By Tristan Jacquel
Why some headphones are closed and others open or semi-open? Does the circum-aural or supra-aural positioning of the pads only vary wearing comfort or is it of acoustic interest? Are headphones subject to the same constraints?
Within the range of a manufacturer, it is not uncommon to find several models of helmets using the same speakers, but having an open structure, semi-open and closed. It is not for simplicity that the helmet manufacturers decline three models around the same speaker. Indeed, the structure of the hull (closed, open, semi-open) considerably influences the sound reproduction. In fact, it changes completely at all depending on the structure of the hull.
The open structure
When a headset is said to be open, the speaker diffuses as much the sound to the listener’s ear as to the room in which it is located. In practice, the helmet shells are open and let glimpse the speaker, usually through a thin grid. This configuration allows the loudspeaker to operate unstressed and as close as possible to its natural response curve. The sound nirvana therefore, provided you can listen to this type of headphones in a very quiet environment, because the open structure lets hear all the surrounding noise.
• sound stage of considerable width
• significant frequency descent
• little tortured impedance (simplified amplification)
• no sound insulation
• significant sensitivity to ambient noise
• you hear what you’re listening to
The closed structure
A so-called closed helmet has hulls without any opening to the outside. The loudspeaker thus works in closed charge, in the same way as in some speakers. The main advantage and not least is an important sound insulation toward external noises. With at least 20 dB of attenuation, the signal-to-noise ratio is with the closed helmets quite excellent and logically favors the perception of micro-information sound. But, in fact, the speaker is put under pressure by the air trapped in the hulls. The membrane is not free of its movements as in an open helmet. The mechanical stresses exerted by the trapped air cause overvoltages and resonances on certain frequency ranges and the loudspeaker loses its linearity. If the speaker has been developed taking into account this constraint – it takes a lot of research and development! – the refund may be exceptionally good. Some manufacturers incorporate damping materials to compensate for the colorations obtained, sometimes in combination with an internal resonator that reduces or increases the volume of low frequencies. Moreover, the stresses exerted by the air modify the impedance curve of the loudspeaker, provoking inflations and deflations, which complicates the task of the associated amplifier.
• excellent signal-to-noise ratio (ideal public transport)
• potentially powerful serious register
• less wide sound stage (with open structure)
• accidents in the frequency response
• more tortured impedance
• requires sharp development
The semi-open structure
This implementation combines some advantages of closed and open structures. The shells of a semi-open helmet are most often “pierced” with one or more small holes, which are similar to the vents of bass-reflex speakers. Of two things, either, these holes are simple perforations and the only benefit is to free the speaker from any
pressurization by trapped air, or the orifice is extended inside the hull by a tube and in this case it is a resonator that is – which amplifies the low frequencies. The multiplicity of simple ports widens the sound’s stage but reduces the signal / noise ratio, while the presence of a vent / resonator releases the speaker, increases the performances in the bass and maintains a very good sound insulation, except however of a short range scene.
The distance between the ear and the speaker of a headset is a sensitive parameter. The supra-aural coupling (where the headphones are placed against the ear) offers the shortest distance between the speaker and the ear canal. The supra-aural coupling promotes good transmission of low frequencies for open helmets, whose structure naturally promotes acoustic leakage. With both open and closed helmets, supra-aural coupling offers the highest level of detail.
• audio resolution
• sum of details
• Weak sound stage with closed headphones
• sometimes average comfort during long listening sessions
The circum-aural coupling
The way the helmet is positioned on the ears of the listener largely influences the perception of sound. The circum-aural coupling allows the earlob of the ear to be totally free of any pressure and stress, since the foam pads surround the ear without (too much) touching it and press directly against the skull. The wearing comfort of this type of helmet is excellent. Only here, the speaker is a few inches away from the ear canal and this necessarily has consequences. These depend a lot on the structure of the helmet. If it is open, the low frequencies will be considerably attenuated compared to supra-aural coupling. In addition, the sound volume reaching the ear will be less. To be convinced, just press with your hand on the hulls to crush the foam and bring the speaker to the ear. It is easy to take 3 to 6 dB more. A circum-aural helmet is therefore less sensitive than a supra-aural and this can be problematic with a little powerful player, especially when the helmet is of high impedance and / or low sensitivity.
• great wearing comfort
• long sessions of listening possible
• less sensitivity and resolution
• Serious indented when the helmet is open type
And the headphones?
Headphones usually have closed shells, their nomadic nature inclining manufacturers to maximize sound insulation against external noise. There are nevertheless open headphones, but the very small size of the shells forces to proceed only with small openings, which function as resonators and increase the amount of low frequencies. On the other hand, inserting directly into the ear canal of the headphones promotes a high perception of the micro-details of the music.