RBH Sound 21-SF/R 4kWatt 21" Subwoofer Review!

Originally published July 8, 2023

RBH Sound hit the brown note with their 21-SF/R 21" Infrasonic Subwoofer!

RBH Sound is no stranger to monstrous bass output as we saw in our review of the SX-1212P/R. Even though that model is a bit older, it still has the ability to rattle your fillings, and its performance is still very high by modern standards. However, instead of just resting on the performance accomplishments of that model, RBH sought to raise the bar once again with something so extreme that Audioholics needs a new Bassaholics room rating to accommodate it. They have just launched the UNRIVALED 21-SF/R subwoofer, a 373 lbs. behemoth with a mammoth 21” driver that has to be one of the highest displacement subwoofer drivers that can be had in any commercial subwoofer to date.

Update: 6/22/24 - This article includes a technical overview of the 21-SF/R subwoofer, along with a comprehensive review of our listening impressions and system integration experience. If you're serious about infrasonic bass and want the ultimate musical experience to match, this review is a must-read!

Design Overview

How does RBH go about creating such a powerful subwoofer? Let’s start to answer that question by talking about the driver. This 21” behemoth uses a carbon fiber cone that attaches to a massive cast aluminum basket with a correspondingly large half-roll rubber surround. It has a staggering 38mm of Xmax (one-way linear travel) and 50mm of Xmech (one-way mechanically limited travel). The motor uses eight neodymium slugs for the magnet since iron ferrite would have been too large and heavy to support within the enclosure. Even though neodymium is very expensive, RBH did not want to compromise on their performance targets for the 21-SF/R, and the resultant magnetic flux is tremendously powerful at 35 Tesla Meters. It has a ton of power handling courtesy of an enormous 4” diameter copper voice coil wound on an aluminum former. Cooling is done through a vented pole piece. It is just as well that the driver should be so powerful since the sub is a sealed design so it does not get any assistance from ports or passive radiators (not that it needs any). All that oscillating copper is bound to cause a fair amount of inductance on its own, so RBH has mitigated that by way of an aluminum shorting ring and full-length copper sleeve.

This beast of a driver needs a beast of an amp to achieve its potential, and RBH has supplied that in the form of the DA-2602DSP: a 4,000-watt class-D amplifier that likely will need its own power circuit to those users intended to run the system at full-tilt. The DSP control has a huge amount of flexibility with phase control, crossover slope control, delay control, XLR inputs and outputs, and an incredible 30-band equalizer for those who want to get obsessive about tweaking their system to perfection. 

Along with a monster amp, the 21-SF/R has a monster enclosure as well. It uses a 43” wide heavily-braced cabinet that has a 27” height and a 21” depth. As an oblong-shaped sub, it exchanges depth for width, and that should allow it to fit in tighter spots than if it was cube-shaped. It is made from high-density fiberboard (HDF), a much tougher and heavier material than the standard MDF. One unique aspect of the 21-SF/R construction is the G.A.D.S. system which stands for Granular Acoustic Damping System (patent-pending). This involves chambers on both sides of the enclosure that contain granular material where the acoustic energy created by the rear motion of the driver can be dissipated. Something else that we don’t see on monster subs are premium outriggers so the 21-SF/R has a more stylish footprint. It also has a heavy-duty curved magnetic grille for those users who want more protection for the undoubtedly expensive driver. Of course, all of this extensive cabinetry does not come without a substantial weight penalty, and the 21-SF/R weighs in at a whopping 371 lbs. Prospective buyers will need to have a serious plan for getting this sub into place. 

What does all of this design amount to? In terms of numbers, the 21-SF/R knocks burst test measurement out of the park so far that it helped to push us over the deciding whether to add a new category for our Bassaholics Room Rating. We call our new rating the Maximus Room Size Rating, and any sub that can achieve it should be able to handle any domestic room size at all. Obviously, only larger home theaters need subwoofer systems that can supply such tremendous SPLs, or perhaps users who are looking for a huge amount of thump in a normal-sized room. 

So what does the 21-SF/R do? Here are the 2-meter (RMS) CEA-2010 burst numbers supplied by RBH Sound:

RBH Sound 21-SF/R CEA-2010 Output (2-meter, RMS)

Note: If you wish to know the 1-meter peak output as called out in CEA-2010, simply add +9dB to each number in the table above. RBH Sound reports their data as 2-meter peak so their figures will be +3dB higher than how we report CEA-2010 data.

Infrasonic Bass for Maximus Bassaholic Awesomeness

Those are some BIG numbers, especially in deep bass (infrasonics). In fact, compared to the many subwoofers we have reviewed and tested, the 21-SF/R would be among the most powerful ever tested at 25Hz and below. It is no slouch in providing mid-bass punch, but a design like this particularly excels at infrasonic grunt. The science of psychoacoustics has shown that as frequencies go lower in bass, a lot more output is needed just to make it audible, and extremes below 16Hz need major SPLs just to be heard. Owners of the 21-SF/R certainly won’t have to worry if their system can surpass the threshold of audibility, because the ultra-deep bass on display here is colossal. When we factor in room gain, triple-digit SPLs in single-digit frequencies become a very real possibility for many owners. Gene is already reporting flat in-room extension to 10Hz in his theater room!

This tremendous level of performance does not come cheap, and the 21-SF/R starts at $11.9k each, in versions with a built-in plate amp or outboard amp for rack-mounting. This sub can also be customized to different enclosure sizes and porting options too. That is a lot of money, but the 21-SF/R is a LOT of sub with performance and fidelity justifying its cost as you can see in our Youtube review and subjective listening tests.

RBH 21-SF/R Listening Impressions

 by: Gene DellaSala

RBH Sound Hits the Brown Note 21" Subwoofer Review!

The sonic impact of the RBH 21-SF/R subwoofer on music was a pleasant surprise to me. As you know, music is everything to me, and if I can’t get a home theater system to play back music with the utmost excellent fidelity, I consider that a failure. I would never compromise my system’s musicality for added bass extension or output. This was a concern of mine when adding such a large subwoofer like the 21-SF/R. However, due to the low distortion, low group delay, and excellent corresponding transient response, these concerns quickly evaporated with the first bass notes while listening to music.

Music Listening

In fact, I’ve discovered infrasonic bass extension in music I didn’t know existed prior to installing the 21-SF/R. Songs like "Fair Trade" by Drake really showcased the bass prowess of the 21-SF/R towards the end, when Travis Scott kicks in with some sub-20Hz bass that added new depth to the experience. "Queen Mary" by Francine Thirteen is a fun song to listen to, with lots of decorrelated effects that give you a surround sound experience with just two channels in a system with great imaging characteristics, like my RBH Sound SVTRS active speaker system. With the 21-SF/R in the mix, the bass drops hard and deep without sacrificing tightness. In fact, I felt the 21-SF/R actually tightened up the system's bass response when it was engaged.

Movie Listening

Watching movies with lots of LFE content with the RBH 21-SF/R engaged was a transformative experience. The car chase in Ready Player One is one of the most iconic Dolby Atmos demo scenes to show off a system's capabilities. The tactile stomping of the dinosaur and Kong’s destruction of the city were experienced with lifelike realism, with true infrasonics being reproduced by this 21” beast of a subwoofer. The finale of the movie, with all the players charging at the end, was a visceral experience that had me fully immersed. Top Gun Maverick was another great movie to showcase the capabilities of the RBH 21-SF/R sub. This is one of those rare movies today that masterfully integrates great visual and audio effects with an engaging story. The impact of the Aurora test flight and the dog fights through the valley were riveting without the 21” sub, but adding it into the mix provided that next-level experience that rivaled anything I’ve heard at even the very best cinemas. Dune 2 has instantly become top-tier demo material for both UHD video and Dolby Atmos. The worm battle scene will send lesser subs to their grave. I heard no misbehavior or mechanical distress from the 21-SF/R at and above reference levels. This sub just wanted to show off its essential place in my system to justify its existence. I was scared for the structural integrity of my house and danced with my own fate, wondering if my neighbors would be knocking at my door. Luckily, I have good relations with them and gave them fair warning of what I do for a living. Bribing your neighbors with good demos and occasional movie nights goes a long way :-)

Conclusion

RBH Sound builds all of their cabinets in the USA, and the products are assembled in-house at their main facility in Layton, Utah. RBH offers a 10-year warranty on the drivers and 3 years on the amp. Personally, I recommend choosing the external rack-mount amp option over the internal amp for better durability and flexibility. If you can supply 220Vrms to the amp, you can achieve up to 4 kilowatts of power, which the driver can handle without issue. If not, based on my bench tests, 120Vrms will still provide you with 2 kilowatts.

In-room Frequency Response at MLP with/without RBH 21-SF/R Subwoofer

The cost of entry for the RBH Sound 21-SF/R and its enormous size and weight may be off-putting to most buyers. However, if you want to take your system to the next level and experience infrasonics while doing no harm to your system's musicality, then I can’t recommend this subwoofer enough. Yes, you can buy multiple 18” subs from reputable brands for the same price and have the bass seat-to-seat consistency advantage of multi-sub setups, but the same could be argued for dual 21” subs from RBH Sound if budget isn’t a concern or if you already have multi-sub and want to add a layer of infrasonics your subs just can't provide. The 21-SF/R subwoofer has transformed my expectations of bass and has convinced me to move my entire system towards sealed high-output capable subs. RBH Sound offers many variants of this subwoofer, including a dual sealed model and a very large ported version for large Cineplex applications. I didn’t need this sub for more output, considering I already have six (6) RBH dual 12” subs in my system, but integrating it provided infrasonics below 20Hz that were just not possible from ported subs in my room. As you can see in the before/after graph, adding +10dB of bass output below 20Hz allowed me to experience infrasonics unlike anything I’ve heard before in my own space. It’s an experience I will never forget and one that I plan on continuing to enjoy for many years to come.

Unless otherwise indicated, this is a preview article for the featured product. A formal review may or may not follow in the future.