- Written by Dr. David A.Rich
- Published on 27 March 2013
Introduction to the NHT B-10d Subwoofer
The world is saturated with subwoofers, many of which have a peak in frequency response to enhance movie sound tracks at the detriment of music. Tuned to have a bandpass response, the movie-inclined subwoofers limit the low frequency extension at usable SPLs. The frequency peak may persist, even if a first-class electronic room EQ is in the loop, especially if the room is sized and the subwoofer is positioned so a modal resonance peak occurs near this frequency.
NHT B-10d distinguishes itself by its performance among units of comparable size and weight. Its anechoic frequency response is much flatter than most subwoofers, which is an advantage for music. My measurements show the bass extends to 29Hz (-3dB) and those frequencies are reproduced at 92dB SPL RMS (1 meter in an anechoic environment) with less than 10% distortion. As we will see in the measurement section, the 10% SPL level increases as we move up from 29Hz. At 40Hz, the 10% distortion limit is 100dB SPL RMS at 1 meter. In practice, the SPL output at 10% distortion is sensitive to room geometry, room construction, speaker, and seat placement and would likely be higher and the -3dB point slightly lower.
NHT B-10d SUBWOOFER SPECIFICATIONS
- Design: Subwoofer, Sealed Enclosure, 18mm MDF
- Driver: 10" Paper
- Amplifier: 300 Watts RMS Bash
- Inputs: Two Line-level, Bypass
- Low-pass filter: 50Hz – 140Hz; 24dB/octave
- Phase: 0-1800 (Two Position Switch)
- EQ: Flat (Music) or Elevated in Mid-bass by 3dB (Movie)
- Protection Audio System-on-a-Chip with DSP in the Signal Path
- Dimensions: 12.6" H x 12.6" W x 12.6" D
- Weight: 28.5 Pounds
- MSRP: $679.99 - Street: $599.99
- SECRETS Tags: NHT, Subwoofer, Audio
NHT has a larger, top-of-the-line, B-12d ($700) with 2Hz additional bass extension and 3dB of additional pressure output for 10% distortion output at a given frequency. I own the discontinued NHT XdW, which would have been the next rung on the ladder ($1,200), but it was specifically designed as a companion to the $6,000 Xd active DSP based sat/sub/amp system. NHT abandoned the pricey segment of the market, which is a shame since the Xd system still yields state of the art performance thanks to tri-amplification and DSP crossovers.
The size and price of the B-10d make it ideal for multiple subwoofer deployment. Multiple subwoofers reduce the variation in frequency response due to room modal response before room equalization (provided each subwoofer is properly placed).
The B-10d has an audio system-on-a-chip (SOC) in the signal path. It includes an analog-to-digital converter (ADC), a DSP core and a digital-to-analog converter (DAC) as well as the required anti-alias and reconstruction filters. The DSP provides a filter to equalize the speaker system (driver in the box) flat in an anechoic chamber. The DSP also incorporates a programmable low pass filter used to set the crossover frequency. How could that be since a rotary potentiometer is on the back of the subwoofer? The level control is one leg of a voltage divider that changes the value of a DC control signal. A special ADC on the SOC IC converts the voltage to a digital word representing its position. The volume control is implemented in a similar way allowing the DSP to perform precision level changes. Eliminating analog signals from flowing in the potentiometers improves reliability. The phase inversion switch involves only a DC voltage change when selected. The value of DC at input pin on the SOC instructs the DSP to invert the phase.
The DSP is programmed to provide the limiter function which protects the subwoofer from being damaged by an excessive signal level. Additional filtering can be provided by the DSP to protect the driver below the subwoofers passband from being overdriven. The DSP is not part of the over temperature sensor protection for the power amplifier which is implemented using standard techniques.