Since we're (I'm) talking about levels (as I was in last night's post), it occurs to me that I've noticed much confusion in folks over the years regarding perceived loudness and level metering. People think that when they hit 0dB on their output converters their mix should be officially 'loud,' but are mystified by the fact that it still feels quiet. The meters are solid in the red, but it just isn't kicking like it should.
Audio level meters, be they RMS or peak, show amplitude only. They do NOT, however, tell you what frequencies make up this overall volume, or the shapes of those frequencies. This is why the HPF I wrote about last night is so important. It removes unwanted LF energy that pushes those meters up without giving you the benefit of perceived volume.
To demonstrate this point, take a signal generator, and make a 100Hz sine wave at any output level, and give it a listen. Then switch to 1kHz at the same level -- a LOT more annoying, no? It feels louder, but it's actual amplitude isn't any higher than the 100Hz tone. Now, change that sine wave to a triangle wave and see which one feels louder.
What do we learn by doing this? Simply that our perception of volume is based on the frequency and shape of the audio content as much as by actual output level being produced.
Ever watched late night TV and been blown away by how much louder than your show the shouting of your local used car salesman feels during the commercials? There is a reason for this: there are FCC rules on how strong a broadcast signal can be (absolute amplitude), so in order to produce the highest perceived level the audio content is sent through a limiter before being broadcast. This limiter isn't concerned so much with frequency content, just the level being transmitted. The LF rumblings of traffic and ambient noise of the city in that detective show re-run uses up the available headroom during the show, but during the commercial there's nothing but the purely mid-range voice of the car salesman to assault your ears. Same maximum amplitude for both (because of the station's limiter), but the frequency content is so vastly different that the perceived level is as well.
Your VU meters are critical in your work (more about metering in a future post), so keep using and trusting them, but in the quest for 'loudness' you must make sure to carve away any and all unwanted content to make room for what your really want your listeners to hear.