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Out­stand­ing sound qual­ity and mea­sured per­for­mance from this small two-way de­sign.


Furby any­one? Prob­a­bly an ob­scure ref­er­ence, con­sid­er­ing that the Furby (a small elec­tronic ro­botic toy with big ears) craze peaked more than 20 years ago, but when I first un­packed Krix’s Eso­terix Al­tum and saw those enor­mous ear-like wave­guides on the front baf­fles, a Furby was the first thing I thought of… pos­si­bly be­cause of my daugh­ter’s pas­sion for them when she was a child. For jour­nal­is­tic ac­cu­racy, I Googled a pic­ture of a Furby to re­fresh my vis­ual mem­ory and dis­cov­ered the Al­tum looks noth­ing like one… but first im­pres­sions are first im­pres­sions, even if they’re not par­tic­u­larly faith­ful!

My first real in­tel­lec­tual thought was that Krix must have had a very good rea­son to fit such a large waveg­uide to such a small speaker, not only for rea­sons of cost (wave­guides like this cost a motza!) but also be­cause the ap­pear­ance of said waveg­uide is likely to alien­ate a sig­nif­i­cant per­cent­age of buy­ers. Show this page to your other half and ask what she thinks of it. If she likes it, you win! And it’s not like you can hide the waveg­uide with a grille… though I do un­der­stand this is some­thing Krix is ac­tu­ally work­ing on, so the best of Bri­tish with that one guys! (Ed­i­tor’s Note: A grille is now avail­able, but it wasn’t at the time of this re­view.)

ElE­phant in thE Room

Yep, I just have to start with that waveg­uide. It seems that the rea­son for it harks back to fun­da­men­tal re­search on sound qual­ity ini­ti­ated by the fa­mous acous­ti­cian Floyd E. Toole (now re­tired) when he was work­ing at the Na­tional Re­search Coun­cil of Canada (be­fore he was head-hunted by Har­man to im­ple­ment his dis­cov­er­ies on speaker brands such as JBL, Revel, In­fin­ity, and Lex­i­con).

Toole was one of the first re­searchers to prove that if a loud­speaker does not have a flat fre­quency re­sponse, pretty much noth­ing else the de­signer does to try to im­prove its sound qual­ity will mat­ter.

How­ever Toole then went on to prove that once a de­signer has en­sured that the speaker has a flat fre­quency re­sponse, then the next big­gest con­trib­u­tor to the per­cep­tion of sound qual­ity (af­ter bass ex­ten­sion) is well-con­trolled lat­eral dis­per­sion.

Con­trol­ling lat­eral dis­per­sion is par­tic­u­larly dif­fi­cult to do in a two-way de­sign be­cause as the bass/midrange driver de­liv­ers higher and higher fre­quen­cies, lat­eral dis­per­sion nar­rows pro­gres­sively, but then, when the tweeter takes over, the dis­per­sion in­stantly widens, so there’s dis­con­ti­nu­ity between the two which is au­di­ble. This is just one rea­son why many man­u­fac­tur­ers opt for three-way de­signs, de­spite their cost and com­plex­ity… and, it must be said, their short­com­ings.

Krix’s so­lu­tion was to fix a waveg­uide to the tweeter to con­trol its dis­per­sion in such a way as to create fewer lat­eral re­flec­tions in the lis­ten­ing room, the re­sult be­ing, ac­cord­ing to An­drew Ben­nett, one of Krix’s en­gi­neers, that lis­ten­ers would: ‘ Hear more of the mu­sic and less of the room’.

The waveg­uide wasn’t ex­actly sim­ple to de­sign. An­other of Krix’s en­gi­neers, David Mur­phy, says he went through nearly 100 com­puter sim­u­la­tions be­fore nar­row­ing the de­sign down to the six he thought would work the best, af­ter which he ac­tu­ally con­structed all six for full ‘real-world’ test­ing and au­di­tion­ing by the com­plete Krix team. It’s sig­nif­i­cant to note that Krix is ac­tu­ally one of the few com­pa­nies in the world that could do this, not sim­ply be­cause of its ac­cess to the nec­es­sary test equip­ment, but also be­cause it has its own wholly-owned R&D fa­cil­ity and fac­tory in Ade­laide, South Aus­tralia, where it man­u­fac­tures all its own cab­i­nets, acous­tic horns… and wave­guides… and crossover net­works. As a world leader in the de­sign, man­u­fac­ture and in­stal­la­tion of com­mer­cial cin­ema speaker sys­tems for the­atre com­plexes around the world, Krix has be­ing man­u­fac­tur­ing horn-loaded sys­tems for many years.

The sound waves Mur­phy’s waveg­uide are con­trol­ling are gen­er­ated by a Reve­la­tor tweeter built specif­i­cally for Krix by fa­mous Dan­ish man­u­fac­turer ScanS­peak. Rated with a di­am­e­ter of 26mm, it’s a ring ra­di­a­tor de­sign with a patented phase plug at its cen­tre, a patented ‘Sym­met­ri­cal Drive’ (SD-2) neodymium mag­net and a non-res­o­nant rear cham­ber.

Bass and midrange fre­quen­cies are de­liv­ered by a 165mm driver that has a coated wood-fi­bre cone that is driven by a 38mm-di­am­e­ter voice coil and a neodymium-iron-boron mag­net whose size is much larger than one would usu­ally expect on a driver of this cone di­am­e­ter. All driver el­e­ments are mounted on a cast alu­minium bas­ket. The driver is fixed to the front baf­fle from the in­side of the speaker, so I could not ac­tu­ally see the bas­ket to mea­sure it for my­self, but the di­am­e­ter of the mov­ing part of the driver is 140mm, so I’m happy to ac­cept Krix’s mea­sure­ment of 165mm. How­ever the im­por­tant mea­sure­ment is the Thiele/Small di­am­e­ter of the cone, which I mea­sured at 130mm, which gives an ef­fec­tive cone area of 132cm². The cone has a con­ven­tional out­wardly-rounded dust cap made from the same ma­te­rial as the cone it­self, which I think gives bet­ter sonic re­sults than when the dust cap is made of a dif­fer­ent ma­te­rial. It’s harder (and thus more ex­pen­sive) to make a wood-fi­bre dust cap than a syn­thetic one, but it seems pretty ob­vi­ous to me that Krix is not try­ing to cut costs in the build of the Eso­terix Al­tum.

The Krix Eso­terix Al­tum is a bass-re­flex de­sign, but you’ll be look­ing in vain for the bass re­flex port if you only check out the front baf­fle, rear panel, sides and top of the cab­i­net, be­cause Krix has hid­den it away on the bot­tom of the cab­i­net, to­wards the rear. A side view of the cab­i­net will im­me­di­ately show how it’s man­aged to do this with­out block­ing the port, be­cause the rear of the bot­tom of the cab­i­net slopes up­wards, so only the front sec­tion of the cab­i­net makes con­tact with the sur­face on which it’s sit­ting.

That sur­face can be a book­shelf or wall mount or some other flat sur­face but—as with all other small two-way de­signs—for best per­for­mance you should be plac­ing these Eso­terix Al­tums on stands. How­ever, be­cause of the lo­ca­tion of the port, this means that many stands won’t be suit­able, so Krix has op­tional stands avail­able for it ($1,195 per pair in satin fin­ish). The tops of these stands fit neatly into a squar­ish cut-out in the base of the Al­tums, af­ter which the speak­ers can be screwed firmly to the stands to give a truly seam­less look, ex­actly as if the speaker is an in­te­gral part of the stand it­self and not just sit­ting on it. The stand has ca­ble man­age­ment in­ter­nally, so that your speaker wires will be hid­den from sight. Un­like some stands with ca­ble man­age­ment, you don’t have to spike the base of Krix’s stand to al­low space for the wires to run out, be­cause it has four cus­tom rub­ber feet that el­e­vate the base 15mm from the sur­face on which the stand is sit­ting. (Of course you can al­ways use spikes rather than rub­ber feet if you’d pre­fer.)

If you had gone look­ing for the bass-re­flex port, you cer­tainly would have no­ticed the very large metal (not plas­tic, mind you!) mount­ing plate on the rear panel of the Eso­terix Al­tum which holds two pairs of speaker ter­mi­nals, the up­per pair ac­cess­ing the high­pass sec­tion of the crossover di­rectly, and the lower pair ac­cess­ing the low-pass sec­tion. (This would seem to be the ob­vi­ous strat­egy and there­fore not worth men­tion­ing, ex­cept that I have run across de­signs where the opposite is the case, so I think I should men­tion it.) The ter­mi­nals them­selves are very high-qual­ity gold-plated multi-way types. If you don’t bi-wire or bi-amp, the ter­mi­nals must be shorted out with the gold buss-bars that are pro­vided with the speak­ers.

The Krix Eso­terix Al­tums are not large, and the an­gu­lar shape of the cab­i­net (Krix refers to the shape as ‘strik­ing pris­matic ge­om­e­try’) means that the di­men­sions listed in the spec­i­fi­ca­tions (390×260×380mm HWD) don’t tell the whole story. Best to see a pair in the flesh, so to speak. Also, the de­sign of the cab­i­net (and Krix be­ing able to build its own cab­i­nets cost-ef­fec­tively) means that all the joints in

The sound waves the waveg­uide are con­trol­ling are gen­er­ated by a Reve­la­tor tweeter built in Den­mark specif­i­cally for Krix

the Al­tum are fully mitred, so there are no vis­i­ble joins (at least there were none that I could see on my re­view sam­ples).

You’re cer­tainly not left want­ing for cab­i­net fin­ishes, be­cause Krix of­fers the Es­to­terix Al­tum in Black Ash, At­lantic Jar­rah, Wal­nut and Black­wood (all real wood ve­neers) plus a very hip colour it calls ‘Cola’ (which, once again, you have to see in real life to ap­pre­ci­ate. The word alone doesn’t give any inkling of what it looks like, and the photographs I’ve seen don’t do it jus­tice). You can also or­der ‘spe­cials’ in any tim­ber ve­neer or high-gloss colour you like, but cus­tomis­ing the speak­ers by spec­i­fy­ing a pre­mium fin­ish in­creases the price to $7,895 per pair. (Match­ing stands in the same pre­mium fin­ish would then cost $1,745 per pair.)

Lis­ten­ing ses­sions

Wow! That was my very first thought when I first heard the Krix Eso­terix Al­tums in my lis­ten­ing room. I’m pre­pared to bet se­ri­ous money that that’s what you’ll think too when you hear a pair for the very first time. What caused this re­ac­tion? The sound­stage. Note par­tic­u­larly that I have not said ‘stereo sound­stage’ but just ‘sound­stage’, be­cause there is no ‘left’ or ‘right’ with the Al­tums… in­deed there are no speak­ers in front of you at all, just the per­form­ers—and that’s as true for a sin­gle gui­tarist as it is for a full or­ches­tra. It’s just as if the mu­si­cians are right there in front of you… and the sound­stage is not just wide, but also high and deep. The au­ral vista these speak­ers create is just stun­ning. I don’t think I’ve heard its like be­fore from any pair of small two-way stand-mount speak­ers.

What makes this per­for­mance even more amaz­ing is that I hadn’t yet got around to op­ti­mally plac­ing the speak­ers in the room… I’d just un­packed ‘em, hooked ‘em up and was about to start the process of find­ing out where in the room they sounded the best. It turns out that they will sound their best al­most any­where in a room, and also in any room, a fab­u­lously use­ful trait that ap­pears to be a di­rect func­tion of the waveg­uide. Yes, the bass did gain some added ex­ten­sion and also some rather use­ful up­per low-fre­quency boost when I moved the Eso­terix Al­tums fur­ther back to­wards a rear wall and, un­like most small speak­ers, the depth of the sound­stage did not di­min­ish when I did this.

An­other sur­prise was just how loudly I could play the Eso­terix Al­tums with­out over­load­ing them. One of the Achilles Heels of small two-way speak­ers is that there’s only the sin­gle bass/midrange driver to create the bass/midrange sound (which is where most of the en­ergy in mu­sic pro­gram ma­te­rial is lo­cated) and this same driver also has to dis­si­pate the power from your am­pli­fier that’s wasted as heat in the voice-coil, rather than be­ing con­verted to cone move­ment. As a re­sult, it’s not only usu­ally quite easy to over­drive a two-way, but also al­ways very easy to cause it to com­press dy­nam­ics. Now I am not say­ing you can’t over­drive the Al­tums—you can—but you will be able to play them re­ally, re­ally loud if you so wish.

As noted pre­vi­ously, the bass from the Krix Eso­terix Al­tum de­sign was re­mark­ably deep and pow­er­ful for a small two-way stand-mount de­sign. But I was even more im­pressed by the clar­ity of the bass, such that lis­ten­ing to the ti­tle track of Dido’s ‘Life For Rent’, the low drum sounds are im­pact­ful, and it’s easy to dif­fer­en­ti­ate the sound of the ‘live’ drum­ming from that of the drum ma­chine (and not, like the old joke, be­cause the drum ma­chine is keep­ing cor­rect time!). This track is also a per­fect ve­hi­cle to demon­strate the su­perb clar­ity of the midrange with fe­male vo­cal. Dido’s voice just hangs in the air between the speak­ers, ex­cept for those mo­ments when there’s some out-of­phase vo­cal ef­fects (de­lib­er­ate or un­in­ten­tional?) which the Eso­terix Al­tums then de­liver with al­most un­be­liev­able ac­cu­racy.

The Al­tums’ out­stand­ing ca­pa­bil­i­ties across both the bass and the midrange were demon­strated per­fectly play­ing Beethoven’s Sonata No 32 Op.111 with Richter at the key­board. I flinched at the power of his hands as he ab­so­lutely smashed it out in con­cert. You can also be im­pressed by the way the Eso­terix Al­tums de­liver the same pi­ano tone from the very low­est notes on the key­board to the high­est.

Krix of­fers the Es­to­terix Al­tum in Black Ash, At­lantic Jar­rah, Wal­nut and Black­wood plus a very hip colour it calls ‘Cola’

But the Eso­terix Al­tums could do more than re­pro­duce just one pi­ano pre­cisely: they also eas­ily man­aged two; am­ply proved when I played Fan­ta­sia on a Theme of Thomas Tal­lis, as per­formed by Mark Beb­bing­ton and Re­beca Omor­dia. All the pi­ano sound on this new disc (‘The Pi­ano Mu­sic of Ralph Vaughan Wil­liams’) is su­perb, and was su­perbly de­liv­ered by the Eso­terix Al­tums, but please lis­ten par­tic­u­larly care­fully to Wil­liams’ amaz­ing ar­range­ment of Bach’s 6 Schübler Cho­rales to hear the sonic com­plex­i­ties these speak­ers are ca­pa­ble of re­veal­ing.

The Reve­la­tor tweeter in the Eso­terix Al­tums de­liv­ered the per­for­mance that no doubt gave it its name, be­cause it was cer­tainly rev­e­la­tory. The very high­est har­mon­ics of bells and tri­an­gles were re­pro­duced with per­fect ac­cu­racy… so ac­cu­rate that it re­quired no stretch of the imag­i­na­tion to sup­pose that some­one was in the room strik­ing them, rather than the sound hav­ing been recorded. If you reg­u­larly play high-res mu­sic which has mu­si­cal con­tent recorded above 20kHz, this is the tweeter you should be lis­ten­ing to.


I had been won­der­ing why Krix had called this model ‘Al­tum’ be­cause it seemed out of kil­ter with Krix’s usual con­ven­tion for nam­ing its mod­els. Af­ter I’d con­sulted a Latin– English dic­tio­nary and dis­cov­ered the word ‘al­tum’ means ‘on high’ or ‘from above’, I had to smile, be­cause the Eso­terix Al­tums truly do de­liver sound qual­ity that seems to have come down from the heav­ens.

not bother any com­pe­tently-de­signed Class A/B am­pli­fier or any mod­ern Class-D de­sign, but some older Class-D am­pli­fiers might re­act un­pre­dictably be­cause of the low im­ped­ance. You can see the elec­tri­cal crossover between the low- and high-pass sec­tions takes place at 39kHz, and the bass res­o­nant peaks are at 23Hz and 65Hz, at 35Ω and 28Ω re­spec­tively. The fre­quency of the ‘sad­dle’ between them pre­dicts that you should not expect any sig­nif­i­cant bass be­low 39Hz.

New­port Test Labs mea­sured the sen­si­tiv­ity of the Krix Eso­terix Al­tum, us­ing its stan­dard test pro­ce­dure, as be­ing 87.6dBSPL at one me­tre for a 2.83Veq in­put. This is an ex­cel­lent (and above-av­er­age) re­sult for a small two-way stand-mount de­sign—even though it falls a lit­tle short of Krix’s spec­i­fi­ca­tion of 89dBSPL—that means the de­sign will make good use of the avail­able am­pli­fier power.

As we have come to expect from Krix, the Eso­terix Al­tum is an out­stand­ingly well-de­signed loud­speaker, with a very flat and very ex­tended fre­quency re­sponse and above-av­er­age ef­fi­ciency for its size. Steve Hold­ing

The Krix Eso­terix Al­tum is an out­stand­ingly well-de­signed loud­speaker

Kris has its own wholly-owned R&D fa­cil­ity and fac­tory in Ade­laide, South Aus­tralia, where it man­u­fac­tures all its own cab­i­nets, wave­guides and crossovers

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