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Equalization In Room Acoustics

MikeSorensen June 19, 2012 1 Comment
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EQ Not First Tool

Equalization in small room acoustics is never a first line of defense and even then should be used sparingly. After a room is analyzed individuals will see bumps and dips in the frequency response curve of the room. They will then use the equalizer to smooth out or reduce the amplitude of the peaks and troughs in the response curve. Unfortunately, this approach does not take into consideration the physical issues of the room that is causing the peaks and troughs in our frequency response. Equalization is not going to be able to compensate electronically for physical abnormalities in the room. Equalization can also make some of the issues worse.

If a room has a 10 db dip at 300 cycles because there is an axial mode issue at that frequency because of the physical size of the room, then adding more energy with an equalizer at that frequency, will only add to the problem. If you keep pouring energy into a hole in the frequency response curve all you get is a deeper hole. One can apply the same rational to bumps in our room response curve. Bumps in our room’s frequency response occurs because frequencies bounce up and down upon themselves in phase as a direct result of the physical dimensions of our rooms.

Fine Tuning Only

Equalization should be used as a fine tuning method after the physically correctable issues in the room have been dealt with. Lets start with the low frequency issues. Without proper low frequency control and management of our low frequency resonances in our room, no amount of EQ will help. Room dimensions must be chosen that reduce and space out the resonances in any given room width, height, and length. There must be budget enough to allow for a room that is acoustically adjusted by choosing the correct ratio of room height,width, and depth that can minimize low frequency resonances. Starting structural and dimensionally correct, will save us time and money in the long run and also allow us to take those dollars and spend them on sound isolation technologies.

Room Modes

Room modal issues can occur between two of our room’s parallel surfaces. These are termed axial modes and are the strongest of the three room modes that are present in small room enclosures. Other resonances can occur between four surfaces and these are termed tangential. Oblique modes are between six surfaces. All three of these room modes can create bumps and dips in the frequency response of our room by restricting certain frequencies based on the dimensions of the room.

low Frequency Boundary Reflections

Other areas of the room can cause different issues. Room boundary reflections can cause abnormalities in our frequency response. The area behind the speakers and in front of the front wall can cause peaks and troughs in the room’s response. This is an area of high pressure when low frequency drivers are involved. All room boundary intersections are places where excess energy can build up and cause peaks and dips in our frequency response.
These areas must be dealt with by using physical absorbers that are tuned at the proper rate and level to absorb excess sound pressure energy. No amount of EQ will help us with this.

Tuned Sound Absorbers

Tuned physical absorbers must cover two main frequency groupings that represent real low frequencies and real middle low frequencies. These two absorbing class distinctions require different solutions. Lets define low frequency as any frequency below 100 Hz. This area must be dealt with using absorption technologies that have the necessary rate and level of absorption to make a sonic impact on low frequency energy. Low middle frequencies can be handled with today’s acoustical foam technology. EQ absolutely no assistance with any low or mid frequency issues.

Rate and Level

Manufactures of acoustical products will say that they have a low frequency absorber and they may have one that can achieve some absorption above 35 Hz. The next and most important question is at what rate and level do they absorb frequencies above 35 cycles. One has to absorb at certain rate and level minimums in order to have a sonic impact on the low frequency issues in our room and do it without filling the room with absorbers which only has an impact on small amounts of levels versus amount of absorbers needed.

Reflections/Reverb

Our primary, secondary, and tertiary side wall reflections can interfere with our listening or monitoring position and can create a different response at the listening or monitoring position. Too many reflections at the listening or monitoring position will confuse the direct signal from our speakers. If our monitoring or listening chair is in a room mode some frequencies will be exaggerated producing peaks in our room response curve and some will be attenuated which will produce a dip in room frequency response and not be heard at all.

If we take all of these reflections with our room and follow each ones decay and attack signature we would get an idea of the reverberation time in the room. Is it neutral enough, so that we can use the room for its desired purpose. Now, EQ can assist us with an electronic manipulation of room reverberation, but having a room that is good from the start is a big plus.

Comb Filtering

Comb filtering can result from reflections off of nearby objects such as a table or chair or the proximity to a side wall. These particular reflections muddle and confuse our mid range vocals and instruments. They are easy to spot on an analyzer screen because there sonic signature looks like a comb. Comb filtering adds a delayed signal of its own which interferes with the original signature. This has constructive and destructive effects upon the original signal. EQ can assist us a bit here, but not really its intended use.

Using equalization in our rooms has both good and bad values. It is good if it is used sparingly and in small amounts. It is not proper when it is used to attempt to correct for peaks and valleys in the room response. All room acoustic issues from low frequency resonances, middle and high frequency control and room boundary surface treatment should be done first before any EQ is considered. If considered use only +/- 2db.

MikeSorensen

I am a structural engineer as well as a master furniture maker. I design cabinets for low frequency, activated carbon absorbers. Connect with me on Google+

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