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Pushing the limits of small speakers - The Reference Mini build thread
Pushing the limits of small speakers - The Reference Mini build thread
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Old 14th March 2017, 12:30 AM   #1
bcodemz is offline bcodemz  Canada
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Default Pushing the limits of small speakers - The Reference Mini build thread

This is gonna be a very different thread that big speakers DIYaudio typically builds. This is going in the other direction, and my curiosity to see how far can you push the performance of small speakers. I think you'll be surprised just how unbelievably good a small speaker can be. I started DIY because I am away from my home for extended periods of time and I need a great sounding small speaker to bring with me. Turns out, you can make a small speaker sound just as good, if not even better than large speakers. So, I decided to just go all out and skip the expensive upgrade cycle. This is a documentation of my journey of curing "upgraditis".

Design goal: Build the most accurate, highest fidelity, and highest bass output small speaker possible without any budget limitations.

Shout out to JavadS and Kevin K for being incredibly gracious in helping me out with the construction of this speaker. I couldn't thank them enough for bringing this project to life. Thank you to Josh Ricci and many others that gave a hand in the design of this speaker.

Forget about any preconceived notions of small speaker. This entire project is about figuring out how to get all of the normal sized bookshelf/floorstanding speaker performance from a small speaker. Originally it was thought it will still be inferior and a compromise to normal sized speakers, but turns out it is possible to do significantly *better* than normal sized speakers.

Design summary: this speaker is 6.25" x 11.5" x 7.5" HWD. It is a 3 way, 4 driver, 2x passive radiator internally amplified active DSP design. It is specially designed so it can be used horizontally or vertically with no performance degradation. A pair can play over 100dB at 40Hz and 110dB when paired with a sub.

Here's a little video demonstrating its bass capabilities.

YouTube

Tweeter: Scanspeak Illuminator D3004/6040-10 1'' Beryllium Dome Tweeter
Midrange: Scanspeak 10F/4424G 4'' Neodymium midrange
Woofer: 2x Custom Audio Technology 5.5" woofer
Passive radiator: 2x Tang Band PR14 oval passive radiators tuned to 42Hz
DSP: MacBook Pro with JRiver
Amplifier: 2x ICEpower 100AS2

Here's a picture of the test enclosure to give you an idea of what the speaker looks like. There is a second woofer behind the speaker that you don't see in this picture.

Click the image to open in full size.

I'll start by listing out the features of this speaker that makes it superior to most normal sized speakers. This is very long. Read the paragraphs below each feature if you're interested for the details.

Controlled directivity with cardioid radiation pattern

You can have the perfect speaker, but put it in a room and the room acoustics will muck it up and prevent the speaker from performing its best. Therefore, controlled directivity is a must to achieve a very high level of performance in a room to reduce the room interactions. This speaker uses a heavy DSP approach to achieve controlled directivity instead of using waveguides. It uses multiple drivers playing variable phase shifted signals to achieve a cardioid radiation pattern down to around 100Hz. A waveguide that can do that would need to be several feet wide!

This is more important in a small speaker because the baffle step loss happens quite high, close to 1000Hz, and the distance to the wall would drastically change the sound of even a properly designed small speaker. By controlling directivity down to ~100Hz, there is no need to worry about the varying baffle step loss depending on the distance to the wall since the sound is only radiated forward.

A 3 way that acts and measures like a single driver point source speaker

Unlike pretty much any multi-way speaker, there is no horizontal or vertical lobing error. Two sound sources couple as one when spaced between 1/4'' of their wavelength.

By using crossover frequencies equivalent to 1/4'' of the wavelength of the CTC distance between the drivers, the 3 drivers acts as a point source, and we get the massive advantages of multi-way and point source without the disadvantages of both (or coaxials).

In order to do this, the spacing between the tweeter and midrange must be minimized to the extreme. The frame of both the tweeter and midrange are truncated in order to place them as close as physically possible to achieve a CTC distance of about 2.2", which still dictated a 1300Hz crossover. This tweeter has exemplary distortion performance, and with a steep crossover the tweeter can be made to handle this.

Stunning bass extension and SPL for its size

As a bass head, a huge priority for this speaker is to get as much bass out of a small speaker as possible. I want the speaker to play so deep that Adele can roll in it! This is perhaps the most fun part of designing (and listening) to this speaker, and perhaps the biggest shocker when people hear this speaker. The result? This 4L speaker should produce 96-111dB from 40-100Hz, 1M outdoors! Add 3-6dB for a pair (gain depends on distance between the 2 speakers and the frequency), and another 3-6dB when placed in a room! I expect a pair to produce a peak of 105-117dB of bass from 1M away in a typical room. Small speakers can't have bass? This one should be able to outperform some floorstanding speakers.

To do this, a custom woofer is needed, and Audio Technology has been fantastic to work with to make a custom driver. Most drivers that are good for high output deep bass have low sensitivity and does not sound good up high. So Audio Technology made me a driver with high sensitivity (90dB) that can also do deep bass. It also has a very low Fs with a large 2" voice coil for extra power handling needed. The voice coil is made of a combo of kapton and aluminum to reduce eddy currents that cause distortion for better sound quality. The frames are truncated to reduce size to fit the biggest woofer possible in such a small speaker. This driver made it possible to maximize both low bass and upper bass/low midrange output.

Multi-band compressor

In order to get 38Hz extension from this tiny speaker, up to 17dB of bass boost is required. This is usually problematic because the speaker quickly runs into excursion limits, and very much limits the maximum volume for the speakers to be safely played at (by as much as 17dB). Therefore, a multi-band compressor is used to limit the woofer's deep bass output to below its limit without affecting the rest of the woofer band. This allows me to get the most out of the speaker at any volume by having very deep bass extension at normal to high volumes and automatically limit bass to prevent distortion when the volume gets really high.

Equal Loudness Contour

I want a speaker that sounds its best at ANY volume. I want to have full sounding bass and flat treble at low/normal listening volumes and a reduced bass and treble at high volumes to prevent listening fatigue from the ear's increased sensitivity to treble at higher volumes. The solution? A dynamic EQ (sort of similar to Audyssey's Dynamic EQ) that automatically adjusts the bass, low midrange and upper treble levels depending on how loud the speaker is playing. Notice the midrange and lower treble is kept flat. This means the bass and treble is gradually reduced as the volume increases. Low volume listening is very full and enjoyable while high volume listening is stunning and fatigue free. One dangerous aspect though is that the speaker is so smooth this way that you can play much much louder without even realizing it, causing hearing damage, potential speaker damage, and pissed off neighbors. Screw that, I like my music loud and clean!

Superb dynamic performance

Small speakers have restricted dynamics and can't play loud right? Not this one. The high efficiency of the system (93dB on the woofer, 90dB on the midrange and tweeter) and active crossover with powerful amps allow this speaker to play incredibly loud and dynamic. A pair should be able to produce peak SPL of just under 115dB 1 meter away. Unbelievable? Well, this speaker is really a 3 way, 4 driver floorstanding speaker packed into a small speaker. All that extra air space in a floorstanding speaker just increases the <150Hz efficiency of the speaker. I can achieve nearly the same bass performance in a small speaker, I just need way more power.

By using 3 way, there is much less intermodulation distortion than a 2 way simply because the woofer isn't asked to do large excursions and play midrange at the same time. By using passive radiators, the woofer's excursion is greatly reduced for bass. This improves efficiency as speakers lose Bl under greater excursion levels as well as face a stiffer suspension with higher excursion levels. Both leads to reduced sensitivity at high volumes and a loss in dynamics.

Virtually no cabinet vibration or resonance

Cabinet resonance is an interesting issue. Unlike what intuition would tell you, a thicker wall causes bigger resonance issues! More accurately, a thicker wall causes the resonance frequency to shift higher up in frequency, and for normal thicknesses (3/4", 1"), this falls in the crucial midrange frequencies. Bracing simply pushes the resonance frequency higher.

So, the solution, is actually to use a thinner wall. In fact, for a 1/4'' wall, the resonant frequency is around 100Hz, according to BBC's research. This is completely out of the midrange band, so the midrange enclosure suffers almost no audible resonance issues within its band. For the woofer enclosure, the midrange enclosure acts as bracing, pushing the resonant frequency up above the woofer's band. Therefore, we have a box that has no resonance issues.

Even though the wall is only 1/4'' thick, the dual opposed woofer configuration cancels out almost all of the mechanical cabinet vibrations. Same for the 2 passive radiators. This way all their mechanical forces cancel out, leaving a vibration free cabinet.

The woofer enclosure is also too small to have standing wave problems, so only a small amount of Acousta-Stuf material is used, and they wouldn't be effective for <400Hz anyways. It is mainly to absorb the harmonics. A layer of Fatmax dampener is lined around the cabinet. Fatmat is lined around the woofer enclosure to provide extra damping to reduce sound from leaving the box. The midrange chamber is actually made of a molded ABS plastic that is even stiffer than plywood while being thinner to save 7% of the enclosure volume. It is stuffed with Acousta-Stuf to absorb as much of the rear wave as possible.

Minimal baffle diffraction

With the combination of asymmetrical driver spacing to nearby edges for all drivers, as well as wool felt treatment on the front baffle, this speaker should have very minimal amounts of diffraction induced frequency response ripples. The speaker grill is also treated with wool felt on all edges and hard surfaces, so this speaker can have a grill on without affecting sound quality.

Linear phase and time alignment

The use of such low crossover frequencies necessitates a steep crossover, but there is a price paid for the nasty phase shift that comes with steep filters. By using FIR filters in the DSP, I can have a linear phase 4th order (or 8th order if I want) crossover. Additional processing can be done to reduce the pre-ringing caused by the steep linear phase filters. Time alignment is trivial to do on a DSP. The result is a time (mostly) and phase coherent multi-way speaker.

And there it is! I spent way too much time and money on this. But with everything this is a speaker that should outperform most traditional "Hi-Fi" style speakers. I should check myself into an addiction centre after this .

Next post will have some pictures of the build progress.

Last edited by bcodemz; 16th September 2017 at 11:11 PM.
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Old 14th March 2017, 12:30 AM   #2
bcodemz is offline bcodemz  Canada
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Finally, let's get to the details of the speaker components.
Driver selection:

Tweeter: Scanspeak Illuminator D3004/6040-10 1'' Beryllium Dome Tweeter

This is the finest small format tweeter available with exceptional distortion performance. While very expensive, at $250 (from Solen) it is actually cheap for a Beryllium tweeter and it performs just as well as the large (and much more expensive) Scanspeak Beryllium tweeters. It has good sensitivity, dispersion, and exceptionally low distortion down to 1000Hz. Best of all, all that performance is in a tiny 2.44'' face plate tweeter that can be further cut down to ~2'' to reduce size even further.

Midrange: Scanspeak 10F/4424G 4'' Neodymium midrange

This is the best sounding ~3.5'' cone midrange available, and the largest midrange I can accommodate to achieve a 1/4'' wavelength crossover frequency with the tweeter. While there are 3'' domes, they are all bigger than the 10F, and it must be crossed much higher at around 600-1000Hz when I want the midrange to be crossed low closer to 300Hz to take advantage of the clarity of the midrange as much as possible. A woofer capable of the same clarity as the Scanspeak 10F would be extremely costly and may not be suitable for bass duties. Even if I did use a driver of that quality, the woofer compartment won't be fully stuffed like the midrange chamber to absorb the rear wave, which will result in an inferior sound.

Woofer: 2x Wavecor WF152BD05 6'' Glass-fibre woofer

A *TON* of work was put into this task. The woofer search gave me a lot of trouble due to the sheer number of choices and trying to find one suitable for bass. This is a classic compromise problem of bass and midrange clarity. Not only many of the high quality and high fidelity 5-6'' midwoofers do not reproduce bass very well at high excursion levels, most are not designed for small enclosures and have "high" Vas and low Fs, both are not "ideal" for small PR enclosures, which leads to very poor PR efficiency and less overall bass output of up to 3dB for the same input power. A driver that is ideal for high output bass reproduction, such as the Tang Band W5-1138SMF, does not have high fidelity midrange, which would be unacceptable given the best in class sound quality of the other components.

I almost gave up and just used a high fidelity woofer and sacrifice bass output, but after some serious digging, I found a little known driver from Wavecor that has almost the best of both worlds. It had pretty much the ideal T/S parameters suitable for my design, while having extremely good sound quality comparable to Scanspeak Revelators. The result is a woofer that is 2-3dB more power efficient in the 40-55Hz range than nearly all high fidelity midwoofers, and just 1dB less than the W5-1138SMF, the ideal miniature subwoofer driver for this application, and avoiding the 7dB less midbass because of the dreadful sensitivity. A high efficiency pro driver has great midbass output, but not nearly enough excursion for low bass. The Wavecor does it all, high sensitivity, high Bl, high compliance that results in about as much low bass and mid bass output as possible from a 5L speaker.

Passive radiator: 2x Tang Band PR14 oval passive radiators

The passive radiators are special. This is a special oval passive radiator from Tang Band with ribbed surrounds to reduce rocking modes. It is the only passive radiator that has enough excursion and linearity to only use two PR to keep up with the 2 woofers, tune them to a very low 42Hz, and still fit in the enclosure. It should have much more linear excursion than the 5.25'' Peerless PR, as Tang Band recommends just 1 of these PR's to go with the W5-1138SMF, a woofer with higher excursion than the Wavecor. It is a shame Tang Band absolutely refuses to sell these PR's separately. So the only way to get them is to buy a bundle with either the W5-1138SMF or the W6-1139SIF, from Europe! This was not an easy (or cheap) PR to get to say the least.

DSP: external 4x miniDSP 2x4 HD

This is the heart of the system that does the crossover, driver response manipulations, phase correction, time alignment, bass boost, high pass, limiting, and more. 4 is needed instead of 2 because 2 of them are used to create a multi-band compressor the dumb and expensive way because miniDSP is so feature limited. A custom DSP will eventually be made to rid of the annoyingly feature limited miniDSP's and to implement the above features even better.

Amplifier: 1x ICEpower 50ASX2 SE, 2x ICEpower 50ASX2 BTL

I will be internally amplifying using the ICEpower 50ASX2 SE (for stereo) for the midrange and tweeter, which will provide 50W @ 4 ohms for each driver. The two woofers will be powered by the bridged version of the 50ASX2, one woofer each, which will deliver up to 170W @ 4 ohms to each woofer. The woofers need a LOT of power to overcome the tiny airspace. Even though they only need about half the power, but its important to have spare headroom so the amp isn't straining. The ICEpower amps are very high performance class D amplifiers that should sound much better than most of the class D amplifiers on eBay and Alibaba that may have questionable designs and parts quality. There will be no doubts on the excellent sound quality, power output, and build quality of ICEpower amps. They have integrated power supplies, the dimensions are incredibly small and they're not horribly expensive like the Hypex NCores.

It's going to be another month before I can finish building this and another few months to get it tuned properly. Love to hear your comments in the meantime. I hope this has been interesting as it has been for me.

Last edited by bcodemz; 14th March 2017 at 12:43 AM.
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Old 14th March 2017, 12:34 AM   #3
bcodemz is offline bcodemz  Canada
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Here are some pictures of the test box

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Last edited by bcodemz; 14th March 2017 at 12:39 AM.
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Old 14th March 2017, 12:42 AM   #4
bcodemz is offline bcodemz  Canada
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Feel free to make any suggestions even though I've started with the build. This won't be the final speaker. I expect more improvements as I learn more. Most likely most of the improvements will be on the DSP side. However, I won't discount hardware improvements either.
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Old 14th March 2017, 03:37 AM   #5
Brisso57 is offline Brisso57  Australia
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Are you intending to put a good radius on all of the box edges?

cheers

Doug
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A speaker-builder's parable: "That burned down, fell over, then sank into the swamp. But the fourth one stayed up. And that's what you're going to get, Lad, the strongest castle of all."
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Old 14th March 2017, 08:15 AM   #6
ewollowe is offline ewollowe  Australia
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Lovely, lovely build. Reminds me of Kii audio.
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Old 14th March 2017, 08:36 AM   #7
Brisso57 is offline Brisso57  Australia
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Looking at your first cab, why did you not move the tweeter closer to the woofer? ... given you said you wanted the tightest grouping? Looks trump performance?

Doug
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A speaker-builder's parable: "That burned down, fell over, then sank into the swamp. But the fourth one stayed up. And that's what you're going to get, Lad, the strongest castle of all."
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Old 14th March 2017, 09:46 AM   #8
chris661 is offline chris661  United Kingdom
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For what it's worth, I really would've gone with one of the 5-6" mini-subs on the market. You talk about them being low-efficiency, and then apply 17dB of boost to get the frequency response of your chosen drivers flat. The mini-subs would have a lot more excursion available, which means more mechanical headroom and greater potential SPL. You'll need to test distortion figures, though.

If loud bass is a priority, I'd consider using some aluminium plate in the construction, perhaps with an extrusion that thermally connects to the magnets of bass drivers. They're likely to see some real power, and small confined spaces aren't good for that.

I've never heard a dynamic loudness contour done well, even with fairly nice commercial HiFi. We'll probably have to agree to disagree with this one, but for a reference piece, I wouldn't want the speaker doing anything other than taking the signal that's going in, and making it into sound as linearly and accurately as possible.

Chris
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Old 14th March 2017, 10:08 AM   #9
brig001 is offline brig001  United Kingdom
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Reading the text, you have two woofers, but I can only see one in the pics. Did I miss something? Love the concepts of this design.

Brian
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Old 14th March 2017, 01:27 PM   #10
USRFobiwan is offline USRFobiwan
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Pushing the limits of small speakers - The Reference Mini build thread
Stunning research for such a small little expansivebox. The dsp is also inside the box?
Do you have any measurements yet?
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