Overview:
The SB Acoustics SB15NRXC30-8 is an 8Ω 5″ mid-woofer from SB Acoustics.
The mid-woofer has a 30.5mm voice-coil with a healthy 10mm (p-p) linear coil travel. It has a very low voice-coil inductance and is fitted with a copper sleeve on the pole piece to ensure a low distortion performance.
Key features:
- Vented cast aluminum chassis for optimum strength and low compression
- Proprietary cone material with natural fibers made in-house
- Soft low damping rubber surround for improved transient response
- Non-conducting fiber glass voice coil former for minimum damping
- Extended copper sleeve on pole piece for low inductance and low distortion
- CCAW voice coil for reduced moving mass
- Long life silver lead wires
- Vented pole piece for reduced compression
Considering its features, build quality and measured performance this mid-woofer has a very high price/performance.
SB Acoustics Spec sheet: SB15NRXC30-8
T/S Parameters:
The four driver unit samples are from two different manufactoring batches. At this price range not much to complain about. Good consistency between the driver unit samples and it match the official spec sheet reasonably well, with a higher Mms and lower sensitivity compared to official specs.
The mid-woofer has a low “fs” for its size and has a high mechanical Q-factor (Qms) and low mechanical loss (Rms).
Impedance:
Driver unit samples 1-4:
Note the very low inductance. Impedance minimum of 6.3Ω @ 300Hz.
Frequency:
Frequency measurement conditions:
The mid-woofer is measured mounted on a baffle in an 14 liter closed enclosure with the following conditions:
Baffle size (WxH): 21,6×35,6cm with 12mm round-over on the baffle edges (r=12).
Driver position: Mounted on center-line with driver unit center 22cm from the top of the baffle.
Mic position: 1m distance, on mid-woofer-axis.
Smoothing: 1/24 octave smoothing applied.
(click on picture to zoom)
Left: Frequency response on-axis (Blue=sample#2, Red=sample#4).
Right: Frequency response 15deg off-axis (Blue=sample#2, Red=sample#4).
The two mid-woofer samples measures virtually the same, which indicates good consistency in the manufacturing process.
0deg = Blue
15deg = Red
22.5deg = Green
30deg = Blue
45deg = Red
60deg = Green
Due to the mid-woofers very low inductance and flat impedance response, the on-axis frequency response has an extremely linear and extended response up to 15kHz.
The broad hump between 0.5-1.4kHz is partly due to the shape of the baffle and the baffle step. We have a notch centered at 1.65kHz due to the low loss surround causing a cone edge resonance. It has a very smooth frequency response between 1.8kHz and 4kHz
3kHz is about the highest suitable cross-over frequency for this mid-woofer.
Distortion:
Measurement setup:
- Mid-woofer near-field measurement at 15cm
- Frequency Range Mid-woofer: 200-10000Hz
- Baffle size WxH: 21,6×35,6cm
The distortion measurements are done in near-field and the amplifier output level was adjusted for each driver so that the fundamental is 90dB and 95dB at 1m. This setting simulates medium-high and high listening levels.
90dB @ 1m (click on picture to zoom)
Left = Sample #2
Right = Sample #4
95dB @ 1m (click on picture to zoom)
Left = Sample #2
Right = Sample #4
Overall this is a low distorting mid-woofer. We have the expected relatively harmless elevated second-order harmonics due to the cone edge resonance, but in this case we also have some elevated third and fifth-order harmonics around the same frequency.
Great performance for a low priced mid-woofer!