Room Acoustics 101

By Leon Sievers
Sound Professional
July 31, 2023

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Nothing can improve the quality of your church sound system as much as determining the proper placement for your loudspeaker system. 

Upgrading your speakers can make changes in the range of 1 to 5 decibels. But simply changing your speaker position can make differences in excess of 15dB in response! Who would buy a loudspeaker product with a +/-15dB swing in response? Regardless of what speaker system you currently use, a basic understanding of the room & loudspeaker interaction and the applied fundamentals can help you make a substantial improvement in your systems performance.

There are no magic formulas for ensuring great sound in every room. We can however, identify and gain an understanding of basic acoustical principles. Some basic equipment including a 20' measuring tape, a test CD with a variety of test tones, an inexpensive analog sound pressure level meter and a calculator can reveal more information than you may want to know. 

Like a speaker, every room has its own frequency response. To make things more complex, the response varies with the listener's location and the room's dimensions construction, and furnishings. Room dimensions determine standing wave frequencies. In general, rooms with dimensions that are divisible by a common factor, like 10' x 20' x 30' tend to compound standing waves at one frequency. 

Room dimensions with non-equal or divisible dimensions are best. Vaulted ceilings, non-parallel walls and irregular surfaces help reduce slap echoes, but have little effect on low frequency standing waves. Room construction affects bass reinforcement, the noise floor, and adjacent room noise. The average drywall wall resonates around 70Hz. Doors rattle, windows sing, air vents whoosh. Just grab your test CD or tone generator, play a sweep tone and listen. The difference in sound you will hear during the sweep is almost entirely due to room coloration. 

Let's begin by discussing how the length of a wave relates to its frequency. This understanding will allow you to take a methodical approach to understanding room response problems. Sound nominally travels at about 1130' per second. The human ear can typically detect frequencies from 20 vibrations per second (Hertz)to roughly 20,000 vibrations per second. We can calculate the wavelength ("l") of any frequency by simply dividing 1130 ("v" or velocity) by the frequency ("f"), using the formula l = v/f. 

Frequency (f) Hz.  20 50  100  150  200  250  500  750  1,000 5,000  10,000  15,000  20,000 Wavelength (l) Ft./In. Rounded 56' 5"  22' 6"  11' 3"  7' 5"  5' 7"  4' 5" 2' 3"  1' 5"  1' 1"  2"  1"  .08"  .06" 

From the previous table we can see, for example, that trying to dampen a 100Hz bass wave that is 11'3" long with a pillow 12" x 12" x 1.5" stuck into a corner is futile. 

We can also use the formula to determine the fundamental standing wave frequency for a given room dimension by dividing the round trip of that dimension by 1130 (v). That is, if our room is 20' long, the round trip distance is 40'. Divide 1130 (v) by 40'(f) for a fundamental standing frequency of 28.3Hz (l). Let's go one step further with our formula and newfound wavelength knowledge and see how we can apply it to understanding the problems of room acoustics.

Early reflections are signals that have bounced off the walls, ceiling and floors and arrive at our listening position later in time, mixing with the direct signal. They are called early or "first" reflections because listening tests have shown that when multiple reflections are received within 20 milliseconds of the direct sound, they are perceived as part of the original. This alters the tonal balance and confuses vocals and dialog. 

Sound panels are available from various acoustical material suppliers and can be obtained in a variety of fabric and finish options to blend with or complement most interior schemes. When the budget can't afford them, attractive homemade sound panels can be constructed easily using compressed fiberglass (Owens-Corning #703) covered with fabric. For improved low frequency effectiveness, use 2" thick panels and stand them off the wall a bit, or use thicker material. 

Slap echoes are reflections that bounce back and forth between bare parallel walls. They can be easily identified by clapping your hands and listening for ringing. You'll find, as you clap and move from the middle of the room towards one end, that the slap echo pitch and ring duration will change. This relates to the different round trip distances the sounds travel as they leave your hands, head off in different directions, bounce off the front and back walls, and pass you by. 

Some common methods of treating slap echoes are the "live end, dead end" scheme and the "dead end, live end" scheme. Both methods involve treating one end of the listening room, leaving the other end "live" for a natural room ambience. 

It is preferable to diffuse slap echoes with diffuser panels. Slap echoes may also be absorbed with carpets, fiberglass panels or drapes. When treating sidewall slap echoes near the sides of the loudspeakers and/or listeners, it is desirable to treat both walls evenly, left and right, to provide a balanced sound field. When treating reflections and echoes, best results are obtained from a proper mix of direct and diffused sound. That is, a balance of diffusive and absorptive materials strategically placed in throughout the room. The key in trying and applying all types of room treatments is to utilize test equipment which is designed to measure the time, energy and frequency relationshipwith the room. Or you can just listen as you go, use the proper treatment for the identified condition, and experiment, experiment, experiment! 

Standing waves are high and low pressure energy buildups, which are determined by frequency and room dimension. They are so named because they do not travel or propagate. Instead, they become anchored at various spots in a room determined by boundary conditions. Although standing waves occur at all audible frequencies in a contained space, our focus will be on the widely spaced, low frequency waves. These low frequency standing waves cause severe peaks and dips in the system's in-room bass response, creating the dreaded "one-note bass" while obscuring truly deep bass. All rooms except those very large rooms or halls (whose wavelengths are so low in frequency that they can be ignored) have low frequency standing waves of consequence. 

You can graphically show which standing wave frequencies will affect a given room by plotting some simple calculations on graph paper. First, calculate the frequency of the lowest fundamental peak (f) for each room dimension (length, width and height, represented by "d"), by dividing 1130 ("v", or the speed of sound) by twice the dimension in feet, using the formula f = v/2d. The dimension ("d") is doubled to account for the sound wave's round trip. Plot your results on separate lines for the room's length, width and height, one above the other, on your graph paper. Scale the horizontal axis from 20Hz to 150Hz in 10Hz increments. Now plot the additional peaks caused by the even multiples of the fundamental frequency up to 150Hz. For example, if your fundamental standing wave frequency is 50Hz, you will also have buildups at 100Hz and 150Hz. Beyond 150Hz, the spacing of standing waves becomes close enough to ignore. On your graph, any points close to each other will indicate an excessive buildup of energy at that frequency. Ideally, standing waves should be evenly spaced, which will provide a flatter in-room response from most locations. A spreadsheet file on a computer could easily be reated to make these calculations and plot your graphs automatically. 

After you've plotted the graph, play tones to verify your standing wave calculations. Use a sweep tone generator or a test CD recording of a sweep tone, along with your SPL meter or real time analyzer. As you tone sweep from 20Hz to 150Hz, you will notice that the volume from your listening position increases and decreases at different frequencies. Pause the sweep tone at a frequency where the volume is at it’s loudest from your listening position, then slowly walk around the room. Notice how the volume of the tone changes dramatically as you move around, louder in some areas, barely audible in others. These high and low pressure zones in different positions around the room are standing wave fundamentals and their multiples. These "room modes" are dependent on the room's dimensions in relation to the frequency of the fundamental and its multiples. Note the standing wave energy distribution. 

The sweep tone test is a great way to prove your predictions and observe very real proof of the standing wave phenomenon. Remember to use caution when playing test tones through speakers, as you can easily drivers during this test. If you're using a tone generator, use only sine waves for testing, not square waves. 

Due to the length of low frequency standing waves, they cannot be minimized by applying foam, insulation or carpet to the walls. Typically, fixed bass traps are not practical either, being massive and complicated to build, and portable traps are only minimally effective unless used in large numbers. Now that we know how room dimensions and the fundamental frequencies and their multiples interact, we can position the speakers and listeners to minimize the effects of standing waves. Using our graphs, we can determine the most desirable speaker, subwoofer and/or listener locations, based on the locations of the energy buildups standing
waves) from the fundamentals and their multiples corresponding to each room dimension.

Speakers placed in a corner will excite the greatest number of room modes (standing waves), while speakers placed in the locations indicated will excite the least number of room modes. Listeners will experience the smoothest response when the speakers and listening location are placed away from room modes. 

Give additional consideration to same frequency cancellation ("suck-outs") by ensuring that the speaker's distance from the sidewall is not the same as its distance from the front or rear wall. Keep in mind that it is rarely possible to set up speakers in the ideal position for all three-room dimensions. Therefore, try to position the speakers in at least one of the recommended placement locations by room dimension mentioned above. 

Another weapon in the standing wave knowledge arsenal is the conservative use of equalizers to help flatten system response. However, equalizers are not a cure-all, just another potential tool when used properly. Always exercise caution, as equalizing a particular frequency from one listening position can cause a severe dip or boost in another, severely altering the overall frequency balance. The potential also exists to blow up amplifiers and woofers by trying to apply excessive bass boost to compensate for a low frequency suck-out or cancellation point. Before applying any equalization, first invest time in determining the best physical speaker placement. A qualified “sound consultant” will have the necessary test equipment to take level measurements from several listening positions and average your results by frequency before making adjustments. 

Fortunately, the wide popularity of subwoofers can be a blessing when dealing with standing waves. Separating the bass driver from the mid and high frequency drivers allows placement flexibility. Also, the ability to separately adjust woofer phase can help tame excessive standing waves to a dull roar. 

Standing waves can be the most problematic audio dilemma to solve. A combination of all the above mentioned methods may be necessary to achieve the flattest possible frequency response in a room. With some time invested, you can immensely improve the sound and the entire listening / worship experience.

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