a pair of bookshelf speakers. The cut-sheet requires only one ~32″x49″ piece and even with that, there is some scrap for the bracing and corner cleats. You can download the basic plans by clicking on the pics and download the cut-sheet here.

I won’t go into details on building the enclosure as it’s pretty much just a basic box. I made all the cuts on my table saw and with some Gorilla Glue and my nail gun I just popped the thing together. You can use screws or clamps or whatever your joinery method of choice. After getting 5 of 6 sides together, I paused to add the 3/4″ x 3/4″ cleats that run front to back in each of the corners. My intent here was to further secure the butt joints and because I really wanted to create a larger round-over on the edges and if I can get my hands on a larger bit, then one day I am going to cut those corners down in which case most of the butt joint will be cut away. I will need the corner cleats to really secure the enclosure. For now I just did a 3/4″ round-over so the cleats just help to strengthen the fairly week butt joints as they are. I added a center beam brace going front to back on all four sides and added the cross bracing which connects the beams to the sides and the top and bottom. Given how small this enclosure is, the interior panels are only about 1 square foot, this feels like just enough bracing without being too much and taking up precious internal volume. We’ve cut the panels in half and then cross braced them basically about 7″ back (not totally centered). The only other thing to do may have been to run a another set of beam supports around the inside of the box, thus cutting the panels in half again. But this seemed fine to me so I left it at that.

I added the front baffle and then sanded down all the edges and prepared the box for the 1/4″ MDF paneling to finish it off. The extra 1/4″ MDF provides a nice smooth finish for painting and covers up the MDF edges which can be problematic to paint. I only covered 4 of 6 sides, so the bottom and back in the end looks less finished, which you can see in the pics. This was basically because I only bought enough 1/4″ MDF to do the 4 sides and not all 6. But if you’re a purist and want to back and bottom to have a nicer finish, then by all means, cover all 6 sides. This has the secondary effect of beefing up the the total wall thickness to 1″. This helps make the cabinet more stiff and more inert. I used a liberal amount of Liquid Nails (about 1/2 thing worth) per side. I cut the pieces slightly oversized and then used a flush-trim bit on my router to make the edges perfect. After that I ran a 3/4″ round-rover bit going front to back on the four edges. This is a little SVS-like, but to me looks really good. I kept the front and back baffles with a sharper corner and just took off the edge with an orbital sander. Then I sanded sanded and sanded some more.

About this point it was time to cut the recess for the driver so it would sit flush to the front of the baffle. The diameter of the RSS265HF-4 ends up being just under 10.50″ so a recess that is exactly 10.50″ is just about perfect. It leaves just enough of a gap so that the driver will drop in without rubbing but not so big that it looks dumb. I don’t remember the depth I did though. I measured the thickness of the basket with about 80% compression on the gasket and I think that landed me around 0.350-ish on the depth? I was basically shooting for the basket to be essentially flush to the baffle. I used a 1/2″ router bit and made two passes, one at 5.25″ and one at 5.00″. Then I cut the hole at 4.625″ (9.25″ dia) with a jigsaw. I test fit the driver and crossed my fingers – it was a perfect fit! You don’t want to screw this part up, so I was happy all my measurements and triple checks worked out. Plus, I’ve gotten a lot better at using the Jasper Jig and getting the cut diameters to turn out. But it does take a little practice. It helps to have a pair of calipers for this part too since a measuring tape just doesn’t have the resolution to to get this part perfect. If you’re not sure, underside the first pass by a 1/16th or so and then measure the diameter you get. You won’t be able to do a fit check at this point, but if the measurement indicates that the diameter is too small then you can increase your radius by one tick on the Jasper Jig and make a second pass. Each hole adds about 1/16th of an inch to the diameter. If that looks like it will be too big, then you can always drill your own hole somewhere in-between. I’ve done that a couple of times for smaller holes.

Testing out the SPA300-D – Does it Deliver?

I got to this point and the shipment for the amp was delayed until Wednesday (it was Sunday) so I took a break and waited for the amp to arrive. Which actually ended up being on Tuesday. I was pretty excited to get the new Dayton Audio SPA300-D Class D 300W plate amp and run it through its paces. I’ll have to save the details of my DIY amp measuring rig for another post, because it certainly warrants a post all on its own. But the essential breakdown is this: Room EQ Wizard (REW 5.20), a custom 400W 4 ohm load box with balanced inputs/outputs, a 45:1 step-down converter with DC block, a DMM and a Presonus FireStudio audio interface. The loopback on the Presonus will do 0.00042% THD (-107 dB SINAD) with a noise floor of -130 dBc. So far for everything I’ve measured, this setup has not been the limiting factor. Everything I’ve tested is at least an order of magnitude worse than this, so I’ve been happy with this setup overall as means for measuring pre-amps and amps (and of course it does speakers too). But details on the setup will be in another post.

The results of the amp measurements I will share in detail. We’ve got a few things of interest we can measure with my setup: frequency response with and without bass boost, 0 and 180 phase reversal, low-pass filter roll-off frequencies, low-pass filter slope, in-band flatness, high-pass filter corner, high-pass filter slope, bass boost response (magnitude and Fc), bass boost Q, THD at varying power levels, max RMS power over frequency and THD at max power. Fortunately REW provides the ability to measure all of these parameters. There really is no standard way to present all of these plots, so I will do my best to make sure they are meaningful and present the data in a way that is easy to read and understand. The graphing scales may change from plot to plot, this is not done to deceive or out of malice but to simply showcase the data better. In some cases, I may have just messed up and plotted them differently and don’t feel like going back and re-doing them all. So there’s also that. Feel free to just click through all the graphs at your leisure you can come to your own conclusions about the amp’s performance. I’ll add a little blurb here as well on my thoughts.

First things first: the amp delivers a solid 300 watts RMS using 40 Hz, 60 Hz and 80 Hz test tones at no more than 1.41% distortion, worst case. At 20Hz the amp was not to reach rated power but for some reason can’t find the measurement details. It seemed like it was just shy of the 300W mark. At 60 and 80 Hz the distortion was actually less than 1% at 300W RMS. Therefore the amp definitely delivers overall on its specified power rating and THD. At no time in my testing did the amp shut down or go into protect mode and the total temps stayed under 100°F. The amp performed admirably in all of my testing. Across frequency the output power stayed consistent all the way up to just under 300W RMS, but there is a slight depression or compression occurring right around 40 Hz at max power. But the overall frequency response stays consistent from 1W basically up to rated power.

Second: there is a high-pass filter on this amp. The FR is down -6 dB at 20 Hz created by a slightly under-damped 12 dB/octave filter when the bass boost is off. With the boost off, there is still about a +1 dB boost present, caused by the high-pass filter not being perfectly critically damped. Though it’s tricky to guess the Q, it’s higher than 0.7. I modeled up some alternate high-pass filter options from 0.7 to 1.5 and this thing is by no means critically damped with the boost disabled. The intent surely is to provide some kind of rumble or subsonic filter to prevent driver over-excursion and taxing the amp. But -6 dB at 20 Hz is pretty aggressive for a subsonic filter. I’ve added the .frd plots of the responses so you can model it with your driver and see how much it will affect the response in your enclosure. Click here to download noboostSPA300-D_response_normalized. Click here to download with30HzboostSPA300-D_response_normalized. Depending on your application, the high-pass may not make an audible difference to you. For a 25L sealed enclosure, this actually works out quite nicely, though combined with the 12 dB/octave roll-off of the enclosure, you end up with a 24 dB/octave slope, which is quite steep, notes below 30 Hz are being filtered very quickly so we are actually at -18 dB down at 20 Hz with this 10″ driver in a 25L box. So keep that in mind when designing your sub with this amp. This is great for keeping things in check, but not so great if your thirsting for some 20 Hz rumbling bass.

Third: The bass boost works as specified. Although with the +1 dB boost already present when disabled, I’d call the bass boost actually only about +5 dB at 30 Hz. The Q is roughly 1.5 and is created by the same high-pass filter used for the subsonic filter but tuned to be even more underdamped. This is basically done by changing the resistor values around a single op-amp circuit to any ratio greater than 1:1. This looks like it’s sitting at about a 12:1 ratio to create this kind of peak. Note with the boost disabled there is probably still a 3:1 ratio between the resistors, which is why the response is never truly flat. They didn’t have to make it this way. This was clearly a design choice. Which could easily be fixed by changing at least one or two resistors in this circuit. I didn’t really get in there and trace out the circuit. I did make note of each IC on the pre-amp board, so I have a general idea about what they are doing. But the option is certainly out there to make adjustments if you really want to DIY this thing. Alternatively, Dayton Audio could provide details around this circuit allowing the user to make mods as necessary. Hint, hint. ;-)

Fourth: low-pass filter, phase switch and auto-on circuits all behave as expected. Though I’d say the upper and lower frequency corners of the LPF are more like 45 Hz and 150 Hz. Still, no reason to complain here. Most people are going to set this thing somewhere between 60 Hz and 100 Hz for most applications. Though oddly enough I measured an even 24 dB/octave slope and not the specified 18 dB per the manual. Not sure why the discrepancy, especially because I think the steeper slope is actually preferred as it will have less bleedover into your main speakers. The phase switch was tested and performed the intended phase reversal of exactly 180 degrees with no other unexpected results. The auto-on circuit worked perfectly in the few cases where I used it. It’s specified for >10 mV and will turn off after 30 minutes if voltages do not exceed this threshold again. There is no audible turn on or off thump. I know this one thing grates on so many people nerves, fortunately they did their homework on this part of the amp.

Fifth and final thought: the last thing I have to note is the one disappointing thing about the SPA300-D and that is that the distortion is higher with the bass boost disabled. Yes, you read that right, with the bass boost off, anywhere below rated power, the THD is up to 100x higher than with the bass boost on. And I say up to rated power because once you reach rated power, the amp distortion takes over and it doesn’t matter whether the boost is on or off. But below rated power at 1W, 10W, or 100W, the distortion is significantly higher with the bass boost turned off. I modeled this circuit (using MultismLive) and fundamentally I cannot understand what would cause the distortion to rise that much given that you’re essentially running the same circuit, just with a different set of resistors. I mean that’s at least one way to do the circuit. I don’t know how they designed this. But however they did it, they did it very poorly. In effect, the distortion should not change at all, other than possibly when the boost is on you might have more distortion just because you have more gain. But to have more distortion with the boost turned off is completely insane to me. So much so that I thought I was crazy and figured maybe there was something weird going on in my setup, that the Class D outputs on the amp didn’t like the resistive load I was presenting (though they worked fine with the boost on) I knew I had to get into the pre-amp stage alone and measure the actual pre-amp without the amp. This is a much simpler test and takes out all the Class D stages and it’s straight line-level for the entire measurement loop.

So that’s what I did. I grabbed an RCA cable and spliced the end and soldered it straight to the output of the pre-amp board right where it goes into the amp board. I’ve got a picture showing the connection here. At this point I basically went through and re-measured a lot of the measurements I had already done looking at frequency response, filter responses, THD and everything else sans actual output power. This is where I was able to confirm the high-pass filter responses, the bass boost responses, the low-pass filter responses and everything basically tracked the power amp measurements, except for the distortion. With the bass boost enabled, the distortion was all the way down at 0.0019%. Excellent! The 2nd harmonic was all the way down at -105 dBc. This bested the best THD I got out of the amp which was only 0.017% at 40 Hz and the 2nd harmonic hovered around -78 dBc. While not state of the art for a pre-amp, it’s an order of magnitude better than the amp stage, which theoretically is all you need. But once I disabled the bass boost, this is where things got ugly – the distortion with a 40 Hz test tone jumped all the way up to 1.42%! The 2nd harmonic sitting right around -37 dBc. That’s a 68 dB degradation in 2nd harmonic performance and a 747x degradation in overall THD! You could literally fly a Boeing 747 blindfolded through this wall of harmonics. In this case the amp stage has no choice but to amp up what it gets and is no longer the dominate noise contributor to this signal.

So what gives? Is my measurement wrong? Is this just a necessary evil given the implementation of the bass boost as a single convenient switch? Is my unit defective? I contacted Parts Express with my findings, gave them the details of the setup and everything I’ve shared here. I have not heard a response back. I was hoping to wait to post my review of this amp until they had contacted me with some kind of explanation, but alas that has not happened yet. So the post goes live. I am very eager to hear back from them. Personally would like to get in touch with the engineers at Dayton Audio to discuss this circuit design better. I’d like to think that this is not the intended function of the bass boost circuit, however I can see how it could be either overlooked or just thought of as not that big of a deal. Seeing as how the pre-amp stage, normally should be cleanest, is ruined by the amp stage, which is ultimately ruined by the driver. I have not taken a measurement of the distortion of the sub in-room to see if this difference is measureable to the listener. I am guessing that sub at any significant volume has greater than 1.42% THD so why fuss over what the pre-amp stage is doing? Which I get and that would be a plausible argument, though not a fair one and one I wouldn’t buy. Anyway, I’ll leave it at that and update this post when I do hear back from PE. I did tell them that that I was very happy with the amp otherwise and intended on keeping it, mainly because in this application the sub response was designed around having bass boost enabled all the time and in this configuration the distortion is at its lowest.

Parting Thoughts

I don’t want to end on that note, so I’ll end with this: the SPA300-D is a great plate amp and the RSS265HF-4 driver is a beast. And this combo absolutely kills it. After watching several movies with this sub in my bedroom and setting it up for music in my living room, I can easily say this subwoofer is one of the best subs I’ve built to date. It bumps, it rumbles and shakes the whole house and everything in it. It’s possibly more impressive with music, since this thing is basically flat to 30 Hz, once you corner-load this puppy and crank up some AJR, it just moves like nobody’s business. But even with movies, it’s no slouch. I’ll get some max SPL measurements shortly as well as some in-room measurements at some point. For now I’m just playing around with it in different locations in my house and seeing what it can do. Once I get a better feel for how this sub works and where it works best, I’ll get some additional SPL measurements. Until then feel free to check out the massive gallery of design graphics, response measurements from REW, pictures of the amp and sub and build pics as well as what this thing looks like in a couple of my rooms. Enjoy!

Design Graphics

Build Montage

Measurements

About Dan

For a complete biography, stop by www.danmarx.org and click around for a while.
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