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Woofer driver positioning on baffle - in front or behind baffle?

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  • Woofer driver positioning on baffle - in front or behind baffle?

    First i have to thank you for making such great speakers. Previlosi i ovned some Chartwell PM 110 speakers who I loved, but the woofer made a noise after 20 years of use, so i was lokking for a replasement. I have looked for years, until i discovered Harbeth. They have the same "sound", but they have more air and detail. The Chartwell also used thin wall construktion(but in a vented disign), so there has to be something with it I like. I Think they have a fullness in voices(around 100 - 300Hz) that other speakers lak. It also creates the big holografic room of the speakers I think.

    Then to my questions. In many designs you please the woofer on the back of the baffle. What is the advantages of doing that?

    Also, I was wondering about some of the crossoverparts you seem to use.I recon you use them for their sound, but it looks like polyester capasitors and I have always heard they were infirior of polypropylens. So can polyestercaps sound as good as/better than polypropylens? Pardon for my bad english, it is not my first language.

    Thank you for this great website and for all the information you give about speaker designing. Its great!

    Best regards Mattism

  • #2
    Welcome to the HUG. I understand you perfectly. Good questions, thanks for the opportunity to do some research for you and to reply.

    Yes, I think that there is a common origin for the Harbeth and Chartwell brand. There is also a connection with the Rogers brand. They are all intellectually rooted in the BBC 'school' of monitor speaker design. Harbeth was formed in 1977 and I believe that the Chartwell company was already in existance by then. I looked-up in my archive 'HiFi Choice, Loudspeakers' No.26/1881 and I see that by 1981, the Chartwell speakers were listed as Chartwell/Rogers and the factory address was that of the Rogers factory, Swisstone Electronics Ltd.. So I guess that by then, Chartwell had been absorbed into Rogers. I will see if I can find more information later.

    Anyway, your question was about mounting the driver on or behind the baffle. First. let's show some pictures of this so we can be clear about the concept of flush or rear mounting.

    TO BE CONTINUED;
    Attached Files
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


    • #3
      Sorry for the delay. I thought I'd do a proper little CAD drawing first which I've now done. I need a day or two more on this please.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #4
        Drive units on baffles - sound waves to the listener (Tech Talk video)

        I've made another TechTalk item. This is just part of the explanation as to why the bass/mid driver is mounted behind the panel for some models and in front for other. It's about path length adjustment. There are other factors such as on/off axis response, but this is a major reason.

        Hope this is of interest - I made five 'takes' to try and reduce the length, but I just couldn't without leaving out what I thought was valuable information.

        New TechTalk here.
        Alan A. Shaw
        Designer, owner
        Harbeth Audio UK

        Comment


        • #5
          Time dalays etc.

          Thank you for a great answer, but I still wonder why you put the woofer on the inside. I understand that the time alignment has to be right, but the moving of the woofer into the cabinet do not compensate for the delay, it makes it worse. Would it not be easier to make the right delay in the tweeter and not move the woofer, you still have to make a delay in the crossover, dont you?

          Comment


          • #6
            Ah! Remember - what matters is the relative delay between the woofer and tweeter as I tried to illustrate in my video. By placing the woofer behind (or flush on the front of) the baffle it may well assist aligning the relative delay with the tweeter. Every speaker system design will be different, and this aspect will have to be considered afresh. As with all things relating to quality loudspeakers there are no hard and fast rules.

            Absolute alignment is impossible - relative alignment over a narrow frequency band is (sometimes) possible with a lot of design effort. And providing that you are not frightened by a complex crossover and have the measuring tools and techniques to attack the problem of alignment. It's one of the most difficult aspects of quality speaker design - and not one I can go much deeper into here.
            Alan A. Shaw
            Designer, owner
            Harbeth Audio UK

            Comment


            • #7
              Tilting stands

              Alan, enjoyed the TechTalk video. I was thinking of investing in a pair of heavy studio grade speaker stands that can both be tilted ( forwards and backwards at any angle) and adjusted in height. Do you think being able to adjust my speakers ( M30s) in this way could help with their alignment and relative delay in relation to my seating position. Thanks Philip.

              Comment


              • #8
                Designing speakers and off axis listening

                Originally posted by P.C. View Post
                ... Do you think being able to adjust my speakers ( M30s) in this way could help with their alignment and relative delay in relation to my seating position...
                Thanks for the feedback which is extremely rare and I appreciate. Without feedback I have no idea if I'm pitching at too high, too low or at just the right technical level. I think I've had only two or three comments on these TTs in the past few years so I must assume that they're of zero interest, incomprehensible or boring. They may look shambolic but they do take time to self-produce!

                Ok, thanks to your feedback I can see that maybe I didn't convey quite what I intended. Yes, from one perspective, if you have a flexible stand solution for height and angle you are surely going to give yourself the maximum performance possible from your speakers. You can trim the stand height and/or tilt to compensate for your own height, different speakers and different listening seats so you can 'gear' the speaker optimally to whatever your personal set-up requires.

                On the other hand, at the design stage I have to take a view on all the height and angle factors impinging on listening at home and inevitably settle on what will be to many users, a compromise. The fact is - as I hope I showed in the TT video - that the poor old designer can sort-out the time alignment (in the crossover electrically or physically) but at only one vertical point in space. And that point is usually on or about level with the tweeter's axis. But how many users are willing to either put their speakers on appropriately tall stands to place the tweeter at or about ear level and/or tilt the speaker back? Few I think. But if they don't, then they are, inevitably, not getting the best from the speakers by a substantial margin. They will, inevitably, be listening off-axis and off-axis means, in the crossover region that the amount of energy at the top end of the woofer/bottom end of the tweeter is many dBs below my design intention. In short, the speaker is going to sound less detailed and somewhat 'dryer' in the all-critical presence region than it is capable of listended to on-axis. That's not by any means unique to a Harbeth design - all multi-driver speakers have the same issues of different path lengths from the drive units. Coaxial speakers don't have this issue but they have others which can be even more serious problems.

                I could go into more detail about this with a follow-up TT, but I'm extremely busy and without more feedback as to whether man-days of my effort to make TechTalks are doing any good for the users (or for Harbeth) I'm reluctant to steal the time from other important activities. Sorry.
                Alan A. Shaw
                Designer, owner
                Harbeth Audio UK

                Comment


                • #9
                  Importance of ear on axis listening ....

                  I totally agree (but then again, why argue with Alan) that for best results you need the appropriate height stands which place the tweeter at ear level. It makes a big difference to the presence and focus of images

                  To my knowledge, all but a couple of pairs of Harbeth I have sold, have been supplied with tailor made stands. I get the customer to measure from ear to floor level, I calculate the required height and get the stands made to suit.

                  The correct height is well worth achieving to get the very best from your speakers, even if they don't quite fit in with the decor. As a second best, the speakers may be tilted slightly back to direct more HF energy towards the listener but I would alwas opt for the correct stands.

                  Comment


                  • #10
                    TechTalk - an appreciation

                    Alan,
                    On a slightly different note, we do appreciate your 'tech talk' but comments from us are not always necessary as you have usually explained the points with great clarity. We read and learn.

                    As to mounting the main driver behind or on the baffle, I have been reading my copy of 'Loudspeakers' by Gilbert Briggs for the umpteenth time and he describes the various mounting arrangements including having the driver standing off of the baffle to provide the appropriate venting without having a separate hole in the cabinet. This looks like a neat arrangement but I wonder how effective this might be ?

                    Comment


                    • #11
                      TechTalks ....

                      Originally posted by hifi_dave View Post
                      ... we do appreciate your 'tech talk' but comments from us are not always necessary as you have usually explained the points with great clarity. We read and learn.
                      Thanks for that. I'm not actually fishing for compliments, but I must know if I'm conveying the explanation at the right technical level. An explanation that fails to convey the key message is worthless. Less than worthless because it just further confuses the viewer. After planning, recording, editing, packaging for the internet, conversion to Flash and uploading all by myself late into the night I want to know how the next one can be better, simpler, more intelligible, smarter graphics, better sound or whatever.

                      Obviously, there is a limit to my own creative abilities - we're not video professionals and it's the content that matters more than the slickness - but there must be room for improvement. Or are they just about right? I'm acutely aware that English is not the first language of many - most - of our visitors. Do I have a good sense of what's needed? Is the audio clear enough?
                      Alan A. Shaw
                      Designer, owner
                      Harbeth Audio UK

                      Comment


                      • #12
                        Time discrepancies ....

                        Just had a look at the TT which is very informative, just a couple of points though:

                        I was curious, at a very simple level, about the maths involved: what sort of time discrepancies are we talking about?

                        Moving a driver from the front to the back of the baffle would be a distance of 20mm at most I'd guess, or 0.02 of a metre. If sound travels at 340 m/s then 0.02m takes 340/0.02 of a second or a 1/17000th - is that right? Doesn't sound like much, but presumably in this context it is!

                        Secondly, the critical issue with respect to time alignment is the vertical axis isn't it? Yet as audiophiles we seem far more preoccupied with the horizontal - this is for different reasons but have we got the balance wrong?

                        Comment


                        • #13
                          Dimensions

                          Originally posted by weaver View Post
                          ... Moving a driver from the front to the back of the baffle would be a distance of 20mm at most I'd guess, or 0.02 of a metre. If sound travels at 340 m/s then 0.02m takes 340/0.02 of a second or a 1/17000th - is that right?
                          Ummm. You've missed a trick here! Have another go-round on the dimensions involved. Have a really good look at my little CAD drawing. Your starting point is incorrect .......
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • #14
                            Small difference in positioning ....

                            Do you mean where you move the bass/mid such that the magnet is flush with the baffle? ie the cone is actually outside the cabinet?

                            In your post #2 above you show a driver mounted on the face of the baffle and a driver mounted behind the baffle. I understand the point that the sound is actually coming from down inside the cone in the case of a base/mid, but relative to it's starting point and in the case of the M30 and C7 above, any point on the cone is only being shifted by 20mm or so isn't it?

                            Comment


                            • #15
                              The velocity of sound waves coming from the bass/mid and tweeter are the same regardless of frequency. That is, approx. 340mS (very, very, very slow compared with the velocity of light).

                              At 340Hz, the wavelength of each cycle of a sine wave is 1.0m. At 3400Hz (3.4kHz) the wavelength is 0.1m (10cm) .....
                              Alan A. Shaw
                              Designer, owner
                              Harbeth Audio UK

                              Comment

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