I am hesitating between the PCM 25.22i
After seeing measurements of several different PMC models posted in this thread you are still hesitating?
Tread carefully. Can you get a home demo?So yes, facing a certain dilemma that is very time-consuming!
The problem with doing that is that sound won't do as it's told and follow the foam. It'll go just as happily sideways, and fill the entire box. What you're suggesting is pretty much what's done with sealed bass boxes, which are filled with foam/wool or whatever.I think we're on to something here - if the problem with TL design is creating sufficient length due to the difficulty in forming long enough 'tunnels' using rigid wall construction, why not use a soft material that folds and can pack tightly into the cabinet?
REW in Charing Cross Road?…a higher end audio store in 1974…
…we had master tape copies to play…
…we sold Revox and other large-spool tape recorders including the fancy Technics model with oval head-block
Tread carefully. Can you get a home demo?
We're experiencing some confusion here. The nautilus has a long line to almost completely absorb the rear radiation of the drivers. This is a good thing, and it is an unusual design. Linkwitz's PLUTO or whatever he called the last iteration of the sewer pipe speakers work in a similar way. The principle is simple - if the rear radiation has to go through 3 feet of polyfill, it will eliminate that energy from bouncing back through the cone. Of course if you want to absorb bass, you need a long line.
The term 'transmission line' also refers to a certain class of woofer loading where the bass output of the speaker is augmented by using the back-wave of the speaker. The idea is very simple - you create a labyrinthine path which delays the rear wave of the speaker, and when it comes out the box, it reinforces the front radiation at a certain frequency set by the length of the path. This is called 'quarter wave' loading sometimes, because the rear radiation is delayed by 1/4 wavelength compared to the direct radiation. As I'm sure most know, if you delay something by 1/2 the wavelength, it will be out out phase; 1/4 wavelength and you get a bit of a boost.
There are other ways of using the rear radiation of a speaker to boost bass. The most common is a bass reflex, where you have a resonating mass of air which really likes vibrating at a certain frequency. A passive radiator does the same thing. This principle can also be combined with a transmission line, where you have a short transmission line which 'loads' a port. This is called a mass loaded transmission line, also called an MLTL. It allows you to get more bass in a smaller box than a transmission line. You will also notice that sometimes the line starts big and gets narrow, or starts narrow and gets big. The former is called a tapered quarter wave tube (TQWT). The latter is sometimes called a back loaded horn.
Quarter wavelength augmentation is not without problems. First of all, the claims that it has less group delay are somewhat misleading. Group delay is a function of the roll-off of the bass; it doesn't really matter what means you use to achieve it. A QL speaker will have a roll-off somewhere between a sealed and ported enclosure, and group delay will be somewhere in between. The idea that group delay is signficiant in bass indoors is sort of ridiculous anyway, we're talking about frequencies which bounce around the room a dozen times before they even reach one period.
The biggest problem with QL augmentation is that the line needs to be big, but for DIYers and expensive speakers this can be overcome.
There are some issues with resonances in the line; these manifest as a series of sharp peaks in the response in the midbass. This is where stuffing comes in. By adding acoustic absorption to the line, you can let the bass pass but kill all the other stuff. This is necessary for good performance.
In my opinion, the best thing about QL augmentation is not the bass but the handling of the back wave of the speaker.
PMC speakers aspire to use quarter wavelength augmentation. I have no idea what they think they are doing, but with their smaller speakers, the lines aren't long enough, and to make them as long as they are, the sectional area of the line is quite small. Both of these factors compromise the performance of the line.
Most DIY designers trying to make reasonably sized transmission lines will use the MLTL topology, which offers reasonable size, good back wave absorption, and good bass efficiency. See this link for an audioxpress article about a DIY speaker by Paul Kittinger which uses MLTL loading.
REW in Charing Cross Road?
When I lived in London in the 70s, I visited an audio dealer called Studio 99 (I think), at Centre Point, several times. They had all that gear and I remember being very impressed with the Amcron hybrid speakers driven by, of course, the Amcron amps. I think the source was TEAC reel-to-reel. This was before the whole Linn-Naim thing took off, so probably 1974-5. Thanks for reminding me.When I started in a higher end audio store in 1974 (as a Saturday boy originally), Crown driven IMF speakers were the dogs danglies (excluding the Crown/Amcron electrostatic hybrids, JBL L200's and AR LST's)and we had master tape copies to play into and through them, as 'vinyl' wasn't judged any good (well, most direct drives fed back due to bad siting and springy belt drives suffered wow quite often back before the Linn LP12 took the UK over ). An IC150- D-150 driven pair of IMF pro Monitor III's could sound awesome with a good source and they did an excellent job with a tape from Angus McKenzie (a notorious reviewer of the era) of Hendon Brass band playing Stars & Stripes Forever
We bought the final batch of III 'Improved' speakers (half a dozen pairs in around 1981 I think) and driven by 'bolt up' Naim and an LP12 source, they were horrible (the amp had a higher output impedance and Lord knows what an LP12 was doing to the bass back then, but we didn't care in our ignorance with the bass light boxes we started to sell and almost brain washing at the time - the master tapes had been 'mislaid' over the intervening years, although we sold Revox and other large-spool tape recorders including the fancy Technics model with oval head-block.
In a smaller room, the very low bass of the IMF could give uncomfortable 'pressure waves' on vinyl sources of the times and the RSPM mk4 model from 1976 offered considerably tautened low bass with lf filter options too. I'd love to hear a pair today driven from a decent modern amp and digital source (to be fair, the ancient Crowns I still use were measured by ken Rockwell and still offer a balanced solid result even today I think).
PMC make several ranges of products all aimed at a slightly different market. The 'hot' ehf is common to most I've heard I think and a definite design intention going back to the LB1 original, which sounded great on a Quad 606 amp but horrible on the then market leading Naim 250 (why my then employer turned the brand down as 'we' were so Naim centric back then). The top domestic model Fact Fenestria are awesome sounding boxes though, but totally overpowering in a smaller space and a chat with Pete Thomas, whom I've known a little bit since the late 70's when he was at the BBC, he indicated that prospective purchasers should look to DSP to blend the speaker to the room better.. I think in fairness the domestic PMC's have been improved over the nearly? ten years since the speaker reviewed here was introduced. The market for these doesn't look so much at price and Naim is routinely used with them locally...
As for an impending ATC review. I'm dreading it myself as I've held a brightly lit torch for the brand for thirty years now and owned and loved a few of their products, only giving up some much cherished 100A's for love and marriage (not saying more on that one ).. Hopefully the 19's are current issue with the latest tweeter design. Listening to so many bling boxes in recent times, I return to ATC's (11's are fine in smaller UK rooms) with a huge sigh of relief! I also fell in love with the Kii Three's as well and going against pre-conceived ideas of Class D amp modules, they sounded so clear and sweet in the dem I sat in on.
Forgive the lengthy post and hope some of the above experiences from the mid 70's bring back memories for some of you.
When I bought my BC1s from Audio T in West Hampstead in 1976, they were using Crown/Amcron as their reference amp.No, the more rural KJ in Watford
There were only three 'domestic' Crown dealers in the UK back then, the third being The Audio Centre in Sheffield from memory..
My brother in law had a pair. Great sounding speakers. I told him if he ever wanted to sell them, I would buy them. Of course they were sold and I never got a chance to purchase them.
Are you still using the Richmond 3 as surrounds ?My speakers, Castle Harlech S2, are of twin pipe quarter-wave design meaning each woofer is laoding it's own pipe. They are 2 way design, app 55L of internal volume, 2*130mm woofers and 28mm soft-dome tweeter. Crossover is at 1800Hz, LR24. Pipes are tappered and stuffed with port at the base of the plyinth.
Here is the uncorrected in-room response, measured from LP which is 4 meters from the speakers. As you can see it is relatively smooth for a speaker of such age and no spikes there:
View attachment 71398
Another thing why I like these speakers is this:
View attachment 71399
As you can see they are able to achieve THD of 1.75% while playing 40Hz tone at SPL of 100dB measured at LP, so 4 meters from the speakers. At 95dB measured at 1m they give 0.45% THD. Not often you can find such clean bass and my guess is this is due to twin pipe quarter wave TL design.
That is not high fidelity!
Thanks. I'm sure you know that this can be tricky to get right. ;-)By eye usually anchored at 200 hz.
I do indeed. I am not sure a mathematical solution is ideal though as weighting of the frequency bands is not equal audibly speaking.Thanks. I'm sure you know that this can be tricky to get right. ;-)
Thanks Amir! Wow, that was much worse than I expected. I’ve always liked the clean look of PMC speakers. This was a myth buster review. These speakers are held in high regard by many audiofools. Measurements don’t lie. Hope the company sees this review. Poorly engineered speakers with low performance at a HIGH price. Not good.