# Perspectives in Control for Multiple Frequency Regions: Part I



## ejbragg (Dec 13, 2009)

Hello, everyone,

In this thread, I intend to to relate some of the concepts described in Frequency Regions to a manageable layout of controlling these frequencies, using today's available building materials. Note that I have no plans to mention supplies marketed by other companies. I have visited different studios in Nashville, and have mused through numerous photographs in magazines looking for clues for studio construction and have noted that I rarely see special "audio foam" hanging on the walls, or fixed bass traps mounted in a special place in the wall. Why not? Usually, acoustic architects design these rooms and pass the specs off to engineers who have these items custom built.

Some info to consider:

*Acoustic Foam*
Acoustic foam might look sexy, but why do you seldom see it in the big studio glamor shots?

In fact, foam, whether open or closed cell, cannot _and will not_ likely do what you want or need by simply placing it on a particular place on a wall. The same is true about egg cartons. The problem is, there is a much deeper understanding required to truly treat a room, and although such techniques might get you by, this is a "shot in the dark" approach at best. Most treatment in commercial studios reflect simple materials bought at hardware stores - most of it in the form of 2 X 4s, insulation, cloth, and paneling (wood or other). It is many times more cost effective to build your own than to throw your money at expensive audio foams, and once you have an educated approach, can be extremely rewarding.

*Bass Traps and Diffusers*
This is another area where most people either simply ignore, or spend a lot of money purchasing generically designed items made by other firms which don't necessarily conform to a design conducive to your needs. The only way to know what you need is to first of all decide what your requirements are, measure the rooms' acoustic properties, and then apply these needs to a methodical, planned out approach. You can design your own treatment. You can then hire out or build the devices yourself. :T In the end, it's about how effective the room meets your needs, rather than having some expensive treatment units that you aren't even sure how they work.

*General*
It is likely we will be dancing on a line between construction principles and actual application in this forum. Once we start crossing the line into design of acoustic treatment, I will defer to the *DIY Projects* Forum.


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## ejbragg (Dec 13, 2009)

So what do we do about many different regions? And how do we apply them to a control room vs. a tracking room? Of course there are different tracking room types - piano rooms, drum rooms, etc. Each has different needs, and each should be built for the specific application in mind. Furthermore, there are different genres of music, which are tracked (recorded) differently than other genres. For example, jazz drums are traditionally recorded with very few microphones, the main mix from a distance from the drums, giving it a more organic (raw) color. Such a room might need to be somewhat reflective. Progressive rock is much more "in your face", which often equates to a close-miked, fairly dry recording environment.

Most people who know anything about audio will understand that different microphones need to be used for different applications. Microphone placement is an art, and especially for drums, due to the fact that there are often so many microphones in the same room. Carefully chosen and placed mics are perhaps the most important first step in professional recording. However, it most certainly doesn't end there. Standing waves, early reflections, comb filtering, distortion, and many other things can plague an otherwise excellent recording despite these precautions. It is my strong opinion that one of the things that sets professional, commercial studios apart from the rest is in the "magic" of the rooms themselves.

Before we begin, we need to know the different categories of room treatment and and separate them. In a nutshell, there are 3 major categories, under which _most_ other topics will likely fit. There will inevitably be areas that need their own category, but here are the major categories for commercial studios:


*Acoustic Architecture* (creating / converting rooms into shapes conducive to listening and working; including modal analysis and planning.
*Soundproof Construction* (Transmission Loss - materials for blocking or reflecting sound)
*Interior Acoustic Design*:
- Modal Treatment
- Reflective Treatment
- Diffusion Treatment

These basic categories are the building blocks of every pro studio. Certainly this list is not exhaustive, but it's a start.


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## ejbragg (Dec 13, 2009)

*Part I: Acoustic Architecture*

This is really the first step in designing your rooms. I will avoid the issues of overall floor plans for now - that is a very broad, very subjective topic. But we'll concentrate on the mindset and method for designing a room, and this applies to all rooms in your studio.

In the event that you are limited to an existing building, you can still use this method to understand the fundamentally existing weaknesses and modify each room as needed, BEFORE setting out to treat the room with acoustic material.

Much of the methods discussed in Part II (and Part III) can be computed automatically using some of today's readily available software applications. These programs are an excellent resource for designing your studio. However, it is strongly suggested that you understand the principles that are used in the software. It is my hope to shed some light on this subject for those who have never learned, or are brushing up on the subject. _(And by the way, you CAN design and build an excellent studio without such software, if you choose. On the other hand, once basic construction is complete, you will inevitably need to test the room before, and between surface treatments. It is very difficult to analyze your room for room modes, overall room reverberation, and transient response without proper software.)_

*Section 1: Room Modes*

First of all, what IS a room mode?

Have you ever noticed, while hanging out with your friends somewhere with the music cranked, that when you walk into a corner, the bass gets louder? Furthermore, have you ever noticed that some frequencies, in different parts of the room, seem to "sing out" louder than all the rest? These are due to room modes, and they are more apparent in untreated rooms (bare walls), if you can even notice this phenomenon over the natural reverberation in such a room. It happens when a bass guitarist hits a certain note and the room seems to "sing along" with the note, making it louder than the rest of the music. In fact, the room is doing exactly that - "singing along"!

At the room mode, there is a frequency that matches the dimension of the room, causing the frequency to _bounce_ back and forth (in layman's terms), between parallel, untreated walls, without a place to escape. The longer the note is held, the more energy is fed into the standing wave and its strength grows.

*Section 2: Modal Calculations*

As discussed in Part I, there exists the potential for standing waves in every room. Any two facing parallel walls have a potential for a modal wave that is equal to 1/2, 1, 1.5, etc. wavelength, if excited. For example, if two parallel walls are separated by 10 ft, the most fundamental room mode appears at 1/2 wavelength. This means that if _half_ the wavelength can resonate in the room, we have a resonance (or mode) at 10 * 2 = 20 ft. A wavelength this long is equal to

f0 = (1130 ft/sec) / 20 ft = 56.5 Hz,

where f0 -> fundamental room mode frequency,
1130 ft/sec -> the speed of sound in air, roughly

The second room mode between these walls is at wavelength = 1...

f1 = (1130 ft/sec) / 10 ft = 113 Hz.

Then we have more room modes predicted at:

f2 = (1130 ft/sec) / 6.67 ft = 170 Hz (10 ft / 1.5 wavelengths)
f3 = (1130 ft/sec) / 5.0 ft = 226 Hz (10 ft / 2.0 wavelengths)
f4 = (1130 ft/sec) / 4.0 ft = 283 Hz (10 ft / 2.5 wavelengths)
f5 = (1130 ft/sec) / 3.33 ft = 339 Hz (10 ft / 3.0 wavelengths)
f6 = (1130 ft/sec) / 2.86 ft = 395 Hz (10 ft / 3.5 wavelengths)

Once we rise above 400 Hz, we are fairly well out of the low frequency zone. No more room modes are needed for two reasons:


Middle and Upper frequencies (Regions III-V) are easily dealt with using acoustic treatment
Generally speaking, the more wavelengths it takes to create a room mode, the less powerful it's likely to be, due to the tendency of such waves to shift and bend out of shape.

A good rule of thumb in locating problems is that the closer to the fundamental you are, the stronger the problem.

Once you have listed these modes, the process needs to be repeated for any other parallel walls, and this includes the floor-to-ceiling surfaces, if they are parallel.

If your room is square, say 10 ft X 10 ft, you'll notice that the room modes along one direction will be identical to the modes in the other direction. This poses a serious problem. The room modes will have to be treated in all cases, but building a room with parallel walls of the same dimension is asking for trouble, because those modes will become much more apparent and your treatment must become more extreme. And the worst of these cases is a cubicle room, 10 X 10 X 10. It would be wise to avoid matching room dimensions, when possible.

_*Side Note about room modes*
The type of room modes described herein have been restricted to *axial* modes. There also exist *tangential* and *oblique* room modes. Because of the tendency for the latter type modes to be typically weaker, they have been left out of this discussion. However, for those who are interested in being thorough in design, these mode types should be considered as well. If there is enough interest, I'll post another sticky on the subject, and go into more depth on all 3 mode types._

*Section 3: Analysis of the Modes*

Once you have computed all the modes and listed them, look for frequencies that are closely related. If, for example, you have one dimension of 10 ft, and another at 15 ft, you will find in the calculations that mode 3 of the 10 ft dimension matches mode 2 in the 15 ft direction. This is almost as big a problem as the matching dimensions of the 10 X 10 room. You will find more such matches further up in frequency, for these dimensions. You will see that choosing your dimensions is not straightforward. If an existing room is to be renovated, and such a problem arrives in these computations, consider building one wall out a bit to offset these frequencies.

Furthermore, if your modes are just within 15% of an octave apart, there are potential problems. In fact, 20% is a safer distance, if you can manage it. It really helps to use logarithmic graph paper to plot your results, since frequencies are naturally logarithmic to our discerning ears. Be warned that this can be an iterative process, until you find peace of mind. It is most important that the fundamental modes stay (ideally) equally apart. And if possible, the 2nd and 3rd modes should be protected from each other, as well. If modes must tread in each others' turfs in the upper end of the spectrum, it may be unavoidable, but certainly preferable to allowing these troubles at the lower end of the spectrum.

*Section 4: Splayed Walls*

There is another practice that can be used, called splaying of your walls. This is where a wall is purposely set at a slight angle from its opposing partner. People used to claim this prevents standing waves; however, modern 3 dimensional computation software has proven otherwise. In the Region III-V frequencies, this practice does help, and it has been shown to also assist in attenuating comb filtering problems. However, at Region I & II frequencies, standing waves are still guaranteed to exist - their frequencies are merely more difficult to calculate.

Nevertheless, feel free to insert splayed walls for the positive reasons mentioned above. Furthermore, ceilings can be splayed or stepped from the floor for added treatment against modes, as well.

*Section 5: Specific Room Designs - Some General Notes*

Generally speaking, any room used for tracking or listening to music should have a decent volume, greater than 1500 cubic feet (42 cubic meters). Anything smaller than this is usually deemed inappropriate for a commercial space unless it is considered a "booth". Booths are usually treated heavily with acoustic treatment to prevent the problems of modes and reflections at a large band of frequencies.

For a control room, there are general shapes suggested by many different sources with some great ideas. The control room is for the commercial studio, the most sacred room, requiring the most careful planning. When building a house, people often say, "Pick the room you'll spend the most time in, and _spend the most time planning it_!" For the control room, nothing could be truer. Ergonomics, work flow, access to other rooms, listening environment, lighting, temperature control, noise control... all these things should be at the highest level in this one room, if you plan for yourself (or any hired hand) to spend hour after hour in focused concentration. The space must be comfortable and impervious to distractions.

Drum Rooms are a beast of their own. The first step in good design is in soundproofing anything that might make a loud noise in the building. Let's face it: drums are just plain loud. Stopping the problem at the source is a lot more efficient than trying to stop it everywhere else in the building. This room needs heavy, airtight walls, ceiling, & floors, and room modes will definitely be a problem caused by the kick and floor tom, if not handled on the front end. Furthermore, unless mounted on a concrete pad in the basement, consider building a vibration absorbing platform for the drums to keep those Region II frequencies from getting in the rest of the building.

Piano Rooms, and any room, in fact, that will be used for tracking acoustic instruments, should be well-planned. In this case, massive walls are also a great idea - for keeping sounds OUT of the room. Acoustic instruments are often recorded with more sensitive microphones, which pick up even the quietest noises. Furthermore, modes will be quite apparent in such rooms, because:


The sensitive microphones on a quiet instrument will give a truer picture of the sound in the room
Acoustic instruments produce their sound by resonating. Pianos, guitars, and anything with a decently sized resonant cavity will likely be affected by room modes, aggravating the dilemma.

Also, I see a growing number of "piano rooms" that are barely large enough to contain a piano. If you truly appreciate music, and if you can afford the space, please don't do this to the piano! A piano is large, and thus requires some decent space in order to sound good. They were designed to project sound outward. Keeping a piano in a tiny room will most certainly affect the recording quality of the instrument.

Break rooms, offices, and waiting rooms should just be placed together, outside the studio space. Phones ringing, people talking, coffee brewing, toilets flushing, .... all these things really need to be blocked behind a heavy, soundproof wall. Even the HVAC units should be kept out there; if not possible, seal it up good with a heavy door. (Sometimes you must rent a commercial building and retrofit.)

Booths for bass guitar may not need to be so large, but again, heavy walls are key, when using amps. And if you or your clients like to track electric guitars with tube amps, you might as well plan for some decently massive walls for those, as well. These rooms, along with vocal booths are traditionally smaller, and may not require a lot of space, especially if the rooms are treated to be acoustically dry. However, certain genres often necessitate flexibility in this area, and some instruments or vocals might ought to be tracked in live rooms.

If you can afford a decently sized live room, make it big, with a high ceiling. About 5000 cubic ft is arguably a minimum for a decent, live tracking room. Such a space will likely see a lot of use for many applications. In fact, you can stick your piano and other acoustic instruments in such a room.


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## stephendbarnes (Jan 29, 2012)

Thank you very much for this article. I know it was posted a couple of years ago, but the calculations you presented for room modes will assist me in treating my space.


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## rocksure (Mar 4, 2012)

Far out there is just so much information contained in these threads on acoustics it is going to take me a fair few visits and reads to try and get my head around much of it. But it's an excellent resource and I will certainly be coming back and reading/digesting what I can whenever I can. Thanks so much for posting all this information ejbragg


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## Capt. Ahab (Jul 7, 2012)

Thanks for posting. I'll need to take more time to digest this a bit more.


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