Posted Nov 03, 2009 09:44 AM
You never forget that first proper guitar amp and the excitement of cranking it up the first time and playing until our ears bleed or if you play bass feeling your internal organs shifting inside you in time with your beat. We love it LOUD!
This article though is about how loudness should affect your choice of an instrument amp and what sort of power you need. This is one of the most asked questions in the forums and one of the most controversial. Interestingly the guitarists seem to be currently looking at smaller amps and discussing whether you can get by with 30W valve amps whilst bassists are asking whether 600W is really enough or should you go for 1200W. I'm not going to talk about brands but I will try to shed some light on the truth about loudness and explode a few myths by explaining some of the technical side of choosing an amp. In the end though it is the quality of the sound that will make you as a musician and not its volume and you will have to judge that for yourself.
What Is Your Amp For?
Your amp has two jobs to do. If you play electric guitar or bass it is part of your instrument, the bit that makes and shapes the sound as much as the guitar itself. You've got to decide for yourself the sound you want and you have to audition the amp with your guitar to make a final choice. Your amp has another job though; it will sit at the back of the stage and will be the way the rest of the band and the audience hear what you are playing. To do this job efficiently it has to be capable of sounding great at exactly the right volume to blend in and match your mates in the band. Too powerful and you will drown out the others or start a volume war. Too weak and you won't be able to hear what you are playing over the other band members. Practice amps can have great tone but they won't be loud enough to work as backline amps. The rest of this article is about what sort of power you should aim for and why. There isn't a simple answer to this but understanding a little theory should help you to make a great choice to achieve the sound you want.
Loudness Isn't About Watts
Sound is created by vibrations in the air which compress the air and then stretch it. This creates changes in air pressure. Measuring this gives you the sound pressure level in decibels. Decibels aren't watts and because most of the power delivered by your amp is wasted by the speakers the number of watts won't tell you how loud you will be. In fact fairly typically only 2 watts coming out of your expensive 100W amp end up as sound the rest is lost as heat. But, some speakers are ten times more efficient than others. So, sound levels depend as much on the efficiency of the speakers as on the amplifier watts. You shouldn't be looking for a 100W amp but an amp and speaker which will give you enough decibels.
Even sound levels are not loudness levels. Our ears are designed to be most sensitive to the frequencies which are most important to us ( the pitch of women's voices) we are much less sensitive to extreme lows or highs, which is bad news for bassists and good news for guitarists. Loudness depends upon frequency, speaker sensitivity and also amplifier power.
Why Are Valve Watts Louder Than Transistor Watts
They aren't, not really, but loudness isn't just about watts. There are three good reasons however why valve amps seem louder than transistor/solid state amps. The first is about overhead, transistor amps don't have any, when they reach their rated output they run out of volts and just stop. They chop off the top of the sound waves and this sounds really horrid. Valve amps are current limited and for brief periods they can usually produce a little extra power before they run out of puff.( In the olden days I once measured a 120W valve amp producing 300W peaks(a Sound City with 6xEL34's)). The biggest advantage of valve amps though is that when they do reach their distortion limits they sound OK and for guitar they even sound great as they distort. This means that we routinely push them to higher average levels so they sound louder.
The final reason that valve amps sound so loud is historical. Valve amps are expensive, they always have been and the price per watt is very high. Early designs like Marshall and Vox got round this by using really efficient loudspeakers with light cones and lots of resonance emphasising the midranges where our ears are most sensitive. These give the classic British sound of speakers like Celestions. Valve amps are still coupled with super efficient speakers and so sound really loud.
Here Comes The Technical Bit
To understand much more you need to get to grips with the decibel scale. It's a strange measurement because it isn't a straight scale like length or power. Doubling the power always increases the sound by 3dB so that an increase from 1W to 2W is the same as an increase from 100W to 200W. We use this scale because this is how our ears work. We can hear things a million million times quieter than sounds that will damage our ears, so tiny sounds are picked up easily and sound is compressed as it gets louder to protect our hearing and to stop smaller sounds being swamped.
A 1dB increase is the smallest change in level we can detect. A 3dB increase (doubling the power) is noticeably louder but isn't twice as loud, to double the sound you need to get a 10dB rise which needs ten times the power. This means if you want to be twice as loud as your 100W amp then you need 1000W if you use the same speaker. It also means you won't hear a difference between a 120W amp and a 100W amp.
The final technical bit you need is the absolute dB or dBA scale where sound pressures are measured compared to atmospheric pressure.
On this scale:
0-10dB =threshold of hearing
20dB = rustling leaves
40dB =quiet home
80dB =noisy workplace
90dB =loud hi-fi
100dB =loud rock concert
130dB =threshold of pain
Any level of sound above 80 dB will progressively damage your hearing depending upon how long you are exposed to it.
How Loud is Your Amp
Now you have a working knowledge of the dB scale you can calculate the loudness of any amp if you have two bits of data: the power of your amp and the efficiency of the speakers. Let's say you have a 100W amp and speakers that have an efficiency of 96dB/watt at 1metre. If you double the power just add 3dB and for 10 times the power add 10dB. This means that 2W will give you 99dB and 10W will give 106dB. Going from 10W to the full 100W will give another 10dB or 116 dB at a distance of 1m from the speakers.
As a comparison I will compare this with a valve amp (AC30?) producing 32W into a 4x12 with a 102dB/W sensitivity. 2W produces an extra 3dB, 4W gives 6dB, 8W gives 9db, 16W gives 12dB and 32 gives 15db. This combination gives 102+15 or 117dB. This combination is louder than the 100W system above but only by 1dB which is just noticeable. Because it is a valve amp though it is more likely to be driven into distortion so it should sound noticeably louder.
Manufacturers don't often give the dB's of the amps they produce and it can be difficult to find the sensitivity of the speakers they use. It is almost as if they don't want you to know you can buy more bang for your buck! A little word of caution here, only use rms watts in your calculations and be aware that a lot of combo's give the amp power into 4ohm speakers but fit an 8ohm speaker in the cabinet which only takes half the quoted watts, Ignore any figure that says peak power or peak watts as this is often just ad-speak.
How Loud Should You Go
It used to be so easy. Valve/Tube amps were always expensive and solid state amps struggled to produce 100W reliably. PA's were pathetic so you simply went as loud as you could. With modern developments transistor amps can now go up to 1000W levels for a couple of hundred pounds and digital amps are about to further reduce the price of amps even further. We are going to have to start exercising restraint and start thinking in dB's not watts. So how loud should you go?
There is lots of great stuff out on the internet on this and it was a fascinating topic to research. My recommendations are based upon measured levels at concerts and potential damage to your hearing. The maximum sound level you should be exposed to for two hours in the United States is 100dB or it was in 1990. The same is more or less true in Europe with laws obliging the wearing of ear defenders if you are exposed to average levels of 85dB at work. The World Health Organisation actually talks about concerts and recommends a maximum exposure of 100db for 4hours. In the UK planning law means that increasingly bars and theatres have to be fitted with electronic devices which cut the power if the sound goes above certain levels. Effectively we have sonic speed cameras.
Measurements by officials at a well known festival in England showed the bassist suffering at 101dB, the drummer 104 dB, the bouncers 108dB and the sound engineer 99dB and wearing earplugs. I think the loudest you should aim for onstage considering both your health and the need to hear and be heard is 100dB. If there are four of you in the band then there is four times more energy or 6dB so each amp is aiming at an average sound level of 94dB.
If your onstage levels are above this you will sound s**t. This is because your amp will be coming through every mic' on stage and so will everybody else's all with slight but audible time delays which will really muddy your sound. You will also have the experience we have all had of being very loud but not being able to hear everything. At these levels your body will protect your ears by tightening up and dispersing as much acoustic energy as it can. You will find it harder to hear the other members of the band so as a group you lose cohesion, meanwhile the singer will not hear their own voice properly and their pitch won't be accurate.
Now to produce a sound level of 100db at 1 metre my very ordinary Peavey speaker needs just 1W. That's right one! If I want this level at 2metres I need 4W and at4 metres I need 16W. Now these are average levels and we need a bit more power to cover the dynamic range (the difference between loudest and quietest sounds). Bass has more dynamic range than guitar and if you play with a lot of distortion you need less dynamic range because effectively the difference between your loudest sound and your average sound is less. There are lots of variables but in the end bass amplification probably needs to achieve no more than 120dB at 1 metre and a guitar amp 114-117dB. This ties in with the outputs of the AC30 amp we looked at earlier so the science ties in with reality which is nice.
So here is the buying advice (at last I hear you sigh). Find out the sensitivity (efficiency) of your speaker. Assuming your speakers are a typical 98dB per watt you need a valve amp of about 50W for a guitar. If you have a trannie amp you really ought to at least double this just to avoid clipping and for bass you need 200W. If your speaker is 95dB sensitivity you will need twice this power and if it is 102dB then you need only half the power. Make sure that your amp will give its full output into the speaker though as a solid state amp usually only gives half its power through 8ohm speakers. Follow this advice and you will have enough power for almost any situation. If you need more power then go through the PA don't try to blast everyone else off the stage.
I hope you found this article useful. There is loads more technical stuff I could have dealt with but I decided to miss out any use of formulae and complex maths and keep the picture fairly straightforward. I'm off to practice; at sensible volume levels, of course. Cheers.