Head start

 For all the time, ingenuity and investment that have gone into the development and refinement of loudspeaker systems, they remain at the mercy of their set-up and operating environment. While it is sometimes possible to build entire rooms specifically with loudspeaker performance in mind, it is more often the case that a good sound is achieved in spite of the surroundings rather than because of them. Welcome, then to the alternative world of the headphone.

Here, instead of echo and reverberation, attenuation due to air and furnishings, approximated splay angles and uncertain throw considerations, we have an intimate arrangement of loudspeaker drivers and ears that offers an alternative range of design possibilities – one that can deliver consistent and sometimes surprising results. Headphones also offer a solution to audio monitoring in situations where a loudspeaker is inappropriate, impractical or simply ineffective. Often overlooked or looked down upon, headphones are an essential, if frequently misunderstood, tool of the professional audio trade.
After such a build-up, it will come as no surprise that there are various different design considerations available – headphones to suit a variety of working situations and preferences.

Circumaural, supra-aural and intra-aural

The first distinction to make between the most commonly occurring headphone designs is between open and closed-back models. Professional applications more often favour a closed design, as this also isolates the listening environment. Indeed, closed-back designs are essential in certain applications. Neither live sound engineers nor DJs have much use for open-back designs, as the ambient sound levels they encounter require a design that isolates their ‘working’ sound from the noise of the outside world – even if (and often because) the two are closely related. DJ models in particular require a significantly higher power handling capacity in order to be effective over the high sound levels typically found in nightclubs and large-scale dance events.

More formally termed circumaural, closed-back headphones are designed to completely enclose the outer part of the ear (the pinnae), and offer a very high level of acoustic isolation from the outside world. Although intra-aural devices (those that fit in the ear, such as earphones/ear-buds and in-ear monitors) can be positioned more accurately, circumaural headphones can be more comfortable to wear and offer better external noise rejection, as well as making it easier to incorporate additional features. These can offer attenuation of in excess of 30dB of ambient noise (32dB in the case of Sennheiser’s HD380 Pro).

The obvious use of this type of headphone extends from late-night mixing sessions in a project studio through follow-spot operators at large-scale concerts to DJs leading a dance party. All of the headphone manufacturers offering ‘professional’ headphones include closed-back models in their catalogues, and there are various proprietary designs and refinements to chose from.

Among these is the RP series of headphones from Fostex, which uses a proprietary diaphragm design with a zig-zag pattern coil, neodymium magnet assembly and copper (as opposed to aluminium) foil etched polyimide film diaphragm to handle input peaks of up to 3000mW. AKG, meanwhile, uses its own Varimotion two-layer diaphragm design with a 200mW power handling capability.

Properly called supra-aural, open-back headphones are designed to allow ambient sound to enter the ear pad, providing a mix of the headphone and surrounding sounds. This type of headphone is particularly suited to use where it is dangerous to be unaware of your surroundings, but is also common in domestic and broadcast applications. They are not included in all manufacturers’ product ranges, however. Among those manufacturers that do offer them are AKG, beyerdynamic, Sennheiser and Ultrasone.

Ear pad headphones are quite similar to full-sized headphones but are smaller and sit on the ear rather than over it. As well as being made more portable by their smaller size, these tend to have a poorer sound quality than closed-back designs, but still offer advantages in certain situations.

In-ear headphones have seen a tremendous rise in popularity in live sound applications over recent years. This is partly due to their ability to offer excellent sound quality and to block external noise, and also because they free performers from the limitations of monitor ‘hot spots’ on the stage. In addition, they can greatly reduce the level of sound on stage, eliminating feedback problems and easing strain on performers’ hearing. Intra-aural designs sit directly in the listener's ear canal, forming an acoustic ‘seal’, with the transducer on one side and the outside world on the other. Even though professional systems normally use custom inserts made using moulds of a performer’s ears, prolonged use can be uncomfortable, however. Generally, in-ear designs are offered as part of wireless systems, such as those from Audio-Technica, beyerdynamic, Nady, Shure, Sennheiser and Trantec.

Surround sound

If you think that surround sound reproduction relies on a circle of loudspeakers arranged around a static listening position, it is time to think again. For while it may seem unlikely that the same results can be achieved by a pair of headphones, this exactly what beyerdynamic’s Headzone and Ultrasone’s S-Logic systems aim to do.
Before looking more closely at these systems, it is worth noting that there are domestic ‘simulated surround’ systems available, but these are far removed from a professional monitoring reference and are intended simply to offer a more spacious listening experience to accompany movies or game playing.

Ultrasone describes its proprietary S-Logic technology as a ‘natural surround-sound system’. Where normal headphones address each ear directly and similarly, the S-Logic system gives the impression that you are listening to speakers metres away. Even at very low levels, this adds ‘space and distance’ to individual sounds that is not possible with conventional heaphones. Significantly, S-Logic does not require any additional processing but works through its loudspeaker/earcup arrangement.

Instead of hitting the inner ear directly, with S-Logic the signals are reflected off the surface of the outer ear before entering the auditory canal to create a natural three-dimensional sound.

‘It is clear that S-Logic is the only way to influence stereo or channel tone signals to produce a three-dimensional effect without any digital or binaural processing apart from the human Head Related Transfer Function (HRTF) of a dummy-head,’ says Florian Koenig, the man behind this technology. ‘The human outer ear fluctuates more than 15dB at frequencies lower than 1kHz and a standard deviation of 6kHz equates to 7dB. Since the tone recordings offer no three-dimensional information regarding common pan-pot stereophonic recording techniques, S-Logic is the only solution.’
Where conventional headphones create the impression of a stereo image within the listeners´ head, S-Logic produces a stereophonic surround sound field that is perceived to be broad, detached, and located in front of the listener. Ultrasone claims to be the only headphone manufacturer to date to directly address the physiology of the ear rather than use electronics to generate a stereophonic sound field. ‘Our headphone transducers are not aimed directly at the auditory canal, so listeners perceive that they are receiving the same volume but with up to 40 per cent lower sound pressure levels,’ Mr Koenig says.

The technology behind S-Logic is based on several principles. First, for circum-aural headphones, the speaker needs to be placed at an angle of 30° to 40° de-centric (lower or frontally based on the ear channel) to the pinnae and the buffer board needs to be acoustically hard to reflect low frequencies, yet damp for the higher frequencies. Second, for supra-aural headphones, the speaker needs to be positioned near centric or slightly lower with respect to the ear canal, where the buffer board should be acoustically hard over the whole frequency range. The upper area of the speaker is to close at about 50 per cent of the diaphragm-reflecting zone, creating a de-centric sound source in front of the ear canal. Further, the ear pad (a cloth surface is preferred for open and leather for closed headphones) is used to provide maximum tone while being free of colouration. Ultrasone’s final consideration is that the speaker should be made of a gold or titanium Mylar diaphragm, developed for an extremely hard burst response (with a THD spectral delay time at frequencies greater than 1kHz of about 0.9ms), in which the drivers allow for very low frequency tone signals (<40Hz) without phase-masking effects and without perception of the real tone signal (not THD) first. (The transmission frequency could be offered up to 35kHz.) These tone-neutral aspects regarded as very important in the reproduction of natural spatial sound effects, as lateral wall reflections work as a function of high dynamic audio signals (>100dB).

The approach adopted by beyerdynamic’s Headzone system is to model the parameters of an ideal auditory space and loudspeaker set-up of up to 5.1 channels – outside the head and in fixed positions regardless of head movement. In contrast to the approach adopted by Ultrasone, this is achieved by processing each surround channel in DSP. The product of this is a series of separate, virtual loudspeakers – one per channel – that can be positioned as required by the user.
Key to its ability to do this is the adaptation of a technique called binaural recording – but where this uses microphones mounted in a ‘dummy head’ to replicate the human ear at the recording stage, Headzone reckons to model the listening environment in DSP. Called Binaural Environment Modelling technology, this takes into account a number of essential aspects of human physiology and psychoacoustics. With two ears, humans always hear in stereo; the fact that we can also hear spatially is due to the ability of our brain to evaluate the signals in relation to each other. This creates both the left/right information and a genuine 3D effect.

The main hearing functions begin with what is known as the HRTF. For each audio event, this encompasses differences in the incidence of sound arriving at the listener’s ears, differences in level and frequency content at each ear, the relationship between direct sound and diffuse sound, patterns of early reflections in a room (a sound source that is very close typically generates different reflection patterns from one that is further away), and finally head tracking. In a natural environment all animals unconsciously use the ability to turn their head in order to find out exactly where a sound source is located. These are often minute head movements, but are extremely important in natural hearing, as it is by this mechanism that we are able to distinguish clearly between sounds occurring in front of and behind us (particularly in spatial hearing).

If a mono audio signal is reproduced via a loudspeaker at a distance of 3m and an angle of 30° to the left of the listener, this signal reaches each ear with characteristics that enable the brain to correctly determine the position of the loudspeaker. With Headzone, this information is imprinted on the audio signal using DSP to give the same spatial impression via headphones, and making it possible to position an audio signal virtually anywhere in the space around the listener and create a ‘virtual loudspeaker’. Headzone also constantly tracks the head movements of the listener, and incorporates them in the audio reproduction to create a stable listening environment.

The next task is be place these loudspeakers in a room, as a sound image is composed of both loudspeakers and acoustic characteristics added by the room they stand in – a ‘virtual control room’.

While the recording industry has to agree on a perfect control room, the qualities of a good listening room are determined by its reverberation time (250ms to 400ms), room geometry, low background noise level, an absence of strong reflections and a highly diffuse reverberation tail. The Headzone virtual control room is based on these qualities, but it also allows customisation of the characteristics of the loudspeaker system in a control room. Headzone also offers adjustable parameters for some of the factors that additionally affect the sound image in a real room and that are absent from a conventional stereo headphone – the size of the room, the distance of the loudspeaker to the listener and the characteristics of the control room.

In summary, whether you simply need to hear audio over a high level of background noise or are looking for an alternative to a large and expensive surround-sound room, modern headphones have a great deal to offer. As with any other critical piece of audio equipment, however, they must be chosen with care, and an understanding of both the available technologies and the considerations of your use.