How loud does it go?

Francisco from Portugal writes:

I’m trying to do a project for a rehearsal studio for my band, and so I’m studying acoustics. I’m trying to determine the sound level in dB that we will have inside the studio.So I’d like to know if there is a way to convert an output power, for example of a guitar amplifier with 100 W (average signal), to decibels (dB).

Unfortunately there is no easy answer to this question. The rating of a guitar amplifier is almost certainly the maximum electrical output it is capable of producing. But in the guitar world, ‘amplifier’ means amplifier and speaker/s. And it is the efficiency of the speaker at converting electrical energy into acoustic energy that largely determines the volume level.

Radio Shack analogue SPL meter In hi-fi we talk of speaker sensitivity, which is the sound pressure level in decibels the speaker will produce at a distance of one metre when a 2.83 volt signal (equivalent to one watt of power into eight ohms) is applied. This averages around 89dBSPL, but is commonly somewhere between 79 and 99dBSPL. If the speaker impedance is eight ohms, and if the amplifier will deliver 50 watts, we can calculate a maximum output level of 106dBSPL at one metre for an average (89dBSPL) speaker. [Level equals the 89dBSPL for one watt, plus the ratio in decibels of 50:1, equals 10 * log(50).]

Since the efficiency of the speaker built into a guitar amplifier is not known, we cannot calculate this in a conventional way.

In any case, the actual volume level is complicated by the environment in which the speaker is operating, since a more reverberant room will keep the acoustic energy in play for a longer period than a dull room.

The best way to deal with the problem would be to take an inexpensive Sound Pressure Level meter (I use an analogue one from what is known in Australia as Tandy and in the United States as Radio Shack — I imagine there’s a similar type of shop in Portugal), don some ear protection, and play the guitar loud. Turn it up until it starts distorting beyond what you think it should (a large part of the character of an electric guitar is the distortion produced in the speaker/amp combination) with the meter switched on and set to a ‘Fast’ response. This won’t accurately show the peaks (the meter won’t respond quickly enough) but it ought to give you a rough idea of the average level.

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