Some Technical Information:

RMS in music, average loudness, punch

The Root Mean Square (RMS) in audio production terminology is a measure of average loudness and is found widely in software tools. A smaller RMS number means higher average loudness, i.e. -9 RMS is 2 dB louder than -11 RMS, 0 is the maximum. The loudest records are -7 to -9 RMS, the softest -12 to -16 RMS. The RMS is no absolute guarantee of loudness though; the perceived loudness of signals of similar RMS can vary widely since the perception of loudness is dependent on several factors, such as the spectrum of the sound (see Fletcher-Munson) and the density of the music (e.g. slow ballad or fast rock).

Since most major pop releases have been for a long time in the -9 to -11 RMS range, the loudness war (the trend of new records sounding louder) has practically ended simply because the physical and artistic limit of loudness has been reached. In pop music, the louder the average level gets, the less punch you will have since the punch depends largely on how loud the kick drum is in relation to the average level of the music. The RMS can be thought of as being the volume difference between the kick drum and the rest of the music - the larger it is, the louder the kick drum is.

Compressed higher RMS vs clipped higher RMS, density

Some experienced listeners feel that around -12 RMS in general or during loud parts and -14 to -16 RMS during soft parts is a "sweet spot" for optimal punch and loudness, neither too loud nor too soft. This perception is still valid considering that the extra loudness (usually 1-3 dB) has often been achieved by simply clipping the smoothly curved tops of the waveforms resulting in flat topped square waves, which may or may not result in a subjective improvement of the sound. Prior to clipping, usually the last procedure in audio production, the "natural" RMS of many songs is in fact just around -12 RMS. Thus, in many cases where the final RMS is -8 to -11, the RMS has not "really" been increased over the -12 RMS "sweet spot", only the tops of the waveforms have been clipped by 1-3 dB; the music is not any thicker or denser, but merely played louder with less punch and more distortion.

In contrast, a "true" higher RMS, is achieved by increasing the density (usually by compression) of the sounds contributing most to the average level, i.e. everything else but the drums, so that their volume as a group can be lower in relation to (usually drum) peaks, yet retaining the same RMS and perceived average loudness as the clipped one, but without clipping, often with a stronger sense of density and pressure. However, in practice, this, too, would probably be subjected to some clipping, resulting in even higher loudness and pressure than the one that was merely clipped.

So it's not how loud you make it, it's how you make it loud.

Limits of maximum RMS in music, average loudness

If the kick drum is hitting 0 dBFS (the maximum of digital sound) on most of its cycles (typically 2 or more) and/or processed (e.g. clipped) to sound louder than 0 dBFS, −8 RMS can have good punch. With precise mixing and separation mastering techniques, perhaps −7 to −6 RMS is achievable without distortion, with an audible but not very punchy kick drum.

Bass punch, kick drum, bass drum, frequencies, waveform

Usually the loudest sound on a pop music recording. The physical punch of the sound waves the listener feels is caused by the short burst (around 20-175 ms) of low frequency energy of the kick drum or some other low frequency sound. The majority of the feelable energy is in the 0-200 Hz range, often the peak, the loudest frequency is in the 40-80 Hz range, often about 65-75 Hz. The punch you feel in the chest is around 120-170 Hz, often about 160 Hz.

Its visual shape, the waveform, is 1 or more, up to 7-10 (nearly) full amplitude (0 dBFS) approximately sine or square wave cycles. The more cycles hit 0 dBFS, i.e. the longer the sound stays at maximum level, the stronger the perceived punch, up to a limit.