The Moods of Modes
Let’s talk about room modes and why they are one of the biggest challenges to accurate sound reproduction within our rooms.
In Our Sound Fields
Low-frequency pressure is everywhere in our small rooms. It is all along the side walls. It is all through the area from the floor to the ceiling. It is between the front and rear walls traveling back and forth through our listening or monitoring positions. We have three sound fields within our rooms and unwanted low-frequency pressure is in all of them. What does it look like? What does it do to our sound? How can we avoid or if we can’t avoid, how do we treat it? Does it require special treatment types and amounts of treatments?
Low- & High-Pressure Areas
What does it look like? First, you need to picture in your mind a weather map. Let’s use the USA map for this discussion. On our map of the states, we have both high and low-pressure areas throughout the country. A low-pressure system has a lower pressure at its center than the areas around it. Winds blow into the center lower pressure area. A high-pressure system has higher pressure at its center than the areas around it. The wind blows away from high pressure. Winds of a high-pressure system swirl in the opposite direction as a low-pressure system – clockwise north of the equator and counterclockwise south of the equator.
We do not have winds in our rooms but we do have pressure variances as with the high and low pressure of the weather. Low-pressure areas on our frequency response curves in our rooms are the dips or troughs. High-frequency unwanted pressure areas are the peaks. In our rooms, we have both peaks and troughs throughout the three sound fields present in any small room. The peaks exaggerate frequencies and amplitudes. The troughs reduce amplitudes down to a level that you may not be able to hear certain frequencies that are drawn into the trough. In one case too much. In another, too little. Neither are welcome.
The Weather Map of Acoustics
Low-frequency pressure is in a constant change of flux within our small audio rooms. The pressure is dependent on frequency, amplitude, room size and volume. As amplitude increases within the dimensions of our rooms, certain frequencies and octave bands are exaggerated or suppressed. These changes in pressure can be affected with as little as a 1dB increase in signal gain. What does this mean to our sound quality? The answer is everything because we do not seek flux, we seek balance. We seek an equal representation of all frequencies regardless of amplitudes. We don’t want our room sounding good at certain pressure levels. We want our room sounding good at all pressure levels.
Room Sizes & Modal Structures
The graphs to the left show three room sizes and their resulting unwanted room pressure issues. We have the first one showing unwanted pressure levels in a room that measures 12′ w x 8′ h x 13′ l . The second graph shows a room size and volume of 14′ w x 8′ h x 17′ l. Graph number 3 shows us what happens in a room of 17′ w x 8′ h x 21′ l. The red lines are major modal pressures that can and do exceed + 10 dB over baseline. The purple or shorter lines represent secondary unwanted pressure issues but they must also be treated. Notice how close the secondary issues are in the 14 w room. When they group that close together, we have extra work to do because there is no separation and spacing between them and their close proximity to each other produce even more issues to work with. The more lines on your room size and volume graph, the more treatment requirements. The more treatment requirements the greater your costs. We want to choose room sizes and volumes that minimize these issues, so we can have less treatment requirements.
Can You Treat Your Peaks and Troughs With An EQ?
The level of a peak can be + 15 – + 20 dB SPL over flat. The level of a trough or lower pressure area can be – 15 dB to – 20 dB. I have seen peaks and troughs that can be 30 points over acceptable baseline numbers. Can we treat these peaks and troughs electronically? The answer is no. You can not EQ a + 15 dB peak or a – 15 dB dip with signal processing. This much electronic compensation will produce audible artifacts. Even with the best EQ, you will hear the “hole” in the mix. Electronic signal processing can be used but if and only if, we have done our job using correct room size and volume along with the proper type, amount, and positioning of treatment.
We have two types of treatment technologies to choose from. We have membrane and diaphragmatic absorption technologies. Membrane is welcome at + 5 – + 8 dB over baseline. In our example above, we must use diaphragmatic. We must place it in all boundary surfaces of our small rooms. This is a good start. Next, we should consider the floor and ceiling surface areas. Remember that low-frequency peaks and troughs are located throughout our rooms. No surface area is exempt. We must have the horsepower of diaphragmatic absorption distributed over all boundary surfaces and we must have a lot of it.