Before you can consider your construction you must consider your isolation requirements. Pages could be written on this subject but you must consider how much isolation you really want. The idea of perfect isolation from external noise started in the days when loose miking techniques were used. One microphone suspended over a string section meant the mic was wound up fully and was extremely sensitive to ambient noise. Nowadays a mic 6" from a marshal amp is a totally different story. At Big Toe Studios I often have a window open and the artist will say -" Hey I can hear the birds, should I close the window?" To which I reply, "No, the only person who will hear it is some stoned out freak with headphones on who will remark excitedly - wow man I can hear birds on this track!" But if you have problem neighbours who don't like drums pounding all day I suggest you apply a certain amount of sound isolation.
The acoustic term here is Transmission Loss. When sound hits a wall there is a certain proportion of the sound reflected back into the room, some is lost in the absorption of the wall and the rest travels through the wall and is called the transmission loss.
The amount of sound that is transmitted through the wall is called the:
The transmission loss obviously varies relative to frequency - the STC is a specially weighted reading across all frequencies and is centred around 500Hz.. Every different wall construction has a different transmission class.
When sound hits a wall the energy is transferred through the plasterboard to the other side via the connection to the stud. This problem can be reduced via two ways:
It is also interesting to note here that the higher the transmission loss the less reflected sound. In other words in a tent there is a high transmission loss but also a low amount of reflected sound so a tent makes a good recording room!! So people in the country who can afford a high transmission loss because there's no close neighbours can allow their sound to get away thus reducing the amount of treatment required to handle the reflected sound.
The standard gypsum wall in a house has a high transmission coefficient at 100Hz as well as a high absorption figure because the gypsum panel's resonate frequency is around that figure. Therefore the reverb in the room is low around 100Hz but higher around 300Hz where the transmission and absorption are lower. That is why most rooms in a house have a reverb peak around 300Hz. (You know the one you keep taking out of kick drums and toms.) Check it out on the reverberation calculator.
Perfect isolation can cost heaps because there is only one thing that will stop sound and that is MASS. The following solutions apply:
Double Wall with Floating Floor
In the drawing above I've drawn a single gypsum (pale blue) layer but adding to the layer can dramatically increase the transmission loss. The options are
This really works well, a double wall with a triple layer as described above on a floating floor will create a room that will allow you to set up a band and not hear it outside the room! Just remember that all the sound is now trapped inside the room and heavy acoustic treatment is required to control it all.