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Chapter 7 : Loudspeaker design: achieving a smooth image transition between left and right loudspeakers

Pages: [4] - Current page: [2]

Date created: 13 December, 2005 - time: 23:03

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The ear uses subtle cues in the characteristic sound of loudspeakers to create a mental image of the performers in space, and they can convince the listener that the musicians are spaced left to right and closer or further away.

The 'X-plane' represents the left to right spread of musicians across the sound stage. The 'Y-plane' describes the apparent depth of the musicians from the front of the sound stage.

There is nothing that a speaker designer can (or indeed should) do to change the positions of performers across in the sound stage's x-plane; that is completely fixed and encoded into the recording. However, there are substantial differences in the way that loudspeakers resolve and present the depth perspective in the z-plane. Your apparent position in the Z-plane (depth) is equivalent to viewing the stage through binoculars: you may choose not to use them (the natural, 'Harbeth sound'), you may use them one way round to move your listening position forward (positive Z-plane movement ) or the other way round to recede from the orchestra (negative z-plane movement). In practice, this depth perspective is a function of crossover design and driver directionality.

When we talk of the 'phantom image' we are referring to those sound which appear to be solid and placed between the loudspeakers. Of particular note is the character of the (dead central) phantom image - this is where vocals are often placed.
 

Fig 7-2A: Ideal loudspeakers, wide sound stage
- musicians spread along the x-plane between the loudspeakers and with
appropriate depth (the way the recording was intended to be heard)

 
 
Goto downloadable MP3 audio examples.

PLEASE READ THIS ENTIRE PAGE FIRST TO MAKE SENSE OF THE MP3 EXAMPLES!

A Harbeth loudspeaker spreads the sound stage equally and evenly between and beyond the speakers creating the illusion that the performers are playing as you would see and hear them at the recording with 'air' around them. Depending on microphone technique, this approximates to a seat in the 5-15th row in the hall - a combination of direct and reverberant sound and a real sense of depth.

The sound stage illusion is usually enhanced listening with dimmed lighting or in the dark where the sense of hearing is heightened. With a first rate recording and credible loudspeakers, you should be able to pick-out, or localise performers in the front row to the back row, left to right side of the stage. With acoustic music, the sound images will be solid and not drift across the stage or up and down. Some listeners even report a realistic sense of height. Of course, in pop-music and film sound, the producer can steer images around the sound stage to excite the listener.

When used in a small or medium sized room of average reverberation time, Harbeth speakers are balanced to provide this natural perspective no matter how far the listener sits from them. At the top of the design specification of all and every Harbeth speaker is that it must not be fatiguing to listen to hence, by implication it must not have an over-vivid phantom image
 
Fig 7-2B: 'Recessed' loudspeakers, weak central sound stage
- musicians appear to have slipped back-stage
 

In this situation, the loudspeakers have a characteristic which create a concave image, well behind the physical plane of the speaker themselves and with a recessed phantom image. Sounds at the extremities of the sound field (hard left, hard right) appear to be magnified in importance. The overall presentation can be described as 'distant', 'uninvolving', 'lush' or 'boring' but not especially unpleasant on acoustic performances such a large-scale orchestral recordings in a big auditorium. On more lively music it is impossible to accurately localise performers across in space and vocalists are sometimes described as 'dark'. When listening extremely close to the speakers, almost wearing them as headphones, this perspective gives the impression of cavernous depth, but at a more normal listening distance the listener is clutching for the missing energy and 'air'.

Despite its limitations, this relaxed presentation will work in moderately reverberant acoustics with glass paneling (especially very close to the listener) as the lower midrange energy level reduces the splatter of reflected energy off those surfaces. The traditional 1940-1980 "BBC monitor sound" has been biased towards this relaxed perspective which makes for a fatigue-free, if not especially 'involving' listening with a perceived warm bass end.

   
Fig 7-2C: 'Pinched', 'pushed', 'shouting', 'coloured'
- musicians at centre of stage bulge towards you like a searchlight beam
 
In this design, the phantom image in the middle of the sound stage is pulled out of the plane of the speakers towards the listener. This may be the result of a deliberate design decision and/or from latent colouration in the drivers and/or from peaks in the frequency response. Consequently, as vocals are normally presented centre-stage, the listener's attention is now riveted to the overly-vivid phantom image to the detriment of a smooth left-right spread across the sound stage. One listener describes this effect as "the vocalist almost nibbling my ear", another of a 'cup-like megaphonic quality'. Technically, this speaker presents a "high-Q" sound stage. When used in a contemporary and hence acoustically hard environment (glass walls, TFT screens) the reflections combine to exacerbate the already ringy presence band.

Under blind listening tests and without a frame of reference (a very critical point) listening far from the speakers, this design can sound initially impressive because of the attention it demands from the inexperienced or untutored listener (see page 7-4). An instantaneous A-B switchover between loudspeakers under evaluation is mandatory to ensure that inexperienced listeners are on-guard against selecting loudspeakers exhibiting this balance, which will all too soon prove irritating and unnatural. Evaluated in single-speaker-mono (not as phantom mono over a pair of speakers) in an acoustically dry space, this speaker may even be initially described as 'detailed' and 'articulate', 'dynamic' and 'impressive'. The A-B switchover concept is covered on the next page, Chapter7-3. When this colouration is beyond a certain minima, the serious listener's fight-or-flight response is sensitised; in stereo, the over-vivid phantom image is unsettling, and results in fidgety and stressful listening.

Reports of this type of loudspeaker needing 'an extended burn-in period, maybe hundreds of hours' this have nothing to do with any ageing process in the speaker but of the listener subconsciously reprogramming his fight-or-flight response to adapt to this sound. A tell-tale confirmation of this over-intensity is that as the overall replay volume is reduced, the phantom image barely decreases in strength remaining prominent and coloured even when the full sound stage has substantially collapsed.

This fatiguing design is usually the consequence of design oversights: 'design in mono and it'll be OK when duplicated for stereo'; neglecting speech as a test material through the design process and not having an adequately developed mental memory of the intensity and dispersion characteristics of real, live instruments and voice.

This peaky, although initially compelling presentation is completely unsuitable for use in even moderately reverberant acoustics with glass paneling because the higher perceived midrange/presence energy level splatters off those surfaces. There is nothing that can be adjusted in this design to ameliorate the hard sound - it is fundamentally coloured despite the initial listening promise.
 
Fig 7-2D: 'Pinched' but also 'dark', 'airless', 'recessed', 'furry', 'confused'
- musicians trapped in an acoustic fog
The worst of all sonic worlds - an odd combination of the extremes of contrasting designs 7-2B and 7-2C: the sound stage through dirty rippled glass. Listeners described instruments as 'oddly dark' with no sense of air around them, akin to a low-bitrate MP3, all low-level detail erased. It's impossible to size the acoustic behind and around the performance despite the vivid, pushy phantom image. Analysis suggests problems due to cone materials (the 'darkness'), misjudged crossover integration and uneven dispersion off axis. . The designer may have been subconsciously aware of the lack of 'air' and often attempts to compensate by lifting the high frequency (tweeter) level. This does not disguise the underlying lifelessness. There are many examples of this coloured sound amongst hi-fi and 'studio monitors'

The extreme left/right edges of the sound stage can take on a strange disembodiment, creating an initial illusion of acceptable - even wide - stereo which, to some listeners, counterbalances the other characteristic defects. The dark, overall lackluster cast gives the impression that a blanket has been thrown over the microphones, rather thicker in the mid-left and mid-right positions. This design leaves the listener with a creeping sense of dissatisfaction: the longer he listens, the worse it becomes until he is so presensitised to the colouration that serious listening ceases to be enjoyable or even possible.

This presentation can not be made to work properly in reverberant acoustics with glass paneling as the higher presence energy stands out in relief against the thin midrange energy level.

Owners of these loudspeakers are often caught up in vicious cycle of equipment upgrading to try and recover the missing 'air' and freshness of live sound which finally ends when they become Harbeth RADIAL users.
Original orchestral photo courtesy of Michael Chang modified by Harbeth
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