Application: A Guide for AKG Pro Dealers and Users

Transcription

1 Headphones & Specifications Application: A Guide for AKG Pro Dealers and Users

2 Table of Contents Introduction...1 The Basic Transducer...1 Basic Headphone Types...1 Hygiene and Hearing Protection...2 Electrical Considerations...3 Main Usage Categories and Model Recommendations In-Studio/On-Stage Monitoring...3 Vocal Booth/Drum Cage Monitoring...4 Broadcast and ENG (Electronic News Gathering) Monitoring...4 Personal Listening...4 Model Recommendations K K K K270S...5 K240DF...6 K240M...6 K141M...6 K K K K55...7

3 INTRODUCTION Headphones are taken pretty much for granted in professional audio, and most sound engineers make little or no distinction among the hundreds of models that are available. Many people, both engineers and non-engineers, have a casual approach to headphones, rarely stopping to think about headphone impedances and what is exactly on the "other side" of the standard 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 are determined 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 for a purpose. In some cases the differences between models are slight, while other models are truly unique. Our purpose in this applications paper is to describe the models in detail, helping the pro dealer and user to make the most appropriate choice for a given job. AKG has a number of distinct advantages over many manufacturers. We have the experience of producing many types of transducers, including dynamic headphone elements, for over fifty years, and there are a number of features common to all AKG headphones that distinguish the line from all other brands. The patented self-adjusting headband allows short work of 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 invent and pioneer the use of high-test plastics in headphone construction. Most AKG headphones can be contorted and twisted well outside their pre-formed shape with little fear of damage. THE BASIC TRANSDUCER The vast majority of headphones are of the electro-dynamic type, in which a small voice coil is placed in a magnetic gap attached to a lightweight diaphragm. You may think of the headphone transducer as analogous to a dynamic microphone acting in reverse. Over the years, AKG has developed and refined both dynamic microphones and headphones, with many of the improvements in manufacturing benefiting both product groups. AKG headphones have traditionally represented an outstanding 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 and its on-the-ear sensitivity. So-called low impedance headphones may vary from 75 ohms up to about 150 ohms. Phones in this impedance range may be directly plugged into the headphone jack routinely found on recording and playback equipment. Higher impedances, such as 600 ohms, are more useful in studio installations where many units may be wired in parallel for studio monitoring applications. Headphone sensitivity is usually stated as the in-the-ear sound pressure level produced by one milliwatt (mw) of audio input. Typical sensitivity ratings of AKG headphones run from 88 db per mw to 105 db per mw. You can see that very little 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 signal will produce 8 mw in a 75 ohm load, a difference of about 9 db. So it is clear that 600 ohm phones and the lower impedance models should not be intermixed in the same application. The 600-ohm models are more rugged than low impedance models in that the higher resistance coils are less susceptible to burn out than low impedance models. BASIC HEADPHONE TYPES Headphones may be differentiated in terms of how they fit over the ear. If the earcup fits snugly around the ear it is known as circumaural. Such phones are comfortable to wear, provide good isolation, and result in a consistent sound from one wearing to another. Bass extension is excellent with the circumaural design since a very good air seal can be made against the head. Somewhat smaller in diameter, the supra-aural design has an earcup that rests on the outer ear, or auricle. This has the benefit of being quite light and comfortable. Response may vary, depending on the precise positioning of the earcup. Bass extension 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 communicates with the outside. Phones may be characterized as open-air, semi-open, or sealed. In the early days of stereo headphone listening, most models were sealed, probably because designers felt that approach resulted in the best frequency response. 1

4 But many listeners tire of such phones after long listening sessions, and headphone designers began to experiment with open cell structures. The open-air designs provide essentially a direct connection with the outside; as such, they provide little isolation in either direction. A person wearing such phones can engage in conversation (if the phone volume is not too high). If such phones are operated at very high levels, they may be annoying to persons nearby. One important performance advantage of the open structure is that it provides excellent acoustical damping, reducing the effects of standing 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 development engineer a number of performance advantages unique to AKG. These models combine the best features of both sealed and open air designs, with none of their disadvantages. Through careful attention to the control of resonances, a surprisingly realistic stereophonic sound field is developed, coupled with low distortion and extended low frequency response. HYGIENE AND HEARING PROTECTION Some persons, for various reasons, may refuse to wear a set of headphones that others have worn. When you encounter this, 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 routine replacement 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 much higher levels than they would ever play loudspeakers. There is some speculation as to why this is the case. It probably relates 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 been too high. OSHA (Occupational Safety and Health Act, 1970) publishes the following data: Permissible Exposure to Noise Levels SPL (A-weighted): Daily Exposure: 90 db 8 hours db 0.25 hours The industry does not generally publish maximum operating level capability of headphones, inasmuch as, even the lowest cost 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 can see 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 a real danger in going any higher than this in playback level. 2

5 ELECTRICAL CONSIDERATIONS A typical portable DAT recorder may specify its per-channel headphone output as: Rated output: 15 mw Impedance: ZL 32 ohms Connector 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 ohms will 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 the power availability dropping with increasing impedance. In any event, the 26 mw available for 600-ohm phones will be ample 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 ohm headphones operating in parallel, the power delivered to each phone would be 6.7 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 resulting impedance and make sure that it falls within the allowable range. This can easily be done, by dividing the impedance of a single phone by the total number of phones loading the line. For example, ten AKG K100 headphones operated in parallel will have a net load of 90/10, or 9 ohms. This load would be allowable on the console discussed above, but certainly not on the DAT recorder. MAIN USAGE CATEGORIES AND MODEL RECOMMENDATIONS In this section we will discuss briefly the major usage categories of stereo headphones, underscoring the specific technical features that support a particular usage. We will then move on to a concise discussion of each headphone, detailing their suggested uses and the reasons why. Primary Usage Areas for AKG Stereo Headphones 1. In-studio/On-stage monitoring 2. Vocal booth/drum cage monitoring 3. Broadcast and ENG monitoring 4. Private listening 1. In-Studio/On-Stage Monitoring Perhaps the majority of AKG headphone sales are to studios and live performance venues, where they are used for headphone cueing for musicians. The important attributes here are: 1. Reliability 2. 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 acoustical seal in order to keep click tracks from leaking into microphones. Click tracks are widely used in recording film scores that are being played in synch with a picture that has already been partially edited and where musical events must match screen events 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 an impedance of 600 ohms are often chosen, since that enables multiple sets to be loaded in parallel across a standard stereo amplifier without problems. 3

6 2. Vocal Booth/Drum Cage Monitoring The vocal booth is a necessary evil in the modern recording studio. It removes the performer from the live acoustical ambience of the studio setting where high studio sound levels may leak into the vocalist's microphone. The balance engineer must somehow create a stereo mix for the vocalist that conveys the essence of the music, including the voice itself along with any pertinent signal processing, such as reverberation. This is an art in itself, and needless to say the quality of the 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 may encounter a vocalist with a small voice who likes to monitor at high levels, and this can cause problems that only a sealed set of headphones can solve. Just the opposite of a vocal booth, the drum cage is designed to accommodate a player who generates more sound level than can normally be contained by the studio. In short, the sound from the drum set tends to leak into all of the open microphones in the studio. The drummer will invariably have to wear a closed circumaural set of phones in order to hear what is going on in the studio. Like the vocalist, he will need his own dedicated monitor mix. 3. Broadcast and ENG (Electronic News Gathering) Monitoring Many on-the-air announcers prefer to wear headphones as they talk. Long-term comfort and lightweight are essential, as is reliability and compatible sound quality. ENG (Electronic News Gathering) normally implies on-location work outdoors, and headphones chosen for this application must be sturdy, lightweight, and of semi-open design so that the user is in constant touch with the outside world. Extended LF response is not necessary, since the purpose is primarily for speech intelligibility. Slightly boosted midrange and HF will enable many users to better understand speech under noisy conditions that are often encountered in ENG. 4. Personal Listening Many people who live in crowded quarters, or who like to listen to music while other family members are sleeping, have naturally gravitated to headphones. For the serious headphone listener, a sizeable financial outlay for a truly exceptional set of headphones is justified. The desirable performance characteristics in these phones are extended, smooth frequency response, the ability to produce high levels with very low distortion, and the ability to recreate a spatially accurate soundfield. Another requirement is long-term wearing comfort. AKG has brought fifty years of acoustical and audio expertise to these specific problems and has designed some of the finest products available for reference monitoring, both in the home and in the studio. MODEL RECOMMENDATIONS In this section we will review all models in the AKG headphone lineup, emphasizing the performance features of each one and detailing the specific performance areas in which a particular model excels: K501 Headphones Uses Varimotion TM large diaphragm design for good spatial separation and outstanding sound resolution. Response extends from 16 Hz to 30 khz. Ideal for lovers of classical music. Uses circumaural earcup with open acoustical design. Classical listening Home studio applications FM broadcast monitoring Multimedia applications K501 4

7 K401 Headphones These headphones provide all that rock and pop music enthusiasts want to hear: tight, powerful bass, crystal clear sound quality and impressive power. Uses Varimotion TM diaphragm design for outstanding sonic and spatial resolution. Pop/rock listening Home studio applications Multimedia applications Broadcast monitor applications K401 K301 Headphones These phones have recently been redesigned, using a large diaphragm with high internal damping for smoother overall response. The open design makes for excellent, fatigue-free listening. Home listening of all kinds Multimedia applications Home studio applications K301 K270S Studio Headphones These phones have a closed design and are ideal where leakage from the phones into nearby microphones must be avoided. Motion picture recording 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 an unattended pair of headphones. Two transducers are used in each earcup for added output capability. Low impedance (75 ohm) design Studio and vocal/drum booth usage Use in high ambient noise environments ENG operations Sound reinforcement monitoring K270S 5

8 K240DF Monitor Headphones The 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 internationally can 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 in parallel. These phones sound slightly flatter than the model K240M. Broadcast transmission monitoring High accuracy home listening Recording studio monitoring K240DF K240M Monitor Headphones The K240M is one of AKG's top selling models and is a virtual standard in recording and broadcast studios. The design is circumaural, semiopen, and the response extends down to 15 Hz. Excellent wearing comfort. General recording and broadcast studio use Home listening ENG operations All headphone monitoring activities K240M K141M Monitor Headphones According to Billboard Magazine, the K141M phones are the most widely used headphones in U. S. studios. The design is supra-aural (on the ear) semi-open; response is extended, with exceptional bass response. High impedance (600 ohms) facilitates paralleling of phones for multiple monitoring activities in recording studios. General recording and broadcast use Home listening ENG operations All headphone monitoring activities K141M 6

9 K100 Headphones AKG's lowest costing professional format headphone. Single side entry high 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. The studio on a budget Home listening Multimedia applications K100 K70 Headphones The K70 is a lightweight monitor quality supra-aural (on the ear) headphone that offers many of the features found in AKG's professional format headphones. Lightweight, self adjusting, comfortable over many hours of use, and replaceable earpads, all in a semi-professional cost effective design. The K70 offers premium audio quality at a price affordable to most customers. Recommended for Home music and video listening Small project studios Multimedia applications K70 K66 Headphones A self-adjusting headband, single cord entry and circumaural earpads provide a very pleasant fit for excellent wearing comfort. A large diaphragm element in an open back design offers an exceptionally smooth frequency response in an affordable headphone package. K55 Headphones A self-adjusting headband, sealed circumaural design and single cable entry offer good user comfort. A large diaphragm element in a sealed design combined with AKG s Turbo Bass function exhibit a punchy low end suitable to pop and rock music. K66 7 K55

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