The Lark-DX seeks to address the bass quality of the SB16PFC25-08 by loading her onto a 18 liters box. At the same time, I’m also evaluating the $20 Peerless DX25TG59-04 tweeter. It’ll be interesting to see how well she compares with my Peerless XT25TG30-04 ring radiator tweeter.
Fig 1 – Black plot=SB16PFC25-08 RAW Response • Baffle Width=9-1/2″
The Black plot in Fig 1 is the RAW response of the SB16PFC25-08 in this 18 liters bass reflex. She is flat all the way to 3kHz, after which there’s a fairly sharp peak brought on by cone breakup. This is quite an impressive response for SB Acoustics entry level midwoofer.
Measurements below 500Hz includes my room reflections. Ignore the deep notch at 150Hz. That’s caused by a floor bounce. The reason I’m not splicing in a nearfield plot is because I need to see the bass level. After tuning the SB16PFC (Blue plot), the midrange sits just above the bass. This will ensure that the bass doesn’t overwhelm the vocals.
Fig 2 – Blue plot=SB16PFC25-08 Low Pass • Red plot=Peerless DX25TG59-04 High Pass (Flush Mounted)
Fig 2 shows the Peerless DX25TG59-04 with her high pass network crossing over to the SB16PFC. I have not made any attempts to remove the SB16PFC peak at 4.5kHz. Reason being that for a budget speaker, I intend to use as few components as possible. I don’t want to end up with the crossover costing as much or more than the sum of the drivers.
Fig 3 – Black plot=Lark-DX Passband
The Black plot in Fig 3 is the passband of the Lark-DX crossover. The summation looks good as no cancellations are recorded. It’s encouraging to see the SB16PFC cone breakup did not cause any severe cancellation at 4.5kHz.
Fig 4 – Lark-DX Frequency Response
Fig 4 is the final frequency response of the Lark-DX. She has a beautiful flat response from midrange to treble, save for a bite at 5kHz. That’s cause by diffraction at the baffle edge.
Fig 5 – Lark-DX Null
Fig 5 is the null response when I flipped the tweeter wires around. It shows there’s some crossover mis-alignment between the SB16PFC and the DX25TG59. I am willing to accept this as long as I can’t hear any phase issues. For a better crossover alignment, I will need to use more components which will increase the cost. A cheaper way is to use a step or tilt the front baffle backwards. In doing so, the acoustic centers of the two drivers will be closer, resulting in better phase alignment.
Fig 6 – Step Response
The Step response of the Lark-DX throws up some interesting behavior. We can see some raggedness in the lower tip. That comes from the crossover phase mis-alignment. The SB16PFC is actually quite fast and smooth. At 90%, she is at 300 microsec. She then slows down for the balance 10%, hitting the apex at 400 microsec. There’s no over-hang as the tip is sharp. Her decay is very graceful. All in, the SB16PFC exhibits an impressive step response.
Fig 7 – Waterfall
Fig 8 – Toneburst Energy Storage
The Waterfall (Fig 7) and Toneburst (Fig 8) plots recorded excess energy from 1kHz~2kHz. These are the ones that can cause smearing. Those light blue slices after 2kHz may look harmful but their wavelengths are much shorter. Note that the Tonebust plot is referenced to cycles in the z-axis.
Fig 9 – Spectrogram
The Spectrogram in Fig 9 shows the excess energy in the time domain. We can see some strong delayed hot spots at about 1.5kHz. I have no idea whether they are caused by the SB16PFC. Fortunately, they dissipated by 7 msec. During auditioning, I did not detect any smearing.
The treble from 2kHz upwards is largely very clean. The bunch of light blue slices above 2kHz seen in Fig 8 is inconsequential when plotted in the time domain.
Bass Reflex Alignment
Fig 10 – SB16PFC25-08 box modelling
Fig 10 is the bass reflex alignment of the SB16PFC25-08. I tuned her for a bit of bass emphasis (+1dB). Her F3 is at 50Hz. This is low enough for most music.
Sound of Lark-DX
The SB16PFC bass sounds great in an 18 liters bass reflex. There’s bass punch which was sorely lacking in the Lark-DCR2. When compared to the 13 liters Lark-XT, the DX has a slightly deeper bass. Having tested the SB16PFC25-08 with different boxes, she sounds best in boxes of 13 liters to 18 liters.
Moving on to the midrange, female voices sound smooth. No shrillness. Midrange clarity is above average. Not superlative like in the Audax HM130CO. But to be fair, that Audax is so much more expensive.
Higher up in the spectrum, the treble is quite impressive. There’s no harshness nor brittleness. What I like about this DX25TG59-04 tweeter is that she has the right amount of brightness. Not overly bright where she becomes distracting. Frankly, she sounds as good as my XT25TG30-04 ring radiator tweeter. As with most dome tweeters, the DX25TG59-04 must be flush mounted to avoid nasty diffractions.
The Lark-DX is one of the best budget speakers I have. Her performance far exceeded her cost. Overall, great value for money.
The Lark-DX should be used with speaker stands of 24″ or taller. Anything below 24″, the bass will start to sound bloated. Furthermore, she should be placed well away from the back and side walls, at least 3 ft.
If her bass sounds bloated for whatever reason, tuning the box lower may help. Presently, the port tube is 2-1/2″ long. Extending to 3″ or 3-1/2″ may tighten up the bass. You’ll need to experiment with this because bass is room sensitive.
Crossover is available on request. Free for DIY. Not for Commercial use.
February 8, 2021Projects