How loud is it at the back? Speaker level and distance

The first question in any room is how much quieter the back row is than the front. The answer starts with one rule.

The 6 dB rule

In open space, a speaker’s level falls by about 6 dB every time you double your distance from it. This is the inverse-square law expressed in decibels: level at distance d = level at 1 m − 20 × log10(d). Datasheets give a maximum level at 1 m, so you can work out the maximum at any other distance.

For a speaker that reaches 128 dB at 1 m (a typical figure for a 12 inch powered top, used here only as an example):

Distance	Maximum level on axis	Change from 1 m
1 m	128.0 dB	0 dB
2 m	122.0 dB	−6.0 dB
4 m	116.0 dB	−12.0 dB
8 m	109.9 dB	−18.1 dB
16 m	103.9 dB	−24.1 dB
32 m	97.9 dB	−30.1 dB

The loss between the front row at 2 m and the back wall at 32 m is 24 dB. That is a large gap, and it is why a single speaker on a stand struggles in a long room: turn it up for the back and the front becomes uncomfortable.

Maximum level is a ceiling, not a setting

The figure printed on a datasheet is a maximum, and manufacturers state it in different ways (peak or continuous, with different test signals). Speech and music are run well below the maximum so that peaks have headroom. The planner lets you set the level of each speaker and pick a target level for the room, and shows where the room is below, inside or above that target.

Adding speakers

Two identical speakers playing different parts of the room do not double the level everywhere. Where the sound from two uncorrelated sources meets, the levels add by power: two equal sources give 3 dB more than one, four give 6 dB. Identical signals that arrive in phase can add up to 6 dB for two speakers, but also cancel at other points. The planner adds by power and ignores phase, so treat the result as an average, not a prediction of every seat.

Rooms are not open space

Indoors, reflections and reverberation mean the level falls more slowly than 6 dB per doubling once you are far from the speaker, and the sound becomes less clear. This planner calculates only the direct sound from the speaker to the listener. It is a way to compare layouts and find coverage problems, and it does not replace measuring in the room.

Try it in the planner

Next: what a coverage angle means.