Sound Isolation / Soundproofing Doors
Part 1 - Use Heavy, Well Sealed Doors
Well, the first thing that needs to be noted is that for any room where sound isolation is important – don’t use a hollow core door.
Even if you are building a basement theater, and don’t care how much sound escapes sideways into the rest of the basement, you should utilize a solid core wood door, or at least a heavy steel door. If you don’t, enough sound might escape up stairs, or through lower-quality partitions in the rest of the basement to cause disturbance where you want it to be quiet. The higher mass of these heavy doors allows STC’s (of the door alone) as high as 30 or so (sometimes a bit higher), with good low frequency performance (better than most types of walls, in fact).
Solid wood core or heavy steel door?
The performance of these doors is fairly similar, so long as they are both “solid”. “Solid” would be steel doors in which the two sides are connected by a typical honeycomb structure, or rigid insulation.
If the two sides of the door are not connected, but rather mechanically decoupled, STC (a measure of middle and high frequency sound isolation) may improve, but low frequency isolation will worsen.
Regardless of your choice of door, pay great attention to the seals.
To get the maximum performance that is possible from a heavy door, you will probably have to pay some special attention to the seals.
Consider upgrading with weatherstrip available from your hardware store or some of the engineered seals some of the engineered seals available from companies such as Zero International, you have to make sure that the door is very well sealed.
A simple field-test of seal quality is to press putty into the seams of all 4 sides of your door, and compare the noise levels. If the noise gets considerably quieter with the putty in place, you still have a way to go. If it doesn’t, then you’ve done your job well.
Improving the Isolation of a Door
The mass of a heavy door is a great starting point for sound isolation, especially at low frequencies. But typical doors are far from perfect. Typically, the performance of doors is limited by a resonance called “coincidence”, which results in a big dip in performance at vocal frequencies.
Damping materials, such as Green Glue, can help. In walls, lab tests have shown that Green Glue essentially eliminates this dip, and the same effect can be realized in doors.
By adding Green Glue and a layer of MDF to a door, you can gain a double benefit – mass addition + the damping performance of GG. Considering that the cost of treating a door in this fashion is extremely low, it is a sensible choice for many.
Mechanical fasteners should be used lin most situations, Screw the panel to the door. One screw in each corner, and three across the middle of the door. Also, make sure not to overload your door hardware – always respect the weight limits.
Part 2 – Exotic Doors and Communicating Doors
If you require very high isolation through a door, you should seriously consider options such as doors specially engineered for sound isolation or communicating doors.
Engineered doors. Specialty doors with STC’s into the 50’s are available from various makers. Costs can be very high, often may thousands of dollars. These doors are typically very elaborately engineered – containing very dense materials such as lead, specialized and highly effective seals, and various other measures to help the doors improve isolation.
When selecting a door, remember that STC doesn’t consider low frequency isolation at all. And often, the very things that raise STC can work to WORSEN low frequency isolation, so make sure to ask the manufacturer for detailed transmission loss data below 125Hz before investing your hard-earned money.
For many, engineered doors might be the preferred solution. They are neat, pre-packaged, and don’t require a lot of hands on work, and can perform better than any DIY single door. For others, however, or for situations where ultimate performance levels are needed, you should consider communicating doors.
Communicating doors. Communicating doors involve the use of two doors that form an “airlock”. Here we show a sketch of communicating doors mounted on each side of a double stud wall:
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The Enormous Advantages of Communicating Doors Include Are Enormous
1. Twice the mass of a single door
2. Can include absorbing material in the cavity between the doors
3. Can be decoupled, to preserve the performance of the best walls, such as staggered or double stud walls
4. Greatly improved tolerance to imperfect seals (especially if you have absorbing material in the cavity as shown)
5. Highest possible all-around performance of any door option
Keep these General Guidelines in Mind When Building Communicating Doors
1. You should still, for both sides, utilize a heavy door
2. If you are going to this much trouble, you should still pay extra attention to the seals
3. The improvements suggested in the section on single heavy doors above still apply, and will still help.
4. Preserve as much of the air cavity depth as you can – in other words, strive to have as much space between the two doors as possible
A well-constructed pair of communicating doors can maintain the performance of all but the very best wall, and for the vast majority of applications, it can maintain a performance level that is more than satisfactory.
For Situations Requiring Ultimate Performance – Dedicated Entrance Rooms
For situations where an absolute level of performance is required, you might consider a dedicated entrance for your room.
When utilizing a dedicated entrance, it is wise to include sound absorbing material inside the entry room.
The absorbing material will cut down on sound bouncing around, and minimize the amount of sound that reaches the second door.
The doors should be as far apart as is practical. Also consider sound isolation construction in these walls.
With a dedicated entry room, any level of performance can be attained. If you so desire / require, you can even utilize communicating doors at both entry points.
The same basic guidelines shown above are applicable here.
Conclusion:
We’ve taken a look at what to do with doors – typically the weakest link in any interior wall that contains them.