Headphones: sound just for you

Headphones are, in short, transducers that, like loudspeakers, convert electrical energy into acoustic energy, but at a much lower level, which is why they need to be placed very close to our ears. Or rather it is the other way around: since we are going to place these loudspeakers close to our ears, minimal radiation is enough. We use them so that the sound reaches only us, isolating us from outside noise and even achieving a musical immersion that is impossible to obtain by other means or, at the very least, achieving it more quickly. But they also allow us to listen to music at whatever sound pressure level we want: from a whisper to the most outrageous volume (without disturbing, in most cases, anyone around us).

The short distance between the earpiece and our ear makes all of the above possible, and even overcomes some of the shortcomings we encounter with traditional transducers: it is easy to achieve high sound pressure with hardly any distortion. But they also have harmful effects: they can damage the listener's eardrum and cause hearing loss.

Yet headphones are a very good solution for avoiding some of the problems caused by listening rooms. Since they are not tied to the architectural vagaries (reflections, vibrations, volume...) of any room, the sound is always clean thanks to direct radiation. On the other hand, with headphones we lose the sense of "spaciousness", something that can only be achieved by listening to music in perfectly treated rooms.

There is probably nothing better than enjoying music using a good system in a good room. But any shortcoming, however small, can disappear with the use of headphones. Another example: with headphones we will always, absolutely always, have a perfect stereo image, even if we turn our head. And another: a woofer needs to move a significant amount of air to faithfully reproduce a low-frequency transient, whereas headphones, which are much smaller, have hardly any trouble reproducing the same sound for the same, and more perfect, perception. Less mass and therefore faster response.

Depending on their construction, headphones fall into three types: dynamic (the most common), piezoelectric and electrostatic. They can also be of closed, open or semi-open design (closed designs are also the most common).



DYNAMIC HEADPHONES



A dynamic headphone is, in short, a miniaturized version of a conventional loudspeaker. Its design is based on the same principle: a magnetic body that attracts a metal membrane (usually steel or soft iron) or diaphragm separated from it by just 0.4 mm. The system is closed at the back and there is only a hole in the center through which the air forming the sound waves escapes. The magnet always has an attractive force (it is permanently magnetized), which applies a certain pull to the membrane for life. This is necessary so that the membrane does not vibrate at twice the frequency of the applied voltage, which is precisely what we do not want (we already have enough energy).

Unlike a traditional loudspeaker design, the central hole we see in headphones (whose diameter is smaller than that of the membrane) is absolutely necessary. Every time the magnet moves the membrane, it compresses the air between the membrane and the front wall containing the central hole.

This first design is improved when the membrane itself, made of Mylar or a similar material, incorporates a one-inch-diameter moving coil. The coil is immersed in a magnetic field, also constant, which moves the membrane to generate the varying air pressure. At rest, the membrane touches the front cover. The advantage of this design is that it avoids the distortion caused by the residual air pressure remaining in the cavity between the membrane and the opening in the earlier design. It also considerably reduces the weight of the transducer, which is important for long listening sessions and for the user's comfort.

Conventional headphones use two of these drivers, and each of them needs its own amplifier channel, which is normally not located in the headphones themselves.



ELECTROSTATIC HEADPHONES



Regular readers of the magazine are well aware of how an electrostatic element works: it is based on the attraction and repulsion of the electric charges present in a capacitor. The secret lies in designing a capacitor with one fixed plate and one moving plate, which is the one that ultimately moves the air. The main problem with this type of headphone is that the force of attraction is directly proportional to the square of the voltage applied, and not just to the voltage. This means that, unless the design is very precise, distortion will take center stage. One way to solve this problem is to apply a fixed DC bias voltage to the capacitor. Technicalities aside, an electrostatic headphone is probably the one that achieves the greatest sonic perfection, although its drawback is that its price is usually quite high.

Halfway between these two types of headphones we find hybrids, or dynamic/electrostatic models. One of the drawbacks of electrostatic headphones is that it is difficult (or expensive) for them to achieve good low-frequency reproduction. This type of hybrid headphone uses a dynamic driver for this purpose, leaving the high and mid frequencies to the electrostatic element. In this way, a dynamic/electrostatic headphone can achieve a frequency range from

16 Hz to 25,000 Hz.

The difference between open and closed headphones is that the former allow outside sound to "pass" into the headphone. Closed headphones let no sound out of or into the headphone. The key role in achieving both goals is played by the ear pad. In an open model the ear pad is acoustically transparent, whereas in a closed design it may even contain a liquid material, not only to prevent sound from passing from one side to the other, but also so that the headphone fits the user's head perfectly. Of course, halfway between the two are semi-open headphones, another AKG patent. These headphones use the ear pads of a closed design, but the front side of the transducer is open.



SPECIFICATIONS



On paper, a headphone, as we have seen, is like a loudspeaker. So, as far as technical specifications are concerned, most of the values are almost the same. To begin with, we note that frequency response is still one of the most important values, since it allows us to discover the dynamics of our candidate. It is expressed in hertz (Hz) together with the tolerance applied in decibels (dB). It is very easy for a headphone (especially an electrostatic one) to far exceed the frequency response of many loudspeakers.

The sound pressure level is exactly the same as for loudspeakers, so values of 94 dB are sufficient. As usual, the lower the sound pressure level, the more current is needed from the amplifier.

The impedance of dynamic headphones ranges from 8 to 2,000 ohms. This impedance is usually always referred to a frequency of 1 kHz. Even so, values between 100 and 600 ohms are the most common. The higher the impedance, the greater the need for amplification.

Of the other values that may be specified, there is one in particular that is hard to find but directly concerns this type of device: clamping pressure. This figure tells us how comfortable the headphones will be to wear. Excessive pressure can cause discomfort, whereas too little makes the headphones move around and lets the ambient noise of the room be heard. Clamping pressure is given in newtons (N), and normal values are between 1.5 and 3 N (approximately 150 to 300 g).



AMPLIFICATION



Like any loudspeaker, headphones need amplification (there are a few models with built-in amplification). Most receivers and amplifiers usually include a headphone amplifier, as do many sources such as CD or DVD-Video players. The standard connection is a jack-type connector, the old DIN connection having been forgotten. Normally, when we connect headphones to an amplifier, it switches off the amplifier channels for the loudspeakers. Headphone amplifiers begin by reducing the signal voltage to achieve suitable operating levels. This is done using voltage dividers, so the signal reaching the headphones does not come from the amplifier's output but from a 10-ohm resistor terminal. Another option is to use a specifically designed amplifier capable of delivering a 400 mV line signal, with an output impedance of up to 2.5 kohms (these are typical values that may vary depending on the amplifier design).



EXTRAS



As with everything, there are models that differ in some way from the norm. In this very magazine we have reviewed Bose's well-known Quiet Comfort. These are conventional headphones with an attractive feature: they offer perfect cancellation of ambient noise. To do this, they include a microphone in each earpiece that picks up the ambient sound that may interfere with listening and reproduces it in antiphase inside the earpiece. In this way, any sound that might interfere with the music is canceled out.

To do this, these headphones (and others from different brands) need a power supply, which means adding some weight to the device because of the battery (in this case, an AAA type).

We have also seen wireless headphones that include, in addition to amplification, an RF or other wireless receiving system. In this case, the weight needs to be evaluated.

There are also amplifiers designed specifically for headphones, as well as signal distributors (a device that amplifies the same audio signal for several pairs of headphones). This type of device is very common in recording studios, for example.

Earbud designs are very popular among MP3 player users. They do not use the headband that applies pressure to make the headphones fit our ears perfectly since, in fact, they are placed inside the outer ear. They require even smaller transducers, so they normally offer somewhat lower low-frequency performance.

Finally, there are "surround" headphones, either using audio processors that, by means of phase shifts and delays, "trick" the ear into thinking that sounds are coming from different directions, or models that use several strategically placed transducers to achieve a more enveloping sensation. This type of headphone, especially the latter, is usually paired with its own processors and amplifiers.