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I want to get a tube amplifier, but the tube amplifiers I fancy are generally at least 22W.

I live in an apartment with thin walls, so I have to get it down to conversation/TV volume.

This brings me to a series of questions:

  • How to measure the volume of an amplifier? Sound Pressure Level in Decibels?
  • Where to get a reference point? "Ordinary" conversation is 55dB SPL? Piano is 60dB SPL?
  • 22W heads are loud right? If the head has "volume" control how loud would it be at the lowest setting and highest setting in SPL decibels?
  • What would be the loudness of 1W, 5W, 50W or 100W head (at various volume/gain settings)?
  • How much volume attenuators can bring down realistically -10dB(~half volume), -20dB(~quarter volume)? What are the choices?

Yes, ideally I would try the whole setup in a music store, not an option, unfortunately

  • 1
    Things are unfortunately not as easy as comparing watts and dB's. I have a 5W Blackstar which is slightly too loud to sound good at apartment volumes. My Mesa/Boogie Studio 22 (22 W) can be played at lower volumes while still sounding acceptable. The Blackstar sounds great if just pushed slightly over comfortable limits. So it will, in my experience, be a process of trial and error until you find something that works. It's better to make educated guesses though, your question is a good one. – Meaningful Username Dec 17 '14 at 12:38
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If you really need a power attenuator depends on what you want to achieve. A 22W amp can usually be turned down sufficiently to be played in an apartment. However, if you want it to sound as if it were cranked up, i.e. you want the power amp to saturate, then you'll need a power attenuator. And in this case any attenuator that matches the output impedance of your amp will do. If you're after a good overdrive sound and if you can't get that with your amp at low volumes, you might also want to consider getting an overdrive pedal in addition to your amp. But note that many amps can produce a good overdrive sound at low volumes by saturating the pre-amp instead of the power amp.

As a final note, it's important to realize that amp power alone does not define the actual volume that will be perceived. The efficiency of the loudspeaker(s) plays an important role too. The loudspeaker's efficiency is usually described by its "sensitivity", which is its sound pressure level (SPL) in dB at 1 W input power measured at a distance of 1 meter. A difference of 3dB sensitivity between two speakers means that you need twice the electrical power to achieve the same volume with the low sensitivity speaker. Typical guitar speakers have a sensitivity of about 94-100 dB, so you see that there are differences of up to 6dB resulting in power differences of a factor 4 (for achieving the same volume)!

  • Very good point about speaker efficiency. Maybe you can elaborate some on this? – Meaningful Username Dec 18 '14 at 9:03
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    @MeaningfulUsername: I added some info on loudspeaker sensitivity. – Matt L. Dec 18 '14 at 11:05
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Before you go out and buy an attenuator, ensure you really need one. @Matt's answer makes very important observations.


Typically attenuators have variable attenuation level; you should be able to get the volume you really want without problems.

(Yes, this is the big knob).

enter image description here

Weber MiniMASS

The only real caveat is maximum power dissipation; they are typically rated in maximum amp power they can reasonably take. For a 22W amp you'll most probably gonna look for a 25W or 50W attenuator.

This is a brilliant article if you want to read more about them.

Note: I'm not affiliated with Weber in any way. I just know about that particular brand existing, and the photo of the attenuator panel says more than a 1000 words.

  • Can you comment on how thing thing connects into the amp? Will it work with all amps, or only ones with specific ins and outs? – GreenAsJade Dec 17 '14 at 13:18
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    @GreenAsJade You need to be able to put yourself in between the amp and the speaker, basically. If the amp is soldered directly you might need to break it apart and solder your own plugs to allow that, which is a fairly easy mod. That being said, most tube combos are connected via a detachable cable, and all heads are so, too. – Bartek Banachewicz Dec 17 '14 at 13:29
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How to measure loudness

  • Sound pressure level (SPL) is used to measure loudness in this case. The unit of measurement is dB(SPL).
  • Sound pressure meter is an instrument used to measure dB(SPL). Cheap sound pressure meters can be bought starting at 20 USD.
  • The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity. The physical quantity is sound pressure measured in Pascals(Pa). Sound pressure level is defined as Lp = 20 log10 p1/p0 where p0 is the reference value.
  • The reference value used is threshold of human hearing (sound pressure of air).
  • It's important to note that there's much more to subjective perception of loudness than sound pressure alone.

Reference values for loudness

    Sound source                          Sound Pressure Level dB(SPL)
    ------------------------------------------------------------------
    Hearing threshold                     0             
    Rustling leaves, light breathing      10
    Calm room, bedroom at night           20-30
    Quiet library                         40
    Normal conversation at 1m             50-70 (avg 60)
    TV (at home level) at 1m              60
    Vacuum cleaner at 1m                  70
    Busy street at 5m                     80
    Heavy traffic, diesel truck at 10m    90
    Jackhammer at 1m                      100
    Chainsaw at 1m                        110
    Threshold of discomfort               120
    Threshold of pain                     130

From thereon to clear any confusion I propose that apartment friendly volume would be in 50-60 dB range. And 70 dB an upper bound for not pissing anyone off. 40 dB being whisper quiet.

Loudness of cranked tube amplifier

  • To calculate the sound pressure level of an amplifier you have to know the Speaker Sensitivity. Speaker sensitivity is expressed in dB(SPL) per 1 Watt measured at a 1m distance.
  • The off-the-shelve speaker cabinets usually don't have this value on the spec-sheet. Generally this value is in the 87-92 dB range. For professional grade, highly efficient cabinets speaker sensitivity is in 100-110 dB range.
  • Assuming the speaker sensitivity of 90 dB and a single 1x12, a cranked 1 Watt amplifier will be 90 dB at 1m distance. Cranked 1W amplifier is VERY LOUD.
    Amplifier power   Sound Pressure Level dB(SPL)
    ----------------------------------------------
    1 W               90 dB
    5 W               97 dB
    10 W              100 dB
    22 W              103 dB
    50 W              107 dB
    100 W             110 dB  
    120 W             110.8 dB

In other words, 10W amplifier will be twice as loud as 1W amp, and a 100W four times as loud as 1W amp.

Attenuating to apartment volume

To bring a cranked 1W amp to 60 dB we have to lose roughly -30 dB. That would be 1/8 of the volume. The power of the amplifier have to be attenuated down to 0.001 W. That's 0.1% of power output.

In other words, to power a speaker at 60 dB sound pressure level we need 0.001W of power.

    Amplifier power   Sound Pressure Level dB(SPL)
    ----------------------------------------------
    0.001 W           60 dB
    0.01 W            70 dB
    0.1 W             80 dB

For a 22W amplifier we have to lose roughly -40 dB, 1/16 of the volume, 0.01% of power output.

Conclusions

The attenuator suggested by Bartek (Weber Mini Mass) should do the job as it can attenuate down to -50 dB.

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