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An Interesting Read - What Makes a Good Rig

Discussion in 'Amps and Cabs [BG]' started by ZenBass, Aug 31, 2005.

  1. Q.What makes a good Bass rig ? - The ultimate question.
    I think most of us would settle for an amplifier-speaker setup that just made our instrument very loud, being of course perfectly clear - just like putting your ear up against the bass body but very loud, so whats the problem in the real world ?

    The modern amplifier made for bass is by most standards a pretty good piece of gear and will reproduce your bass with good fidelity, the differences between one amplifier and another being of a relatively subtle kind in the nature of control preferences, presentation, audio power output, weight, cost, fashion etc.
    There are differences, and things to watch because the bass guitar, working at the extreme low end of the sound spectrum still produces great demands on an amplifier due to the continous nature of the bass signal ie there are few gaps in it for the amplifier or its internal power supply to recover in. However, to a large extent amplifier performance can be made more or less 'perfect' within a given role if enough money is spent on it.
    Part of the problem is the characteristics and deficiencies of the loud speaker system they are connected to and the large powers required to drive this system.
    So we get back to an ever present problem for all audio/sound reproduction systems, the weakest link in the chain is still the loudspeaker system. Yes - if you get enough of them together they can deliver the goods, but in terms of variability of performance and lack of evoloution of the basic principle, the speaker still seems primitive. They've been mathematically analysed and are at least now characterised by their pysical constants but it is still very difficult to produce alot of evenly dispersed high quality sound.

    You may have seen or heard that speaker specifications go down at the extremes of our hearing range. This means that they produce less sound even though you put the same amplifier power into them. This is a physical characteristic due to the small size of the driving diaphragm (your loudspeaker cone) compared with the large size of the soundwaves it is trying to produce, the cone having little 'grip' (known as coupling) on the air at these low sound frequencies and the lower the note the worse the problem gets. This is reflected in the greater movement of the loudspeaker cone hence the truth in the statement about needing to shift alot of air at low frequencies. All the paraphernalia of loudspeaker enclosures, boxes, cabinets, baffles, couplers, horns etc are there to try to address this and other problems.
    Practical performance at your average 'gig' is always compromised by the relatively small volume of the performing space compared to the large size of bass sound waves. The sound waves reflect off the room boundaries and set up patterns where sounds reflected of boundaries meet - so called standing-waves because certain areas seem to have little sound and others too much - though this will vary from note to note !

    Why is bass so difficult to produce 'in quantity' ? - As you have probably heard, that which we perceive as sound is in this world, actually fluctuations in pressure in the air around us. These fluctuations are started off by someting producing a force on the air. If the force and hence the fluctuations are regular in time, we hear a definite tone or pitch.
    The force on the air implies enery being transferred to the air and the air propagates (carries off) the induced fluctuations. In fact the air naturally transfers these fluctuations which have a definite distance between them (the wave length). The faster the fluctuations the higher the pitch heard and the smaller the distance between each of the repeating fluctuations. So bass waves in air are very big. The practical consequences of these wave sizes is that the small high pitched sounds are easier to direct and produce so you can put all the sound where you want it - in front of the speakers - bass waves due to their size are difficult to control and quickly revert to their natural omnidirectional spreading out - dissipating your bass energy every where. Also due to the wave sizes being similar to our living and meeting spaces they reflect off walls and interfere with themselves and don't have space to produce a nice even 'sound-field'.

    Loudspeaker Impedance - You will have heard of it, What the hell does it mean - do I need to Know ?
    Well, knowledge is power, so from the begining, the arrangement is something like this:

    Your bass guitar strings resonate and vibrate at a given pitch, these movements are converted by your pickups or other transducers into electricity which is vibrating at the same rate. The amplifier 'amplifies' ie makes larger these electrical fluctuations until there is sufficient electrical energy to drive a small linear electric motor that is coupled to a light diaphragm that is itself coupled to the air (ie the loudspeaker) and the movements of this diaphragm again produce vibrations in the air which we hear as sound waves. This little linear-electric motor is the coil/magnet assembly in the speaker. This motor will absorb a certain amount of electrical power and the impedance is the measure of this 'absorbing factor'.
    Loudspeakers tend to come in standardised values of impedance and this figure can give you a general idea of how much power your 'cab' will try to absorb, NOT how much it can take, this is power handling which is how much electrical energy can be absorbed before performance declines or damage ensues.
    Luckily some things are proportional. So from a standard solidstate amp (technically: with constant voltage output) a 4 Ohm cab will try to absorb twice as much as an 8 Ohm cab and therfore try take twice as much power from the amplifier and consequently be louder.
    This assumes that the Sensitivity or efficiency of the cabs is the same. This is the figure which shows how much sound power you get out for a given amount of electric power in.

    Now the problems start. In practice nearly all the factors mentioned vary with frequency - ie to you and me the pitch of the note being played, - you may eve notice some of these variations as you play.
    Now as impedance varies with frequency it is quoted at a specific frequency - usually in the mid range somewhere eg 400Hz. The impedance basically decreases through out the bass range, so a 4 Ohm cab will actually have an impedance less than 4Ohms at low bass so if your amp can't cope with this extra demand you will lose some oomph. This sort of thing explains why some amplifiers give different results in practice especially when you add the effects of protection circuitry and the like.

    Things to know about and watch for:

    Amplifiers with output transformers (usually valve/tube types). These have a Definite or changeable output impedance to which their load (the speaker) has to be matched to eg for a 4Ohm cab the amp must be set to 4Ohms. The advantage is that you get the full power of the amp into your speaker. Main disadvantage, if not set correctly this may damage the amplifier. Never operate amplifiers of this type with no speaker connected. (Technically : With no load connected to an output transformer the transformer appears as a large inductor across the output stage and very large voltages may be produced which will cause transformer or output device breakdown).

    My bass amp was alright but now it distorts a bit when up loud.
    This scenario is quite common not only among bassists but all amp users.
    Much of the time there is no actual fault but it is difficult to diagnose completely.
    It often seems to be that the amp/speaker combination is being used near but within its real limits most of the time. This may not be obvious because many people mistakenly think that because the amp controls are not at full then the amp is not working at full.
    It then takes from this point only a little extra demand on the amp with a small twist of the volume control and bingo ! you are over the amps clean output range. A trial with a more powerfull amp/speaker setup or extra speaker cabinet is the only way to be sure if the amp/speaker is proved clean on the test bench.
    Because of the variability of the sound output due to interaction with the space configuration of the performing place this can happen much more easily than you might expect.

    Not mentioned.
    All speaker systems have uncontrollable areas caused by resonances.
    Resonance is a physical phenomena observed in all material structures to one degree or another. It is the property of an object to want to vibrate more at certain repetitive rates (frequencies) than others. You can imagine what difficulties this causes vibration producing equipment like loudspeakers !

    Yes its a veritable minefield out there which is why it is so difficult to be specific about equipment. I think in general you get what you pay for in terms of a better or more rigorously engineered product.
    But at the end of the day it's what sounds and feels good to you regardless of all else.

    Source : - www.gbguitars.clara.net
  2. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    So when will he publish the answer to the question? :D