Room Acoustics

So, I'm going to pick up here where lesson 2 left off, and start explaining the basics.
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Sound is a wave, which can be measured as wavelength.

If you remember from lesson 9, which explains phase and comb filtering, if you duplicate a sound, put a delay of a few milliseconds on the duplicate copy, then blend it back in with the original, you get comb filtering. Well, that's what happens when you record into a microphone, and sound bounces off the wall. Except, often, the sound is bouncing off several walls, a floor, and a ceiling, so you have several layers of comb filtering. This causes muddiness in the sound quality, especially in the frequency range between 100hz - 400hz. I call this the mud range, because it's difficult to prevent recordings from getting muddy in this frequency range, and it's something I hear quite often. The low frequencies also have a lot more energy than high frequencies, which is part of the reason they are more problematic. They are much more difficult to absorb, and they have the greatest detriment to the sound quality. But understanding how they behave can help you know which shape and size of room will sound best.
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Although reflections are the enemy, it's important to understand that sound doesn't reflect the same way that light does, especially lower frequencies, and this is because of the drastically larger wavelength of sound compared to light. Sound, like anything, will take the path of least resistance, and if it can, it will simply go around an obstacle, rather than go through, or be reflected. To prevent the soundwave from going past an object, such as an absorption panel, the widest dimension of the object needs to be at least half the wavelength just to begin having an effect. So, a 75hz sound wave has a wavelength of 15 feet. Half that wavelength is 7.5 feet. Most acoustic panels are 4'x2', and therefore will not do anything to a 75hz wave. On a 4'x2' panel, the widest dimension is 4', so it won't do anything to a wavelength of less than 8', which is 140hz or lower. And even higher than 140 hz, it gradually becomes more effective as the frequency gets higher, 140 is just the starting point. So even with 150hz or 160 hz, that panel won't do much.
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But the same is true if there's a wall or a cabinet in a studio and then an opening behind it. That wall might not reflect the lower frequencies.
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For high frequencies, like 1000 hz and up, you can think about sound the same way as light. It will reflect in a similar way, but for frequencies of 100 hz and down, don't think about it as sound. Think about it as a fast pressurizing and depressurizing of the room. These rapid pressure fluctuations need to go, or else they build up. The best way is to have a large enough space for the waves to disperse. In order to disperse a sound wave, the largest dimension needs to be longer than the wavelength. So a 50 hz sound wave is 22 feet long, and therefore needs a room longer than 22 feet to even begin dispersing it.



For good sound quality, it's essential to remove or reduce the reflections.
The larger the room, the better it is, and less treatment is needed because the sound waves get dispersed more. Smaller rooms have a higher concentration of reflected waves, and therefore need more treatment.


Many pro studios have a large room for recording that is specifically designed to disperse the sound waves in a way that is not harmful to the recording, and even produce a pleasant reverberation. This is done by having a room that is fairly large, has lots of diffusion, non parallel walls, high ceiling or arched ceiling and angled walls. For everyone else who simply wants to convert a room in their house for recording, acoustic treatment is essential.

What does treatment do?
The primary goal of treatment is to make a small room "sound" like a big room. This is done by placing panels near the walls that absorb sound and do not reflect it. Panels can be made or purchased, and the good quality ones basically act like a black hole for sound. They are made of fibers that vibrate at the right frequencies and transfer the energy of the sound into a tiny amount of heat.

Can I skip the room treatment and just get a good E.Q. instead?
No! The effect of reflecting waves interacting with each other does more than just change the E.Q. curve. It creates complex comb filtering as well as it literally changes the shape of the waves. Both of these issues are impossible for an E.Q. to reverse, it would be like trying to use an E.Q. make a piano sound like a trumpet. This same principal applies to a monitoring environment. There are some studio monitors on the market that offer room analysis and correction. While it does work a little bit, it is mostly a gimmick, and the real solution is proper room treatment..

There are 2 areas of prime importance for acoustical treatment
1- First reflection points
2- Bass buildup in the corners

- First Reflection Points

As you play guitar into a microphone, the majority of the sound going into the mic travels in a direct path from the instrument into the mic. However, as the sound of the instrument is dispersed throughout the room, the microphone also picks up the reflections.
Sound travels about 1 foot per millisecond. So, if the guitar is 5 feet away from a wall, some sound will travel from the guitar, bounce off the wall, and reach the microphone about 10 milliseconds later. This doesn't seem like much, but it disturbs the wave pattern. Plus, if there's 4 walls, a floor and a ceiling, that's 6 points where the sound directly reflects off and comes back to the microphone. While there's not much you can do about the floor (do not use carpet), the other 5 "first reflection points" can be fixed by placing sound absorption panels at these locations.

- Bass buildup in the corners

In a studio, lower frequencies are more problematic than high frequencies. Since lower frequency sound waves tent to build up in the corners, the most effective way to alleviate them is by placing treatment in the corners, called Bass Traps. Bass traps should be optimized for absorption in the lower frequencies. I would recommend Owens corning 705 rigid fiberglass....more on this later.

Another important consideration in the placement of acoustical panels is to never place them directly against the reflective surface, such as wall or ceiling. Always leave at least a few inches of space. This way the panels will catch a wider section of the wavelength, and will absorb lower frequencies better. The less dense a material is, the more important this space is.


What materials to use

There is a wide variety of acoustic panels on the market. A lot of them don't actually work very well, or only absorb high frequencies, which will make the sound worse. It's important to absorb the sound either evenly across the frequency spectrum, or absorb more bass than treble. Remember, it's the lower frequencies that cause the acoustical problems, so absorption that's heavy on the treble will only take away the part that sounds good, and leave all the lower frequency that cause problems. This is why carpet and studio foam are bad, they absorb the desired high frequencies, and leave the unwanted low frequencies.
The challenge is that bass is more difficult to absorb. It requires thicker panels or specific material. The best acoustical material available is actually sold as thermal insulation. The very best is "Owens Corning 703 rigid fiberglass insulation", often called OC 703. You cannot go wrong with this stuff, however there may be better options for more specific purposes. This gets us into broad band and narrow band absorption, as well as density.

Broad band absorption will absorb all frequencies from high to low a moderate amount. Since high frequencies are more easily absorbed, broadband absorption panels generally absorb them more.

Narrow band absorption panels will absorb a specific frequency range very well. Bass traps are an example of this. Special panels can also be made to absorb one specific frequency.

Density affects how much absorption a material will have, and also which frequencies it absorbs best.
- Lower density materials - Absorb upper frequencies while the low frequencies can pass through if it is not thick enough. Lower density materials also tend to be more broadband.

High density materials- As a material becomes more dense, less thickness is needed. Since the high density material will also be more firm, it becomes slightly reflective in the higher frequencies. Owens Corning 705 rigid fiberglass insulation is an example of a high density material that excels at absorbing bass frequencies.

Pro's and cons of different materials and how to use them:

- Egg cartons- Don't ever use these! they do nothing and are ugly!

- Studio Foam- Generally inneffective. It is low density and too thin to absorb the important lower frequencies. However, it can be used effectively by being glued to the front a thicker more dense panel.

-Heavy moving blankets- Better than nothing, just not hung from the wall. Drape them on mic stands and place closer to the instrument. They are broadband, but too thin to be very effective.

- Open door or window- Remember, the goal is to get rid of the sound, and an open door or window allows sound to escape and not return. While this may cause other problems and may not be worthwhile, it is a consideration.

-Furniture - Couch, bed mattress, pillows... these provide broadband absorption and although they're low density, they're often thick enough to be effective. Furniture also provides diffusion, which scatters the sound waves, and is helpful.

-Normal house insulation, sometimes called "the pink stuff" or "the fluffy stuff". This type of insulation works, although is not very dense so it needs to be at least 2 feet thick to be effective. If you take 2 foot thickness and squish it down to 1 foot thickness...that counts as 2 foot thickness because it would be twice as dense.

-Acoustical house insulation- This type of insulation is designed to go in the walls of homes to reduce the amount of sound travelling from one room to another. It is designed to absorb sound, and it works well. It is a medium density and an effective broadband absorber if there is at least 5" of thickness. Roxul Safe'n'Sound , or rock wool is a good example of this type.

-Owens Corning 703 Rigid fiberglass insulation - This is a very effective material for sound absorption, and is probably the most common material used in high end sound absorption panels by the best manufacturers. It is considered a broadband, but the frequency range that it is optimal at absorbing is that of our hearing. It is quite dense, and 2" thickness is effective, although 4" thickness is ideal, with 4" airspace between the panel and the wall.

-Owens corning 705 Rigid fiberglas insulation - This is very dense, and is the best material for absorbing bass frequencies. It also absorbs midrange frequencies very well, and becomes slightly reflective towards the upper frequencies. Even 1" thickness is effective, although 2" is recommended. OC 705 is is ideal for bass traps in corners, but can also be used as wall panels, or as a cloud where the ceiling is low and less thickness is desired. Feel free to glue studio foam on the surface to alleviate some high frequency reflections.

Tip - Owens Corning rigid fiberglass insulation comes in batts. It is yellow coloured, and fairly firm. If you touch it, fibers will rub off onto your skin. It needs to be covered in fabric. Go to the hardware store and get some aerosol spray on glue (I use 3M super 77). Spray the glue onto the insulation, place the fabric on and push out the wrinkles before the glue dries.

This site only covers the basics, for more detailed explanations of room acoustics and treatment, visit: http://ethanwiner.com/acoustics.html
General Recommendations
For a small to medium sized rectangular room. This is appropriate for both mixing and recording.

-Place 2'x4' panels of 2" thick Owens Corning 705 at 45 degree angles in all the corners.
-Hang 2'x4' panels of 2" thick Owens Corning 705 from the ceiling. Leave about 4" airspace and cover 80-90% of the surface area of the ceiling where recording or mixing takes place.
-Place homemade panels of acoustical house insulation, as shown below, along the walls. Or,
-Place 2'x4' panels of 4" thick Owens Corning 703 at the first reflection points, 4" away from the wall.

Hard flooring throughout the room, a rug or area mat for comfort is acceptable.