Lark-SM (SB16PFC25-08 with BC25TG15-04)

Lark-SM (Studio Monitor)
15L Bass Reflex

The Lark-SM came about when I was refining the first Lark that I did in 2017. I felt the frequency response can be improved vastly now that I got the Peerless BC25TG15-04 sorted out.

After a few attempts, I ended up with a super flat response (Fig 1). It wasn’t something I was aiming for. The best part with it is she doesn’t sound dull or lifeless like some flat response speakers do.

Fig 1 – Lark-SM Frequency Response • Nearfield below 500Hz • Brown plot=Port • Baffle Width=8-3/4″

The Black plot in Fig 1 is the on-axis response of the Lark-SM. Measurements below 500Hz are in nearfield. From 500Hz upwards, there’s virtually no deviation until 5kHz. After that, there are a few dips of -2dB. The Lark-SM treble is actually smoother than that of the G1. And this is from a $15 tweeter, the Peerless BC25TG15-04.

The Brown plot is the output of the rear port. Apart from seeing the bass tuning, I wanted to check on the pipe resonance. There is a peak at 900Hz but it is well below (-20dB) the port fundamental. It is quite benign because I didn’t hear any “honking” in the midrange during playback.

Fig 2 – Lark-SM RAW Frequency Response • Blue plot=Nearfield

The Black plot below 500Hz is my in-room response, that is with reflections. What I am looking for is the bass loudness relative to the midrange. On average, it’s about -2.5dB less. I am quite satisfied with this. In fact, I prefer it this way. Vocals will have better clarity.

The ugly notch at 150Hz is an anomaly in my setup. It’s a floor bounce that my mic picked up. The nearfield measurement doesn’t show the notch, so it’s not a defect in the woofer. 

Fig 3 – Lark-SM Crossover Passband

The Black plot in Fig 3 is the crossover passband. The summation is beautiful. No cancellations are observed in the passband. The Red plot is the Peerless BC25TG15-04. The tweeter must be flush mounted otherwise the response will not be so smooth. The Blue plot is the SB Acoustics SB16PFC25-08. I did not eq out the peak at 4.5kHz because I wanted to keep the crossover components low. As it turned out, the peak did not interfere with the tweeter.

Fig 4 – Lark-SM Null

The Violet plot in Fig 4 is a beautiful null when I wired the tweeter in the opposite phase. The null is quite deep, indicating almost perfect time alignment. The tip is at 1.6kHz, exactly where the woofer and tweeter are crossing.

Fig 5 – Lark-SM Step Response

The Lark-SM transient is actually quite fast. At 95%, she’s at 300 microsec. It is only in the last 5% that she slows down, ultimately hitting 400 microsec.

Fig 6 – Lark-SM Waterfall

Fig 7 – Lark-SM Toneburst Energy Storage

The Waterfall (Fig 6) and Toneburst (Fig 7) plots show minimal artifacts in the treble. This is an impressive performance for a $15 tweeter.

Fig 8 – Lark-SM Spectrogram

The Spectrogram in Fig 8 recorded some streaking at 1kHz. During playback, I didn’t detect any smearing.

Fig 9 – Lark-SM Excess Group Delay

The Lark-SM has an Excess Group Delay of 2.30msec at 47Hz (Fig 9). This is quite acceptable for a bass reflex, more so when the Lark-SM is meant for nearfield monitoring.

Fig 10 – Lark-SM Impedance

The Lark-SM nominal impedance is 8Ω. The lowest she goes to is 5Ω and that is at 20kHz. Her electrical phase is almost linear. Power amplifiers will have no issues driving the Lark-SM.

Fig 11 – Lark-SM Bass Reflex Alignment • Fb=46Hz

Fig 11 is the bass reflex alignment of the Lark-SM in a 15 liters box. I tuned her to 46Hz (Fb) resulting in an F3 of 55Hz. In the frequency response plot in Fig 2, the Lark-SM does roll off at 55Hz. It goes to show the importance of measuring the Thiele/Small parameters instead of simply relying on the published specs.

Summary of Lark-SM (Studio Monitor)

The Lark-SM is a dual purpose speaker. She can be used as studio recording monitors or in the home for music. She is an exceptional value for money. The SB16PFC25-08 is only $28. The Peerless BC25TG15-04 on the other hand, is an incredible $15. It’s not often I can get this level of performance with so little money. Highly recommended for those on a budget.

Crossover is available on request. Free for DIY. Not for Commercial use.
Unless otherwise stated, all measurements were made in Full Space (4 pi) with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.