The standard ISO 23591 [1] on the acoustics of music rehearsal rooms was published in 2021. It gives upper and lower limits for the reverberation time as a function of volume and type of music. The standard also contains an annex in which a method is outlined for estimating the sound pressure level (SPL) in the room. If the sound powers of the sources are known, the connection between SPL and sound power is the sound strength of the room, G, as defined in ISO 3382-1 [2]. Music rehearsal rooms deviate from concert halls in several ways. Since there is no need for an audience, the volume is normally significantly smaller. The number of musicians playing in the room can be the same as in the concert hall in the case of an orchestra rehearsal room, but often the room is meant for considerably fewer musicians, and the smallest case is a practice room for a single musician. The volume of a room is one of the most important parameters for the acoustics. When the volume is reduced, it is necessary to find a balance between increase of SPL and decrease of reverberation time.