Headphones Specifications&&Application:A Guide for

AKG Pro Dealers and Users

Table of ContentsIntroduction ................................................................................................................................................................1

The Basic Transducer..................................................................................................................................................1

Basic Headphone Types .....................................................................................................................................1

Hygiene and Hearing Protection ................................................................................................................................2

Electrical Considerations ............................................................................................................................................3

Main Usage Categories and Model RecommendationsIn-Studio/On-Stage Monitoring ........................................................................................................................3Vocal Booth/Drum Cage Monitoring................................................................................................................4Broadcast and ENG (Electronic News Gathering) Monitoring ...........................................................................4Personal Listening ............................................................................................................................................4

Model RecommendationsK501 ...............................................................................................................................................................4K401 ...............................................................................................................................................................5K301 ...............................................................................................................................................................5K270S..............................................................................................................................................................5K240DF...........................................................................................................................................................6K240M............................................................................................................................................................6K141M............................................................................................................................................................6K100 ...............................................................................................................................................................7K70 .................................................................................................................................................................7K66 .................................................................................................................................................................7K55 .................................................................................................................................................................7

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INTRODUCTIONHeadphones are taken pretty much for granted in professional audio, and most sound engineers make little or no distinctionamong the hundreds of models that are available. Many people, both engineers and non-engineers, have a casual approachto headphones, rarely stopping to think about headphone impedances and what is exactly on the "other side" of thestandard ring-tip-sleeve headphone jack found on almost all recording and playback devices, consumer and professional.Actually, very little information on headphone specification and application has been published, and we at AKG aredetermined to correct that state of affairs with this technical note.

Consider the AKG line of headphones; there are 10 models listed in the current catalog, and each one of them is there fora purpose. In some cases the differences between models are slight, while other models are truly unique. Our purpose inthis applications paper is to describe the models in detail, helping the pro dealer and user to make the most appropriatechoice for a given job.

AKG has a number of distinct advantages over many manufacturers. We have the experience of producing many types oftransducers, including dynamic headphone elements, for over fifty years, and there are a number of features common to allAKG headphones that distinguish the line from all other brands. The patented self-adjusting headband allows short workof fitting any model to any head. The patented flexible twin conductive rails above the headband eliminates the traditional"cable yoke" found on many other brands, and allows for single cable entry into one earcup. AKG was the first to inventand pioneer the use of high-test plastics in headphone construction. Most AKG headphones can be contorted and twistedwell outside their pre-formed shape with little fear of damage.

THE BASIC TRANSDUCERThe vast majority of headphones are of the electro-dynamic type, in which a small voice coil is placed in a magnetic gapattached to a lightweight diaphragm. You may think of the headphone transducer as analogous to a dynamic microphoneacting in reverse. Over the years, AKG has developed and refined both dynamic microphones and headphones, with manyof the improvements in manufacturing benefiting both product groups. AKG headphones have traditionally represented anoutstanding value, considering their consistent response and lightweight, durable construction.

The two important electrical specifications for a pair of headphones are the impedance of the phone transducer itself andits on-the-ear sensitivity. So-called low impedance headphones may vary from 75 ohms up to about 150 ohms. Phones inthis impedance range may be directly plugged into the headphone jack routinely found on recording and playbackequipment. Higher impedances, such as 600 ohms, are more useful in studio installations where many units may be wiredin parallel for studio monitoring applications.

Headphone sensitivity is usually stated as the in-the-ear sound pressure level produced by one milliwatt (mW) of audioinput. Typical sensitivity ratings of AKG headphones run from 88 dB per mW to 105 dB per mW. You can see that verylittle power is needed to drive a stereo headphone pair to very high listening levels.

Doing a little bit of math will show that a signal of .775 Vrms will produce one mW in a 600 ohm load. That same signalwill produce 8 mW in a 75 ohm load, a difference of about 9 dB. So it is clear that 600 ohm phones and the lowerimpedance models should not be intermixed in the same application. The 600-ohm models are more rugged than lowimpedance models in that the higher resistance coils are less susceptible to burn out than low impedance models.

BASIC HEADPHONE TYPESHeadphones may be differentiated in terms of how they fit over the ear. If the earcup fits snugly around the ear it is knownas circumaural. Such phones are comfortable to wear, provide good isolation, and result in a consistent sound from onewearing to another. Bass extension is excellent with the circumaural design since a very good air seal can be made againstthe head.

Somewhat smaller in diameter, the supra-aural design has an earcup that rests on the outer ear, or auricle. This has thebenefit of being quite light and comfortable. Response may vary, depending on the precise positioning of the earcup. Bassextension may suffer in the supra-aural design inasmuch as a good air seal cannot be made.

There is further differentiation among headphones in terms of the "open-ness" of the structure, or how it communicateswith the outside. Phones may be characterized as open-air, semi-open, or sealed. In the early days of stereo headphonelistening, most models were sealed, probably because designers felt that approach resulted in the best frequency response.

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But many listeners tire of such phones after long listening sessions, and headphone designers began to experiment withopen cell structures. The open-air designs provide essentially a direct connection with the outside; as such, they providelittle isolation in either direction. A person wearing such phones can engage in conversation (if the phone volume is nottoo high). If such phones are operated at very high levels, they may be annoying to persons nearby. One importantperformance advantage of the open structure is that it provides excellent acoustical damping, reducing the effects ofstanding waves in the outer ear/earcup cavity. The result is far more accurate, peak free sound.

The semi-open design is used in three of AKG's most popular headphone products. This approach provides the developmentengineer a number of performance advantages unique to AKG. These models combine the best features of both sealed andopen air designs, with none of their disadvantages. Through careful attention to the control of resonances, a surprisinglyrealistic stereophonic sound field is developed, coupled with low distortion and extended low frequency response.

HYGIENE AND HEARING PROTECTIONSome persons, for various reasons, may refuse to wear a set of headphones that others have worn. When you encounterthis, be prepared to break out a replacement set of earpads. AKG provides a new style of easily replaced, twist-lock earcups.It is a good idea to maintain extra stock of these on hand, not only for the situation discussed here, but also for routinereplacement of any that may show wear and tear.

Another matter of great concern is the preservation of hearing. Most listeners to headphones tend to play them at muchhigher levels than they would ever play loudspeakers. There is some speculation as to why this is the case. It probablyrelates to the fact that there is no physical (body) component to headphone sounds as there is with loudspeaker listening.If anyone complains or comments on even slight ringing after a listening session, you can be sure that the levels have beentoo high.

OSHA (Occupational Safety and Health Act, 1970) publishes the following data:

Permissible Exposure to Noise LevelsSPL (A-weighted): Daily Exposure:

90 dB 8 hours92 695 497 3

100 2102 1.5105 1110 0.5115 dB 0.25 hours

The industry does not generally publish maximum operating level capability of headphones, inasmuch as, even the lowestcost phones can play much louder than anyone should ever need. (We have published rated power for the models K501,401, and 301 in our most recent literature.) With an industry average sensitivity of 94 dB for one milliwatt input, we cansee that 10 mW would produce a level of 104 dB, while 100 mW (0.1 watt) would produce a level of 114 dB. There is areal danger in going any higher than this in playback level.

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ELECTRICAL CONSIDERATIONSA typical portable DAT recorder may specify its per-channel headphone output as:

Rated output: 15 mWImpedance: ZL 32 ohmsConnector type: Stereo phone jack

The value of ZL, the load impedance, is taken to indicate a minimum value, indicating that any value higher than 32 ohmswill work satisfactorily, with only a slight reduction in output. Any nominal low impedance headphone set would work,but clearly, 600-ohm headphones would not be indicated here.

A typical home recording project console may specify, per channel:500 mW (8 ohms)26 mW (600 ohms)

In this case the manufacturer is stating that the console can handle a wide range of headphone load impedances, with thepower availability dropping with increasing impedance. In any event, the 26 mW available for 600-ohm phones will beample enough, considering the relatively high sensitivities of headphones.

A typical one rack unit stereo headphone line amplifier may indicate: one watt maximum into four ohms; operational range,4 to 600 ohms (output voltage 2.83 Vrms). This specification implies a wide range of loading and output level capability.Under worst-case loading of 150 600-ohm headphones operating in parallel, the power delivered to each phone would be6.7 mW, enough to drive a pair of 96 dB sensitivity phones to a level of 102 dB.

In general, be careful about paralleling too many headphones on one circuit. Always make a quick check of the resultingimpedance and make sure that it falls within the allowable range. This can easily be done, by dividing the impedance of asingle phone by the total number of phones loading the line. For example, ten AKG K100 headphones operated in parallelwill have a net load of 90/10, or 9 ohms. This load would be allowable on the console discussed above, but certainly noton the DAT recorder.

MAIN USAGE CATEGORIES AND MODEL RECOMMENDATIONSIn this section we will discuss briefly the major usage categories of stereo headphones, underscoring the specific technicalfeatures that support a particular usage. We will then move on to a concise discussion of each headphone, detailing theirsuggested uses and the reasons why.

Primary Usage Areas for AKG Stereo Headphones1. In-studio/On-stage monitoring2. Vocal booth/drum cage monitoring3. Broadcast and ENG monitoring4. Private listening

1. In-Studio/On-Stage MonitoringPerhaps the majority of AKG headphone sales are to studios and live performance venues, where they are used forheadphone cueing for musicians. The important attributes here are:

1. Reliability2. Robust sound (not necessarily flat response)3. Economy

In a studio where film soundtracks are recorded, an additional requirement will be for the phones to have a good acousticalseal in order to keep click tracks from leaking into microphones. Click tracks are widely used in recording film scores thatare being played in synch with a picture that has already been partially edited and where musical events must match screenevents in exact synchronism. This usually means a closed circumaural design.

In a normal recording environment, the demand for a good acoustical seal is much less stringent. Models with animpedance of 600 ohms are often chosen, since that enables multiple sets to be loaded in parallel across a standard stereoamplifier without problems.

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2. Vocal Booth/Drum Cage MonitoringThe vocal booth is a necessary evil in the modern recording studio. It removes the performer from the live acousticalambience of the studio setting where high studio sound levels may leak into the vocalist's microphone. The balanceengineer must somehow create a stereo mix for the vocalist that conveys the essence of the music, including the voice itselfalong with any pertinent signal processing, such as reverberation. This is an art in itself, and needless to say the quality ofthe headphones is paramount in keeping a singer happy.

Normally, leakage from open or semi-open headphones into the microphone is not a problem. But occasionally you mayencounter a vocalist with a small voice who likes to monitor at high levels, and this can cause problems that only a sealedset of headphones can solve.

Just the opposite of a vocal booth, the drum cage is designed to accommodate a player who generates more sound levelthan can normally be contained by the studio. In short, the sound from the drum set tends to leak into all of the openmicrophones in the studio. The drummer will invariably have to wear a closed circumaural set of phones in order to hearwhat is going on in the studio. Like the vocalist, he will need his own dedicated monitor mix.

3. Broadcast and ENG (Electronic News Gathering) MonitoringMany on-the-air announcers prefer to wear headphones as they talk. Long-term comfort and lightweight are essential, as isreliability and compatible sound quality.

ENG (Electronic News Gathering) normally implies on-location work outdoors, and headphones chosen for this applicationmust be sturdy, lightweight, and of semi-open design so that the user is in constant touch with the outside world. ExtendedLF response is not necessary, since the purpose is primarily for speech intelligibility. Slightly boosted midrange and HF willenable many users to better understand speech under noisy conditions that are often encountered in ENG.

4. Personal ListeningMany people who live in crowded quarters, or who like to listen to music while other family members are sleeping, havenaturally gravitated to headphones. For the serious headphone listener, a sizeable financial outlay for a truly exceptional setof headphones is justified. The desirable performance characteristics in these phones are extended, smooth frequencyresponse, the ability to produce high levels with very low distortion, and the ability to recreate a spatially accuratesoundfield. Another requirement is long-term wearing comfort. AKG has brought fifty years of acoustical and audioexpertise to these specific problems and has designed some of the finest products available for reference monitoring, bothin the home and in the studio.

MODEL RECOMMENDATIONSIn this section we will review all models in the AKG headphone lineup, emphasizing the performance features of each oneand detailing the specific performance areas in which a particular model excels:

K501 HeadphonesUses VarimotionTM large diaphragm design for good spatial separation and outstandingsound resolution. Response extends from 16 Hz to 30 kHz. Ideal for lovers of classicalmusic. Uses circumaural earcup with open acoustical design.

Recommended for:• Classical listening• Home studio applications• FM broadcast monitoring• Multimedia applications

K501

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K401 HeadphonesThese headphones provide all that rock and pop music enthusiasts want tohear: tight, powerful bass, crystal clear sound quality and impressive power.Uses VarimotionTM diaphragm design for outstanding sonic and spatialresolution.

Recommended for:• Pop/rock listening• Home studio applications• Multimedia applications• Broadcast monitor applications

K301 HeadphonesThese phones have recently been redesigned, using a large diaphragm withhigh internal damping for smoother overall response. The open designmakes for excellent, fatigue-free listening.

Recommended for:• Home listening of all kinds• Multimedia applications• Home studio applications

K270S Studio HeadphonesThese phones have a closed design and are ideal where leakage from thephones into nearby microphones must be avoided. Motion picturerecording using "click tracks" for synchronization is an example of this.When the phones are removed, a switch is opened that mutes the phones.This switch is a big advantage in eliminating feedback and leakage from anunattended pair of headphones. Two transducers are used in each earcupfor added output capability. Low impedance (75 ohm) design

Recommended for:• Studio and vocal/drum booth usage• Use in high ambient noise environments• ENG operations• Sound reinforcement monitoring

K401

K301

K270S

K240DF Monitor HeadphonesThe K240DF meets specific criteria of the IRT (German Radio Institute)for diffuse-field equalization, which is ideal for broadcast monitoring,setting a standard by which program material traveling internationallycan have a common listening reference. The design is circumaural,semi-open for listening comfort, and the high impedance (600 ohms)facilitates studio monitoring, with multiple phones connected inparallel. These phones sound slightly flatter than the model K240M.

Recommended for:• Broadcast transmission monitoring• High accuracy home listening• Recording studio monitoring

K240M Monitor HeadphonesThe K240M is one of AKG's top selling models and is a virtual standardin recording and broadcast studios. The design is circumaural, semi-open, and the response extends down to 15 Hz. Excellent wearingcomfort.

Recommended for:• General recording and broadcast studio use• Home listening• ENG operations• All headphone monitoring activities

K141M Monitor HeadphonesAccording to Billboard Magazine, the K141M phones are the mostwidely used headphones in U. S. studios. The design is supra-aural (onthe ear) semi-open; response is extended, with exceptional bassresponse. High impedance (600 ohms) facilitates paralleling of phonesfor multiple monitoring activities in recording studios.

Recommended for:• General recording and broadcast use• Home listening• ENG operations• All headphone monitoring activities

K240DF

K141M

K240M

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K100 HeadphonesAKG's lowest costing professional format headphone. Single side entryhigh quality cable with gold connectors, self adjusting leather headband,and typical AKG linear response. Semi-open, supra-aural (on the ear)design allows for hours of use free from fatigue and discomfort.

Recommended for:• The studio on a budget• Home listening• Multimedia applications

K70 HeadphonesThe K70 is a lightweight monitor quality supra-aural (on the ear)headphone that offers many of the features found in AKG's professionalformat headphones. Lightweight, self adjusting, comfortable over manyhours of use, and replaceable earpads, all in a semi-professional costeffective design. The K70 offers premium audio quality at a priceaffordable to most customers.

Recommended for• Home music and video listening• Small project studios• Multimedia applications

K66 HeadphonesA self-adjusting headband, single cord entry and circumauralearpads provide a very pleasant fit for excellent wearingcomfort. A large diaphragm element in an open back designoffers an exceptionally smooth frequency response in anaffordable headphone package.

K55 HeadphonesA self-adjusting headband, sealed circumaural design and singlecable entry offer good user comfort. A large diaphragm elementin a sealed design combined with AKG’s Turbo Bass functionexhibit a punchy low end suitable to pop and rock music.

K100

K70

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K66 K55

AKG Acoustics, U.S. • 1449 Donelson Pike • Nashville • TN 37217 • Tel: (615) 360-0499 • Fax: (615) 360-0275Visit our Web Site at www.akg-acoustics.com