September 2007

World Premiere!
Nomad Audio Sentinel
An open baffle that may be head and shoulders above the vast majority box loudspeakers.
Review By Scott Faller
Click here to e-mail reviewer

  For this article, I’m stepping away from my (loosely) self-imposed writing guidelines. After a lot of soul searching, have found the things that trip my trigger most often have gain stages that glow, have low power or high efficiency. These are the things in my audio life that get my juices flowing. There is no point in trying to fool myself (or anybody else) about this because if I did, I’d have a miserable time trying to write the articles. Sorry, that’s just part of my personal bias. I’ve tried and tried writing about gear that isn’t in my ‘sweet spot’, but ultimately think I’d rather be lashed to chair and forced to listen to Diana Krall on a shitty CD player stuck on repeat, than write about something that doesn’t turn me on. It’s absolute torture.

So why exactly have I chosen to write about a speaker that is 84dB/W/m (on a good day) and has a crossover in it? Well, let me tell you. A few months ago Paul Hilgeman decided to make the relatively short trek south to St Louis and perform a demonstration to our local audio guys, the Gateway Audio Society. Paul’s company, Nomad Audio, is a speaker manufacturer whose base of operations is in the greater Chicagoland area.

Paul brought a pair of his larger speakers, the Ronin’s. The Ronin consisted of a sealed cabinet, which housed a powered woofer and an open baffle midrange driver that has a concentrically loaded, a tweeter for the highs. That evening about fifteen of us were treated to one of the better sounding, low efficiency speakers I’ve (personally) had the pleasure of listening to. While Paul was there we tried a couple of different types of amplification, tubes (ala VTL) and one of the big Niagara Class D amps. As you might imagine, I preferred the VTL’s to the Class D but that (again) boils down to my personal preference.

As I intently listened to Paul’s creation, I couldn’t help but think to myself that he is really onto something here. These speakers had tons of good stuff going on for them, huge soundstage, great depth and imaging like Maggie’s ….. but they had tremendous bass. Not that Maggie’s don’t do bass (because they do), these just had significantly more grunt at high SPL’s. Needless to say, I was extremely impressed with what I’d experienced. Impressed enough that when Paul contacted me and let me know he was releasing a ‘smaller’, less expensive version of the Ronin’s, I jumped on the chance to write about them.

The Design

When you look at the Sentinel, you can see it is a direct descendant of its big brother the Ronin. They share the same open baffle midrange and sealed woofer design. There are several differences though. The Seas driver used as a midrange in the Ronin is now used as a woofer in the Sentinel. Replacing the Seas Excel W22EX series for the midrange duties is the T18REX series of coaxial drivers. This uses a 26mm soft dome tweeter, the same as the Ronin.

The Sentinel comes standard finished in a light Bamboo. Paul does offer several other wood finish options for a slight up charge. The back side of the sealed woofer cabinet is sloped presumably to help knock down the internal reflections and buildups but also to give a slightly larger surface to give the midrange a ‘lift’ in its open baffle arrangement. On the backside of the Sentinel is a single pair of nice quality, gold plated binding posts.

Inside, fixed to the bottom of the speaker cabinet you will find the crossover. Here Nomad Audio has chosen a combination of air core and iron core chokes along with nice sounding pair of metalized polypropylene capacitors. The style of chokes (air or iron core) is carefully chosen to avoid saturation at the loudspeaker’s power limits.

Paul’s chosen crossover points are 400Hz for the woofer to midrange and 2000Hz from the midrange to the tweeter. The slopes are 3rd and 2^nd Order respectively. Nomad Audio does offer impedance compensation (a Zobel network) as an option to the speakers in case you are using a tube amp. The particular pair I have here does not have this option. All of the internal wiring is Furutech OF copper with a Teflon wrapper.

As you might expect, the woodwork being in Bamboo and having a light finish is quite attractive. The overall build quality is first rate. This pair of speakers requires a stand. With my low seating (Futons), I needed an 18” speaker stand. The ones I chose were solid wood spools (8” diameter stanchion) that were originally designed to be plant stands. They work exceptionally well and are pretty much resonant free, just as you want in a stand.

Open Baffles or Dipole’s

I don’t know if any of you have been paying attention to the online forums over the past couple of years but there is a quiet revolution taking place in the audio market. Besides the absolutely remarkable sounds coming from computer based music servers, modified Squeezeboxes and inexpensive DAC’s, the open baffle speaker is making huge inroads on the audio fringe. There is a feeding frenzy going on right now by the DIY crowd over open baffles. Some of the smaller speaker manufacturers are starting to take notice and are now offering OB’s in their product lines.

The open baffle or dipole speaker is nothing new. They have been around since the early 1900’s. Over the years, a number of speaker manufacturers have designed and marketed dipole speakers. Some of the more well known dipole speakers are Magnepan’s, Martin Logan’s, Quad’s, Carver’s, Innersound’s and Apogee’s.

If you aren’t familiar with dipole speakers, essentially what they are is an open backed driver that is mounted on a single piece of wood (a baffle) without an enclosure around it. This driver radiates sound towards you (in phase) and then is also allowed to radiate sound from the back side of the driver (180 degrees out of phase). This out of phase sound is then reflected off of the wall behind your speakers, which is then brought back into phase because of the reflection (when sound bounces off of a flat rigid surface at a right angle, the phase is reversed by 180 degrees). This ever so slight delay in the reflected sound causes the sound waves (mostly in phase) to trick your mind (psychoacoustically) into thinking that the recorded sounds have greater depth and spatial qualities. It tends mimic the sound of a large recording studio or hall. When done right, the sound can be utterly breathtaking.

In recent years the DIY crowd (I consider myself part of that group) has really latched onto the dipole/open baffle concept. With their modern mentor Siegfried Linkwitz at the helm, they have created quite a stir on the audio boards and the DIY speaker builder pages out in cyberspace. I myself am a re-convert from the conventional monkey coffins. Straying at first from box speakers to open baffles (ribbons) to back loaded horns (and front loaded horns), I’ve finally settled on Lowthers in an open baffle using a boxed 15” woofer to (actively) drive the bass. I’m here to tell you, there is nothing like the sound of an open baffle speaker. It is completely free of all the compromises associated with box speakers.

The ironic part of this adventure for me is, I popped my audiophile cherry on an original pair of Maggie’s way back in the early 70’s. Even back then I was absolutely taken with the concept of the dipole speaker. Back then I made my own crude imitations of the dipole. Rather than mounting the midrange to a baffle, I used to mount a tweeter on the back of my DIY speakers to get some of those spatial cues and depth of image that the rear reflections provide. Rudimentary as they were, they captured part of the essence of a good dipole.

In more recent years I again played with dipole designs. I used some 30” Carver ribbons to construct my own dipoles. I used them for a number of years in a bi/tri-amped system until I found my musical soul mate, the Lowther PM2A’s. I now use the Lowthers in an open baffle configuration.

Moving Forward

As usual, I tried these speakers out in each of my three rooms, each using varying amplification. Feel free to click here and see the lowdown on each of my rooms including the size, layout and the different gear used.

The Redrum

This is where the Sentinel’s spend the most time. They really seemed to mate with this room extremely well. The initial placement and setup was truly a snap. I (literally) guesstimated where they would sound decent, plopped them down and started listening. I’d bet I didn’t move them more than about 6” in any direction to get them dialed in (though I did move them around just to make sure this placement sounded the best). This really reminded me of how the Ronin’s interacted with my friend Steve’s room in when they were in town last fall. There, we did the same thing, plopped them down and started listening. They too weren’t overly picky about placement at all.

As it is with any open baffle speaker, you need to have them out from your front wall at least three feet so that you get that great depth of image. In my case and since I’ve got the room, I decided to pull them out about four feet into the room. They are about two feet from the sidewall. This position seemed to give me the best depth and most cohesive image in this room.

As I listen to the Sentinel’s as driven by my big Radii tube amps, I was definitely reminded of the sound I heard back last fall. The Sentinel’s project an exceptionally large and deep soundstage. Since these speakers are coaxial and radiate in a 4pi manor, the sweet spot is quite expansive. You can literally get up from your listening position and move around in your room and still maintain a reasonable image. Sure, you might get slightly more (or less) off axis information as you move around but the basic stereo image stays remarkably stable. No ‘head in a vise’ sweet spot with these speakers, that is very refreshing for a change.

Let us start with the audiophile stuff first this time, shall we? As I listen to the Sentinel’s ability to project a soundstage, I start with Pink Floyd’s Signs of Life from Momentary Lapse of Reason. The lapping water on the shoreline is coming from about four, almost five feet from outside the speaker boundary. This is actually quite exceptional. Not many speakers I’ve had in my little laboratory have been able to pull that one off. I know I’m jumping ahead of myself but the first bass notes of this song are quite deep, solid and impactful. I can’t help but mention how well the Sentinel’s do in reproducing this quality of bass in this room.

As I listened for the placement of the performers on this virtual stage, I played several tracks. The first was "Throwdown at the Hoedown" from Left of Cool by Béla Fleck & The Flecktones. Towards the end of the song, the recording engineer has Bela and Victor Wooten panning back and forth between the left and right channels. The Sentinel’s did a fine job making that handoff without leaving a hole in the middle of the soundstage. The next track I used was "Daraijin" from Mondo Head by Kodo Drummers. In several parts of this song, there is a single row of over ten drummers that stretches across the stage. The drummers trade individual strings of beats, all the while peppering the image from far left to far right with evenly spaced performers. The stereo mic used to record this picked up those minor placement differences almost perfectly. In turn the Sentinel’s gave me those spatial cues that told me there was distance between each of the drums rather than that one lump of sound coming from between the speakers.

Another thing that the Sentinel’s excel at is soundstage depth. When placed at a decent distance out from the head wall, the soundstage extends about four to five feet behind the speakers and often deeper, providing that is the way the music was mixed.

Since I’ve mentioned the bass reproduction of the Sentinel’s several times, let’s go there next. Here I decided to try something a bit different. I cued up Eric Serra’s soundtrack to The Fifth Element, the second track called Mondoshawan. If you remember from the movie, this is the music that is playing when the spaceship lands next to the ancient ruins at the beginning of the movie. This song has a synthed bass track that goes down to 8Hz fairly evenly from about 200Hz. As Paul has mentioned on the Nomad Audio website, you can expect that these speakers will do about 30Hz in room. Well, I measured things a bit differently, at least in my room. On the soundtrack, the Sentinel’s died off at about 18Hz (-3dB, measured and verified with my Sencore SP-295c). Between that point and the 200Hz, they were relatively even with some minor boosts and dips due to room nodes and nulls (partially do to my seating position). I reconfirmed this with several other musical tracks including some test tones. That’s pretty impressive for a sealed speaker.

The bass I heard coming from the Sentinel’s was quite impressive. It was very solid with very little overhang. I really think this is attributable to the sealed box design of the woofer. On your typical vented speaker, its ported tuning frequency (if not properly calculated) can cause all kinds of weird harmonic overtones that make the speaker (woofer) sound slow and undefined. This is not an issue with the Sentinel. Its bass is well defined and quite deep in this medium sized room. The 8-inch Seas woofer sounds reasonably large. It doesn’t quite fill the room like a big 15-inch but they did a fine job on nearly everything I threw at them.

When we move onto the ultra critical midrange, here the Sentinel’s faired quite well also. Playing both male and female vocals to gauge, there was only a hint at the lower midrange being ever so slightly thin. Some of my opinion might be due to the ultimate reference gear I use, Lowthers and SET’s. Nothing gets the midrange more ‘right’ than that combination. My reference system is an extremely hard act to follow. Some of what I’m hearing could attributable to this room and my seating position (most likely culprit). I’ll know more when I take the Sentinel’s and try them in my other rooms.

Staying on the midrange frequencies, the mids are reasonably transparent. As I listen to tracks like Eric Clapton’s One Track Mind from his Back Home release, this CD is very clean sounding. The Sentinel’s only show hints of not retrieving the lowest levels of details. As I played something on the opposite end of the musical spectrum, Stravinsky’s Rite of Spring, Maazell and the Cleveland Orchestra, this piece revealed how open sounding these speakers can be. The open baffle design transformed this piece from a simple recording to a piece that gives the illusion of the concert hall. The sound was big, open, dynamic and reasonably accurate.

The timbre of the speakers overall was quite good. They had no problem convincing me that a piano was a piano and an oboe was an oboe. I didn’t notice much coloration at all. As it is with most rooms, speaker placement and bass reproduction can be an issue. These speakers are no different. If you get them too close to the room boundary, they can embellish the bass notes. This really isn’t much of an issue as a quick repositioning of the speakers solves those issues. Although with rock and roll, moving these speakers back a little bit made for some darned fun listening.

When it comes to the high frequency reproduction of the Sentinel’s, they did quite well also. The highs were smooth and extended without being harsh or gritty at all. The handoff between the midrange driver and the tweeter is relatively seamless, as I didn’t notice anything of consequence sonically. As you would expect from a nice sounding soft dome tweeter, the high frequencies reasonably detailed without being overbearing or sizzley. The highs came across smooth and accurate providing a nice amount of ‘air’ and transparency to the presentation especially when played with a good vinyl rig.

Before I got out of the Redrum, I decided to try an amp that seems to be an unlikely combination. I swapped out my high-powered (as far as tube amps are concerned) modified Radii’s for the little factory Modified JoLida 102b at somewhere near 25 watts per channel. Though slightly underpowered in this room, I easily reached 90+dB peaks, which for some, is louder than they normally listen.

I have to say without reservation, this combo sounded extremely good, especially with vinyl. This little amp continues to amaze me. Regardless of a speakers current draw demands and sensitivity, the little 102b continues to do a fabulous job with all. Sure, you aren’t going to rock your brains out with 95dB but what you do get with this combination is extremely good. The soundstage projected is even deeper and wider. As a little bonus to this new combination, I’m actually starting to hear a bit of the ‘presence’ that I normally only hear in my SET/Lowther/Vintage Sub system.

The Grape Room

Next up is the smallest of my rooms. I had some serious reservations about how these speakers would perform in this room. On numerous occasions I’ve tried slightly larger speakers in here and nearly every time they just didn’t work without adding some really heavy acoustic treatments. This time I’m hoping that since the Sentinel’s are an open baffle, they will load the room differently and work a little better with these room dimensions.

After I moved the Sentinels into place in this room and got all of my wiring secured, I hit the play button on the latest Crosby Nash release, Highlights. I did some very minor repositioning of the speakers (I might have moved them 6” in a couple directions) and I had what was to be their final position. After a bit of fiddling with the positions of room treatments, I was ready to listen.

Upon restarting the CD, I was immediately taken by how well they worked in this small room. From mids on up to the highs, the Sentinel’s performed nearly as well as they did in the Redrum. When it came to the bass, it really wasn’t as bad as I expected. Sure, I had some buildups (and a suck out or two) as one would expect from a small room but they weren’t nearly as bassy as I had envisioned. A quick trip to my stock of mineral wool and I had myself a makeshift bass traps that I promptly placed in the room in a LEDE configuration. All of a sudden, the Sentinel’s sounded quite even from top to bottom.

This confirmed my thoughts that these large open baffle speakers would work fine in this room even though they appear to be too big at first glance. The end result of these minor efforts gave me a system that sounded very good. The bass was solid and deep without any major sonic anomalies. The mids through the highs were clean, open and spacious without any hints of congestion.

As I popped on some more familiar reference music, I found the placement of the performers on disc’s like the Adam Rafferty Trio’s release Three Souls, to be as precise as when the Sentinels resided in a much larger room. The other item of note is the stability of the image. Just as it was in the Redrum, the image stayed remarkably firm as I repositioned myself on both ends of the Futon. That said, as intimate as the confines of this room are, I must say that the overall image projected is smaller than that when it was in a larger room. That too is very reasonable expectation considering the small room dimensions.

The Blue Room

This is a room that I’ve never had any issues getting a pair of speakers to mate well with. The confines are very speaker friendly, especially open baffles. Since the Sentinels are all of 84dB, I went the solid state route. I brought the Odyssey Audio Khartago down from my Grape Room. Having bipolar’s for amplification and being very conservatively rated at 100wpc, I knew this should be a very good mating.

Just as I anticipated, these speakers fit this room like a hand in glove. The extra power and current drive the Khartago let me really see what these speakers are made of. As a tube fan, you might imagine I’d have to slide in some snide remark about solid state but you won’t find it here (believe it or not). I did use my highly modified Korato KVP-20 tubed pre along with my Paradisea tubed DAC in front of the Odyssey amp but the real credit for the sound goes to Klaus Bunge for making a great sounding amp that can drive just about anything.

As you can imagine in this room, these speakers were freed from the constraints and confines of smaller rooms and lower powered amps. In turn, the Sentinels were fed the current they tend to like. As I cranked the volume ever upwards, the Sentinels opened up even more. As good as they sounded with the high powered tubes, I have to admit that they performed even better with the drive of solid-state.

There were some definite advantages to having them in a much larger room. The soundstage got noticeably deeper and wider. At the same time the dynamics seemed to improve a fair bit. The sound became more relaxed and easier to listen to. I suspect that this is due in part to the way the Sentinel’s load a larger room. Music played at high SPL’s seemed to have a better overall flow in this much larger space.

The only drawback that I noticed to having the Sentinel’s in this considerably larger room was a roll off in the lowest octave of bass. That shouldn’t come as a surprise to anyone though. Unlike their big brother the Ronin’s, these use a smaller 8-inch woofer in a sealed enclosure. When you move them out from the headwall to get the deepest, widest soundstage, the lowest octave of bass will drop off slightly. That’s not a big issue if you don’t mind compromising and moving the Sentinel’s back towards the wall to utilize its gain. The other issue was loosing the midrange fullness of an all tubed system. Again, this is a compromise. All of the great qualities of tubes that I love relented to the sheer authority, raw power and much better overall control of solid-state. This was a seriously good mating.

In The End

Before I finish up, let me wander for a bit. Through the years I’ve had good exposure to dipoles of nearly all flavors. Some have been extremely good, some have been just average. To date I think my favorites have been either the Maggie 3.6’s or a pair of highly modified pair of IIIA’s that a good friend of mine owns. I’ve recently heard the original prototype’s of the Martin Logan Monolith’s that were pretty amazing. I’ve always felt that an open baffle speaker has stood head and shoulders above the vast majority ‘box speakers’. They do ‘audiophile’ things that your typical speaker could only wish it could do. Of all speakers, I think open baffles are my favorite.

What I heard in each of my different sized rooms confirmed that an open baffle speakers tends to be a fair amount more room friendly than your typical box speaker. Sure, you will have bass issues to deal with in smaller rooms but from the midrange up, they load a room in a way that is far friendlier. This seems to make placement much more forgiving. Don’t forget that if you want a big, deep soundstage you will need to allow yourself some distance from your headwall.

The Sentinel’s give you a couple of advantages over your conventional dipole speaker such as a Maggie. First, is sheer SPL’s. These can play considerably louder than a ribbon. At high SPL’s ribbons start to compress and slap. Also the bass from the Sentinel’s (subjectively) has more slam. Again, this is because of the limited excursion of a panel. On the opposite side of the spectrum, the downside of the Sentinel over a ribbon is transparency. Ribbons have a cleaner, slightly more accurate sound to them (to my ears). The bass from a ribbon is significantly ‘tighter’ sounding. In turn, a ribbon tends to have a more ‘presence’ in its presentation. Neither of these are ‘right’, neither are ‘wrong’, both are merely preferences and tradeoff’s in the way you want your music served up.

In my time with the Sentinel’s I found that they do like a bit of juice. They really start to come alive after about 90dB. Tubes worked just fine when they were driving them (without the Zobel option) but I have to admit they did come to life when I fed them with a solid-state power plant. I can only imagine what they might have sounded like with a couple of hundred watts of pure Class A power. I’d bet good money they would perform even better.

The Nomad Audio Sentinel’s are quite the speaker in my opinion. They might not do everything perfectly but at just over $2000, I’d much rather live with them than most any other speaker in that price range that I’ve heard. If you get a chance, you really owe it to yourself to give them an audition.

My Ratings

Please keep in mind this rating system is used to compare the Nomad Audio Sentinel’s against absolute perfection, or a money no object speaker design. If you see what you think may be a low(ish) score, it’s because there are speaker designs that are even more refined but consequently cost considerably more. To top that off, if I assign 5’s across the board, I’ve just painted myself into a corner leaving no room for that ‘ultimate’ speaker. You won’t see me handing out many 5’s. In turn, I feel I need to leave room in the ratings system to accommodate those speakers.

Specifications

Type: three-way open baffle loudspeaker

Frequency Response: 54Hz to 24kHz (+1.25 -3dB). Real bass response will reach into low 30's in typical listening rooms.

Tweeter: 28mm silk dome, Lightweight Ferro-fluid and Neodymium ring motor.

Midrange: 17cm TPX cone 400Hz to 1500Hz.

Woofer: 22cm 2Hz to 400Hz with loading being sealed Box Q=.65

Impedance: 8 to 12 Ohms across entire audio band

Sensitivity: 84dB/W/m

Electrical Phase: -30 to +35 degrees across entire audio band

Dimensions: 22 x 10 x 17 (HxWxD in inches)

Weight: 35lbs each

Price: $2100 for bamboo, $2600 for wood finish

Company Information

Nomad Audio
750 Lee St.
Elk Grove Village, IL 60007

Voice: (847) 847-8525
E-mail: info@nomad-audio.com
Website: http://www.nomad-audio.com