First off, I'd like to thank everyone for the incredible help given to me when I was doing research for my ultimate small speaker build. I call it, the Reference Mini. After too many hours of researching, modelling, thinking, etc, I think I got to a point where the design is mostly complete, and I want to share it here and get some comments on possible improvements from members here that knows much more than I do.
For those interested, I talked about my previous attempt at such a speaker here. What I'm proposing here is a successor that improves upon every performance aspect after learning everything from the previous build.
Design goal: Build the highest fidelity small speaker possible with as much bass output as possible in the 40-60Hz region to cover most music well.
In more specific speaker design goals, I wanted to achieve the following, in no particular order:
1. Excellent clarity, deep bass response, and extremely high dB per liter ratio for bass.
2. Minimize/eliminate horizontal and vertical lobing errors
3. Matched directivity and smooth polar response
4. Minimal cabinet vibration
5. Excellent off axis response
6. Linear and minimum phase response
7. Flat power response
8. Transient perfect response
9. Time aligned drivers
10. Minimize baffle diffraction
11. Ruler flat frequency response
The enclosure will be 14'' x 6'' x 6'' built with 1/4'' baltic birch plywood. In the horizontal configuration, the driver configuration looks like
W P WMTP
The enclosure is a pair of dual opposed configuration. The woofer and woofer will be back to back, and so will the PR's. The tweeter and midrange will be arranged vertically in the horizontal placement. This allows me to use a 1/4'' BB for the enclosure and still have a vibration free cabinet. While this is a convenient configuration, the rear firing woofer is not ideal because it can cause comb filtering. At 6'' deep, it will be 90 degree out of phase at 340/(4d) = 340/(4 * 0.15) = 566Hz. Even though the crossover is going to be 300-400Hz, I want a linear phase speaker, which won't happen. But then the fact that it is vented will screw up the phase anyways. I still would like to reduce this effect, maybe with a delay, but I don't know if it'll work.
Another (big) issue is that it seems adding the PR's don't actually boost output much. It increases the output at tuning by 1dB with the same power vs sealed. It seems like all the PR is doing is reducing the excursion demand on the woofer. On a driver like the Tang Band W5-1138SMF or the Peerless SLS 5.25'', there is a 4dB increase in output vs sealed with the same power, and the PR excursion level is a lot higher. I'm not quite sure why, because the T/S parameter of the L16 and SLS are very similar. Only big difference is the Cms is double on the L16.
A miniDSP 2x4 HD will be used as the crossover and compressor. FIR filters will be used to achieve minimum phase. The speakers will be time aligned. The crossovers will be LR4, and things like baffle step correction, bass boost, high pass, and driver response correction will also be done after the speaker is built as I prefer to measure instead of relying on simulation graphs.
A 2 band compressor will be used to protect the speaker. I'm thinking of leaving the >50Hz band uncompressed, and the 40-50Hz band will be compressed with a ratio of 50 and an attack of 1ms. This is extreme, but so far it has been working pretty well with the previous speaker. I need to learn more about compressors before I experiment to find better settings. Any tips here would be helpful.
I will be internally amplifying using the ICEpower 50ASX2 for the midrange and tweeter, which will provide 50W @ 4 ohms for each driver. The two woofers will be powered by the ICEpower 125ASX2, which will deliver 125W @ 4 ohms per channel to the woofers. 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.
What do you think of the design? Any comments or suggestions for improvement would be greatly appreciated.
For those interested, these are the reasoning behind the design choices.
Tweeter: Scanspeak D3004/6040-10
Midrange: Scanspeak 10F/8424G
This is the best sounding ~3'' cone midrange available, and the largest midrange I can accommodate to achieve a 1/4'' wavelength crossover frequency with the tweeter. I will explain more of this significance later in the post. 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
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.
Passive radiator: Either 2x Peerless 5.25'' passive radiator or 2x DIY PR from modifying the Tang Band W5-1138SMF.
Using the Peerless would be the easiest solution, but there is only space for 1 PR per driver, and the PR's 6mm xmax is not enough excursion. I can use 3 of them, or 1.5 PR per active driver, which would be enough, but the mass needed to reach a 42Hz tuning, almost 2x the Mms, might be too much for the PR's suspension to handle.
So a solution could be to make a DIY PR. The Tang Band W5-1138SMF is a very good candidate as it has a 9mm xmax, so the xmech must be a lot higher than 9mm. The driver has at least 2x more excursion than the Peerless PR, which should be enough for the woofer. The Mms of the driver, at 29g, is 3x higher than a typical 5'' woofer, which means the suspension has the greatest chance of handling the ~60g of Mms needed to hit the tuning. At $30 each, it is not expensive either.
The DIY PR seems like the clear choice, but I'm a little haunted by the miserable failure the last time I tried that. Also, the Peerless PR has ideal PR parameters, while the TB woofer does not, and may be less efficient than the Peerless PR.
Enclosure design and driver placement
The midrange tweeter will be in a vertical configuration to save width. All drivers will have their frames cut off to reduce size and so they can fit closer together, especially the midrange and tweeter. When drivers have a crossover frequency that is the same frequency as the 1/4 wavelength of the center to center distance each other, the sound of the two sources couple as if it is coming from one source like a point source speaker. This eliminates any vertical or horizontal lobing errors, so the speaker can be placed horizontally like a sound dock or vertically when used as a pair with no performance degradation. Imaging and coherence is improved, and the directivity is matched between the woofer and tweeter, improving imaging further.
Normally, this is impossible to do with a tweeter and midrange. With the standard 4'' tweeter frame and a 5.25'' midrange. The best case center to center distance is 5'', which corresponds to a 1/4 wavelength of 678Hz, clearly far too low for any 1'' tweeter. However, with the small format tweeter and small 3'' midrange, with the frames cut off I can achieve a CTC of just 2.2'', which is a 1/4 wavelength of 1500Hz, which is a doable crossover frequency for a capable 1'' tweeter.
Why did I scrap the 4 woofer sealed design?
The speaker will be tuned to 42Hz to achieve useful output to about 38Hz or so. I analyzed many bass heavy pop songs, and most bass is above 40Hz, and mostly around 45-60Hz, so I want to concentrate to get as much output in this region. I'm sorry Josh, I won't be doing the 4 woofer design. The power efficiency of going from 2 woofers to 4 woofers is far too low to the point I can only achieve just a bit over half the xmax at the rated max power of the driver. Realistically I can't put that much power in the speaker to drive it anywhere near xmax. I would also need to use a smaller driver than the L16 because a speaker with 4 L16's would need to be a lot bigger than I would like. So a 2 woofer + 2 PR setup would have equal or greater max SPL than a 4 woofer sealed configuration everywhere except below tuning. The PR would contribute at least 2dB of extra output even an octave above tuning. The only benefit of a 4 driver design would be output to 25-30Hz instead of 38Hz, which would be very cool and brag worthy, but I have to use twice the driver, 4x the amplifier, bigger enclosure, the need for a 5 band compressor rather than just a 2 band for the PR version, which would also be extremely expensive and complicated to do. Over $1000 would be saved, and a lot less equipment. I do intend to actually use this speaker. I don't want to have to hide 6 stereo amplifiers (or an iNuke), 4 miniDSPs, and a 12 channel audio mixer for this little speaker. Plus, even with just 2 woofers I still cannot reach xmax at the 80W max rated power of the L16 (80% there though), further confirming that 4 woofers was wasteful and unnecessary. Josh, I pass the torch to you to build a crazy 4 woofer design to get the ultimate bass performance at any cost. Maybe use the Beryllium tweeter as well?