How to choose headphones

Viabcroce

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What follows is an abstract from an article I originally published in Italian on Accordo.it.
The proofreading and editing for this English version has been made by Christopher Trionfo, to whom my gratitude goes :)


COUPLING?
The term coupling in the electrotechnical realm refers to the interactions which occur when two devices exchange electrical signals, and specifically analyses how the working regime of each is changed by the other one.
Let source be the device sending signals, and load the device receiving them. Note that a load can be a source for another device connected after it, like it happens for example with a power amplifier, which is typically (but not exclusively) a load for a preamplifier and a source for a cab.
When dealing with how a load reacts to a source we talk of input interfacing; output interfacing refers to a source’s behaviour, depending on its load.
To fully understand and manage all the possible coupling issues between an amplifier and headphones, we have to consider only four parameters:

  • For the source: output impedance and output level.
  • For the load: input impedance and sensitivity (how strong the sound is at the same input signal).


INPUT INTERFACING
This refers to the interaction between the amp and the headphones.
Apart from electrostatic models, all headphones connect to the amp’s “headphones output” which, as per the above, is completely governed by its impedance and output level.
There are basically two families of devices capable of driving headphones:

  • Power/integrated amplifiers (i.e. hi-fi amps) fitted with a headphones output;
  • Preamplifiers fitted with headphones output: this family includes any device fitted with a headphones output which is unable to drive a cabinet, such as guitar multi-effects units, modellers, CD players, recorders, dedicated headphones preamplifiers, radios, tablets, computers, smartphones, etc.


Power Amplifiers
A power amp’s headphones output can output a signal of tens of volts (for example 20 V for a 50 W amp). This is a very high value, more than enough for most headphones.
Usually the “headphones output” is a simple resistor (100-700 Ω, but typically 200-400 Ω) put in series to the loudspeakers’ output to lower its voltage.
This circuit has two characteristics:

  • the maximum output level depends on the headphones’ impedance: two pairs of headphones used with a given amp may result in pair A sounding louder than pair B. However, when a different amp is used, pair B may sound louder than pair A.
  • if the headphones’ impedance varies with the frequency, the overall frequency response will depend on both the amp’s and the headphones’ non-linearities rather than just on the former.

The solution to the latter issue is to either use an amp with a lower output impedance, or reduce the actual impedance with a simple (properly calculated) external resistor put in parallel to the output. When the headphones’ impedance is at least 10 times higher than the amp’s output impedance, the overall frequency response will not depend on the load’s impedance curve.


Preamplifiers
Preamps typically output 500-1000 mV, up to about 5 V for a very good preamp or a dedicated headphones preamp. In order to get high volumes with low output signals you’ll need headphones with good sensitivity. Preamps can be classified in two large groups, according to their output impedance:

  • those measured in hundreds of ohms. These preamps behave like power amps in terms of coupling, with the exception that the external attenuator trick won’t work. Because of this you’ll need headphones with high impedance in order to get the best possible linear response;
  • those measured in tens of ohms (like most dedicated headphones preamps). These are the best in terms of linearity.


OUTPUT INTERFACING
A very important and often overlooked issue is the headphones-to-ear coupling. Headphones do not really exclude the room variable from the listening experience: the room is simply replaced by a much smaller environment: the air volume between each of the headphones’ transducer and each ear. This room will be referred to as “the environment”.
For output interfacing there are two basic mechanical issues which determine the sound quality: the environment’s resonances and the acoustic insulation.
Resonances create the familiar “sound of the ocean” we hear when we put a shell or a glass against the ear; these are stationary waves. In order to reduce them we can use absorbing materials (such as glass wool) or reduce the environment’s size (as if using a smaller glass). Now we can think of headphones like a couple of “very small glasses”.
Note: in-ear headphones reduce the environment’s size the most.

When we put the glass over our ear we hear external sounds differently: that is the environmental insulation. The desired degree of insulation depends on the application; more insulation means that

  • we hear less external noise/sounds (which might be a good or bad thing);
  • the headphones’ output will bleed outside. This is not advisable for overdubbing for example, since the mic(s) will catch the playback track(s).


Linear Response At Low Frequencies
If the air moved by the headphones’ loudspeakers escapes from the environment, the result will be a loss in sound pressure, which will most evidently affect the low frequency response.
To easily check this, push the headphones against the ears with your hands while you’re listening to any signal: a low frequency boost will be clearly audible.
One of the advantages of headphones is that they can go as low as 0 Hz. But the response will be the best only when perfect air-retention is attained. With a given model of headphones, environment sealing will depend on our head’s and ears’ shape and how they match the headphones’ shape.
Note: “closed” models do not necessarily perform better than open models under this respect just because air more easily escapes from the latter: open headphones follow a different design and work as a bass-reflex cab. The real difference is the degree of bi-directional insulation, while the quality of the overall performance only relies on the headphones’ design and on their coupling.


Linear Response At High Frequencies
When sound is produced in a more or less closed environment, some frequencies get emphasized while other are weakened. This depends on the generation of bouncing waves, which are summed either in phase, out of phase or at various degrees of phase shift. This is an unwanted phenomenon, which always alters the original sound and depends on how the various wave lengths a sound is made of fit each of the environment’s dimensions. The smaller the environment’s size, the less different size wave lengths will perform according to the rule. In a very small environment such as the one we’re considering, only high frequencies (short wave lengths) will be altered.


Linear Response At Mid Frequencies
In this range of frequencies the environment doesn’t affect the response.


Everyone Hears Differently
It’s impossible, of course, to foresee which static waves will be established, or how sealed the environment will be for a given headphones-to-ear coupling. Nevertheless, it’s important to realize that two people (regardless of their hearing sensitivity) will hear sounds differently when wearing the same pair of headphones.


HEADPHONES
In terms of coupling with the head, there are several kinds of headphones: open, closed, circumaural, supra-aural, and ear-fitting. They all can sound good or bad, depending on the design and on how they couple with the amp and our ears.


Data Sheets
Since each person’s ears and head are different, the industry has defined a standard for headphones measurements: the standard ear. It’s a defined surface wearing a test microphone, against which the headphones’ earpad/cushion is gently pressed in a defined position.
Of course, when we wear those headphones the response will change, depending on our head’s shape and on how well the headphones fit us. To complement the standard ear measures, “open field” measures are usually taken (where the headphones are not pressed agains the standard ear and there’s no contact between the transducer and the mic), to check whether any resonance peaks measured through the standard ear are the result of output coupling or are purely electric.


Playing Loud
If you need high sound pressure levels or dynamics (where a very strong transient is well reproduced and doesn’t make the system seat) you’ll need an amp with a strong output level. Unlike a generic pre, a power amp or dedicated headphones pre can supply a robust enough signal for almost any headphones.
However, when the output signal hits the headphones, the higher the headphones’ sensitivity the stronger the sound will be.
Sensitivity can be measured in dB/V (how many dBs of sound a device can produce for each input volt). 90 dB/V is a low value, 120 dB/V is very high.

Of course, the headphones’ impedance also plays a role here: when output impedance is low (tens of ohms), the lower the load’s impedance the louder the sound. When output impedance is high (hundreds of ohms), high-impedance headphones will sound louder because the way the output works changes.


Headphones Impedance Vs. Linear Response
By definition, an audio device’s impedance (electric resistance to the flowing of the signal) changes with the signal frequency.
Is is worth noting that impedance data reported on technical sheets (nominal impedance) refer to a 1-kHz signal only, which says nothing about the impedance at different frequencies (completely described by the impedance curve). A pair of headphones could have a 50-ohm impedance at 1000 Hz and a 100-ohm one at 100 Hz.
Impedance at a certain frequency determines how loud a signal at that frequency will be reproduced. If the headphones’ impedance varies a lot with the frequency, its frequency response won’t be linear, unless the amp’s output impedance is many times lower than the headphones’ one. As we have seen, a low output impedance results in a signal level independent of the load’s impedance curve.

Let’s make a numerical example: a pair of headphones with an average impedance of 600 Ω but with an 800-ohm peak at 200 Hz would generate a 1.4 dB peak at 200 Hz when driven by a source with a 1-kohm impedance.
The best possible source exhibits a low impedance and a high output level. Of course, headphones with a more linear impedance curve will have less linearity troubles when driven by a high-impedance amp.

Note: headphones’ impedance curves are generally much more regular than those of speaker cabinets: this makes coupling an amp and a pair of headphones much easier than coupling an amp and a cab.


HOW TO CHOOSE HEADPHONES
High-impedance headphones (more than 100 Ω, with the typical value for professional headphones being 600 Ω) work very well with a high-impedance, hi-output source such as a power amp. On the other hand, low-impedance headphones (50-100 Ω) can make better use of a lower output level, such as the one supplied by a generic preamp.
On the whole, high-impedance headphones are the most versatile: they sound good with any output impedance and don’t create non-linearities where impedance variations occur in the load. But when such headphones are used with an amp with a very low output level, their sensitivity has to be quite high in order to provide good dynamics and a high sound pressure level.

If you’re in the market for a pair of headphones for a specific use, you should keep the application in mind and try headphones with a device whose coupling characteristics are at least similar to the amp you’re going to use. Do not check them with a power amp if you plan to use them with a pre or similar audio device.

In regard to the output interfacing, check the following:

  • Do the headphones keep their position on your head and ears even when you move/dance/turn around?
  • Do you feel comfortable after extended use (headphones press too much, your ears sweat and become hot, headphones don’t fit your glasses, etc.)?
  • Does the cord produce any extraneous sound? Hitting or scratching it should not produce any sound in the headphones.
  • Does the kind of insulation to and from the external environment suit your needs?
  • Check the amount of “low end amplification” effect when you press the headphones against your ears: chances are, headphones which generate the less bass increase better fit your head/ears.


USING HEADPHONES
The best option overall for properly driving headphones is a dedicated amplifier. You’ll get enough voltage to drive practically any kind of headphones an high volumes (unless they have an extremely low sensitivity and high impedance). Furthermore, headphones amps don’t have to reach high voltages so can focus on the quality of sound. They are usually extremely linear, can drive a vast variety of headphones in terms of impedance and sensitivity and may exhibit some useful controls, such as selectable output impedance, automatic impedance setting, selectable gain and other services.
 

tomaburque

Member
Messages
584
Dude, that's a Wikipedia article. The best headphone forum I've seen is http://www.goodcans.com/ your content may find a better home there.

I want to quibble with the part about the headphone amps. They are more for current than voltage. The typical iPod or now smartphone does not have enough current to properly drive a big pair of AKGs or other pro level cans.
 




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