B'en, on va vérifier
The design of the miniature monitoring loudspeaker type LS3/5A H.D. Harwood
2. Description
(a) General
Fig. 1 shows the general appearance of the loud-speaker. The external dimensions are 31x19x16 cms (12x71⁄2x61⁄2 in) and the weight is approximately 5.3kg (11.6 lb). Its input impedance is nominally 15 ohms and it will accept the output of a 50 watt amplifier for programme signals and then gives a peak output
-5 2
level of +98 dB with reference to 2 x 10 N/m , measured at
1.5m in a room with 0.4 sec reverberation time. The loudspeaker is of the two unit type using a 127 mm (5 in) low-frequency unit and a high-frequency unit with a 20 mm (0.75 in) dome type radiator. The cross-over frequency is about 3 kHz.
(b) Cabinet
It has been indicated above that one of the objects of the design was to produce as small a loudspeaker as possible consistent with an adequate axial response/frequency characteristic. The cabinet has a free volume of a little under 5 litres (0.17 cuft.) and is of the closed type, as the use of a vent would not be helpful with such a small enclosure.
The construction is of 12 mm (1⁄2 in) birch plywood with fillets of beech. Although no difficulty was found with this construction for the original design (LS3/5), it was found that, for the LS3/5A, it was necessary to specify the wood more carefully than was originally thought necessary. The use of any type of hard wood such as Parana Pine for the fillets was found to be unacceptable as a clearly audible colouration was produced by a resonance of the l.f. unit on its chassis, and these fillets had insufficient mechanical impedance to reduce it; this matter is 2 reported in more detail elsewhere. In order to ensure that this
resonance would not give rise to any further difficulties
the side panels are damped with a layer of damping material and the top and bottom panels with two layers; in addition, a p.v.c. edging isapplied to the chassis of the low-frequency unit so as to decouple it somewhat from the front panel. In order to damp the air modes of resonance inside the cabinet all internal surfaces except the front panel are lined with polyurethane foam; this has a thickness of 25 mm (1 in) on the top and bottom of the cabinet and 16 mm (3⁄4’’) on the sides and back. The cabinet is sealed so as to prevent air leaks which could give rise to extraneous noises resulting from the high sound pressures produced inside the cabinet; even screw holes are made air tight.
Repris par Falcon Acoustics, dans leur premier document sur la construction de la LS 3/5 : du bitume 2 mm cotés et fond, 2 X 2mm dessus et dessous.
http://www.g4dcv.co.uk/ls35a/pics/falcon1.gif
La signature sonore de la LS3/5 dépend de plein de paramètres dont la conception de la caisse. AMHA
Pour avoir jouer lors de la construction des Prima by Kro (HCFR) avec différent matériaux, MDF, contreplaqué de bouleau, bitume ou pas, dosage MDM3, les différences auditives sont nulles à bas niveau d'écoute mais nettement perceptibles dès que l'on monte le niveau d'écoute. Je pense qu'il en est de même avec la LS3/5 .....que j'avais transformé en Daline by Fris en 1989 et là, c'était encore plus compliqué.
Cordialement
Jean louis