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Thread: BBC-style thin-wall cabinets. Why so special?

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    Default BBC-style thin-wall cabinets. Why so special?

    Quote Originally Posted by Gan CK
    ... i still do not quite fully comprehend this lossy cabinet philosophy & how it can be tuned to the advantage of the critical midband... One thing for sure however is that there is totally no trace of cabinet colouration in the midband. How a vibrating cabinet that can have no impact on the critical midband really baffles me...
    What underpins the BBC's thin-wall cabinet philosophy (and I was surprised to read that exact word in one of Harwood's papers recently) is the observation that a perfectly cast bell will ring on for many seconds. Conversely, a bell with a hairline crack will sound leaden and hardly ring at all. It's the same with cabinets: if the panels are all rigidly glued together then at some critical frequency or other a note or notes in the music will trigger the cabinet's natural structural resonance. In such a rigid structure, there is nothing that can be done to suppress the ringing - and each time that note reappears, it tops up the ringing which then becomes a permanent drone underneath the music.

    Conversely, in a thin-wall cabinet, the lossy joints (i.e. removable baffle/back and the generally 9-12mm thin panels used throughout the box) each act as an acoustic hairline crack. They inhibit the build-up of resonance. Simple as that really!

    Now, let's not kid ourself that it is possible to kill cabinet resonance stone dead. It isn't. Not with any approach to cabinet design because the sound pressure inside the cabinet is huge. What the thin-wall approach does is to move unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch. Note that I said adequately. Providing that the resonance, be it from the cone, cabinet or even recording - whatever the source - is x dBs below the fundamental, the BBC proved that it was completely inaudible. Once inaudible to trained listeners on all types of music/speech, that is the end of the matter. Inaudible to the trained listener is as good as the solution needs to be. It is neither necessary nor cost effective (nor good engineering) to continue pushing for a degree of theoretical excellence that nobody can appreciate but everyone must pay for. That pragmatism keeps our speaker affordable - and sounding natural.

    What we seem to be lacking in the industry today is the good old fashioned common sense that was abundant when serious researchers with zero commercial interest (i.e. the BBC) had their hands on the tiller. Thank goodness that they thoroughly documented their efforts for posterity since physics, acoustics and our hearing are the same now as fifty years ago. Now it seems we are all conditioned by marketeers to chase theoretical perfection which is far, far beyond what our ears can reliably resolve.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

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    Default Re: BBC-style thin-wall cabinets. Why so special?

    Quote Originally Posted by A.S.
    What underpins the BBC's thin-wall cabinet philosophy (and I was surprised to read that exact word in one of Harwood's papers recently) is the observation that a perfectly cast bell will ring on for many seconds. Conversely, a bell with a hairline crack will sound leaden and hardly ring at all. It's the same with cabinets: if the panels are all rigidly glued together then at some critical frequency or other a note or notes in the music will trigger the cabinet's natural structural resonance. In such a rigid structure, there is nothing that can be done to suppress the ringing - and each time that note reappears, it tops up the ringing which then becomes a permanent drone underneath the music.

    Conversely, in a thin-wall cabinet, the lossy joints (i.e. removable baffle/back and the generally 9-12mm thin panels used throughout the box) each act as an acoustic hairline crack. They inhibit the build-up of resonance. Simple as that really!

    Now, let's not kid ourself that it is possible to kill cabinet resonance stone dead. It isn't. Not with any approach to cabinet design because the sound pressure inside the cabinet is huge. What the thin-wall approach does is to move unwanted resonances downwards in amplitude and frequency so that they are adequately buried below the music and then pushed down in pitch. Note that I said adequately. Providing that the resonance, be it from the cone, cabinet or even recording - whatever the source - is x dBs below the fundamental, the BBC proved that it was completely inaudible. Once inaudible to trained listeners on all types of music/speech, that is the end of the matter. Inaudible to the trained listener is as good as the solution needs to be. It is neither necessary nor cost effective (nor good engineering) to continue pushing for a degree of theoretical excellence that nobody can appreciate but everyone must pay for. That pragmatism keeps our speaker affordable - and sounding natural.

    What we seem to be lacking in the industry today is the good old fashioned common sense that was abundant when serious researchers with zero commercial interest (i.e. the BBC) had their hands on the tiller. Thank goodness that they thoroughly documented their efforts for posterity since physics, acoustics and our hearing are the same now as fifty years ago. Now it seems we are all conditioned by marketeers to chase theoretical perfection which is far, far beyond what our ears can reliably resolve.
    Thks once again for the prompt reply & a very good illustration indeed Alan. The thin wall cabinet really does work wonders as far as accurate timbre & musicality is concerned. The importance of the BBC findings & the hard work of all the engineers involved back then cannot be denied & taken for granted indeed. I just finished listening to a set of CDs & LPs over the last 2 hours & am taking a short break to read this reply. After which i will continue to listen till the wee hours in the morning. Its really very addictive listening to music played on Harbeth. Ok time to carry on with my musical journey with my wonderful SHL-5s.

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    Default Re: BBC-style thin-wall cabinets. Measurements and observations (1)

    The reason that the BBC began to investigate alternatives to the thick wall enclosure used in the massive floor standing LSU/10 was simple: the increase in outside broadcasts in the late 1950's (an era when the TV service rapidly expanded) needed smaller, more portable, lighter speakers that could be taken around in small OB trucks, the LS3/1 being the first. For this reason alone - nothing to do with technical research, the thin-wall panels philosophy (to use Harwood's own word) was born. It was subsequently discovered that thin-wall had excellent acoustic properties and this was proven by detailed, documented measurement by the BBC and others (Barlow, Stevens). All classic BBC monitors since the LS3/1 in 1959 have used thin-wall cabinets - the exception I can think of was the LS5/12A.

    The attached image (source: Barlow) shows a speaker's frequency response (red trace) hovering along the 40dB sound pressure line - an arbitrary loudness. Also plotted on this graph are three other curves - I've coloured one black, and the other blue, but there is a dashed curve in between them. These three show what a microphone picks-up as the output from the cabinets side, top and back panels alone completely ignoring the useful sound output from the drive unit. You can see that in the middle frequencies, this 18mm cabinet has a peak output around 500Hz which is very nearly as loud as the drive units output! In other words, although the walls appear to be thick and rigid to the eye, to the sound waves inside the box they are as acoustically transparent at this frequency as a sheet of paper. Shocking.

    At the other extreme, the box made of 6mm has its peak output well below 100Hz, somewhere around the port resonant (tuned) frequency, which diverts the cabinet's acoustic output well away from the all-critical midband down to bass frequencies. Note however, that the 6mm cabinet has a peak quite close to that of the 18mm somewhere about 500Hz but it is at a lower level. This is not ideal.

    Now the clever bit! If you look closely at the 12mm curve (the dashed line) you will see that it has only one peak - at about 100Hz. The peaks at the middle frequencies evidenced on the 6mm and 18mm panels are much suppressed with the 12mm panels. This 12mm option looks promising. Now, suppose we could tame that 100Hz peak and reduce it's level ... and maybe pull it down in frequency a little we'd have a great sounding, resonant free, clean-midband cabinet..

    Continued ....

    >
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    Default Re: BBC-style thin-wall cabinets. Measurements and observations (2)

    I illustrated in the previous post how the speaker cabinet's panel thickness had a great influence on its resonant behaviour. I showed that despite the visual impression that 19mm panels are rigid and solid, acoustically they are as transparent as tissue paper at a certain problematic frequency. I also illustrated that the problem frequency is related to the thickness of the panel, and that as the panel's thickness increased, the problem frequency moved upwards in frequency encroaching into the midband.

    The solution that the BBC researchers described involved using a relatively thin panel - say, 9mm, half the conventional thickness for a speaker box, but loading the inner walls with a rubbery bitumatic material such as roofing felt or its industrial alternative. Being rubbery, this substrate panel flexes with the wood panel, and it's flexing causes the sound waves to be converted to heat in the rubber molecules. Once they are safely locked-up as heat, their energy is dissipated and will not cause us any sonic problems. Attached is an example of a (not very good speaker, but that doesn't matter at all for this illustration) with its frequency response in red. The cabinet made from 9mm panels. The black curves shows the output from the raw cabinet excluding and isolating that from the drive unit and the green curve, the output when the panels have had a layer of bitumen attached. Three things can be observed:

    1. The peak frequency where the cabinet is acoustically transparent is about 120Hz. This has been slightly reduced to about 95Hz by the addition of the bitumen counter-layer.

    2. The magnitude of the peak noted in (1) has been reduced by about 10dB (by two thirds) by the addition of the bitumen layer to the raw 9mm panel.

    3. In the critical middle frequencies (marked in yellow) the 9mm panel + bitumen is remarkably inert.

    Conclusion:

    Despite ones preconceptions that 'thick panels must be better' in the critical middle frequencies, it is clear that the opposite is in fact true: 'thin is much better' - thin panels are a better solution to controlling panel resonances, and making a speaker box that doesn't sound boxy. Thin can be tuned - thick can't.

    I am aware of one manufacturer of tower speakers who recently decided to remove the bitumen lining from his cabinet and replace it by using even thicker MDF walls! I can not understand the acoustic logic behind this move. I can understand the cost saving.

    >
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    Alan A. Shaw
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    Default Re: BBC-style thin-wall cabinets. Measurements and observations (2)

    Quote Originally Posted by A.S. View Post
    I illustrated in the previous post how the speaker cabinet's panel thickness had a great influence on its resonant behaviour. I showed that despite the visual impression that 19mm panels are rigid and solid, acoustically they are as transparent as tissue paper at a certain problematic frequency. I also illustrated that the problem frequency is related to the thickness of the panel, and that as the panel's thickness increased, the problem frequency moved upwards in frequency encroaching into the midband.

    The solution that the BBC researchers described involved using a relatively thin panel - say, 9mm, half the conventional thickness for a speaker box, but loading the inner walls with a rubbery bitumatic material such as roofing felt or its industrial alternative. Being rubbery, this substrate panel flexes with the wood panel, and it's flexing causes the sound waves to be converted to heat in the rubber molecules. Once they are safely locked-up as heat, their energy is dissipated and will not cause us any sonic problems. Attached is an example of a (not very good speaker, but that doesn't matter at all for this illustration) with its frequency response in red. The cabinet made from 9mm panels. The black curves shows the output from the raw cabinet excluding and isolating that from the drive unit and the green curve, the output when the panels have had a layer of bitumen attached. Three things can be observed:

    1. The peak frequency where the cabinet is acoustically transparent is about 120Hz. This has been slightly reduced to about 95Hz by the addition of the bitumen counter-layer.

    2. The magnitude of the peak noted in (1) has been reduced by about 10dB (by two thirds) by the addition of the bitumen layer to the raw 9mm panel.

    3. In the critical middle frequencies (marked in yellow) the 9mm panel + bitumen is remarkably inert.

    Conclusion:

    Despite ones preconceptions that 'thick panels must be better' in the critical middle frequencies, it is clear that the opposite is in fact true: 'thin is much better' - thin panels are a better solution to controlling panel resonances, and making a speaker box that doesn't sound boxy. Thin can be tuned - thick can't.

    I am aware of one manufacturer of tower speakers who recently decided to remove the bitumen lining from his cabinet and replace it by using even thicker MDF walls! I can not understand the acoustic logic behind this move. I can understand the cost saving.

    >
    Thks Alan for the above illustration. Its indeed enlightening. To us layman, a thicker cabinet would naturally confer more solid engineering & better build quality. Never knew that a thick cabinet would have such a detrimental effect on the resultant sound. Well at least now i know that the thin wall cabinet has made a very important contribution to the excellent & accurate tonality/timbre aspects that are hallmarks of Harbeth loudspeakers. Of course the contribution from the Radial driver is also crucial to the overall sound of Harbeths. By the way Alan, is the thickness of bitumen critical? Besides lining the internal walls with substantial acoustic foam, have you tried alternative materials like rock wool?

    Oh before i forget, what are the sonic advantages of having the bass or midbass driver mounted from inside of the cabinet? (for eg the M40.1)

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    Default Re: BBC-style thin-wall cabinets. Measurements and observations (3)

    Yes, it is counter-intuitive that thin walled cabinet can have lower output in the all-critical middle frequencies than thick, rigid walled cabinets. Of course, this is not news to anyone who has made a stringed instrument: light, thin walls, carefully secured around the edges and with just the right amount of damping here and there make for a wonderful open sound.

    I have now combined into one graph the previous two. The red trace is the frequency response of the speaker. Don't worry that it's not smooth - it's just whatever was to hand. The interesting curves are the blue one, showing the cabinet output alone (ignoring the output of the bass unit) when the box is made from 18mm (3/4 inch) undamped panels. Note again that in the middle frequencies the output of the box is very nearly as strong as from the woofer. That implies that the cabinet is acoustically transparent and the sound normally constrained inside the box is setting the 18mm panels into sympathetic motion and they are acting as a huge radiating surface.

    Contrast that horrible situation with the box made from 9mm board (half the thickness) loaded on the inside of the panels with a counter-layer of bitumen. I've shaded in white the reduction in box output - down from about 38db to about 12dB, roughly 20 times lower output than from the walls of the rigid box!

    But .... bitumen is a by-product of oil, and the price has risen dramatically in recent years. Our cabinet makers complain that we are one of the very few using it these days on cost grounds.
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    Default

    Alan,

    Do the advantages of thin walled design diminish with the cabinet size?

    I am supposing that a small panel cannot as easily be damped for a low resonance frequency; and I am supposing that a small panel has less scope to add colourations.

    But the whole matter is so counter-intuitive . . .

    Your views?

    The facts?

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    Default Screwed back

    I discovered that the early BBC LS3/5a had a screwed-on back panel, but later ones including Harbeth's, didn't.
    Does this indicate it is less effective on a smaller cabinet ? Mind you, the modern Stirling Broadcast LS3/5a has a screwed-on back panel, as does the Harbeth P3ESR. My old ( 1993 ) HL-P3s don't. I suppose it all depends on the particular design, as Stirling's '3/5a and the 'ESR don't have the same drive units as the original LS3/5a.

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    Default Thin wall cabinets - the secret?

    Alan, you make it sound(sorry) so simple making good speakers. Why are not many more manufacturers using thin wall cabinets? Is it more expensive to manufacture, etc.

    I had a pair of Spendor BC 1 once, I remember my first look inside and the slight disappointment because there was nothing inside just thin slabs of foam rubber and bitumen glued to the walls.

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    Default Great engineering ....

    Quote Originally Posted by LarsS View Post
    Alan, you make it sound(sorry) so simple making good speakers. Why are not many more manufacturers using thin wall cabinets? Is it more expensive to manufacture, etc.

    I had a pair of Spendor BC1 once, I remember my first look inside and the slight disappointment because there was nothing inside just thin slabs of foam rubber and bitumen glued to the walls.
    Ah, maybe not impressive, but beautiful, effective, simple, repeatable, ingenious and cost-effective engineering!

    You can just imagine the head of BBC R&D tasking an engineer to investigate thin-wall speaker cabinets for use in the BBC in the 1960s. The conversation could go something like ....



    Head: "The outside broadcast boys are complaining that they have to make do with small trucks and vans and that sooner or later they will have to make room for two monitor speakers (for forthcoming stereo) when they hardly have the space for one of our large monitors. I'd like you to investigate a way of making a light weight speaker cabinet that saves their back strain humping the speakers around, and, whilst you are at it, see if we can find a way to reach (or even exceed) the current sonic quality standards we know and trust ...."

    Junior: "Of course. Can I ask whether you have seen the latest hifi speakers on the market? They use fancy materials, exotic colours and make great claims about huge sonic advances. Don't you think that we ought to evaluate them?"

    Head: "Are you serious? I couldn't care less. They are making for profit. We aren't. We, the BBC, got into speaker design because there was nothing commercially available that provided the balance of characteristics that we as broadcasters need. Convince me that after we have spent man decades on this that the hifi industry has something truly novel to offer us."

    Junior: (Silence).

    Head: "Oh, and before you ask, let me remind you that we are funded by the TV viewer (in the UK) having to pay a licence to watch BBC TV. Before you get carried away with exotic materials, may I remind you that I, as one of those viewers, do not want to be paying a penny a year more to the BBC because you have selected fancy materials and space-age technology to make these new speakers..."

    Junior: "So how much can I spend?"

    Head: "As little as possible. I don't want to hear that you have been using materials or techniques that require expensive set-up costs, or hard-to-get parts, or audiophile nonsense."

    He reaches into his pocket, pulls out some coins and slaps them down on the desk.

    Head: "There you are. That's your budget...."

    Junior: "(Shocked). What? Are you serious? If you kept up with the hifi market you'd know that progress needs us to spend money. Technology isn't cheap you know ...."

    Head: "You need to get out more. You really have been reading all that hifi guff haven't you. Ok son, here is the challenge. Any fool can get carried away with esoterica when someone else is paying. I'm paying (points to coins). What would really impress me is when you report back to me in six months' time that you've met my criteria. That you've designed new speakers for our broadcast colleagues that are portable, lightweight, much smaller than we're used to, and they think that they sound as good - or here's the challenge for you - better than what they're used to - designed by your illustrious predecessor. It's over to you, but remember, what'll impress me is how much brainpower, insight and novelty you can bring to bear on the problem, not how much of the BBC's cash you can blow on fancy ideas ..."

    Junior: (Pauses, thinking). "OK, I'll take on the challenge. But may I ask, what the best bit of advice you can give me please to get started?

    Head: "That's simple. Remember what I said about keeping the cost down? Good engineering gets results. Brilliant engineering gets results at the lowest possible cost. My rule of thumb is this. If you can find standard materials that can be shaped and worked down at the Portobello Road street market, you beat the costs down to petty cash; if you can prove to me by scientific measurement that those materials and techniques achieve the best possible result for our needs; if you keep a proper daily engineer's log book, if you write up this completed project in a Paper so that neither of us make fools of ourselves when those outside the BBC replicate our work, then you'll go far in the world of real engineering and I'll be proud of you. Now get along ...."

    The young engineer had almost reached the door when the craggy old chief added

    Head: "And after you've finished these cabinet masterpieces, don't forget to bring me the change."



    Not as fanciful as it may seem! The mind set that brought the BBC monitors into fruition was wholly different from that of the conception and creation of any other loudspeakers. With stereo sound on the horizon and with the BBC having several hundred studios, the re-equipping program from mono to stero was going to be extremely expensive for the tax payer.

    About BBC licence here and here. It has always been an extremely sensitive political issue.
    Alan A. Shaw
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    Default Costs?

    So costs were a reasonable big issue?
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    Best, Alex

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    Default Costs + size + stereo performance

    Quote Originally Posted by der_yeti View Post
    So costs were a reasonable big issue?
    Costs have always been a significant factor at the BBC, it being a publicly funded organisation.

    But more than that, the two main drivers behind the BBC's pioneering work on loudspeakers were cabinet size and the dramatic improvements in real sound quality impending in the late fifties. The standard high quality studio speaker throughout the fifties and early sixties was the monster LSU10 (see Harbeth History for some excellent background information). By 1960 it had become apparent that there was a need for quality monitoring to extend significantly higher in frequency than was the case in the medium-wave world of AM radio (and shellac 78rpm discs) and furthermore, with stereo on the horizon, it was apparent that speakers had to become smaller so that two of them could be accommodated in the tiny control cubicles of the time.

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    Default British post-war Utility Furniture

    My wife inherited our Nicosia house with furniture, much of it having been made in the early 1950s by a village carpenter. After world war II wood was in very short supply, especially in Cyprus. The tables and cabinets are thin wall construction using the very minimum of plywood and wood. The material used is minimum, the skill in its use maximum.

    BBC thin wall cabinets would have been designed for construction in this style of cabinet making. The British Utility Furniture of the period was constructed in the same way.

    http://en.wikipedia.org/wiki/Utility_furniture

    Ignore the awful 1970s chairs but look at the almost Art Deco table made late 1950s - thin plywood top with much hidden cross bracing and a wrap round some six inches deep giving more strength. To the left see the coffee table of similar plywood construction.



    Look also at the thin wall cabinet holding the Quad 405 and upon which the TV stands. Nothing is as it seems. What look like solid sides, shelves, door and top are fabricated in plywood with the minimum wood filets. The Art Deco curve at the bottom of the cabinet shows it cannot be solid. Even though it is mostly fresh air it is remarkably strong and very rigid - all that skill gone into matchwood!




    Thin wall was common woodworking practice.
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    Default Laminations

    There are some german measurement that also suggest that lamination is a good thing
    http://www.hifi-highend-selbstgebaut...ersteifen.html
    http://www.picosound.de/D_gehmat.htm
    Google translate works quite well but i think the graphs in itself tells a lot

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    Default Commercial example

    Quote Originally Posted by DrBoar View Post
    There are some german measurement that also suggest that lamination is a good thing
    http://www.hifi-highend-selbstgebaut...ersteifen.html
    http://www.picosound.de/D_gehmat.htm
    Google translate works quite well but i think the graphs in itself tells a lot
    There is a US 'high-end' manufacturer that has used the idea of ceramic tile layering plus a felt coating - it might be Vonschweikert audio, I think.

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    Default Speaker cabinet as musical instrument?

    Quote Originally Posted by A.S. View Post
    Yes, it is counter-intuitive that thin walled cabinet can have lower output in the all-critical middle frequencies than thick, rigid walled cabinets. Of course, this is not news to anyone who has made a stringed instrument: light, thin walls, carefully secured around the edges and with just the right amount of damping here and there make for a wonderful open sound.
    I wouldn't take that musical instrument analogy too far, though I can understand the motivation behind making such associations.
    A stringed instrument body is designed to MAXIMISE its resonant properties across its harmonic spectrum, so that the sound of vibrating a few inches of reinforced nylon string can be amplified to fill an auditorium.
    A speaker cabinet (should) be designed to MINIMISE its resonant properties so that its panels and structure do not amplify the effects of drive unit mechanical vibration and internal sound pressure to create spurious output.

    By all means use thin walls and then damp them effectively, but the cabinet construction has a different purpose from that of Stradivarius. It should be primarily the drive unit diaphragm(s) that produce the "wonderful open sound" !

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    Default Luthierie and loudspeaker cases - my experience

    Quote Originally Posted by richerm View Post
    I wouldn't take that musical instrument analogy too far, though I can understand the motivation behind making such associations.
    1. A stringed instrument body is designed to MAXIMISE its resonant properties across its harmonic spectrum, so that the sound of vibrating a few inches of reinforced nylon string can be amplified to fill an auditorium.
    2. A speaker cabinet (should) be designed to MINIMISE its resonant properties so that its panels and structure do not amplify the effects of drive unit mechanical vibration and internal sound pressure to create spurious output.

    3. By all means use thin walls and then damp them effectively, but the cabinet construction has a different purpose from that of Stradivarius. It should be primarily the drive unit diaphragm(s) that produce the "wonderful open sound" !
    1. Real luthierie is very specific part of handicraft and very demanding or even challenging area for acoustics science. Everything what Alan described is true, although can be interpreted as you did. You are right that stringed instrument shell is to amplify the vibration of the strings (maybe not maximise but amplify in optimal as we like way). The hundreds years of experience and then scientific approaches proved it is impossible to built naturally sounding acoustical stringed instrument with angular shaped boxes - such instruments exist - they are popular e.g. among steppe folks of Asia but their harsh sound would be hardly accepted in performing Bach overtures or Beethoven sonatas, but it is very matching to velvety sound of (guzheng or koto, I don't know Mongolian name) Asian zither, plucked with finger pads.

    If you asked anbody of.... let's choose Khusugtun folk band members (very refined people and musicologists) to play Bach or Beethoven they would pick up with no hesitation classical violin or cello.. and would play excellently, I assure you. The link - http://www.youtube.com/watch?v=NQkrsdjJB2s

    More about modern approach to making stringed instruments you can see from short movie "The Violin Maker" (http://vimeo.com/37749081) about one of the most talented modern luthiers - Sam Zygmuntowicz from New York. Notice the measurement tools in his workshop - they are not so far from those used by most of reliable loudspeaker makers.

    2. Minimising the vibrational effect of loudspeaker cabinet is not so easy task - you always must be aware about some compromises that will inevitably occur when applying one of the most proven prescriptions for cabinet making. It is designers knowledge (also experience and art) to minimise drivers influence on a final acoustical characteristics of the working loudspeaker.

    Approx. three years ago, bored with the sound of commonly offered 2,5- way slim towers, I asked very talented relatively young acoustics engineers, specializing in delivering whole the sets for conference and audience rooms, if they were so kind to measure professionally and advise corrections for two cubic feet three-way loudspeakers built according to Danish designer, consisting of very refined Scan Speak drivers that occupied my cubbyhole for a longer couple of time. I was returned them ready for playing ... after namely one year and two months "you know, the volumes for midrange and woofer are superb, but each type of vibration and standing waves damping involves another problem in interacting between drivers thus some slight alignments in crossover etc. etc. ; we would like to obtain the maximum from these precious drivers; do you mind if we could approach the matter in our spare time" - I did not at all - my hobby (then) occupation got their training ground. They used the layers of bitumen pads with different characteristics in form of constrained damping, also some felt and natural wool pads. The loudspeakers sound excellently.

    Then I bought a pair of SHL5 Harbeths from my friend, who replaced them with rather huge 40.1 monitors. Both of them also sound excellently.

    3. More than a decade ago I had opportunity to listen to concrete shell monitors with very good drivers. Bloody experience, never again.

    The violin itself was aimed to ... imitate human singing :))

    All the best.

    P.S. The traditonal Mongolian zither is called yatga or yatug.

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    Default Thisn wall plus ....

    Don't forget that the secret is not just the thin relatively flexible panel, it is that the speaker panel is tuned downward in frequency by the addition of a heavy, rubbery, counter layer on the inside. Well, Harbeth thin wall panels are, but that puts us in a very small class of those who persist with this expensive and difficult to master technology of thin panels + damping. I understand that one 'BBC influenced' brand has abandoned the counterlayer and thickened up the MDF. An odd choice from a sonic point of view perhaps, but makes construction of the cabinet very much easier and cheaper.

    Cabinet makers hate thin-wall boxes with a passion because they are so unforgiving of error. Which pushes the cost up, and the number of cabinet makers who will even contemplate making samples let alone taking on an order for bulk production, can be counted on a few fingers, even globally. If you can find someone, anyone, anywhere at any price willing to make a success of thin-wall, damped speaker cabinets year in, year out, sign him up immediately and nurture that relationship!
    Alan A. Shaw
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  19. #19
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    Jul 2012
    Location
    Poland
    Posts
    180

    Default Thin walls and physics...

    Quote Originally Posted by A.S. View Post
    Don't forget that the secret is not just the thin relatively flexible panel, it is that the speaker panel is tuned downward in frequency by the addition of a heavy, rubbery, counter layer on the inside. Well, Harbeth thin wall panels are, but that puts us in a very small class of those who persist with this expensive and difficult to master technology of thin panels + damping. I understand that one 'BBC influenced' brand has abandoned the counterlayer and thickened up the MDF. An odd choice from a sonic point of view perhaps, but makes construction of the cabinet very much easier and cheaper.

    Cabinet makers hate thin-wall boxes with a passion because they are so unforgiving of error. Which pushes the cost up, and the number of cabinet makers who will even contemplate making samples let alone taking on an order for bulk production, can be counted on a few fingers, even globally. If you can find someone, anyone, anywhere at any price willing to make a success of thin-wall, damped speaker cabinets year in, year out, sign him up immediately and nurture that relationship!
    Initially, the idea of putting drivers into thin wall cases can seem somehow ridiculous for majority of audio fans, but if we dig deeper into physics, namely dynamics, everything will occur to be OK, more - very OK. How is it possible? The main principle in case of thin walls is to obtain low, more - the lowest possible tuning of the cabinet. What can seem even more strange to many audio enthusiasts is the fact that the bigger size of the cabinet, the ... better results (at least in theory) can be obtained.
    Someone can say: "Yes, but the relatively small stiffness of such thin walls and their long span will cause serious amplitudes of cabinet's vibrations from working (vibrating and pumping the air) woofers, midwoofers or midrangers, thus something pretty well hearable to listener!". We hardly could not agree such an argumentation, if we did not know that there are some proven ways how to reduce these unwanted panels' flutterings significantly and effectively.

    The main are:
    a) the addition of high density, but maximally flexible layers to some cabinet's panels, so that by their additional mass to minimise panels' vibration amplitudes and / or to lower resonance frequencies,

    b) propose such a construction of a cabinet so that (as much as it is technically possible) panels don't interfere mechanically one with another (don't transfer their vibrational energy from one to another),

    c) minimise the mechanical effect of working driver(s) onto the cabinet,

    d) minimise the "acoustical" effect of working driver(s) onto the cabinet (change of air pressure),

    e) proper geometry of an cabinet and / or its acoustical damping to eliminate standing waves, minor sound colourations etc.

    f) maintain the maximal mechanical, thus acoustical repeatability of each cabinet (thin walls "technology" demands neat, careful and sound carpentry, assembly and ... very unyielding quality control).

    These are only few very theorethical (I imagine that main but I can be wrong, I am no professional in this discipline) measures, the rest is long-time experience and hard work on one's own (as usual) :))

  20. #20
    Join Date
    Mar 2014
    Location
    Sweden
    Posts
    2

    Default Science, plus marketing

    When I read the BBC papers about cabinet resonance and monitor development, there are several cases were things go wrong due to softwoood fillets instead of hardwoood, the wrong glue and so on, in line with A.S description on how tricky it is with thin wall cabinet.

    The second thing is that all manufacturers work on a market, they have to make money otherwise it is a hobby not an occupation! The market like cables to play with and expensive looking shiny connectors, I am phasing out all spring loaded terminals, banana plugs and the like for Speakon. This because I build my speakers as a hobby and can use what I think is best even if they look boring next to huge screw terminals with bling factor. In the 1979 and 1980 more than 95% of all loudspeaker boxes were wider than they were deep and many midprice speakers had a 10" or even a 12" bass driver. Now most speakers are deeper than wide and many use two or three 6.5" bass drivers on a narrow front instead of a bigger single driver on a wide front box. So science and acoustics is part of the story but various market driven factors also come into play.

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