Canford has considerable experience in the design of headphone limiters and their use in the operational environment. Whilst virtually any headphone or headset can have a limiter fitted, using one of the seven types of standard multipurpose limiter circuits, this usually has to be in an in-line housing. By designing type-specific circuits for individual ear loudspeakers, these can be made small enough and appropriately shaped to enable them to be built into the earcup housing. Adding a limiter circuit to a headphone alters the input impedance.
For level limited headsets, see CANFORD LEVEL LIMITED HEADSETS.
The Sennheiser HD480 type headphone is an open back type headphone with 1700 ohm capsules. In order to distinguish these headphones from standard non-protected versions, they are distinctively marked on the ear shell. Limiting level is fixed at 85dBA (green trim ring), 88dBA (red trim ring) or 93dBA (grey or blue trim ring) (note 1). Standard connector options are available, and because the limiter is within the capsule, it is possible to use spare or replacement leads, as for standard versions.
All types have interwoven copper/Kevlar cables for extra strength. As with in-line limiters, each headphone is individually calibrated, and as each capsule has its own individually calibrated limiter, spare limited capsules are available.
Single-sided cables connect to the left earpiece, with a loop over the headband to the right earpiece; a fixing kit is supplied to hold the cable onto the headband. Note that in these versions, the earpieces are mounted onto the headband the opposite way up (connector at the top).
The headphones are available as standard in eleven terminated versions:
||1200 ohms per earpiece
||91dB for 1mW
A professional monitoring headphone with great comfort for extended wearing. Supplied with 2.5m coiled cable, 3.5mm jack plug with 6.35mm adapter.
Closed ear design, incorporating Beyerdynamic’s bass reflex system, combines articulate low end response with a lightweight diaphragm. Frequency response equalised to the diffuse sound field. Fitted with 3.5mm jack with 6.35mm adapter.
The closed back DT100 headphones have 400 ohm capsules. Units are available in the black shell version, with distinctive marking to distinguish them from standard non-protected versions. Limiting level is fixed at 93dBA (note 1). As with in-line limiters, each headphone is individually calibrated. Standard connector options are available, and because the limiter is within the earshell, it is possible to use spare or replacement leads, as for standard versions. Due to the method of construction, it is not possible for replacement ear loudspeakers to be user-fitted in these types, units requiring repair must be returned to Canford. For the same reason, replacement limited ear loudspeaker units are not available as spare parts.
The DT100 level-limited headphones are available as standard in five versions - four terminated and one without cable:
Heavy duty ‘closed ear’ general purpose headphones. Very robust construction, hidden fixings hinder tampering whilst allowing easy repairs. 2 metre cable with steel conductors, fitted with robust Neutrik 3 pole A-gauge or B-gauge plug. Impedance 2 x 400 ohms.
Rugged closed ear design and wide frequency response for reliability and accuracy (note 2). 3 metre coiled OFC cable, fitted with gold 3.5mm 3-pole mini jack with 6.35mm 3-pole jack adapter.
Shure SRH440 and SRH840
Closed-back, circumaural headphones designed for recording and programme monitoring, with 3 metre coiled cables. Both types are available with 3.5mm plugs with 6.35mm A-gauge adapters. The SRH440 is additionally available wired stereo with a Neutrik B-gauge plug with black cover.
Sennheiser HD25 Plus
Closed ear designs with extremely high external noise attenuation. Flip-away earcups, can be locked securely in place. High wearing comfort due to a splittable headband and leather-effect earpads. Single-sided, straight 1.5m cable, steel cored for reliability, supplied fitted with a straight 3.5mm 3-pole jack plus adapter to 6.35mm A-gauge 3-pole jack plug. Impedance 70 ohms. Frequency response 16-22000 Hz, 200mW load rating.
1) The quoted limit level of 93dBA, 88dBA and 83dBA is for measurement using dummy head (or mannequin) systems. For measurements using flat plate calibrators the level will differ. Health and Safety authorities are more inclined to use the dummy head type measurement.
2) Due to the enclosed circuitry, the sensitivity and presented impedance of these headphones is different from standard versions.
Certain ear loudspeakers and cables are listed as spares in a group. For further information, see CANFORD LEVEL LIMITED HEADPHONES AND HEADSETS Spare parts.