There are an enormous number of variables involved in the recording of audio and in the reproduction. Perhaps that could be the attraction of the ongoing endeavour to capture a stereo soundstage and retaining as much of the reality as possible in the playback, knowing that it will never be perfect!

The major areas of variability are in the choices of microphones, the variations in the way they are used to capture the stereo soundstage, how many are in use, their placement in the acoustic, the characteristics of that acoustic, and the judgements made in what ever ‘monitoring’ room is available, apart from the loudspeakers in use. Whatever is finally available to the home listener, is then subject to another plethora of variables, and it is with this aspect I want to start.

THE PLAYBACK

A number of years ago I recorded, in dozens of different listening rooms and different playback systems, the sound of one particular CD. In each case, as near to a similar sweet spot, symmetrical to the speakers, at like volume levels, the recordings were made via binaural mics in my ears. Playback comparisons were then made via quality open back headphones.

These playbacks were all entirely different, let alone the one where the speakers were out of phase and another where the tweeters were defunct! Apart from differences in loudspeakers, there was their placement, symmetrical to the room or otherwise, the room sizes, their dimensional proportions and different reflections, low frequency resonances, furniture layout and floor covering.

There is obviously never going to be a ‘playback standard’, but there are perhaps some ways of reducing the multifarious number of effects mentioned above. Inherent in this is the construction of your property - solid brick interior walled rooms are different to timber framed panel constructed interiors, as I noticed when I moved to the latter type, which I straight away preferred.

Traffic resonance was obviously noticeable in the earlier home, but not so now. However this would also be a factor in the LF reproduction of the loudspeakers, as also would be the size of a room, apart from the loudspeaker’s design characteristics and positioning – you can’t win!

Loudspeaker manufacturers ‘favour’ a number of choices of design types. For electromagnetic drivers there are sealed boxes or various ported types, an extension of this being the transmission line. Electrostatic loudspeakers justifiably have their enthusiasts, however you do have to consider their positioning, due to their bi-directional output.

There was a characteristic of a well known electrostatic loudspeaker, that I used to have. This was the ‘point source’ of the full frequency range reproduced. This is also a noticeable facet of some dual concentric coned mid and high frequency drivers I have used. With two or three driver designs, it is likely that they will be vertically in line, or at least the mid and high should be.

These aspects are considered to be an important factor in obtaining clear imaging. So, if your loudspeakers are as symmetrical to the room as possible, away from corners and walls, preferably angled in to the ‘sweet spot’ listening position, it is worth checking out that you get a clear centre image with a mono recording. With passive loudspeakers you could temporally wire both from the same amplifier output to directly test the speakers themselves. All decent playback systems should have a mono push button to easily check all is well, give or take any waxing up of our ear’s listening abilities!

The timbre tonality differences, or colourations, between the original sound and what is reproduced are likely to be more from the loudspeaker playback than due to what the microphones added, and of a different nature also. An aspect to consider is that boxed loudspeakers can radiate a sound from their side and rear panels, and electrostatic loudspeakers are inherently free of this box effect possibility, apart from needing to take into account their ‘out of phase’, rear output.

THE RECORDING

I don’t know if it is definable what we should expect from the reproduced stereo soundstage? My view is that the loudspeakers, as individual sound sources should not be directly noticeable. The stereo should start in the middle and spread out towards the loudspeakers, with a good imaging and balance of instrumental sources between the loudspeakers and a ‘detachment’ of the recording venues ambience behind and wider than the performers.

Good location imaging of the performers in the reproduced stereo soundstage, can allow the listener to choose to focus on a particular performer or group of performers. This asset is less likely if the musical balance is poorly done with an unwanted dominance of some other sources, give or take what the composer indicates in the score, and how that is interpreted by the players, or by the conductors in the cases where they are involved.

It was interesting that in the early days in the move to stereo, there was a ‘ping pong’ approach on some recordings, where you had ‘mono’ instruments coming direct from each of the loudspeakers! Mind you, I have recently heard a 5.1 Surround Sound DVD of a string quartet, where they have placed the violins fully in the front L and R loudspeakers, with the viola and cello coming from the two rear loudspeakers – I don’t know of any concerts where you can get the audience to sit in the middle of a string quartet!

However, microphones are definitely used closer to the performers than almost any of a typical audience. In addition to that there are often difficulties in using the layout, that some groups of performers always want follow, to get a decent recorded musical and stereo soundstage balance. Consider Brass Bands and smaller ensembles like String Quartets.

The former have two major groups of performers directly facing each other, with a third group, at the back of the performing area, with the percussion behind them. If only those two groups to the conductor’s left and right could be ‘opened out’, their instrumental sounds would come over much more directly and ‘naturally’ on the main pair above and behind the conductor!

With String Quartets etc, they are obviously so used to rehearsing in much smaller places, than a concert platform, and are so used to directly hearing each others contributions. In the String Quartet’s case the balance from the second violin and the viola is often compromised, unless the quartet can be persuaded to open out a bit, to suit a single stereo pair mic approach.

There is one potential pit-fall with location recordings, that can be so obvious when you get back to base. You hear the live ambience in the venue while you are setting up, and when you get to hearing it in the ‘control room’, you can easily misjudge the amount of ambience you are getting, because it always seems a lot less than you were used to in the hall.

Back at your usual playback room you can often find you have captured more than you want! One good way around this is to play back, in the location monitoring situation, a known recording of the same genre you are about to record, and have it playing back while you are getting to the stage of hearing what the session microphones are about to provide you with!

Classical music recording sessions are often made in Concert Halls or Churches. Temporary studios, in fact. Monitoring rooms are often almost cupboards! Loudspeaker placement and the room’s acoustics add to the judgements made on the musical balance and also with the amount of ambience being captured, as outlined above.

So, on to variety of approaches which are possible at the recording stage and in the subsequent Post Production, as it is known. There are many different microphone techniques in use. Different recording engineers tend to specialise with a particular approach. It must be said, that their experiences with a given way of doing it, can mean they have really honed their technique over time.

Some believe in a minimalist approach and often get away with a simple pair of mics, suitably placed in a given acoustic, with the layout of the performers often suiting the results, in terms of balance and imaging. This single pair approach can be with directional mics (usually Cardioid) human ear spaced apart and angled, say 90 to 110 degrees. Not many use two vertically coincident directional mics angled at 90 degrees, one above the other – little stereo width is a noticeable result

Others prefer to use non-directional mics (Omni) spaced apart more than the ear spacing, mentioned earlier. There is in fact a basic reason for the use of non-directional mics – they have a more extended low frequency range than directional mics usually have. For some, the down side is poor imaging and a tendency for the stereo to collapse to one or other loudspeaker, if the listener moves even a little from the sweet spot. However, in this ‘horses for courses’ world, the use of Omnis with piano recordings, is fine!

Regarding the use of two microphones, there is a system called Mid & Side and used by some. Here you have a Mid mic facing to the centre soundstage, directional or non directional, and vertically coincident with it, a Side bi-directional mic (Figure of Eight) picking up the sound from either side of the Mid mic. The signals are mixed with the Mid mic in phase to both the left and right of the stereo with the Side mic mixed out of phase to the stereo.

The technique uniquely covers the centre of the sound stage, the stereo starts in the middle and spreads out symmetrically towards the loudspeakers, with a variable stereo width instantly available, with the amount of the Side mic signal fed in the mix!

There is a famous three mic rig known as the Decca tree. Three large diaphragm Omnis are used, two spaced around a metre apart and a centre one out in front, around half a metre forward. Omni directional mics are known to have varying amounts of directionality at frequencies above 5kHz and this could improve the soundstage imaging being obtained.

There is now a sort of renaissance in the use of Ribbon microphones, from a number of microphone manufacturers, many with on-board amplification which does reduce their noise floor. Regarding the mostly commonly used capacitor microphones, with various diameters of circular diaphragms, there is a manufacturer in Sweden who uses rectangular diaphragms which have been judged to have ‘ribbon mic’ qualities.

Regarding the use of ‘special’ cables, I am not aware that these are often involved in Studio or Location recording situations. The crucial feature regarding microphone cables in the professional recording field, is that the feeds are all balanced, in early days with transformers and nowadays, more likely with IC circuitry. Mine you, we don’t deliberately run them close to mains cables, especially if lighting feeds are involved!

Some recording engineers do not like extra spot mics, but there are occasions where they do need to be used to get a musical balance. The skill is that they should not really be noticed in the mix! Going too close can certainly be evident, and an often consequence is that more and more are then put out! If the aim is missing to achieve the important ‘depth perspective’ in the stereo soundstage, then it is likely that there are many close-up mics out there. I don’t know if there is a world record for the number of mics used on an orchestra, but sixty are known to have been used……..!

An interesting development, in the current use of spot mics, is for their feed into the mix, to be delayed in milli-seconds related to their distance from the main mics. More easily done nowadays than in the past, and now practised by many recording engineers in getting a proper musical balance and a depth perspective in the stereo soundstage.

Something that is not fully pursued are the ‘subtle’ mechanical resonances which may ‘colour’ the sound being captured by microphones. Certainly it is obvious with mic stands on wooden floors. Best overcome by using blocks of thick foam under the legs of the mic stand, the weight of the stand being really effective in stopping those vibrations reaching the microphones. However, perhaps we don’t consider the less obvious effects of vibration transmission up the cables to the mics, or do some stands resonate to loud impacting sounds?

POST PRODUCTION

Razor blades, with reel to reel tape, was the skilled way of editing in the past. Nowadays computer program editing, and the use of the visual waveform, is an amazing advance. The amount of editing ‘needed to be done’, varies a lot. It is recognised that too many ‘cuts between takes’ do not always maintain the ‘feel’ of a performance.

There is a particular aspect in post production that is also important to the listener’s experience. Wide excursions of the dynamic range, which are perfectly fine at a live performance, do need to be sensibly dealt with for any domestic playback.

I am certainly not advocating the mad situation, current with most ‘pop’ music, where the levels through out the songs are pushed up to the digital limit, with the whole thing ‘bashing zero’. This is in pursuit of ‘loudness’, as the selling point! The old Beatles stuff and other CDs of that era are not like that!

With classical music it is perfectly acceptable to trim any wide excursions in level, done progressively to keep the sense of dynamic change, and in that way allowing the overall average level of the performance to be raised and more interesting to listen to.

I know of two CDs where the dynamics, as recorded at the sessions, and with the headroom that has to be there to avoid a sudden peak going ‘over the top’, were not allowed to be trimmed. A purchaser of one complained that the levels were ‘all over the place’, and he had get up to vary the levels himself, to enjoy what he had paid for.

Another, which was at a particularly low level for the opening three minutes, had the reviewer’s complaint that he made five attempts to get the level right to suit the 30dB leap at the very pronounced tutti. It is believed that the composer, on receiving his issued CD copy, turned up the initial volume to suit the music at the start, and at the three minute point, his tweeters ceased to ‘exist’!

WHAT ABOUT ‘SURROUND’ AUDIO REPRODUCTION?

Are we missing out on improving our whole audio listening experience by not progressing to having the ‘stereo’ ambience lifted out from just being between our stereo speakers, to it being available all around us, not just horizontally, but including the height aspect that exists in our real world around us?

Future posting on this ‘Saturn Sound’ site will cover the many ways the writer has ‘researched’ this extension to our listening experience, albeit with the very unlikely event that it will ever ‘take off’, as it did with stereo over mono.

Many differing microphone rigs have been used along with the need to have a ‘mastering standard’ and a ‘properly’ set up playback system. You can imagine the greatly increased number of ‘variables’ that this endeavour has revealed!

FINAL THOUGHTS

Here is a sort of ‘Unanswered Question’, to use the title of a work by composer Charles Ives. Is there likely to be an important phase relationship, over the audio spectrum, heard in the live recording situation, that is not maintained at any replay situation, particularly due to microphone imperfections in that area, along with what phase corruption every loudspeaker playback, must contribute to it - just a thought?

To conclude, it is definitely a fact that the number of variables, at the recording process and then even more of them at the listener’s playback, are the main stumbling blocks in always achieving a good musical listening experience. It makes me wonder if the 24bit 96k and beyond will ever be that important – especially when we have DAB Radio at around a seventh or below, the bit rate of the CD Red Book standard. That apart from what the iPod and MP3 deliveries do and what happens with music downloads.

Some years ago I was approached by a well known ensemble, who asked if I could lend them or hire some mics to record their next concert, as they had just acquired a 96kHz sampling DAT recorder. I asked them what kind of mics they would like? They said ‘It didn’t matter’. It was a very short phone call ………!