October 2009
James's Excellent AdventureHi All.
I have owned 3 versions of the PMC IB2 Transmission Line (TL) loudspeaker over the past 7 years. They have evolved over this timeframe and before I give my thoughts on the current version (IB2i) I thought some background in Transmission Line design might be in order. For all those interested in pictures and technical issues please link here:
http://www.pmc-speakers.com/product.php?mode=view&pid=141Advantages of Transmission line design:
The transmission line (TL) is considered the theoretical ideal as well as being what some consider to be the most complex construction required to load a moving coil loudspeaker design. The basic concept is that 'ALL' of the rear wave information coming off of the woofer - or any loudspeaker driver for that matter - would be absorbed by the enclosure (box) behind the woofer. In the real world this ideal is obviously impossible as the speaker box would be the size of Montana!
The most practical implementation is to create a long tunnel within a cabinet by introducing internal partitioning that folds the line up and down the length of the cabinet. At the very end of the line there is a very specific size of hole that vents directly into the room --- (today's secret - the vent is the same size as the woofer). A major benefit to this construction is that the labyrinth (partitioning) braces the entire structure from beginning to end. This reduces cabinet coloration created by the effects of the outer walls of the cabinet flexing. Cabinet integrity and internal panel damping are essential to produce a successful design due to the TL's inherent ability to produce phenomenal low frequency extension, even from a modest size box.
The main driver is placed at one end of the TL, which is heavily damped with absorbent acoustic material. The foam or absorption material used, probably the least visually dramatic of the components is tremendously important. It has to absorb all the upper bass frequencies and allow the lower frequencies to exit the vent at the far end of the line 'in phase' with the main driver. It must also be exactly specified to ensure a consistent and balanced back-pressure on the driver by interfacing smoothly with the column of air within the line. With too high a damping, the driver cannot move freely enough, and conversely too little damping produces a lack of control and the result is a low frequency response similar to that of a regular ported design.
When this equilibrium is achieved between the length of line and the acoustic absorbency, the air density increases by up to 30% making the 'effective' line length far greater than its physical length. This back-pressure holds the main driver in a vice like grip and the control is effective over a huge frequency range, reducing unwanted cone movement which lowers audible distortion. This lack of harmonic distortion in the low frequency creates superb midrange clarity as it eradicates the effects of masking. The consistent air loading also facilitates full audible bandwidth at all listening levels allowing for extended periods of monitoring without the risk of fatigue.
So the main advantages of the TL design:
Lower distortion
Improved driver control
Higher SPL
Lower bass extension from a given box size
Consistent balance at all levels
If you think about music being a transient condition then all this technology kind of falls into place. The ability of the loudspeaker to 'stop and go' on command without overhang has obvious major advantages. The loudspeaker follows the signal from the amplifier much more precisely and the result is much more accurate translation of the original input signal.
The IB2's - My 1st Adventure:
The first pair of IB2's I owned was 7 years ago, when they first came out, and were in a 20x25x8 listening room in the beginning and eventually ended up in a 23x16x8 listening room. They had a very full bodied sound with very impressive dynamics. They could sometimes sound a little thick with certain music and not quite as 'fast' or detailed in the 'micro' dynamic area but never aggressive or in your face. So if your room was a little bass sensitive say below 200Hz it could be an issue when the balance between the bass frequencies and the mid/highs was tilted towards the low end. OK with most classical but maybe a little ponderous with jazz and music with a lot of low to mid transient information. I modified my pair to be a fully active version (not condoned by PMC I might add) which tightened up the bass just enough in my room and I have to say was one superb sounding system. I sold them and replaced them with a second more up to date pair of IB2's a number of years later and used them in my more conventional size room = 16x23x8. I dubbed them 'the world's smallest 'full-range' 3-way point source speaker.'
My 2nd Adventure:
The second pair I tried making active as well and very strangely they sounded two thin and lacked body. I actually preferred these newer versions with their passive crossovers over the active version. I assume changes in the transmission line and the fact that the IB2's were never designed to be active may account for some of these issues. Passively they were very spacious and had a great sense of power and ease throughout the frequency response. Actively they sounded too thin and had a harness about the sound that was quite different from my first pair? Passively they were just excellent and my only observation was that again the tonal balance favored the low end and because they were so good and full bodied in the lows they would sometimes not sound quite as 'fast' with transient material in the same way that other speakers I had would - (Ex: Thiel, Magnepan, Martin Logan). My journey continued and I recently sold pair two and purchased a pair of the new IB2i versions.
My 3rd Adventure:
OK so it has only been about a month since my last pair of PMC IB2's left the building and I already missed them. I setup the new IB2i's on 21 inch Sound Anchor stands in sound room one - 16x23x8. They are 10 feet apart center to center are 4 feet from the front wall and I am sitting 10 feet back from the speakers with the midrange drivers just above ear level and their axis crossing about 2 feet behind my head. The first thing I noticed was they have a huge sound stage... really deep and really w--i--d--e! But they sounded a little sterile (oh my god what have I done!) so I decided to curb my excitement and left them to burn in for a few hours - PMC recommends minimum 15 hours.
SEE BELOW
