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Related question: Is there an amp power formula?

When I look at amps in a shop or online, I ask myself what the amp is suitable for:

  • Can it be played quietly enough for home use without disturbing the neighbours/sleeping wife/etc.?
  • Is it loud enough to play along with acoustic instruments?
  • Is it loud enough to be heard by an audience in a room of a certain size?
  • Is it loud enough to be heard alongside a drumkit
  • etc.

It's clear that wattage isn't an adequate indicator: the Beatles played gigs on Vox AC15s -- I have owned a 15 watt guitar amp that couldn't possibly be heard over a drumkit.

It's also clear that size isn't much of an indicator either -- there are tiny guitar amps that are seriously loud.

When choosing an amp, how do I know how loud it's going to be?

Quick word on scope: - Let's not talk about micing up amps for the PA. - Let's be general and talk about bass, guitar, keyboard amps, and amps for any other electric instrument.

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4 Answers 4

up vote 6 down vote accepted

Although watts actually are a pretty decent way to predict the loudness of an amp, it's worth understanding what the listed wattage of an amp means. Remember that an amp is designed first and foremost to amplify the input signal, and that for many applications, it's desirable that the amp do so while keeping the signal as clean as possible. So an amp's wattage rating is traditionally listed as the amount of power it can produce while keeping the total harmonic distortion (THD) of the output under a certain percentage.

But maybe the guitarist doesn't care about keeping the signal pristine---in fact, maybe he'd like the amp to produce some distortion, maybe even a lot of distortion. In that case, he's going to be running the amp at higher wattages than its rating and thus it will seem louder than its rating would imply. It matters, too, whether the amp uses tubes or solid state transistors, because tube distortion is often thought of as being more pleasant than solid state distortion. This means that a guitarist playing through a 100 watt tube amp may turn it up to get distortion from the amp, while a guitarist playing through a 100 watt solid state amp may rely on pedals for his distortion and leave the amp turned down to avoid the unpleasant solid-state distortion.

You mention the Beatles playing through 15 watt Vox AC15's, which are tube amps. In concert, they turned those amps way up and got a fair amount of distortion from them, meaning their amps were producing way more than 15 watts of power. Perhaps the 15 watt amp you mentioned as being underpowered was a solid-state amp?

As far as amplifying other instruments is concerned, each instrument has its own specific needs, and so its hard to compare wattages across different instrument categories. Keyboard amps, for example, are designed to be as clean as possible, so a keyboardist is not likely to turn an amp up beyond its listed rating. Bassists, as a general rule, require higher-rated amps because their lower frequencies require more power to produce a similar volume. For example, in a typical rock band, the guitarist might play through an amp rated anywhere between 50 and 150 watts (if you're planning on cranking your amp up to AC/DC-type distortion levels, btw, 50 W is plenty loud), while many bass heads are rated in the 300-400 W range. In my own case, my guitar amp is a Fender Twin Reverb rated at 85 watts and my bass rig is powered by a Crown K2 power amp which produces 1,600 watts in bridge-mono mode. I can tell you that the Twin absolutely keeps up with the bass rig in terms of volume---but then, I want to distort the guitar amp, and I want to keep the bass amp as clean as possible.

In any event, wattage is not bad as an indicator of an amp's loudness, so long as you understand what it does and doesn't actually mean.

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Wattage and size are actually the single best indicator, but you are right - they aren't the whole story.

For example a 150W guitar amp is likely to be a lot louder than a 150W hi-fi system as the amp model doesn't require that much accuracy in a guitar amp, whereas a hi-fi amp should reproduce sound very accurately.

I have played some small venues using just a 150W amp for my guitar - turned up halfway even a 100W Marshall through a 4x10 can be nice and loud.

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I wouldn't want to try with anything smaller, as even when mic'ed up I want to be able to hear my monitor over the drums, house PA, even the singer! Admittedly I play rock and metal, but it is blues based rather than thrash based so I want enough headroom that I can accurately decide my clipping and distortion in my pedal, not the amp.

If you want clarity, go for a higher power amp - then you are less likely to hit any headroom/distortion constraints.

I have played in bands where we used 12 kW of amplification between 2 guitars, bass, vocals and a keyboard to halls of up to 1000. Loud, but we had headroom so we never had to turn it up close to clipping. Nowadays I like to have my pedal connected directly to the PA system and just use my wedge monitor. (means we don't need a large van for travelling to gigs, which is nice)

You can always turn an amp down low, but to avoid disturbing sleeping flatmates you want a headphone amp:

enter image description here

That may all seem a bit of a ramble, but what it comes down to is that you just need to try them out. Start at 150W if you have a standard drum kit on stage with you (this is also a good size for mic'ing up for the PA with either a 2 or 4 speaker cab) and work up from there.

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Don't forget that with tube amps you need to take into account the actual power tubes and preamp tubes and if the output tubes are optimally biased. e.g. if you substitute a 12au7 for the 12ax7 in the preamp section, suddenly your screaming amp is a bedroom practice amp. –  filzilla May 16 '12 at 18:23
    
very true - never tried that though :-) –  Dr Mayhem May 16 '12 at 18:28
    
It's great to try when you can no longer lift your vintage 1965 Super Reverb and have confined to it to your bedroom. :> –  filzilla May 17 '12 at 20:49
    
When it comes to guitars, a 40W amps with a single mildly efficient 12'' is already loud enough to overpower a drummer (and I mean a metalhead drummer pounding his own beloved metalhead drumkit). –  bruno desthuilliers Apr 2 '13 at 19:04

Watts output(RMS) and the efficiency of the speaker(s) are just about all you can use to estimate. The rule of thumb is that it takes a doubling of watts to increase output by 3 db.

I have seen speakers with sensitivity ratings of 95db (cheesy no-name '70s 12" speaker I found in an old Fender Amp) to 101db (A quality EV 12" guitar speaker in an Ampeg VT-22... LOUD)

I'm going to try to explain this with an example rather than writing a detailed post because I'll likely get the details wrong...

Take a Fender Deluxe (A 20ish Watt(RMS) amplifier) and put in the 95db speaker and you'll struggle with volume (you'll be at 80% to 100% on the volume control) but you'll get pretty good power amp distortion because the amp has to work very hard to push the speaker to an acceptable volume.

Now, take the same Fender Deluxe and install the 102db speaker. It will be too loud to turn up past 40% or so and the power amp distortion will be minimal because the amp does not have to work as hard to produce an acceptable volume.

I hope this makes sense...

Edit: This link explains the math.

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I find it very surprising that the best answer - the only one to mentions decibels and speakers efficiency - is the least upvoted. –  bruno desthuilliers Apr 2 '13 at 19:06

All I can say is that this topic is complained about a lot, and is controversial. The answers seem to be really complex. But the consensus is, no, there is no clear, simple relationship between the manufacturer's reported wattage and the practical, real-world volume that the amp can produce. Indeed, different models of amps produced by different manufacturers which report the same power wattage will in fact sound different in practical volume level.

Here's a good technical note/blog post, "Watts vs. Volume" by a well-respected guitar amp builder who talks about the issues involved. He has a lot to say about the Fletcher/Munson curve (the frequency response of the human ear at varying levels of volume) and perceived volume, how he designs his amps, and how he tries to report honest numbers to reflect how loud his amps are going to be.

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