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Since its inception ten years ago, the Harbeth User Group's ambition has been to create a lasting knowledge archive. Knowledge is based on facts and observations. Knowledge is timeless. Knowledge is human independent and replicatable. However, we live in new world where thanks to social media, 'facts' have become flexible and personal. HUG operates in that real world.

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Technical questions to Harbeth's designer

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  • #91
    Alternatives to vented cabinets?

    Alan can you please explain the difference between the construction of a Harbeth speaker and an Advanced Transmission Line speaker.
    Thanks.

    Comment


    • #92
      Alternatives to vented cabinets?

      I would try but what is an "Advanced Transmission Line speaker"? Is that a marketing description? I'm not really up to date with what's on offer.

      In my (limited) experience, the problem with loudspeaker design has always been of "one step forwards, two steps backwards" when considering alternative solutions to old familiar problems. When you say transmission line what I think you are referring to is a speaker box in which the rear of the woofer unit breathes into the room via pathway inside the box. That pathway or line is most conveniently made by an arrangement of shelves that force the rear sound to make a journey which finally exits in an orifice on the front or back of the cabinet. It's an alternative to the vented (ported) cabinet or the sealed cabinet but is a woodworking nightmare.

      What are the claimed advantages? The claim is of deeper bass because the rear motion of the drive unit could (theoretically) create a sound wave that reinforces the sound from the front motion hence (perhaps?) 'bass energy for free'. That sounds plausible except there is never energy for free in nature. As you can imagine, there are only certain frequencies where the sound from the rear of the woofer is perfectly in-phase with that from the front of the woofer, and those frequencies are solely related to the length of the pipe. At all other frequencies, to one extent or another, the pipe's output fully or partially cancels the direct sound from the front side of the woofer because is is not exactly in-phase with it. This is a serious and unavoidable problem.

      This design conundrum of pipe length, frequency and phase has been insoluble for centuries which is why church organs are still built with a series of pipes of varying length according to the note to be played. 'One pipe' length doesn't suit all bass frequencies: a pipe is only at it's best over a narrow range of frequencies. This becomes a problem when a loudspeaker woofer is mounted at one end of a fixed-length pipe.

      The only way to avoid the unwanted cancellation due to the out-of-phase sound is to stuff-up the pipe to prevent the rear sound permeating down the pipe and exiting into the room to mix with the wanted front sound; but this would block the pipe at the wanted (in-phase) frequencies. This duality can't be achieved. The pipe cannot be selectively open or closed at specific notes nor can it vary it's length according to the music as a trombonist would slide his trombone. The pipe geometry is totally fixed inside the speaker cabinet.

      In accordance with what you would expect from (acoustically) short pipes in real-world (small) speakers, regardless of whatever is claimed to be gained at say, 40-50Hz, pipe speakers I've measured feature a significant dip in overall energy in the lower midrange due to the pipe's output being anti-phase to the main output. Stuff foam into the pipe's mouth and the midrange level recovers. How does this depression sound? It seems to diminish weight and warmth - and makes male voice sound like female voice. I find the sound thin, unnatural and uninvolving. I just cannot imagine how short pipes can be made to work - but I know that many designers have tried every conceivable variation on the theme.

      If you compare a domestically sized pipe system (they've been around for at least fifty years so there are various examples) with a sealed box or ported system it's apparent that whatever the claimed benefits, the BBC and other commentators concluded that there was no pressing advantage, and some significant disadvantages. If I remember correctly the phrase used was "the transmission line speaker offer no advantages over other well executed designs including vented (ported) cabinets". I guess that if there were worthwhile advantages at a reasonable cost we'd all be using them!

      If you are curious about the tremendous difficulties of TL design, here is a good overview. Fig 3 and 6 demonstrate the problems of the inevitable series of resonances that exists in all pipes and the suckout I've mentioned in the midrange.

      ++++++++++++++++++++++++++++++++++++++++++++++

      P.S. I made a very quick search for some more information in my paper archives because what I read in the reference (marked here above) reminded me of an analysis published many years ago.

      I said above "the transmission line speaker offer no advantages over other well executed designs including vented (ported) cabinets" I think that a less contentious statement (until I can find that exact quote) is 'the transmission line speaker offer no advantages over a sealed cabinet'. [Reference: D.A. Barlow, H.J.Leak & Co., Letters to the Editor re: A.R. Bailey's 'A Non-resonant Loudspeaker Enclosure design' Wireless World 1973/4]
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #93
        Re: Technical questions to Harbeth's designer

        Alan, thank you very much for your reply and your time spent. A friend of mine is now convinced about the lack of advantages of an Advance Transmission Line speaker and the advantages of a Harbeth speaker technology, as he was wavering between a Harbeth speaker and an A.T.L one (a P-- speaker specifically).

        Comment


        • #94
          What is "speaker voicing"

          Dear Alan

          First, you must excuse me if my English isn't quite up to the standards here as it isn't my first language.

          I've been recently introduced to your speakers and I am fascinated by Harbeth's history, or rather legacy, in speaker design but there is one issue regarding the design process which is not yet completely clear to me and after participating in couple of different discussion forums I noticed that there also seems to be great confusion regarding this subject.

          As far as I've understood voicing is mainly frequency response adjustments when designing a speaker but I am sure there's more to it. How do you voice the speakers and how important are adjustments in frequency response in this regard? As I've understood even the chosen materials for the driver affect the tone (obviously not preferable yet inevitable) of the reproduced sound but do you take that in to consider when voicing a speaker? In other words, where do you draw the line when it comes to conscious decisions regarding the overall tonality of the speaker in your design processes?


          Many thanks, K.A.

          Comment


          • #95
            What is "speaker voicing"

            While we await Alan's definitive statement can I just say that Alan must spend an awful lot of time just listening and tweaking to get the best balance between measurements and natural sound.

            I don't think you can make great speakers without actually listening to them. Far too many speakers (especially from larger companies) are designed by computer and commitee. The computer does the work and the commitee tick the boxes at the end of the design process. The result is an awful lot of stylish but boring and bland speakers. Style over substance.

            Apologies Alan for jumping in.

            Comment


            • #96
              Re: What is "speaker voicing"

              Thanks for the question. Your English is completely perfect and you have raised an important question. Actually perhaps it is the question which every loudspeaker buyer should carefully consider. Where can I start to answer you?

              The first point to make is that I really don't know how other speaker designers think or work. I'm perfectly content to work alone - actually I prefer to do so - but I have worked in a large foreign corporation so I have first hand experience of the commercial, emotional and psychological pressures that are brought to bear on designers generally by me and my staff in the commercial departments.

              All commercial loudspeaker products are brought to market with (presumably) some differentiation from other existing loudspeakers. That differentiation may be cosmetic (cabinetwork), commercial (price) or technical. The marketing process takes control of the cosmetics and commercial details, but is at arms length from the technical process - there is usually some friction between these two functions. This division of responsibility between the pure engineering and creation of the product and bringing it to the consumer is a dangerous gap. The marketing people believe (and have to believe) that they can sell anything and the engineering people obviously don't want to create perhaps insurmountable technical hurdles for themselves - they too have a life outside work. So, with these two functions somewhat pulling against each other, the result tends to be products with attractive ergonomics (styling) and mediocre technical performance. And that's the rut that the speaker industry has been in for many years.

              Consider for a moment that a mass produced woofer, the engine, the working heart of a loudspeaker, the part you're really paying for, costs less than a family meal at a burger bar and it's obvious that at that price level technical excellence has been compromised right through the part. How can a loudspeaker system designer, selecting such a unit from a trade catalogue, expect technical excellence? Of course, he can't. Everything in the drive unit design will have been cut-down to the bare minimum to minimise the supplier's costs, maximise profit and enhance speed of assembly. Many of these woofer parts are made in huge factories by the millions every month, and whilst the QC is generally good, they display no technical excellence whatever. They make a noise, they have adequate technical parameters of frequency response etc. but they are not exceptional.

              As noted, the division of responsibility between sales/marketing and design means that there is a reticence for the one function to probe or interfere too much with the other. When marketing visit the design dept., engineers hide papers, bury prototypes in cupboards and make the their commercial visitors unwelcome and uncomfortable. They certainly don't expose intricate technical concerns that they have - perhaps about latent coloration issues in those cheap drive units they've selected - because marketing people by definition need to believe in the design process.

              Yes, the "voicing" process is significantly one of manipulation of the energy levels in various frequency bands, plus very careful consideration of how the speaker fires into the room - its dispersion characteristics. But that's only part of the story. What if the designers we mentioned above discover, right at the end of the design process and now with the cabinets designed and prototyped, the launch photography done, price lists calculated, pet-customers teased for orders .... that "something doesn't sound quite right"? What would they do? What could they do? What should they do? What would you do? Would you be the one volunteered to crawl into the Engineering Director's office and tell him that the design was a failure? What does happen is that the product is launched and comes to market. The marketing boys either know that it's not a good as it could be but have to sell it anyway, or couldn't care. They know that all consumer products no matter how ill designed or poorly conceived or manufactured will sell at a price with sufficient promotion - example the goods sold in an "everything is a dollar" type of clearance store.

              And the solution? As I hold a remit for marketing, design and finance it seems logical to me that to maximise our financial return Harbeth should create products that are at a technical level excellent and market themselves (happy users are our best advert) without needless promotional expenditure dressing mutton up as lamb. As those speakers last for 20+ years you too benefit.
              Alan A. Shaw
              Designer, owner
              Harbeth Audio UK

              Comment


              • #97
                What amplifier(s) and cables were using during design stage?

                Hi Alan,

                I have viewed the long video showing how you fine-tuned your speakers during the design stage and it well impressed how your work was so hard, exhausting and time-consuming.

                I wonder what amplifier(s) and speaker cable(s) you were using during the design stage for M40 or other models? Any change when designing M40, M40.1 or other models?

                I learned in this website that humble 79 strand speaker cable was recommended as a long-term reference when choosing speaker cables. So are 79 strand speaker cable your reference speaker cable in your lab?

                Cheers,

                Ken Wong

                Comment


                • #98
                  Re: Technical questions to Harbeth's designer

                  AS will not give away what equipment is used. Many have tried... ;) I have a very strong hunch to what the equipment (amplifiers) might be, but I will leave it at that, as it is simply not important to the user. Use whatever floats YOUR boat!

                  Comment


                  • #99
                    What amps etc. ....?

                    Thank you. As you say it is completely unimportant what cables, amps, CD player, stands or beer I drink during design! I am listening through the electronics to the speakers. The absolute and most vital thing for me is that the electronics are stable from month to month, decade to decade. This rules out tube electronics which slowly but inevitably degrade from the moment they are first switched on. So even though I like the romance of tubes, I dare not use them. It also rules out brands that do not have a proven long-term commitment to after care, newer brands, brands of unknown origin etc. etc.

                    Cables: I bought a reel of 79 strand or similar some years ago and it works for me.

                    Note: I am most definitely not saying that my choices are the best. I guarantee that with time, care and money you can do better. But they are a stable base line for me, within my budget and use common industrial parts so available for many years. I have many amps of the same brand/model as backups and all can still be serviced to original spec.. That's good enough for me! I hesitate to name names simply because I am not kidding myself or you that what I use is in any sense an audiophile system. It isn't. It's a ordinary bloke's average+ system and no more.
                    Alan A. Shaw
                    Designer, owner
                    Harbeth Audio UK

                    Comment


                    • TechTalk videos .... and long term archiving etc.

                      Originally posted by kckwong723 View Post
                      ... have viewed the long video showing how you fine-tuned your speakers during the design stage ...so hard, exhausting and time-consuming
                      Long the TechTalk video may be (sorry) but what you are seeing is just a few minutes on one day, one of many days spread across months or years. Looking back from twenty or fifty years, nobody will believe that such a painfully slow process was the correct way to design speakers. Surely no designer would voluntarily invest so much mental effort into a design unless there really was no alternative method. And that is the point we are stuck at; we have all the fancy test equipment, it's reliable and fast but it doesn't tell you how the speaker sounds, in a room. It tells you how it measures and that's a completely different thing - or may not be. I just wish there was a better way.

                      I dread to think what it must be like to work as part of a design team. Can you imagine dancing around changing this or that component, asking the panel of listeners what they prefer .... better/worse? yes/no? keep/reject? What would come out of that collaborative effort would be a product reflecting the wishes of the dominant personality of the listening panel. That's human nature.

                      We have recorded about 800GB of raw video footage in the past few years covering many aspects of the design and manufacture including videos of measurement sessions at the BBC chamber, village hall and outside in the garden. I think Trevor has mentioned that now we've figured out how to put videos on the website we will start to roll out more next year. We are now very much aware that what we record here may well have a long term curiosity value for future generations and we're trying to use the best current technology to archive as we go. Video recordings we've made since April 09 (such as at the X-Ffactor studios) have been filmed in widescreen 16:9 HD direct to hard disk in the camera. Before that it was 4:3 SD.

                      I'm aware that as internet speeds increase as we eventually go to an all-fibre network we can re-master from the original source files so we aim for the best possible source quality.
                      Alan A. Shaw
                      Designer, owner
                      Harbeth Audio UK

                      Comment


                      • Re: TechTalk videos .... and long term archiving etc.

                        Originally posted by A.S. View Post
                        ...We have recorded about 800GB of raw video footage in the past few years covering many aspects of the design and manufacture including videos of measurement sessions at the BBC chamber, village hall and outside in the garden. I think Trevor has mentioned that now we've figured out how to put videos on the website we will start to roll out more next year. We are now very much aware that what we record here may well have a long term curiosity value for future generations and we're trying to use the best current technology to archive as we go. Video recordings we've made since April 09 (such as at the X-Ffactor studios) have been filmed in widescreen 16:9 HD direct to hard disk in the camera. Before that it was 4:3 SD.

                        I'm aware that as internet speeds increase as we eventually go to an all-fibre network we can re-master from the original source files so we aim for the best possible source quality.
                        Sign me up as customer #1 for the DVD/Blu-Ray documentary ;-)

                        Comment


                        • Unequal cable lengths - any problem or not?

                          With the amp on one side of the room, is it ok for the speaker cable runs to the two speakers to be of unequal lengths? I thought not, but my recent professional cable installation was done with 3 metres of cable for one speaker and 15 for the other. Decent quality 4mm square section copper cable. Are the unequal lengths a factor?

                          Comment


                          • Screw torques?

                            Should the woofer and tweeter screws be tightened to specific torques? Are there any torque specifications available somewhere?

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