Converting this $6 Goodwill Vizio Subwoofer into a Bumping Beast!

Last weekend I stopped by my local Goodwill hoping to find some audio-related goodies I could play around with. Lately it’s been slim pickins in the used stereo department with mostly junky old Blu-ray players and bits of pieces of all-in-one home theater systems (and those were never any good when they were new). This time I noticed a pretty nice Vizio subwoofer that looked like it had potential. It was at least in good shape. The woofer was hidden behind a grill cover that at the time I couldn’t tell if it was going to be easy to remove. But the nice flared port on the front and the compact size, and decent heft, made me think this just might be a worthwhile buy. It was listed for $12.99, but since it had a blue tag, it just so happened to be a blue tag weekend, which meant it was 50% off. So for a whopping $6.50, I decided it’d be worth taking a shot. If nothing else, the enclosure alone was worth at least that much and I could buy a new driver and amp if I absolutely had to.

Once I got this little subwoofer home I found a model number and looked it up. Turns out it is a Vizio S4251W-B4 Subwoofer that was part of a Vizio S3851W-D4 Soundbar Surround Sound combo back in 2014. I found the old Amazon listing for it and it got surprisingly good reviews. There were no specifications for the sub itself, other that a completely needless output rating of 100 dB. Thanks for that Vizio! So after tearing this little subwoofer down we find a pretty hefty 6″ paper cone subwoofer with a decently fat foam surround. A stamped steal basket and a good-sized magnet make up the motor structure with a label indicating a 4 ohm rating and a 90W power capability. Pressing on the cone revealed it moved just fine with no voice coil rubbing, so at least it didn’t appear to be blown. But man that spider and surround sound were stiff, like this sub has zero compliance. I should have set up REW to measure the T/S parameters (I may still do this later when I measure the amp) but for now I was just checking things out and so far it was looking pretty good.

The enclosure looked decent, made from just 1/2″ MDF, but I really like the long flared port that fired out the front of the cabinet. It reminded me of the old Bose Acoustimass subs from back in the day. Of course this sub just has a regular side-firing driver in a basic 4th-order vented design, but size-wise and visually looked comparable. The amplifier module is mounted in its own smaller enclosure, since they made no attempt to seal off this part of the box, so the subwoofer is in its own enclosure which was really handy for providing options for how we were going to power this thing. So being that it’s part of a soundbar, even though it has an internal amplifier, it doesn’t have any way to connect to it with a regular RCA cable. It’s supposed to connect to the soundbar which most likely uses some kind of proprietary connection. I tried to see if the sub would show up as a Bluetooth device on my phone by using the PAIRING button on the back but to no avail. It wouldn’t show up. Doesn’t really matter, I couldn’t have used it as a Bluetooth device anyway, it just wouldn’t have worked. I needed a direct connection via an RCA to be driven with a regular line-level signal like a normal subwoofer.

I thought there might be a way to bypass the wireless connection portion and get straight to the amp. After poking around this sub actually contains two small chip amps, as the sub was supposed to provide power to a pair of surround sound speakers too. I was able to make out the part number to be a TSA5713 which is a 25W stereo class D chip amp from TI. So let’s take a look at the datasheet and see what we have to work with here. Since there are two amps on this board, one is clearly used in stereo mode (BTL) to drive 25W to each surround sound speaker (assuming they were 4 ohm speakers), which would be connected to the sub via the RCA jacks on the back. The second chip to probably was wired in mono mode (PBTL) to drive the single 4 ohm sub to an unspecified power level. The graph that shows this configuration stops at 40W, just before the amp distortion curve spikes. I’m guessing it can provide around 50W which is respectable, but honestly not great. These little amps are typical of what you find in cheap sound bars and/or TVs and aren’t really anything to write home about. So with that I figured I wouldn’t waste any more time trying to figure out how to tap into a line-level signal and decided just to replace the amp entirely.

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Building a Notch Filter for the Beyerdynamic DT 770 Pro Headphones with Measurements

In this article I’ll go over how I designed, built and tested a passive notch filter for the Beyerdynamic DT 770 Pro 250-ohm closed-back headphones and include measurements using REW of the different filters responses. I bought these headphones over Black Friday weekend last year from Beyerdynamic’s website where they were doing a deal where you got a free pair of black Velour EDT 770 VB ear pads, a $40 value, plus the headphones were another $10 off their usual price of $169 and they did free shipping. I was in the market for some new headphones to replace my trusty old Audio-Technica ATH-M30’s that I’ve had for several years and I thought these would do quite nicely. That same weekend my son and I had stopped by Guitar Center and they had about 8 pairs of headphones on display that you could listen to with sample music of varying styles. We checked each of them out and after a quick listen it was pretty obvious the Beyerdynamic DT 770’s were the clear standout. Good bass, nice treble, clean and clear midrange, not to mention they were the most comfortable of the bunch. Before this I hadn’t even considered these headphones, and up until this point I had already picked up a pair of Sennheisers that were also on sale over Black Friday. Though I wasn’t able to A/B these two headphones directly, I decided to return the Sennheisers and picked up the Beyerdynamic DT 770 Pros instead. A decision that would later send me down this rabbit hole of researching filtering and EQ options in order to make these headphones actually listenable for the long term.

While these headphones do sound great overall, they suffer from one major flaw that could be a deal-breaker for most – they are extremely bright! Doing a direct comparison to some of my other headphones, these suckers have got some highs that seriously sparkle, and it’s not subtle. People say that this is just that classic Beyerdynamic sound and I will say that for some tracks, this does sound fantastic and, in short durations, is manageable. But in general, most songs sounds bright and harsh, making long listening expeditions not that enjoyable at all. In all other regards however these headphones sound amazing and they are super comfortable (even while wearing glasses, like I do) so after giving the DT 770s a fair shot straight up, I decided I had to figure out how to EQ these things into submission. Fortunately there’s a ton of info for these headphones on the internet, and I quickly realized I wasn’t alone in my perception of these headphones. And while there are plenty of reviews that will talk about how these headphones sound, I found three major sources to be the most valuable in providing raw measured data that enabled me to tackle this treble problem a little more scientifically.

Naturally my first stop was Rtings.com. Without question, one of the greatest resources for objective reviews of headphones and TVs. They give the DT 770’s an overall Neutral Sound score of 8.3 which is quite high and favorable. Raw frequency response measurements confirm what everyone can hear, that these headphones are extremely neutral (in terms of matching the Harmon target response) from 20 Hz up to almost 6 kHz and therefore have excellent lower-bass response and mid-bass response up through the midrange, but above 6 kHz is where things get a little wonky. The treble climbs well above the target by over 10 dB and peaks at around 8.2 kHz. Even though they provide frequency responses for all their headphones, they don’t offer filter or EQ suggestions on how to improve them. So you’re kind of on your own to figure out how to fix anything that looks amiss. But this is a good start.

That’s when I ran across another incredibly valuable resource for headphones, plus a second set of measurements for the DT 770s, and that was from oratory1990 over on Reddit. oratory1990 has measured response plots for the DT 770s including EQ options to bring them into alignment with the Harmon target. This is where for the first time I downloaded and played around with Equalizer APO and got my first taste of the possibilities for active EQ in Windows. I entered in the EQ settings he recommended and WOW, what a difference! For the first time these headphones sounded absolutely amazing all the way through, that overly bright treble, the sibilance, the shrill, was all gone. I couldn’t believe it was that easy and with that I had figured I had my solution. While I wasn’t a super fan of having to use a software-based EQ, it was powerful enough, and seamless enough, so, why not?

Well, I quickly learned why not, at least for me, that this solution was not a long-term fix. I planned on using these headphones in my mini-budget home recording studio and I quickly realized that Equalizer APO does not work with ASIO drivers and it does not work with live line-in sources, like a piano, microphone or guitar. Basically it doesn’t work with Reaper (my go-to DAW), as it does not EQ anything recorded in it or played through it or anything live via the line in. The EQ only applies only to pre-recorded music played directly in Windows, like with Spotify or YouTube. I thought I could build a similar filter in Reaper using ReaEQ, but quickly learned how much of pain that was as well. Not mention, the biggest gripe about using a software-based EQ was having to turn it off any time I switched to using speakers. So with the Equalizer APO not working for most of my intended applications, I ended up searching for another solution. Now if your setup is more dedicated to just playing back music, or you don’t have speakers, then Equalizer APO is fantastic and I found it to be super useful.

So then last but not least, I came across DIY-Audio-Heaven and as the name suggests, I was in heaven! I have to give mad props to Solderdude who basically inspired this entire project by being the first person (as far as I could tell) to offer a fully passive solution to fixing the Beyerdynamic treble peak. And it’s so simple! I don’t know why I didn’t think of this before. As someone who has designed and built so many freaking speakers, with crossovers, and filters, even notch filters, I don’t know why the idea to passively correct the DT 770s didn’t come to me sooner. But Solderdude absolutely nailed it with his notch filter design, and not to jump ahead too much, this one fix literally solved the treble peak issue with these headphones and has made them sound just incredible. Amazingly enough, he provides passive filter recommendations for hundreds of headphones, not to mention provides measurements and reviews of hundreds of headphones, for your reading pleasure. As it turns out, a lot of headphones that are bright, or sound bad for a myriad of other reasons, can be improved significantly with a single passive in-line filter. And he’s got options for how to fix a lot of them. Not only does he provide this as a service, where you can buy them from him, he basically gives you the design, so if you’re a little more into DIY, then you can tackle this project on your own pretty easily. This was all I needed to start down this path of how to passively EQ the DT 770 Pros to make them sound better.

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Measurement & Review of the AIMIYA A07 Power Amplifier

A few months ago I purchased the AIMIYA A07 Power Amplifier from Amazon for a whopping $60 during Prime Day. I’ve had the unit hooked up to my simple studio setup and have been very impressed with it so far. The performance-to-cost ratio is pretty remarkable. It’s worked flawlessly over the past months and I have been very happy with it. I currently have it hooked up to my modded Jamo’s and it sounds absolutely amazing. It has plenty of volume and never seems to run out of power. So today I finally decided to pull out my measurement gear and so I could get some real data on its performance, to see just what this amp is capable of. I know there are plenty of other reviews out there for this amp, part of the reason I purchased it in the first place, so I don’t think I’ll be sharing anything earth shatteringly new. But I enjoyed putting this thing on the bench and getting some data of my own just the same. So read on to see how she performs!

The heart of the AIMIYA A07 is a TI TPA3255 High-power Class D chip amplifier. The datasheet for this part can be found here https://www.ti.com/product/TPA3255. This is a very popular chip amp and has made its way into many small home audio amplifiers from a variety of manufacturers. However, worth noting is implementation of this chip is still up to the designer and AIMIYA is no slouch in this realm touting a long line of amplifier options to suit almost anyone’s need. Overall output power is dictated mostly by how much raw DC power you can provide. AIMIYA supplies a very respectable 32V/5A power supply with the unit which for most people will be plenty, as I’ll show here. But note that that amplifier has more to give, if you’re willing to spend a little more for a larger, more powerful DC power supply. And based on the measurements I took with the stock power supply, I believe more power available in the TPA3255.

So a quick re-cap of my measurement setup (which take it for what it’s worth) is based on a simple loop-back measurement using REW, a PreSonous Firestudio audio interface and a custom balanced amp-level to line-level converter/load box that I designed and built for this exact purpose. I’ve played with this setup quite a bit and have basically got it dialed in as best as I can to take as accurate as possible frequency response and distortion measurements of amps that range from 10W to 400W watts. The Firestudio interface is calibrated (via soundcard cal) and is able to provide an overall THD of only 0.00074% and a THD+N of about 0.0063%. While probably not great for measuring high-performance DACs or headphone amps, this setup is fine for measuring most power amplifiers, or at least most of the amplifiers that I’ve come across, as their distortion levels are almost always much worse that this. I’ve included plots of the baseline loopback measurement of just my setup so you can get an idea of where the noise limitations exist. From this you can see that all of the amplifier measurements are higher than the baseline and are therefore going to be fairly representative of truth for this amp. Not to mention nothing I measured drastically contradicts anything already out there already showing what this amp can do.

So first let’s just look at the frequency response of the A07. One of the easiest things to measure, since REW was basically designed to measure frequency response of just about anything. I limited my measurement to 5-22,000 Hz which shows a very nice and flat response from 20-20,000 Hz where the response is down only -0.3 dB at 20 Hz and down -0.8 dB at 20 kHz. This was measured into a 4 ohm load. I did not do any measurements into an 8 ohm load, as I figured most speakers people are going to use with this amp are going to fall into the 4 ohm territory. Including the Jamo’s that I am current using with it. Basically you can assume the total available power will be lower, and the frequency response will peak slightly before cutoff, but it should be minimal. Overall this frequency response is excellent for a Class D amp and represents a decent filter design by the folks at AIYIMA. Take note that the response does start to roll off at around 5 kHz which when combined with a speaker that if normally sounds a bit bright, this roll-off will mellow it out just a bit, but not much, and will likely go unnoticed. The roll-off in the bass response when combined with the type of speakers that most people will use with this amp (ie, small bookshelf speakers) should also go basically unnoticed. That being said, I’d say this amp is about as neutral as you’d expect and should sound great with a majority of speakers.

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Inexpensive Crossover Upgrade for the OSD Nero MB5 Bookshelf Speakers

So I picked up a pair of OSD Nero MB5 bookshelf speakers about a month ago and after giving them a listen and measuring their frequency response, I decided these speakers need an upgrade! In a previous post I updated a pair of Jamo Concert 9 Series speakers where we tackled internal damping, port tuning and the crossover. In this update we’re just going to be tackling the crossover with just one very minor change to the damping. First off I think the Nero MB5’s are great little speakers. I got them on sale over at Crutchfield for only $99. Amazon typically has them for around $159. As of this writing they are still on sale at Crutchfield and for this price, in combination with the updates we’re about to go over, these speakers honestly cannot be beat. These are essentially the passive version of the Nero AB5 powered speakers, ditching the internal amplification (and any DSP) for a simple passive solution with a passive crossover. They even left the LED for the logo on the front baffle, but it isn’t connected to anything.

They have the look of a nice studio monitor, but without the finesse or feel. The box is light, lacks any real bracing and any significant damping, save a single piece of poly fill that covers about half of each side and the top. The soft dome tweeter is framed in a very good-looking waveguide which provides a classic 1st order gain response as the frequency decreases, but any benefit here is not really realized due to the paltry 6 dB/octave filter on the woofer which lets an excessive amount of cone break-up modes to enter the equation. The dual ports provide a tuning frequency of about 60 Hz which is fantastic for a speaker this size. The little 5-1/4″ woofer can produce an impressive amount of bass and depending on how close you set these against a wall, the bass can either be just right or way too much. Don’t expect miracles here, but the bass response is quite punchy. Some port noise (chuffing) can be heard for bass-heavy tracks that don’t have a lot of other music to mask it. But is bearable.

At first listen I really enjoyed these speakers, there was nothing that appeared completely wrong or unmanageable. After a quick measurement with REW and my Behringer ECM8000 mic, we start to see some of the issues this speaker actually has. Measured on axis at about 0.5m directly in front of the speaker midway between the woofer and tweeter, we see a 6-7 dB peak around 5 kHz that screws up any hope of expecting these speakers to sound flat or neutral. Note that this response will look different depending on listening position. Move up or down or left or right and this peak is less predominant. However for purposes of this crossover upgrade, the intent was to make the speaker flat on-axis and ear-level being directly between the woofer and tweeter. As it stands, these speakers will sound a little better either above or below the midpoint of this speaker. But for now let’s open this speaker up and see what makes it tick and see if we can figure out what is causing this uneven frequency response and what it might take to make it better.

Disassembly of the Nero MB5 is pretty straightforward. There are a total of (12) 4mm hex wood screws holding the plastic front baffle and terminal cup/crossover. The only real trick is there are two smaller phillips-head screws hidden underneath the 3M EVA vibe isolation pad in the front left and right corners. You have peel back the foam over about a 1″ x 1″ area to access the screw to remove it, thus releasing a plastic tab attached to the front baffle. This seems holey unnecessary and just makes removal of the front baffle trickier. See pics below for an idea of what this looks like. Once that’s out of the way, the front baffle and as it turns out, the tweeter assembly, come off together. The 5-1/4″ woofer is revealed and can be removed as well. After disconnecting the drivers we can remove the terminal cup and crossover assembly and get our first look at what kind of magic the folks at OSD came up with to tame this beast. Turns out to be just another typical cheap crossover consistent with this type of speaker at this price point. The low-pass filter duties are handled by a basic iron-core inductor creating a simple 1st order, 6dB/octave filter network. Boring, and, ineffective.

I have, in my experience, never found a 1st order filter to be sufficient for a low-pass design to suppress cone break-up modes to a manageable level. The best you might get is with a poly cone as they tend to have slightly better break-up modes, but OSD touts this is a “composite fiberglass woofer” and you aren’t breaking any laws of physics here. Once you get above the fundamental 1st mode of this material, it’s going to resonate, its going to break up, it’s going to like being excited at these frequencies and will respond in kind. If you’re gonna design a speaker and expect to get away with a 1st order filter (which by the way can be turned into a 2nd order filter with a cheap-o 79-cent capacitor), you better be prepared to work out some exotic cone material with specialized composites for better damping to control those break-up modes or you’re going to be fighting a losing battle. 1st order filters just don’t have what it takes to be effective in this regard. So that’s the first thing we will fix. The woofer measured about 3.6 ohms at DC and therefore represents a solid 4 ohm speaker. See the impedance plot of the woofer measured in the box below.

Moving on to the tweeter, we have a 2nd order high-pass filter made up of a 1.2 uF poly cap, a 0.24 mH inductor and an 8 ohm series resistor. This actually sets the crossover frequency to be fairly high, you’d expect a gap going from the woofer to the tweeter, however that’s where the waveguide comes in and does its magic. The waveguide is providing about 6 dB/octave gain all the way down to about 2 kHz. When combined with the filter response, you end up with a decent overall frequency response that actually combines really well with the woofer. And the benefit is that this gain comes acoustically, so the tweeter is able to do less, i.e., less distortion, and still provide a decent output. The only adjustments needed here will be to match the crossover point and level with the woofer once we add the 2nd order filter to the woofer. We’re going to make only a small adjustment here, as I felt like the raw tweeter response with the waveguide was quite good with no serious defects. In which case all we need to do is set the crossover point so it sums well with the woofer and adjust its level so it’s not too bright but not too dull. Special notch or other shaping filters won’t be needed here.

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Big Brother Has Arrived – The Sonorous 12S Subwoofer Build

Earlier this year I built the Sonorous 10S subwoofer and have absolutely loved this subwoofer. So when the opportunity presented itself to build a second bigger version of this sub, I knew I had to do it. And so I introduce the Sonorous 12S Subwoofer! This sub is identical to its littler brother with the exception of the use of a Reference Series 12″ Dayton Audio RSS315HF-4 subwoofer driver and an enclosure that is 10 liters bigger. This sub uses the same Dayton Audio SPA300-D subwoofer plate amplifier which pushes a solid 300 watts. If the 10″ version was an awesome little subwoofer, the 12″ version thing takes it to the next level. Even when driven with the same mount of power, the Sonorous 12S has a touch more extension and just about 2 dB more output than the 10S subwoofer. But for the most part these two subs are nearly identical. This post won’t got into nearly as much detail as my previous post since the amplifier is the same, and most of that post was about the amp, but I will talk through some of the differences as compared to the little 10S version.

First let’s get straight to the technical brochure for the Sonorous 12S Subwoofer:

Dayton Audio RSS315HF-4 12″ 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, 35L Enclosure (1.48 cuft.)
System Qtc = 0.707 (critically damped, perfectly damped)
f3 = ~44 Hz (Bass Boost OFF)
f3 = ~26 Hz (Bass Boost ON, +5 dB at 30 Hz)
Max SPL = ?????? (measurements to come….) mathematical limit at 300W is 111 dB
15″ x 15″ x 16.25″ (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
1/2″ Radius Cosmetic Roundovers (running front to back)
(4) Large Rubber Feet
5 Year Warranty on amplifier and driver (from Dayton Audio)

If you read the datasheet for the 10S subwoofer, you’ll notice that both subs share a favorable amount of details. Mainly because the amplifier is same across both subs. Honestly this was a decision I struggled with though. I kept tossing around amplifier ideas that pushed more power, I really wanted at least 500W. The Yung SD500-6 was an obvious contender as it had the raw power to push the sub to its limits and the 25 Hz bass boost to keep the response flatter in this tiny 35L box. In fact the 25 Hz boost modeled a bit better than the 30 Hz boost, but in the end I couldn’t justify the $100 price increase for an extra 200W of power at the expense of a solid 5-year warranty, a defeatable bass boost, no turn on/off thump and just good old fashioned Dayton Audio quality. Nothing against Yung, but with plate amps, this thing becomes a part of the sub, and if down the road I did have to replace it, it would mean having to figure out how to cut a new hole or fill this hole to make a new amp fit. I figured this way I’m covered for 5 years no matter what. The only other option that was tempting was the SPA500DSP amp which also offers a solid 500W and offers a complete DSP solution so tailoring the bass response would be right at my fingertips. In the end this option was also ruled out simply due to cost. This option adds significant cost to what otherwise is a pretty affordable subwoofer. Money-no-object scenario though, that would be the amplifier to get with this subwoofer in my opinion. External amplification can also work just fine, especially if you can do some DSP.

Up until now I had been using the 12″ RSS315HF-4 subwoofer for Garage Theater duties in a 95L ported enclosure with a 450W amp and that combo was awesome. I ended up swapping out that sub for an older TC Sounds dB-500 sub that was in my daughter’s car that she is no longer using. So after playing a little bit of musical subs, that landed me this 12″ bad boy to play around with. Since I really liked the way the 10S version sounded in my living room paired with my DM-4 mains, and since that sub was moved to my bedroom, I thought it would be really cool to have a dedicated sub for that system. That way for those times when I really wanted to crank up some tunes with bass, there’d be plenty of bump to go around. Plus I really liked the look of that small, sealed, white enclosure with that single black aluminum cone driver emanating on it its front baffle. I knew I could make a bigger version and have it sound at least as good, while still not taking up a ton of real estate, with the RSS315HF-4 12″ driver.

So with the the driver and amp in mind, I designed up the enclosure. In a similar fashion to the Sonorous 10S, I wanted this sub as small as possible, so naturally a sealed option made sense. And with the +6 dB bass boost of the SPA300-D, I could buy back some extension that’s always lost using a compact sealed design like this. But even without the boost, this sub models/sims great. Being a bit of a perfectionist, I went for a perfectly critically damped Q of dead on 0.707 and was able to design the box to be exactly 34.7 liters. This offers an ideal response that is neither underdamped nor overdamped. Though actually turning on the bass boost does mess with the total system Q overall, this is a great starting point to be at as far as volume goes. F3 is around 44 Hz without the boost and about 27 Hz with the boost enabled. The roll-off below 27 Hz is quite steep, due to the high-pass filter in the SPA300-D. It’s roughly -6 dB down at 20 Hz with a 12 dB/octave filter. Combine that with the natural 12 dB/octave roll-off of the sealed enclosure and the response is rolling off at a combined 24 dB/octave. This is apparent in the excursion plots where you can see that at no time is the subwoofer at fear of over-excursion, no matter the source frequency. So that’s kinda nice as I’m always afraid of blowing my subs from too much power.

The enclosure design is pretty basic but has some unique qualities. It’s made from 3/4″ MDF and finished in 1/4″ MDF for a total panel thickness of 1.0″. Four 3/4″ x 3/4″ corner cleats run front to back to strengthen these joints. Additional panel bracing was added to all four panels in the form of 3/4″ x 1.5″ ribbing pieces. This ribbing is then cross-braced to opposing panels using 3/4″ x 2″ MDF braces that are interlocking in 4 locations. All of this bracing does take up some additional volume but was calculated into the initial design from the start. The overall enclosure sits at 15″x15″x16.25″ (LxWxD). This size looks absolutely sweet with that massive 12.4″ subwoofer on its face. This was absolutely deliberate leaving a mere 1.3″ from the edge of the driver to the edge of the cabinet. The driver is also perfectly centered too to provide a pleasing aesthetic. The top and bottom edges are rounded over at 1/2″ front to back while all other edges are simply sanded down lightly to take off the sharp corner. This is opposite to many of the Dayton Audio cabinets which round-over the front and back edges, top-to-bottom. I just think this way looks better but is up for personal taste. Rounding over the edges is purely cosmetic and provides no other sonic benefits given that the upper frequency of this sub will likely never exceed 100 Hz. Making the box slightly deeper than its width/height is also somewhat cosmetic as I think it looks better, but is also to allow the box volume to increase just to under 35 liters.

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Simulating Subsonic Filters and Subwoofer Response-Excursion Effects

So this is going to be a Part II to my original post from 14 years ago where I first discussed the topic of proper subsonic filter design in conjunction with subwoofer enclosure modeling. It’s been a while, but I recently got around to actually doing what I said I was going to do all those years ago. While my specific example incorporates this super-old (and no longer available) EV XEQ-2 crossover, the methods employed herein are practical for anyone looking into subsonic filter design, how it works, and the best way to incorporate it into you next sub project without sucking the life out of your bass. And more recently since the availability of online, web-based simulation tools, such as National Instruments MultismLive, taking the guesswork out of active filter design is a thing of the past. So let’s get into it!

I’ll do a quick re-cap of my previous post on this topic, but the main idea behind this post is to show how to design subsonic filters and then how to model them into a subwoofer design before you even buy or build anything. The two main tools for achieving this are NI’s MultismLive and Unibox 4.08. Unibox is a bit dated for speaker enclosure design and runs in Excel, which not everyone uses, but is my favorite box simulation software of choice. I took a stab at using WinISD to do this, but it doesn’t provide the option to import .frd files to create custom filters. You can create simple filters using the built-in filter tool, but is limited to certain prescribed filter types. Plus it’s not obvious how to go from the simulation to a real-world active filter, unless you’re building a system with DSP. Unibox 4.08 on the other hand will let you import .frd files which can have any shape, slope, Q, response, etc. you can come up with.

So with that let’s start out with this simple example, which is based on a sub I built several years ago, so we can see how it all plays out. First we’ll start with a simple simulation of the driver and enclosure. In this case I’ve got a Dayton Audio RSS315HF-4 in a 95-liter enclosure tuned to 22 Hz. This looks pretty good in the simulation, has a decent f3, a nice and flat in-band response, without any peaking or drooping, but if you look at the excursion below 19 Hz, you can see that Xmax is widely exceeded, as is the assumed Xmech. Regardless of the driver and enclosure, this is a pretty common behavior that will occur with any vented design. Passive radiator designs are bit better and sealed designs are even better yet, but they too can benefit from a properly designed subsonic filter to accompany them. So even though this sub is already tuned pretty low, if you didn’t want to implement a subsonic filter, you could argue three points: 1. there is little source material that is actually going to push the sub to those levels below 20 Hz and 2. most if not all amplifiers are not even flat 20 Hz, meaning below 20 Hz the amp is starting to roll-off anyway, thus limiting the actual excursion at the driver and lastly 3: eh, it’s just modeled data, who really cares? All valid, but flawed points on their own. A subsonic filter should be implemented to keep the driver under control below tuning to prevent possible driver damage, lower distortion, and keep the amplifier happy (i.e., possibly going into protect mode during playback and interrupting your movie/music).

The biggest problem with subsonic filters is by design, they limit excursion, and in doing so if not set to the correct frequency, will limit the output at and above tuning, which we do not want. Steeper slopes can help, but also make the filter more complex. In this case I am using a fairly simple 12 dB/octave active filter that requires only one op-amp to implement. So how do we determine where to set the corner frequency of this subsonic filter? Well, that’s where MultismLive comes into play. I built a model of the exact crossover that makes up the EV XEQ-2 and ran an AC simulation which lets you see the exact response of the filter. I show that circuit here, which is made up of an input buffer stage, which can be set just about any gain with no EQ, there’s an 18 dB/octave low-pass filter which can be set to any frequency desired, and then there’s the 12 dB/octave high-pass filter (the subsonic filter) which also can be set to any frequency and any Q desired. The input stage is unity gain, and provides a good buffer stage to the source, the LPF stage is set to about 90 Hz, but can be adjusted as shown with three resistors and last stage is the subsonic stage which is set by only two resistors. Depending on the values selected, you can actually create a bass boost circuit, which can be used to model bass boost circuits of common plate amps to see how they will sum with your enclosure, which is what I did with my Sonorous 10S subwoofer. But for this vented sub box, we want zero boost, in fact the ideal roll-off is where the Q=0.7 which can be achieved by changing R9 and R11 at a ratio of 1:2.

So I played around with a bunch of resistor values for gain, LPF and HPF and ran an AC simulation for each one to get an idea of different frequency responses that can be achieved. You can click on this link here and play around with the simulation and model some filter cutoff frequencies of your own. But we’re not done yet. MultismLive lets you export the data into a .csv file which this is where the whole design gets interesting. With that exported file, we can now import that into the Frequency Response Active Filter section of Unibox and actually see how the HPF affects the box response and how it affects the driver excursion. I created about 10 different filters, and imported each of them into Unibox and then compared them all see which one looked the best. And by looked the best, basically what I was shooting for is best control over excursion below tuning, but with minimal impact to frequency response above tuning. Go too low, and excursion is still too high, but FR is minimally impacted, go too high and excursion looks great, but FR suffers. The balance is basically somewhere in the middle. By plotting all of the options, we can better choose where we want this subsonic filter to sit. But even Unibox has limitations, one being you can’t compare more than 6 responses, and you can only compare FR plots and nothing else. So I exported the raw data from Unibox into my own worksheet in Excel where I plotted several configurations and overlay excursion and FR plots on the same graph. I’ve shown that here for this example design. Notice that a filter corner (-3 dB) of 20 Hz does the best at keeping excursion in check, but also has the most impact on FR. The 12 Hz corner is better than no filter at all, but still allows over-excursion. The sweet spot is probably right around 15 Hz or 17 Hz. So that’s the optimum subsonic filter corner for this subwoofer. And that’s the design. Short and sweet.

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