06 Sep 2014 à 07:58
VEY a écrit:Bonjour aux heureux Harbethiens,
Ãtant moi-même possesseur depuis quelques mois de 30.1 avec pieds SKYLAN, que mettez-vous entre le support haut des pieds et les enceintes ?
Je crois que ce sujet a dÃ[emoji767]jà Ã[emoji767]tÃ[emoji767] abordÃ[emoji767] mais je n'arrive pas a retrouver le fil !!!
Je crois aussi avoir lu quelque part que les Harbeth aimaient bien avoir le "cul libre" et que donc on pouvait les dÃ[emoji767]coller lÃ[emoji767]gèrement du support (avec des pieds style GEL PAD ATACAMA par exemple), qu'en pensez vous ?
Par ailleurs, est- il utile de mettre des supports sous les pointes des SKYLAN (type plaque de marbre ou autre), sachant que j'ai du plancher dans mon chez moi ?
Au plaisir de vous lire.
Bernard.
09 Sep 2014 à 12:00
09 Sep 2014 à 22:26
10 Sep 2014 à 08:47
10 Sep 2014 à 09:07
10 Sep 2014 à 09:12
Jerome W a écrit:J'ai reçu les Q Bricks.
De simples rondelles de métal.
Mais effectivement, cela paraît un poil plus propre et dégraissé que sans.
Envoyé de mon iPhone à l'aide de Tapatalk
10 Sep 2014 à 10:53
10 Sep 2014 à 11:19
10 Sep 2014 à 11:30
JérômeB a écrit:C'est surtout que je ne comprends pas bien comment, mécaniquement, une enceinte peut être plus "propre" quand elle est posée sur 4 pastilles en plastique. Forcément, dans cette configuration, elle vibre et ce n'aide pas à rendre le grave mieux détouré.
En revanche, si on la fixe ferment au pied par du Blu Tack, elle ne "bave" plus.
10 Sep 2014 à 11:42
Jerome W a écrit:Attention.
L'enceinte vibre peu importe sur quoi elle est posée.
La question du découplage, c'est celle du passage ou de l'absorption / diffusion de ces vibrations dans le support puis dans le sol.
Si tu diminue la surface de contact entre l'enceinte et son support, alors tu diminue physiquement la quantité de vibrations transmises au support
Jerome W a écrit:puisque par définition, le coffret de l'enceinte présente une résistance au passage des vibrations : il n'est pas conducteur à 100%.
Si il l'était, la quantité d'énergie transmise au support serait la même, que les enceintes soient en contact plan / plan avec lui ou qu'elles reposent sur 4 points.
Jerome W a écrit:D'autre part, les Q Bricks sont en métal, pas en plastique.
10 Sep 2014 à 12:27
10 Sep 2014 à 12:30
mambojet a écrit:@Jérôme
Mieux en tous les cas que le blu tac que j'ai également essayé....
10 Sep 2014 à 12:44
10 Sep 2014 à 12:47
10 Sep 2014 à 12:55
mambojet » il y a 7 minutes a écrit:@Jérôme
Le mieux est sans doute de lire ce qu'en dit AS:
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.
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...
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.
La conséquence est qu'il faut, au maximum éviter de contarier les vibrations de la caisse
Pour ma part, La M30 posée directement sur son support ne fonctionne pas bien. La large surface d'appui contrarie le bon fonctionnement de la caisse
J'ai coupé de petits bouts de CP(Environ 15mn x 15mn et 3 par enceinte) que j'intercale entre l'enceinte et son support.
Sur les bords bien entendu...
Le CP n'isole pas et permet l'écoulement des vibrations mais ne contrarie pas le fonctionnement de la caisse qui est "libre"
A mon avis, cela fait bien le job: écoulement des vibrations, non contrainte de la caisse
Mieux en tous les cas que le blu tac que j'ai également essayé....
10 Sep 2014 à 13:00
Jerome W a écrit:Dans ta pièce, pour tes enceintes, il se trouve simplement qu'une transmission intégrale des vibrations au Skylan est bénéfique.
Jerome W » il y a 6 minutes a écrit:Pour les Q Bricks, ils sont bien en métal, la petite pastille silicone ne sert qu'à protéger le coffret de l'enceinte.
10 Sep 2014 à 13:30
10 Sep 2014 à 13:37
Jerome W a écrit:Normal,
Avec une enceinte de poids très modéré comme la LS3/5, les règles du jeu changent.
Jerome W a écrit:La différence en termes de vibrations transmises dans un cas de contact complet et avec découplage n'est plus significative.
10 Sep 2014 à 15:19
10 Sep 2014 à 16:35