Headphone Fundamentals

[AudioStudies]

Initial post will address very basic headphone info, with the hope that subsequent posts can add additional relevant info. Many thanks to those who helped me on the Objective and Subjective thread to better understand headphones, which I didn't get into until this year. Initial post revisions will be on-going for now, as more info comes in from the experts, to whom I am very grateful.

Introduction

Headphones are electroacoustic transducers (speakers) that convert electrical energy to acoustic energy; designed for a single person to listen, rather than a group. In the colloquial, headphones are known as cans. Historically, headphones originated from the telephone receiver earpiece, and were the only way to listen to electrical audio signals prior to the advent of amplifiers that could power speakers to produce sound. In 1958, John C. Koss, an audiophile and jazz musician, produced the first stereo headphones. Previously, headphones had monophonic sound directed to both ears, and were used only by the US navy, telephone and radio operators, and those in similar industries.

Mass market headphones are often marketed with features that can include noise-cancelling, built-in FM radios, waterproofing, and smart technology. High end headphones are luxury products often constructed with high quality components, and are designed to show the state of the art of headphone technology. Even with with high end headphones though, there are some misses and not all of them end up sounding good. Many of the higher end headphones are designed to be used with accessories such as external amplifiers called headphone amplifiers and combination headphone amplifier / digital-to-analog converters (DACs). Some high end headphones are designed specifically to be used with mobile phones. Mass market headphones typically work fine with mobile phones or other low powered devices.

Headphones designed to cover the ears are often characterized as either open-back (with an open grill on the back side of the ear cups) or closed-back, with no openings. Traditionally, open-back designs have allowed for easier production of high quality headphones; however many excellent products are of closed-back design.

The open-back / closed-back distinction, though colloquially common, is not ideal for characterizing all modern headphones. There are arguably two principle volumes from the standpoint of a headphone, the space in front of the driver (between the driver and the ear), and the space behind the driver (typically the cup, open air for some designs). Both of these volumes may be sealed or conversely have some degree of leak relative to the outside air. The connection of these two volumes to each other is of great significance with respect to headphone acoustic performance. Some headphones have a closed back to each cup but isolate noise poorly because both the rear volume and the front volume are not isolated from the exterior air. An open back is found on some headphones that have completely sealed ear chambers.

In most cases, with open-back use, sound will emanate into the nearby environment; and in most cases with closed-back use there is not nearly as much leakage. In general, closed-back has the advantage that they can be used when others are around, such as on airplanes. A more limited number of headphones are designed as semi-open, and the back is only partially open.

Some headphones are designed to fit directly into the ear (not cover the ear); and these are called in-ear monitors (IEMs); and they have become increasingly popular. The better IEMs have excellent sound quality, but may not feel as comfortable as over-ear headphones, especially for long listening sessions. IEMs are lighter, smaller and easier to store and transport than over-ear headphones.

Some of the companies that manufacture headphones include: Focal, Audeze, ZMF, HiFiMan, Sennheiser, Stax, Fostex, Grado, DCA, Denon, Final, AKG and Shure.

Earbuds are small ear pieces that unlike IEM's do not go into the ear canal and do not sit on the ear but are tucked into the concha of the ear.
The sound quality is often lacking as it is very difficult to get a good seal. These types of ear phones are often included as freebies with portable gear; and consequently are not recommended for high quality sound. They have the advantages of low cost (or no cost) and portability; as for example the Apple Earpod.

Another type of headphone is called an on-ear headphone, that doesn’t completely go over the ear, nor does it fit directly in the ear. On-ear headphones sit on the pinnae where over-ear sits around the pinnae onto the skull. Depending on the size of one's pinnae and the available room within the pads some over-ear headphones are partially on-ears for certain people. This usually leads to a poor seal and poor performance. On-ear headphones are not as popular as the other types; and not noted for sound quality. They may not feel as comfortable as over-ear; however they can feel less hot (or warm) for extended use and are more suited for on the go use.

The reason for the many different types of headphones is due to historical reasons and advances in technology that have led to better manufacturing techniques, smaller and more efficient transducers, and marketing for specific types of uses; gaming for example. The invention of many things such as mobile phones and laptop computers have influenced the headphone market, and subsequently the technology, considerably.

Impedance

Impedance is a measure of the opposition to current that a circuit exhibits when a voltage is applied to the circuit. Impedance is similar to resistance; however typically includes other aspects that oppose current, beyond just resistance, such as capacitance and inductance. Most headphones have significant inductance and small amounts of capacitance, in addition to resistance. Therefore, when headphones are connected to a power source, there is a reactive load, and impedance is present; and the magnitude of this impedance is a measure of opposition to current that a particular set of headphones exhibit. Headphone impedance is typically measured at frequencies of 500 Hz or 1 KHz and specified in units of Ohms (Ω).

Knowledge a headphone's impedance can help determine how much voltage headphones will need to produce a reasonable listening volume. High impedance headphones require the use of a headphone amplifier to perform properly. High impedance headphones cannot be powered well from the output jack of a smartphone or other low powered device.

Mobile phones and other low powered devices can typically power headphones up to 32 Ohms (32 Ω). When headphone impedance is between 33 Ω and 100 Ω it may (or may not) be possible to generate enough volume from a low powered device; however it is likely that an improved performance will be attained with the use of a headphone amplifier. Many headphones have impedance specifications in the range from 100 Ω to 300 Ω and these will definitely require a headphone amplifier.

In general, high impedance headphones require more voltage. Although high impedance headphones produce less volume for a given amplifier output level they are coveted because of their ability to handle electrical signals quite well; enabling more accurate and vivid sound. Typically high impedance headphones cost more than low impedance headphones. However, there are some excellent designs of headphones with relatively low impedance.

An amplifier used to power headphones, including headphone amplifiers and amplifiers within a low powered device, has an output impedance specification. The amplifier should have an output impedance less than of the impedance rating of the headphones; and the lower the better with respect to amplifier output impedance. An arbitrary statement that appears on many web sites is that an amplifier should have an output impedance less than 1/8th of the impedance rating of the headphones. Some experts believe 1/10th is a more appropriate ratio. Following the 1/10th guideline will help to ensure a flat frequency response when listening to the headphones.

Headphone impedance can vary with the frequency of the incoming signal; thus in these cases, the actual impedance can vary from the specified impedance when the headphones are in use. However, some headphone designs have little to no impedance change over the audible frequency range.

Sensitivity

Sensitivity is a measure of how effectively headphones (or other loudspeakers) convert electrical signals into sound; thus indicating how loud the headphones will be for a given electrical drive level. Sound pressure level (SPL) is typically measured in units of decibels (dB). There are two methods of measuring and specifying sensitivity:

• In decibels (dB) of SPL at one milliwatt = [ dB (SPL) @ 1 mW ]; or
• In decibels of (dB) of SPL at 1 volt = (dB (SPL) @ 1V):

Regrettably these are often (incorrectly) interpreted to mean the same thing and used interchangeably. Those who do know the difference will often distinguish between them as power sensitivity and voltage sensitivity.

Often voltage sensitivity specs from manufacturers depict only a number and leave out the units of measure (dB/mW or dB/V) for proper interpretation. Some manufacturers show dB/mW whereas others (who like to show high numbers) depict dB/V values. Yet other manufacturers do not even specify voltage sensitivity; rather they post maximum sound pressure levels (SPLs), based on their maximum power rating. All this corresponds to an un-standardized chaotic mess. The end user should be aware that the most convenient efficiency number that can be used to directly compare headphone efficiency numbers is the dB/V number (which if unknown can be calculated from dB/mW rating combined with impedance).

Headphone amplifiers are voltage sources; therefore the voltage sensitivity is most useful for comparison of speakers. However, if only the power sensitivity is known, a form of Ohm’s Law can be used to convert to voltage sensitivity or by using online calculators. When the voltage sensitivity is known, the maximum volume for a pair of headphones can be calculated if the maximum amplifier output voltage is also known. The appropriate voltage specification is root mean square (RMS) voltage, often denoted VRMS .

Consider the case of an amplifier with an output voltage VRMS = 1.0 volts; connected to headphones with sensitivity of 100 decibels SPL per volt = 100 dB (SPL)/V. The maximum volume that can reach the ears for this case is 100 decibels.

Power consumption of headphones is determined by the applied voltage and the drawn current. Headphone amplifier power is typically specified in milliWatt (mW).

Choosing Headphones

The most important considerations in choosing headphones are tonal balance and overall sound quality. Clearly, the impedance and sensitivity of headphones are important considerations; as well as the specifications of the amplifier intended to drive the headphones. There are other considerations as well such as build quality, comfort, aesthetics, value and affordability. The intended use is of great importance, as closed-back may be better for going to the gym or travelling via public transportation, and open-back may be better to achieve great sound for private uses. A gaming headphone has different user requirements than one intended solely for listening to music on a mobile phone. User preferences such as in-ear, on-ear, or over-ear certainly come into play. The reputation of the manufacturer, serviceability, and the warranty for a given set of headphones are also important considerations.

Transducer Technology

The driver elements within headphones are electroacoustic transducers; that is they convert electrical energy to acoustic energy. Some of the more common transducer technologies in modern headphones include: planar magnetic, balanced armature, dynamic, and electrostatic. Less common driver elements include ceramic, air motion transformer, and ribbons. Excellent designs are possible with any of these technologies, and each has pros and cons in the attempt to reach excellent designs.

 

[RayDunzl]

Clearly it can be seen from the above form of Ohms law that sensitivity and impedance are related; the higher the impedance the less the sensitivity

Impedance is not a determinant of sensitivity:

 

[AudioStudies]

Impedance is not a determinant of sensitivity:

View attachment 80607

Thanks, Ray. I will correct.

 

[Chocomel]

Yeah Impedance basically has nothing to do with Sensitivity, it's better to see Impedance as a Measure of voltage/Current ratio required than a Measure of how difficult a headphone is to drive.

Edit: as for earbuds could say they sit on the ear shell (concha) rather than go into the ear canal. Popular examples being the apple earpod.

 

[JohnYang1997]

Impedance is not a determinant of sensitivity:

View attachment 80607

View attachment 80610

It also depends on the unit of sensitivity. I assume you are using dB/mW here. If you use, dB/V the result should be different.

 

[pozz]

High end headphones are designed for the best possible sound quality and very rarely incorporate any of the aforementioned features as a design choice.

This is not a useful distinction IMO. "Sound quality" and "high-end" are vague terms regularly manipulated by mass-market manufacturers and boutique manufacturers alike for their purposes.

Headphones should be distinguished on the basis of their functions, as you've done, but boutique manufacturers should get no special status. Whatever divide exists should be treated in terms of demographics, production techniques, performance (with evidence like FR graphs to back it up) philosophy/outlook and so on.

I also think your descriptions of overear/on-ear/in-ear headphones are too colloquial/commonplace, unlike the way you started the post. The reason for the different types is 1) history and 2) functional/engineering differences that developed over time, including better refinement of goals and manufacturing techniques. The ability to accurately create smaller and more efficient transducers, for example. This doesn't have to be very detailed but writing these sections with that in mind would make the distinctions much clearer. Year they were introduced. And so on.

 

[AudioStudies]

This is not a useful distinction IMO. "Sound quality" and "high-end" are vague terms regularly manipulated by mass-market manufacturers and boutique manufacturers alike for their purposes.

Hopefully there is a way to make the point that phones with noise cancellation and/or FM radios are not as likely to be better sounding cans without being to vague. Or, I could be wrong.

 

[pozz]

Hopefully there is a way to make the point that phones with noise cancellation and/or FM radios are not as likely to be better sounding cans without being to vague. Or, I could be wrong.

DIY Audio Heaven (@solderdude) has measurements of Sony and Bose headphones with/without active noise cancellation. In the Sony headphones, the DSP also corrects for the driver's flaws. This feature will likely be seen more in the future. The advantages are similar to those posed by active vs. passive speakers.

My point is that the lines should be drawn based on function, not the abstract notion that multifunction/pure analog designs are better (edit: or worse). It's enough to say that some headphones have radio, microphones, amplifiers and so on built-in, and that so far dedicated units show better performance in general, although the margins are shrinking.

 

[AudioStudies]

My point is that the lines should be drawn based on function, not the abstract notion that multifunction/pure analog designs are better. It's enough to say that some headphones have radio, microphones, amplifiers and so on built-in, and that so far dedicated units show better performance in general, although the margins are shrinking.

Agree 100%

 

[Mad_Economist]

Though colloquially common, the "open/closed back" distinction isn't really that ideal in terms of characterizing headphones. A Fostex TH900 has a "closed" back to its cup, but isolates noise poorly because both the volume behind and the volume in front of the driver are not isolated from the exterior air. An Audeze LCD2 has an "open" back, but the earpad chamber is completely sealed - to the point that putting on the headphones can cause the diaphragm to flex from the pressure.

There are arguably two principle volumes from the standpoint of a headphone, the space in "front" of the driver (between the driver and the ear), and the space "behind" the driver (typically the cup, open air for some designs). Both of these volumes may be sealed or have some degree of leak relative to the outside air, and their connection to each other is also quite significant in headphone acoustics.

 

[solderdude]

High end headphones are designed for the best possible sound quality and very rarely incorporate any of the aforementioned features as a design choice. Many of the higher end headphones are designed to be used with accessories such as external amplifiers called headphone amplifiers and combination headphone amplifier / digital-to-analog converters (DACs).

High End headphones are luxury products usually made with high quality components. These headphones are made to show what is technically possible. This can translate to good sounding headphones but isn't always the case.
There are also high end headphones designed to be driven from a phone.

Open-back designs allow for easier production of high quality headphones; however many excellent products are of closed-back design. With open-back use, sound will emanate into the nearby environment. Thus closed-back has the advantage that they can be used when others are around, such as on airplanes. A more limited number of headphones are designed as semi-open, and the back is only partially open.

Closed back headphones are designed to keep outside noises out and ensures there is little to no leakage of sound to the outside.
Noise cancelling takes this a step further and actively lowers frequencies (below 1kHz) further than can be achieved in a passive way.

Open and semi-open headphones leak sound to the outside and also lets sound from the outside in.

Both have their specific use cases.

When you want to be more complete there is also something like ear speakers (Jecklin float, AKG K100, Raal SR1A, Mysphere etc., Sony MDR MA900) which are very 'open' headphones and are basically speakers located near the ears.

NOTE: Another type of headphone is called an on-ear headphone, that doesn’t completely go over the ear, nor does it fit directly in the ear. These are not as popular as the other types; not noted for sound quality. They may not feel as comfortable as over-ear; however they can feel less hot (or warm) for extended use and are more suited for on the go use.

On-ear headphones sit on the pinnae where over-ear sits around the pinnae onto the skull. Depending on the size of one's pinnae and the available room within the pads some over-ear headphones are partially on-ears for certain people. This usually leads to a poor seal.

• In decibels (dB) of SPL at one milliwatt = [ dB (SPL) @ 1 mW ]; or
• In decibels of (dB) of SPL at 1 volt = (dB (SPL) @ 1V):

dB/mW: dBSPL @ 1mW
dB/V: dBSPL @ 1V

Regrettably these are often (incorrectly) interpreted to mean the same thing and used interchangeably. Those who do know the difference will often distinguish between them as power sensitivity and voltage sensitivity.

The problem here is that often sensitivity in specs only shows a number and leaves out the dB/mW or dB/V. Some manufacturers use dB/mW others (who like to show high numbers) show dB/V numbers where others do not specify or even post maximum SPL levels (based on their max. power rating) It is a mess. The only handy efficiency number that can be used to directly compare headphone efficiency numbers is the dB/V number.
This can also be calculated from dB/mW rating combined with impedance.

Ear buds - please give ideas for what to say here

Earbuds are small ear pieces that unlike IEM's do not go into the ear canal and do not sit on the ear but are tucked into the concha of the ear.
The sound quality is often a miss as it is very difficult to get a good seal. These types of ear phones are often included as freebies with portable gear.

The output voltage of a given headphone amplifier is essentially constant for most common headphones;

No its not. Headphone amplifiers are voltage sources but there is nothing constant about it.

Although output voltage from a given amplifier to headphones is essentially constant, this does not mean the power is constant because the current is changing. Headphone amplifier power is typically specified in millaWatts (mW).

No its not constant. Power delivery (or rather power consumption) of the headphone is determined by the applied voltage and the drawn current (voltage/resistance). Its not millaWatts but milliWatt.

Clearly, the impedance and sensitivity of headphones are important considerations; as well as the specifications of the amplifier intended to drive the headphones. There are other considerations as well such as comfort, aesthetics, affordability, and open-back versus closed-back. The reputation of the manufacturer and the warranty for a given set of headphones are also important considerations.

The most important consideration is tonal balance and overall sound quality. Another consideration is use case (open or closed) and whether or not they are going to be used portable (jogging, public transport/outside, or indoors) and if one prefers in-, on- or over-ear. Are they going to be used with mobile phones (taking calls etc) or gaming etc. Price, availability and warranty are also deciding factors as well as looks and functionality.
Build quality, fit and comfort are also factors as well as serviceability for those that plan to enjoy their headphones for many years.

 

[solderdude]

Impedance is not a determinant of sensitivity:

View attachment 80607

View attachment 80610

Funny list.
The LCD2C is not 130dB/mW but 101dB/mW.
130dB is the SPL at its nominal power rating.
Here is a list (that also is not always correct) with basic headphone specs of a lot of headphones.

 

[RayDunzl]

Here is a list (that also is not always correct) with basic headphone specs of a lot of headphones.

From that list:

Discard these - wireless? Noise Cancelling?

And the rest of the list:

 

[AudioStudies]

Thanks everyone for the many replies, I will be gradually trying to edit the original document, to account for your thoughts, as best I can.

 

[solderdude]

Discard these - wireless?

I don't know... some of these wireless ones also can be used wired ?
Some wireless ones couple direct on the driver, some on an active input, some digitize, some do not.
It is quite possible the input resistance of these devices is indeed somewhere between 1k and 2k

 

[Mad_Economist]

From that list:

Discard these - wireless? Noise Cancelling?

View attachment 80654

In general Zin > 800Ω in headphones means you're looking at an ADC or internal amplifier rather than the coil. For portable headphones, I'd probably say that's typically the case for anything >100Ω.

 

[solderdude]

There was a 2kΩ HD414 long ago...
And currently the 0.1Ω RAAL SR1A.
Let's call these the odd ones but showing the extremes.

Notice at 1kΩ the dB/V and dB/mW numbers are the same because at 1V a 1kΩ resistor dissipates 1mW

 

[AudioStudies]

For some reason, I seem to have lost the ability to edit my posts. The good news is that I made a lot of the changes that you were all kind enough to suggest. A couple more minor things I wanted to change but it won't let me. I am very curious to hear the response to the question John Yang raised, and it would be interesting to see a plot with the other units of sensitivity. Regards, and thanks to everyone.

 

[AudioStudies]

I have two sets of headphones, the Sennheiser 650 Clone, the HD-6XX (open back) and Audeze Sine (closed back). I enjoy both of them. I think the HD-6XX has a rated impedance of 300 Ohms, thus a high impedance headphone. I am not sure about the impedance rating of the Audeze Sine, but I think it is quite a bit less than the other one. Both of these cans sound great when powered from a dedicated headphone amplifier. The Audeze Sine is the only one that I trust plugging into low powered devices, such as my portable recorders and telephone. I would welcome any comments on these cans.

 

[solderdude]

Audeze Sine
HD650

After a couple of days you cannot edit your posts anymore. When you want it edited you will have to contact one of the admins and PM them the text and tell them (link to) the post you want edited.