Why are some headphones closed-back models, while others feature an open or a semi-open design? Does the circumaural or supra-aural positioning of the ear cushions only impact the wearing comfort, or does it affect the sound restitution as well? Are in-ear monitors also subject to the same constraints?
It is not rare for manufacturers to use the same drivers for open-back, semi-open-back and closed-back headphones belonging to the same range. Designing three different models around one driver is no small feat, and the structure of the earpiece (closed, open, semi-open) has a considerable impact on the sound restitution. Truth be told, the sound changes drastically depending on the design adopted.
Open-back design
With open-back headphones, the driver delivers the same amount of sound towards the listener’s ear as it does towards the room. Since the ear cups are open, the driver is visible, most of the time through a grill. This type of design ensures constraint-free working conditions for the driver, thus keeping it as close to its natural response curve as possible. The ultimate listening experience, if you will. Note that open-back headphones must be used in an extremely quiet environment as the open design lets all the surrounding noises seep into the music.
Advantages:
- Extremely wide soundstage
- Neutral sound restitution
- Extension in the lows
- Easy-to-match impedance (simplified amplification)
Drawbacks:
- No acoustic isolation
- Outside noise very noticeable
- People around you can hear what you are listening to
Closed-back design
Closed-back headphones feature completely sealed ear cups. Just like certain speakers, the driver is loaded in a closed enclosure. The main advantage, and a crucial one at that, is the excellent isolation from outside noises. With an attenuation of at least 20 dB, closed-back headphones ensure an excellent signal-to-noise ratio, which logically favors the perception of micro-information. Since the driver is in a closed enclosure, it has to be able to withstand a certain amount of air pressure, which means the cone cannot move as freely as it can in a pair of open-back headphones. The mechanical effects caused by the air contained within the ear cup include the generation of voltage peaks as well as reverb over a given frequency range, which means the driver loses its linearity. If this is taken into account during the design phase of the driver -which requires an extensive R&D process- the restitution can be exceptionally good. Some manufacturers use damping materials to make up for the sound coloration, sometimes adding a port to adjust the response in the lows. Moreover, the mechanical constraints have an impact on the driver’s impedance curve, and the resulting variability makes the amplifier’s work more difficult.
Advantages:
- Excellent signal-to-noise ratio
- Potential for powerful lows
Drawbacks:
- Less expansive soundstage (compared to open-back models)
- Non-linear frequency response
- Difficult impedance
- Requires extensive research and development
Semi-open design
This type of design combines advantages from both open-back and closed-back designs. The earpieces of semi-open back headphones are often “perforated” with one or several small holes which can be compared to bass-reflex ports. These openings can be one of two things. They can either be a simple aperture whose sole advantage is to relieve the driver from the pressure inherent to a closed-back design, or it can be extended inside the ear piece via a tube to work as a bass-reflex port -thus providing an extended response in the lows. Multiple small openings result in a wider soundstage but diminish the signal-to-noise ratio. On the other hand, the use of a port ensures optimal working conditions for the driver and enhances the response in the lows, while maintaining excellent sound isolation. The only drawback of the port is a somewhat restricted soundstage.
On-ear headphones
The distance between the ear and the driver is a very important factor. On-ear headphones (the ear pads are placed on the ear) offer the shortest distance between the ear canal and the driver. On-ear design favors optimal transmission of lows for open-back headphones -an important feature considering that this design is prone to sound leaks. On-ear headphones offer the highest level of detail with both open-back and closed-back models.
Advantages:
- Sound resolution
- Details
Drawbacks:
- Restricted soundstage with closed-back headphones
- Can become uncomfortable during long listening sessions
Over-ear headphones
The way the ear cups are positioned on or around the listener’s ears has a major influence on sound perception. Over-ear headphones free the ear from pressure and constraint since the ear pads sit around the ear and rest directly on the head. Over-ear headphones ensure great comfort, but the few extra centimeters separating the driver from the ear canal impacts the sound restitution in different ways depending on the headphones’ design. Open-back, over-ear headphones are susceptible to losing more power in the lower end of the spectrum compared to a on-ear design. Moreover, the sound level is also inferior. By simply placing your hand over the ear cup and pressing on it to compress the earpad and bring the driver closer to the ear, an increase of 3 to 6 dB can be observed. The sensitivity rating of over-ear headphones is therefore inferior to that of an on-ear model, which may be a problem with a low-power DAP, especially if the headphones have a high impedance rating and/or a low sensitivity rating.
Advantages:
- Very comfortable
- Ideal for long listening sessions
Drawbacks:
- Low sensitivity and resolution
- Open-back headphones often lack power in the lows
What about IEMs?
Most of the time, in-ear monitors feature a closed-back design. Since they are mainly designed to offer an on-the-go listening experience, they require the best sound isolation possible. Although some open-back IEMs do exist, these can only feature extremely small openings, which work as bass-reflex ports and improve the response in the lows. Semi-open-back IEMs will therefore not provide the same open soundstage as open-back headphones. On the other hand, because they can be inserted directly into the ear canal, in-ear monitors enhance the perception of micro-details.
