Sound Absorbers (Part 3)
Sound Absorbers Part 3. The final part deals with acoustic treatment of the studio floor and ceiling.
In this, the final article of a short series, we deal with the two most neglected surfaces found in all studios, namely the floor and the ceiling.
When considering the acoustic treatment of a room, be it a home studio or a fully-fledged recording studio, many people approach the problem of treating the floor and ceiling in a similar manner; that is to say, they don't do anything! You're standing on the floor and as far as you are concerned that's it.
But what of the fact that throughout this series we have spoken of the dangers and problems of parallel walls? Aren't the floor and ceilings simply parallel walls turned through 90 degrees? Of course they are. So what makes you think the same problems caused by fundamental resonance modes and flutter echo won't be present between these two surfaces as well?
The answer, of course, is that the same problems will be present, so let's have a look at what can be achieved in practical terms to lessen the effects of these parallel surfaces upon the room's acoustics.
In the March issue of HSR the 'Acoustics' article on the design and construction of a floating floor provided a method by which you could isolate the floor of a room from any structure-borne noise travelling through the building. This form of acoustic isolation will not, however, provide any help towards controlling the internal acoustic properties of the room.
It must be stated that the absorption properties of any floor covering provide very little in the way of a contribution towards the control of acoustic problems. The use of carpets as a means of absorbing sound can, at best, only affect the high frequencies and do nothing to remove bass resonance: using thick carpets with plenty of underlay is all you can reasonably do to help ease the problem. It's, of course, impractical to lay too many thick carpets on top of each other as you will soon reach a point where you start to sink into the carpet and the moving around of any equipment on rollers becomes extremely difficult.
Another important factor to bear in mind is that if the floor covering is of carpet, then it should be both rugged and fire resistant (cigarettes). Many studios tend to use rugs rather than fitted carpets as this allows you the option of removing them to produce a very live acoustic when the surrounding floors and ceiling are also of a hard reflective material. However, if you are using one room for both the performance and recording of your sounds then it's better to go for a carpeted floor, which is particularly important in an area where vocals are being recorded, as singers often tap their feet and the resulting noise may be picked up by the microphone. As we can do little to change the floor we must look up towards the ceiling for the real solution to our problems.
Unless you're building a studio in an attic which has a sloping roof, the majority of people face life with an ultra flat ceiling. Two options are available, you can either build a new suspended ceiling parallel to the existing one which has good absorption properties, or you can build an angled ceiling. As you might expect, the angled ceiling approach is the better solution and will enable greater control of bass resonance problems, however, for those who can't make such drastic cosmetic changes to their room the suspended parallel ceiling will perform well and considerably reduce the problems in question. See Figure 1.
The task at hand is to construct a ceiling-mounted sound absorber that will remove both flutter echoes and bass resonance frequencies simultaneously. By referring to the previous articles in this series we know the performance characteristics of the panel absorber design and the Helmholtz resonator. From these two options, we choose the second, for the reason that when it is built as a multiple resonator it will remove large amounts of bass and mid frequencies as well as controlling the high frequency problems by virtue of the holes present in the front surface.
If we first of all consider the parallel ceiling design shown in Figure 1a, we can observe that by fixing joists to the ceiling, spaced a few feet apart, we can then mount pegboard onto the joists with a layer of fibreglass behind the board to fill part of the air gap. This design is constructed in a similar manner to the wall panels covered in Part One of the series, and with the pegboard in place, the whole structure results in a series of boxes just like the Helmholtz resonators described last month.
The performance characteristics of such a design remove a broad band of frequencies and help to cut down the effects of reflections between floor and ceiling. This type of design performs reasonably well and is of some use, especially in situations where you need to retain a near normal cosmetic finish to the room. The best solution, however, is provided by the sloping ceiling design shown in Figure 1b. As with the wall designs, the breaking up of parallel surfaces by the introduction of angles will help to cut down standing waves and the possibilities of flutter echo.
Figure 2 illustrates in more detail the construction of an angled ceiling design which, in principle, is similar to the Helmholtz resonators of the previous design. However, due to the new ceiling being sloped this creates boxes of varying depth; consequently, the absorption characteristics of this type of ceiling are broader than those of the parallel ceiling design.
When building the sloping ceiling several points of construction should be looked at. First of all, the minimum height of the new ceiling should be carefully considered, for rooms with high ceilings this could come down to around seven feet and would produce boxes of good depth across the entire length of the ceiling. For modern houses whose ceilings are normally already fairly low at around seven and a half feet, you won't be able to come down very far and so much shallower boxes will result.
Once you have secured the joist structure to the walls, hardboard sheets should be fixed in place between the joists and the original ceiling to form a box shape. These boards should have fibreglass matting glued to both sides before fixing in place the pegboard covering which forms the new ceiling. When constructing this design, you can improve the isolation properties of the ceiling by placing all the supporting structure on rubber mounts. This will help to stop any bass frequencies from vibrating the ceiling structure as well as cutting down the chances of sound being transmitted through to the original ceiling.
When the support structure has been built the pegboard cover panels can be screwed in place; just prior to doing this a good, deep layer of fibreglass should be placed behind them, six inches or so if possible. One final point to bear in mind is that any light fittings should be brought down to the new ceiling height before all the front panels are fixed in place.
That concludes this short series on acoustic treatment. All the articles have endeavoured to show how with simple and cheap materials you can tackle the job of studio acoustics with the minimum of expertise and yet still produce very acceptable results. Some people may only make small changes to their room's acoustics, while others will. I'm sure, employ many of the designs and techniques presented here to construct their own studios. The key point to remember is 'don't be afraid to experiment'.
Feature by Paul Gilby
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