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.

TRANSMISSION LOSS

The amount of sound that is transmitted through the wall is called the:

Sound Transmission Class- STC

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:

• Staggered Studs. Here you use two studs for each side of the wall. The plasterboard on one side is attached to one stud and the plaster on the other side is attached to the other stud. The two studs are connected to a common base and top plate.

• Flexible Channel. Here a metal channel is attached to the stud and the plasterboard attached to the metal channel thus reducing the connection to the stud. The channels are mounted horizontally at 600mm (2 feet) centres. This system is extremely effective - check out the figures in the STC Chart.

Studs:

• Except for staggered stud systems, substituting timber studs for steel studs generally results in a significant decrease in sound isolation.
• Increasing the thickness of steel studs from 0.55BMT to 0.75BMT or 1.15BMT will decrease sound isolation
• Decreasing the stud spacing will decrease the sound isolation.

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:

• Floating the rooms. A typical construction consists of creating new rooms within your existing rooms. This means building a floor on top of the existing floor with neoprene isolation pads and Rockwool on the underside and then building walls and a ceiling using the new floor as a base. This is the ideal system for total isolation because the new room is not mechanically connected to the main room but it is also the most expensive system. The main advantage of floating rooms is the low frequency isolation it gives. If you are building in a block of flats it would be essential as it is the only real way of achieving total sound isolation but if you are building a garage studio it really would depend on how much isolation you require. If you are an acoustic folk band - forget it - if you are a heavy metal power band it is essential because it is the only way you will stop the bass and kick drum from annoying your neighbours.
• Double Walls. This basically means building two timber/gypsum sheet walls between each room with Rockwool in the cavity. If you wish to go further you can double the layer of gypsum and even further by sandwiching a layer of fibre board between the two sheets of gypsum. (This is extremely effective because sound doesn't like going through changing medium densities). There are some companies who make sound isolation gypsum which is thicker and heavier than normal gypsum sheet. In Australia it's called Soundcheck and is 16mm(5/8") thick. The hollow concrete block -(Besser Block) is an excellent wall construction as it attenuates sound efficiently and cheaply.
• Sealed Environment. There is one important factor that must be understood about sound isolation. If you build a beautifully sealed wall between two rooms you will get good isolation BUT if you put one nail hole in the wall you will loose a lot of the isolation!! Sound is really not the waves we keep on describing but air pressure difference so if you allow the two air masses to join at any point the pressure gradient will transfer, so make sure that all walls are sealed tightly. Also make sure all joints around doors, windows and air-conditioning ducts are sealed.

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

• Adding another layer of gypsum which is glued (not nailed) to the first sheet and should be a different thickness than the first sheet. i.e. 16mm (5/8") and 12mm (1/2")
• sandwiching a layer of fibreboard between the two sheets.

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.