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Sound on Stage

Power, loudness and power amplification

Thin Lizzy

Regrettably, there is nothing very tangible in the relationship between amplifier power, sound level and perceived loudness; this is something many musicians are painfully aware of, after having parted with lots of cash for a casually described 'megga-wattage' which didn't live up to expectations! Broadly speaking, a tenfold increase in sound level or amplifier power corresponds to a mere doubling of loudness to our ears. Thus doubling the power of your instrument amps can be expensive yet disappointing. All other factors being equal, the increase in loudness will be very subtle indeed. But variations in the methods by which amplifier watts are measured, together with the effects of loudspeaker impedance, efficiency and dispersion pattern can all work together to make, say, a nominal '100 watts' sound impossibly loud or puzzlingly mute.

These are objective, measurable effects; others are subjective, but equally real. Colouration (the emphasis of certain frequencies) in the speakers and distortion can suggest immense loudness, whilst clean, uncoloured sound rarely seems loud enough. The general result is that amplifier watts should be taken with a generous pinch of salt; the loudness your ears perceive is always the final arbiter! When a good valve amplifier is overdriven, it produces a distorted sound that is both immense and palatable. Apart from their ability to add ethereal ambience, emotion and warmth to the guitar, valve amplifiers of any given rating invariably seem louder than equally powerful transistor counterparts. The reason lies partially in psychoacoustics — the characteristic distortion of the valve amplifier simply sounds loud — but also in the conservative nature of valve amplifier power ratings. Additionally, the output power of any amplifier is at least doubled by gross overloading, and in good valve amplifiers, this potential extra power is musically acceptable and thus usable, whilst in transistor amplifiers, it is usually very discordant and 'ear-ripping'. As a result, a carefully chosen 30 to 100 watts of valve amplification is adequate for playing loud, raunchy music almost anywhere! It's not surprising that valve amplifiers are the quintessential sound of Rock...

In creative hands, a handful of transistor amplifiers can exhibit 'musical' overload characteristics similar to good valve amplifiers but in general, transistor amplifiers produce tinny, harsh, brittle, metallic effects when they are overdriven; they sound either crystal clear or extremely obnoxious. This doesn't mean that the transistor amplifier is useless — merely that different techniques are needed to coax music out of it. Or, to look at it another way, the transistor amplifier suits some types of music better than the valve amp. In general, transistor amplifiers have to be played clean at levels well below overload to avoid the generation of unpleasant sounds. This means that the potential of full power isn't available, and adequate power ratings will be at least three times those indicated above for valve amplifiers, ie: 100 to 300 watts for lead guitar.

The transistor amplifier is best suited to clean styles of playing and is therefore very acceptable on bass or guitar work demanding a clinical, sterile feel, whilst the harsh, ear-ripping characteristics were well suited to the punk idiom of 76-79. However, acceptable distortion can be introduced via FX circuits and the speakers in order to extend the versatility of the instrument and make it sound louder without discomfort. Neither type of amplifier is inherently 'the best', there are good and bad examples of each. It's only that one is better suited to your music, your skill and your guitar. In short, the best amplifier is the one you can develop a relationship with.

As a potential amplifier purchaser, listen to, or better still, try out as many models as you can — preferably with your own guitar or bass, and bear in mind that the sound of a 'head' (separate) amp will be profoundly influenced by the speakers it's tested with, amongst other factors. It's all too easy to end up with a bland and unsympathetic beast of an amplifier simply because it was demonstrated with a particularly good guitar and speaker cabinet! Find the point(s) where the amplifier overloads, and listen to how the sound changes above and just below the overload point. Is it a useful and potentially musical sound? Or does it rip your ears and shatter windows? If you ever intend to practise with the amp at low levels, then listen to the amp at these levels. The sound may be different, but again, is it a useful sound? Above all, talk to other guitarists, and visit your local group gear shop and try different types of guitar amps with different wattage ratings.

Speakers — an introduction

Two types of speakers are used in conjunction with electro-musical instruments. The first, akin to the hi-fi speaker, is primarily a reproducer, which aims to convert electrical waveforms into facsimile air vibration.

These 'reproduction' speakers are most useful for keyboards and are the foundation of the best PA systems. We'll look at this type of speaker later in the series, particularly as it concerns the bass player. Meanwhile, speakers intended for the guitarist and for vocals PA provide the antithesis of pure reproduction — again they are part of the instrument. Like the amplifier, the speaker can be driven hard to subtly mould the nature of a valve amp's distortion, giving a slight 'edge' to the sound for instance. In conjunction with a transistor amplifier, the speaker's distortion can be dominant at average levels, and helps to give 'roundness', 'punch' or 'bite' to an otherwise neutral sound. Some speakers feature metallic centre domes to enhance the upper harmonics of the guitar; these are best avoided with transistor amps, which tend to emphasise these harmonics naturally. Whilst the centre dome will certainly give extra bite, it will also exaggerate the tinny, metallic sound generated by the amplifier to a fatiguing degree. In this case, a speaker with a conventional paper tweeter cone is preferable.

The prime requirements for guitar and vocal speakers are an efficient mid-range response and a power capacity that's about twice that of the amplifier. The second requirement arises because guitarists invariably overload their amplifiers, and in this case, the power output will be well in excess of the amplifier's nominal rating. Hence it's a sensible and well proven rule of the thumb to allow a 100% power margin for guitar speakers. Although a modern 12 inch driver will often meet these requirements, in the past, high power drivers weren't readily available, if at all, and it was necessary to use several 12 inch or 10 inch drivers to achieve a sensible power capacity, eg: the traditional 4 x 12 cabinet. At the same time, guitarists discovered the unique musical properties of overdriving these marginally rated speakers with valve amplifiers. As a result, another school of thought has arisen — rate your speakers to match the power capacity of the amplifier, enjoy the music and pray that they don't blow up! Fortunately, this philosophy works, provided you're using the right speakers, preferably the ones that the manufacturer supplied. The secret here is essentially a case of causing the cone to produce distortion without burning out the voice coil.

The overall result is that no hard and fast rules can be laid down for guitar and vocal speakers — the 4 x 12 inch, 2 x 10 inch and even 4 x 8 inch are all broadly similar and equally valid approaches. The only definite proviso is that driver(s) with a diameter greater than 12 inches will rarely provide sufficient 'top' and should be avoided; 10 inch and 12 inch speakers are the traditional choice for guitar and vocals rendition, and not without good reason! For bass, the frequency response requirements are shifted down by around two octaves. 'Top' becomes unimportant, whilst the overriding need for good low bass response governs the cabinet size. The traditional sound of the electric bass puts great emphasis on the low notes, as if to mimic its cousin, the double bass. Because the ear is insensitive to low bass, high sound levels are essential for them to become audible. In the double bass, this is achieved by the sheer size of the vibrating panels. Likewise, for the electric bass, to make the bottom notes audible, a large area of speaker cone is called for, together with a high-power amplifier. Thus the traditional 60's bass sound was usually achieved with 24, 18 or 15 inch drivers, singly, in multiples, or by horn loading, which can achieve the same results with a smaller driver, together with a number of other advantages, though at greater cost.

Frequency range of Rock instruments and vocals.

However, the bass has few interesting sounds to offer when the harmonics are suppressed in this manner. The bass becomes much more expressive when the harmonics are emphasised; something that the funky bass players of the '70's extolled. To make these harmonics audible, one can simply use a midrange speaker in conjunction with the large diameter bass speakers to boost the upper octaves, or, alternatively, use 12 inch drivers. These have a natural response which extends into the upper register of the bass but to achieve audible bass (simultaneously), a large cone area is also required — hence 4 x 12 inch drivers again, which elegantly combines good 'top end' response with an area (and hence bass response) more than equal to two 15 inch drivers. Finally, unlike the guitar, a heavily thumbed bass readily causes speaker cones to bang against their end stops. As this is potentially much more disastrous than the voice coil overheating that can occur in guitar speakers, it's good practice to rate speakers intended for electric bass at four times the amplifier's nominal power capacity.

For vocals, the power and frequency response requirements are much the same as the guitar speaker's. Hence 8, 10 or 12 inch speakers, used in multiples to provide adequate power handling capacity. Although a peaky, coloured response will give a useful 'pokey' vocals sound that is just about capable of cutting through wailing guitars, it may aggravate tendencies to feedback, 'howling round' before you can raise the volume control to the sound level you require. Unfortunately, this problem can only be totally avoided by using microphones and speaker systems with a flat, uncoloured response.

Nonetheless, it's wise to steer clear of exceptionally coloured vocals speakers unless you can be sure they can be wound up to workable levels in practice. Vocals speakers also profit from special enclosure techniques, again to avoid premature feedback. The traditional column speaker, although much maligned, is excellent from this point of view. Containing between four and eight 12, 10 or 8 inch speakers in a vertical array, the typical column provides a well defined and 'forward' dispersion pattern, the idea being to keep the sound away from the microphone and the ceiling, from where it's likely to be reflected back to the stage. In many respects, a well designed column speaker is the ideal vocals speaker, short of a very expensive tri-amplified, horn-loaded system. In comparison with the currently trendy scaled down (and therefore compromised) models of large scale PA rigs, a pair of good columns exhibit the fine balance of delicacy and incisive 'punchiness' that is the hallmark of good mid-range and hence a good vocals speaker. Best of all, secondhand columns are available cheaply, being ostensibly 'out of fashion'.

When choosing a speaker cabinet, remember that you will have to carry it around! Today, there is rarely any intrinsic merit in owning a huge speaker stack, unless as a heavy metallist you demand an impressive and machismo stage set up; a well designed and sensibly small speaker enclosure can be just as loud and efficient as the monsters of the '60's. Again, listen to the sound of the speaker; a good cabinet will tend to produce a useful and acceptable sound without requiring much prompting from the amplifier's tone controls. Then stand back from the cabinet and try to judge how well the sound will project from a stage. Projection is all-important — don't forget that your audience also has to hear you! In particular, the low notes have a habit of disappearing as you move away from vented (reflex loaded) bass cabinets.

For many people, loudspeakers are as much a journey of self-discovery as the music they convey. You will only find the perfect speaker when you cease to be creative. Since the speaker is a part of your instrument, it deserves to be changed and toyed with as much as your guitar and your own ideas and feelings about music.

Table 1. Glossary of technical terms
Term Definition
Head an amplifier discrete from the speaker cabinet.
Combo a combined speaker and amplifier.
Slave a discrete power amplifier without tone controls, usually driven with the 'line level' signal from the guitar amplifier. Loudspeaker known variously as a driver, a cone driver, a chassis speaker, a horn-loaded driver, etc, when the essential sound generating component is referred to.
Loudspeaker known variously as a 'cab', cabinet, enclosure, column, direct radiator speaker, horn-loaded speaker, horn, baffle or reflex— when a complete loudspeaker system is being discussed. Direct — also known as an infinite baffle, this type of speaker cabinet is radiator the commonest to be found on stage; the driver is visible and couples directly with the air.
Column speaker a specialised form of direct radiator, where several drivers are arranged in a line to control the sound dispersion.
Horn-loaded loudspeaker also known as in various guises a horn, 'bin', 'bullet' or 'lens', the driver is usually hidden and couples with the outside air via some form of expanding tube.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jul 1981



Feature by Ben Duncan

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