May 2011

Ultrasone Pro 900I’ve been doing a lot of headphone listening over the past few months, and I’m not talking about portable listening. I’m talking about sitting in a quiet room, connecting the 'phones to the best equipment I have, and letting the music wash over me. Relatively little space in the high-end press is dedicated to headphones, even though they can offer an astonishing level of fidelity and musical enjoyment for a fraction of the cost of an equivalent loudspeaker-based system -- but we at the SoundStage! Network have decided to devote to them a few more words than we have in the past. After being impressed with the Edition 8 from Germany’s Ultrasone, I was curious how much of that performance was offered by Ultrasone’s Pro 900 -- the top sealed design in their Pro series. While at $599 USD the Pro 900 isn’t cheap, it’s within reach of many more audiophiles than is the $1499 Edition 8. 

Ultrasone has been designing and manufacturing headphones since the early 1990s, which makes them a relative newcomer -- most headphone makers have been in business since the early years of electronic recording. Unlike those other companies, Ultrasone focuses exclusively on headphones, and 20 years is more than enough time to have built up a solid reputation; Ultrasone has won numerous accolades and a devoted following of audio professionals. 

All of Ultrasone’s professional models share two characteristics: S-Logic and ULE shielding. In a natural listening environment, sounds first strike the outer ear and are then directed into the ear canal. The S-Logic natural surround technology uses decentralized driver locations to direct sound at the outer ear. This is more akin to listening to loudspeakers, and greatly improves spatial perception when listening through headphones. Another benefit of S-Logic is that because it takes advantage of the natural amplification of the outer ear, or pinna, the required sound pressure level at the driver can be reduced by 3-4dB while resulting in the same perceived loudness. This reduction in SPL is safer for your hearing over the long term, though listening to headphones at high volumes is still unadvisable. 

Nor is S-Logic the only way in which Ultrasone demonstrates concern for those who spend many hours each day using their products. You may have read media reports about possible links between the radiation from cellular telephones and brain tumors. Although the frequencies involved are different, the drivers in headphones also produce electromagnetic waves very close to your head. Ultrasone’s ultralow-emission (ULE) shielding blocks up to 90% of this radiation. Whether or not any health benefits are ever proven, I see no need to unnecessarily irradiate my brain while listening to music. I don’t know which of these two design characteristics might be responsible, but I’ve found Ultrasone headphones to cause less fatigue over long listening sessions than headphones from other manufacturers. 

The Pro 900 is manufactured in Germany from a black, nonresonant plastic with aluminum accents. The look is modern and attractive. The padding around the earcups and on the headband is a soft velour material that is comfortable and “breathes” better than leather. As with most other Ultrasone headphones, the earcups can swivel flat, or fold up into the headband for more convenient transport or storage. The Pro 900 uses the same 40mm, titanium-plated driver as the Pro 2900, the difference being that the Pro 2900 is an open-back design. Ultrasone uses titanium in its top models because that element stiffens the driver, helping it to better track the input signal, particularly transients. 

Ultrasone Pro 900The Pro 900 comes in a hard-sided carrying case with a nylon outer covering and with an assortment of accessories. There are two detachable cords, one straight (3m) and one coiled (4m), both terminated in 1/4" gold-plated Neutrik adapters. A 1/4"-to-1/8" adapter is also provided. The problem with such adapters is that the weight of the adapter plus the weight of the original connector puts a great deal of strain on both plug and jack. Over time, that strain can break the solder holding the jack to the circuit board. Since the cable is detachable, I would prefer to see an additional cable terminated in a 1/8" plug. It would be especially convenient if this additional cable were made shorter, for those who might use the headphones with a portable device. The package also includes two speed-switch earpads -- one gray, the other black -- and a CD of demo material. 

The specifications Ultrasone provides for the Pro 900 are a little vague, as is often the case with headphones. The frequency response is stated as 6Hz-42kHz, but with no window given. On the other hand, since a ruler-flat measured frequency response is not generally regarded as desirable for headphones, specifying the response to be within ±xdB wouldn’t make much sense. Some people wonder why the output of a headphone, or any other component, needs to extend beyond the audible limit of 20kHz. There are two reasons. Frequencies beyond that point interact with each other and with lower frequencies to produce some audible effects. More important, high-frequency response tells you how fast the diaphragm can respond to a change in the signal -- i.e., how well it can track transients. The Pro 900’s nominal impedance is 40 ohms, which should work fine with the vast majority of headphone amplifiers and portables. The sensitivity is given as 96dB, but Ultrasone does not state whether that is at 1V or 1mW. Practically speaking, I got plenty of volume from the Pro 900 no matter what device I used as a source. 


Sealed headphones reduce sound leakage in both directions. If you’re in a situation, professional or personal, in which sound from your headphones might disturb workflow or domestic tranquility, a sealed design is the only choice. On the other hand, if you need to sonically isolate yourself from your surroundings, the 15dB of attenuation that the Pro 900 provides will be welcome. Even in a quiet listening environment, that degree of isolation allows subtle details in a recording to become perfectly evident, as they aren’t competing with ambient sounds. Whether it was the whisper of a performer’s breath, the creak of a chair, or the rustle of a turned page, the Pro 900 let me hear it. 

Musically relevant details, too, were preserved. At their entrance midway through Dream of a Witches’ Sabbath, from Marek Janowski and the Pittsburgh Symphony’s recording of Berlioz’s Symphonie Fantastique (SACD/CD, PentaTone PTC5186338), the chimes were given a real shape -- the Pro 900 conveyed the initial stroke of the hammer, the subsequent swell, and the eventual decay. When the chimes enter a second time, a few measures later, the initial attack was much more subdued, as it should be, but the sound still had shape. In the same recording, I vividly heard the sound of bows bouncing against strings in the collegno passages. I was similarly impressed with the clarity of the transients of percussion sounds, such as the hi-hat cymbal in Ali Jackson’s extended drum solos on Wynton Marsalis Quintet & Richard Galliano’s From Billie Holiday to Edith Piaf: Live in Marciac (CD, Rampart Street WME19457-2). 

The lack of extraneous noise also benefited soundstaging. All the sounds I heard were from the recording, which created a fully immersive experience. While no headphones can image like a good pair of minimonitors, Ultrasone headphones, with their S-Logic technology, do manage to push the sonic image outside of the listener’s head, and with the Pro 900 the soundstage changed from recording to recording. Listening to studio albums, I could tell that there was no real space around voices and instruments, but I didn’t get the same claustrophobic feeling I experience with most other headphones. Orchestras in naturally miked classical recordings opened out in front of me with an excellent sense of left-to-right imaging and a credible sense of depth, and the absence of external noise let me hear reverberations off the back wall of the recording venues. Nor did I appreciate this fidelity to hall ambience only with classical recordings. Listening to Bucky Pizzarelli’s Swing Live (24/96 FLAC, Chesky/HDtracks), I felt more of a sense of sitting there in the jazz club than I have with any other headphones or speakers. 

The Pro 900’s extended high-frequency response endowed instruments with real authentic harmonic textures. One of the best recordings of pure violin sound that I’ve heard lately is Julia Fischer’s second volume of Schubert’s music for violin and piano (SACD/CD, PentaTone PTC 5186 348). Through the Pro 900, Fisher’s violin was as sweet as I could ask for in softer passages, and took on a little more bite in louder ones. There was also an excellent balance between violin and piano. Most of these traits were obviously due to the quality of the recording, but the Pro 900 did its job by neither obscuring nor editorializing too much on what the audio engineers had created. Similarly, woodwinds in orchestral recordings had the requisite reediness, and brass instruments had both tone and sizzle. 

Although I remained impressed by the Pro 900’s portrayal of upper harmonics, over time something began to bother me about the overall timbre of acoustic instruments. I concluded that the Ultrasone's lower midrange -- from a few hundred hertz up to around 1kHz -- was slightly recessed. This wasn’t a gross aberration, but it did subtly shift the balance of some instruments. It focused my attention more on the sparkle of a piano’s initial attack than on the tone; similarly, with acoustic guitars, I heard a tremendously clean pluck, but the body of the note seemed slightly diminished. The characters of voices, too, were slightly altered. Sopranos fared well, but altos and tenors were a bit lighter than I thought they should be. Baritones were a little thin in the upper part of their range, but extra-resonant toward the bottom. Basses were tremendously resonant and powerful, if sometimes a bit indistinct. 

This impression of a recessed lower midrange was at least partially due to the headphones’ response in the frequencies immediately below. As is often the case with closed-back ’phones, the Pro 900 had an abundance of bass and midbass energy. That fact was obvious when I listened to music with considerable content in that region, but I also used a series of test tones to develop a more precise understanding of what was going on. The Pro 900 seemed to have rising output below about 180Hz that didn’t diminish until after about 50Hz, and was noticeable down to 30Hz. Although this was not accurate sound, such behavior is often preferred with headphones to make up for the lack of whole-body involvement that one has with loudspeakers. There were certainly occasions when I appreciated the robust sound of the Pro 900. Sometimes, though, especially with rock recordings, I felt that the bass was a bit much. After listening to a few tracks with heavy kick drum, I felt as if the mallet were striking my head rather than the head of the drum. 

I realize that other listeners may prefer heavier bass than I do, and that the Pro 900 isn’t as disproportional in that regard as many other sealed headphones. However, bass articulation is not subjective. The Pro 900 could cleanly delineate the beginning of a bass note, but there was enough overhang to muddy fast runs in the double basses. Bass-drum rolls turned homogeneous, ignoring the mallet strokes that create the roll. Jazz and rock recordings with sufficiently separated bass lines didn’t seem to suffer as much, since in these types of music the rhythmic drive is dependent on the beginning of each note, not the end. If you audition the Pro 900, be sure to pay particular attention to this aspect of its sound. 


Costing only 40% as much, it’s not surprising that the Pro 900 should suffer in direct comparison to the Edition 8. While the Pro 900 delivered a commendable level of detail, the Edition 8 is truly outstanding in that regard. The Edition 8 can also deliver lightning-fast transients that the Pro 900, and no other dynamic headphone in my experience, could quite match. Even though they’re both sealed designs, the Edition 8 lacks the midbass emphasis and slight boominess of the Pro 900. 

These two models present different overall perspectives on the music. The Edition 8 has a very front-row sound, while the Pro 900 gave me the impression of sitting a few rows back. To put it in the context of speakers: The Pro 900 was like listening to a good pair of nearfield monitors, the Edition 8 like pulling the same monitors up right next to my ears. The Edition 8 makes me hear absolutely everything that’s on the recording, something that’s important for audio professionals but can be a bit much for the rest of us. While I appreciate this technical accomplishment of the Edition 8, I found the slightly more distant prospective of the Pro 900 generally more enjoyable. 

For the past four or five years, my preferred headphones have been the Ultrasone Pro 2500, which has now been superseded in Ultrasone’s lineup by the Pro 2900 ($599). There were many similarities in the sounds of the Pro 900 and Pro 2500. They have similarly extended high frequencies, but the Pro 900’s high treble was a little sweeter and more refined than the Pro 2500’s. Since the driver used in the new Pro 2900 is the same as that used in the Pro 900, and has been improved since the Pro 2500, I would expect this difference not to hold in that comparison. The two designs’ midrange performances were also similarly smooth, but the Pro 2500’s frequency response is subjectively flatter than the Pro 900’s, remaining flat through the midbass and down to about 40Hz. The Pro 2500 doesn’t have quite the Pro 900’s level of articulation at the beginning of bass notes, but it also lacks the latter’s overhang. I’m not sure whether it’s the increased isolation of the Pro 900 or its S-Logic Plus technology that was responsible, but I heard a greater sense of space and more precise placements of instruments through it than I do with the Pro 2500. The two headphones conveyed equal levels of detail, but those details were more obvious through the Pro 900 because they didn’t have to compete with sounds from outside. 

Although I don’t listen to them regularly, I’ve hung on to my Sennheiser HD 600 headphones ($519, but available for significantly less) because so many other reviewers refer to them and so many audiophiles are familiar with their sound. The HD 600, an open-back design, sounds significantly more open and airy than the Pro 900 -- but it’s important not to confuse airiness with detail. The Pro 900 was much better than the HD 600 at conveying the little extraneous sounds on recordings, as well as more musically relevant details, such as the precise attacks and decays of notes. The Pro 900 also had better high-frequency extension than the HD 600, which meant that it more accurately reproduced the upper harmonics of instruments -- but the HD 600 has a more even response through the midrange, which gives instruments and voices a little more of their fundamental tones. Choosing between them for bass performance will be a matter of taste -- neither is accurate. As stated above, the Pro 900 was heavy in the bass and midbass, but the HD 600 is light, beginning its rolloff at around 130Hz. The Pro 900 was much punchier, but the HD 600 never exhibits any bloat or overhang. With the HD 600, as with most headphones, all of the sound is confined within the head, which is what drives many people away from headphones altogether. Use of a good cross-feed circuit can somewhat alleviate this impression, but it won’t produce the type of soundstage you hear from Ultrasone headphones, or any other headphones with decentralized drivers. 


The Ultrasone Pro 900 is an excellent choice of closed-back headphone. Its exceptional high-frequency extension can exploit the ability of high-resolution recordings to deliver harmonic textures. Its rendering of fine detail is not quite up to the level of Ultrasone’s own Edition 8, but it comes remarkably close for 60% less money. My only reservations have to do with the Pro 900’s bass performance, which did not fit my priorities, and its effect on the lower midrange. If you crave a little extra bass in your headphone listening, and are willing to sacrifice a little agility that is inevitably required to get it, then the Pro 900 may be your ideal choice. For me, the Pro 900 brought out the technical aspects of listening to recordings while never sacrificing the music, and was a whole lot of fun to boot. 

. . . S. Andrea Sundaram 

Associated Equipment 

  • Headphone amplifier -- Grace Design m902, HeadRoom Total BitHead
  • Headphones -- Ultrasone Pro 2500 and Edition 8, Sennheiser HD 600, Grado SR60
  • Digital source -- Ayre C-5xeMP universal stereo disc player, Apple iPod (fifth generation)
  • Computer -- Laptop running Windows Vista and Realtek HD audio ALC 272 with coaxial digital output running foobar2000
  • Analog source -- Michell Tecnodec turntable, HR power supply, modified Rega RB-300 tonearm, Shure V15-X MM cartridge into Trigon Audio Vanguard II with Volcano power supply
  • Interconnects -- DH Labs Revelation, JPS Superconductor, QED Silver Spiral
  • Power conditioning -- Equi=Tech Son of Q

Ultrasone Pro 900 Headphones
Price: $599 USD.
Warranty: Five years parts and labor. 

Ultrasone AG
Bernrieder Strasse 17b
D-82327 Tutzing, Bavaria
Phone: +49 (0)8158-9078-0
Fax: +49 (0)8158-9078-50 


US distributor:
Ultrasone Inc.
40960 California Oaks Rd.
Murietta, CA 92562
Phone: (951) 677-4600
Fax: (951) 677-8662