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8.9 ROOM SHAPE
To avoid standing waves and flutter echoes, parallel walls and surfaces should be avoided.
In new construction or an extensive renovation this is relatively easy to do. Even with a low budget it is possible to build additional drywall surfaces into a room to create non-parallel walls, and with the addition of some doors, any large spaces behind a new angled interior wall can be usefully repurposed as storage!
If room within room construction or renovation is not possible, various commercially available acoustical products can be applied to existing wall and ceiling surfaces to reduce standing waves, flutter echo, and excess liveness.
8.10 ABSORPTION
Standing waves, flutter echo, and excess reverberation can be controlled by applying absorp-tion products to walls and ceilings to soak up some of the sound hitting those boundaries and reflect less sound back into the room, as shown previously in Figure 8.2. Absorption products are generally applied along parallel axes of a recording room in “equal but opposite” patterns.
Twenty percent coverage using a good absorption product can have a significant effect. It is not necessary to cover every square inch of the surfaces! Over-treatment will produce a room that is too dead and dry for most recording purposes, and the recorded sound will be dull, lifeless, and probably muddy and boomy.
Absorption products have an NRC rating – Noise Reduction Coefficient. This is a measure of how much sound they absorb. Values between 0 and 1 (or more) are typical. The higher the NRC, the better the product is at absorbing sound. NRC is not a simple percentage and an NRC of 1 does not indicate complete absorption – values higher than 1 are possible.
Fi g u r e 8 . 4 Some “room within room” shapes. Rooms A and B have no parallel inner walls, and only one set of perpendicular walls. With a little more engineering, pentagonal shapes can also be used, as shown in room C.
c
B
A
O uter Wall inner Wall}
To control lower frequencies, the absorption product must be thicker, and be appropriately placed away from a boundary – ideally a quarter wavelength away, which is a realistically impossible distance from a wall for lower frequencies! Inexpensive one inch thick foam, with an overall NRC of about 0.5, is only good at absorbing mid and high frequencies (1 KHz and above) – it has little effect on the low-mid and low frequencies. Two inch thick foam is a com-monly purchased broadband absorption product, and has an overall NRC of around 0.8 – but it is only effective above about 500 Hz. This means that a room treated exclusively with this product will still have low and low-mid frequency standing waves and a relative deficiency of high frequency liveness and brightness, causing the room to sound dull, muddy, and boomy.
The effective frequency range of absorption products can be significantly extended downwards by positioning them away from the wall (or ceiling) by a few inches using some kind of spacer. However, control of frequencies below 150 Hz is still difficult, because their wavelengths are so large relative to the 1 to 4 inch thick absorption products typically applied for broadband control. Bass traps are specific products designed to absorb the longer wave-lengths of low frequency energy. They are much bigger and thicker products that are usually placed in corners or along axes where multiple boundaries intersect – locations where bass frequencies can be most efficiently absorbed.
If you intend to use acoustical foam, you should check building codes in your area. Some foams do not meet fire code regulations for construction materials and cannot be used. Mineral fiber or mineral wool products offer more effective absorption and better fire retardancy than foam.
8.11 DIFFUSION
An alternative to absorption is to scatter reflections so that they don’t directly reflect directly back on themselves to create standing waves or flutter echoes. Diffusion, in the form of dif-fusors, randomly disperses reflections (rather than absorb them). Diffusion retains a room’s sense of liveness and open space, and does not change the frequency content of the sound in the room – avoiding the dulling and muddying effects that too much absorption has. Commercial diffusion products are readily available, ranging from inexpensive styrofoam to expensive wooden devices, or they can be home built.
A mixture of absorption and diffusion is typically applied to many medium and large rooms, to absorb excess reflections while randomly scattering a controlled amount back into the room. In some studio designs, some walls are treated exclusively with diffusion and others with absorption. In other designs a mixture of absorption and diffusion is applied to each
surface. An acoustical consultant and some relatively superficial room-within-room construc-tion are the best way to get great sounding recording rooms, and the materials for home-built custom treatment often cost less than buying commercial wall mounted treatments. But if an acoustician or simple construction are not realistic options, most reputable manufacturers of commercial acoustical treatment products offer consultation services to help the end user make informed purchasing and treatment decisions.
Fi g u r e 8 . 5 A home built diffusive wall randomly scatters reflections, opening up the sound of the room, and keeping it relatively bright.
Fi g u r e 8 .6 A generic treatment of a small- to medium-sized rectangular room. Standing waves and flutter echoes are reduced using a combination of absorption and diffusion products, and bass traps are used to tighten up the low frequency sound of the room.
(North Wall)
(West Wall)
1 b B rl
h L 1 u
top□
Bass Trap Absorption
bottom (East Wall) | | Broadband Absorption
D iffusion
(South Wall)
top (Ceiling)
D oor 1 bottom
feature semi-live large ensemble spaces, and others have a mixture of live reverberant drum rooms, semi-live, and dead rooms.
• Drums certainly benefit from being recorded in fairly live, medium to large rooms.
Without the reflections of those sized rooms the recorded drum sounds can be dead and lifeless.
• Acoustic guitars and most other acoustic instruments really benefit from being recorded in at least a slightly reflective room – the life is sucked out of them if the room is too dead. Conversely, a recording made in a room that is too reverberant will have too much potentially inappropriate room sound, and lack clarity and intelligibility.
• Pop vocals and spoken word are usually best recorded in very dry rooms to maximize intelligibility and provide a clean, intimate, up close perspective. The engineer then has the ability to add artificial acoustics and effects during mixing. It is appropriate to cover a high percentage of the surface area of a vocal booth with absorption.
• Rooms can be too live, even though they do not have parallel surfaces and standing wave problems. Minimal absorption can be applied in order to tame the excess live-ness. In medium to large rooms sometimes just one or two surfaces will be treated. In smaller rooms, partial but equal treatment of all surfaces usually produces better results.
• A room that is “too bright” generally has a frequency imbalance in its reflection char-acteristic. Thin boundaries reflect higher frequencies while letting lower frequencies pass through them and escape. Thinner absorptive treatments can be used to “darken” a room’s sound because they are only effective at absorbing higher frequencies.
• A room that is “too dark” or “too dull” does not have enough high frequency reflection – probably due to too much absorptive treatment in the room. Diffusion can be added to brighten up a dull sounding room without adding standing wave or flutter echo potential.