Rule 4: Room acoustics
Apply room acoustic treatments where required
The typical listening room is a hostile listening environment, and it is no respecter of high-end audio components
Getting your room to translate properly
It is pertinent to note that the listening room is part of your listening system and a significant variable in the quality of music reproduction. The room is said to 'color' at least 50% of the music. This is because early boundary reflections interfere with the direct sound that travels from the loudspeakers to the listener. This causes phase cancellation and a phenomenon called comb-filtering. Studies have shown that we only receive 12% of direct sound and the rest are reflections that affect the tonality and richness of the music. Comb-filtering can greatly affect the frequency response of the direct and indirect sound, certain frequencies will be boosted and other frequencies may be attenuated at the listening position. Standing waves is another negative phenomenon that is formed inside the listening room. This is due to the acoustic pressure propagated by the loudspeaker woofers pumping out energy creating distortions at the lower frequencies. Standing waves will influence mostly tonal balance in the bass region which in turn muddies the lower midrange frequencies. Therefore, we need to control both the early boundary reflections and mitigate standing waves where they happen.
Early boundary reflections
Middle and high-frequency energy above 250Hz have wavelengths that are much shorter in length and height than would fit most rooms. They do not produce room modes and are responsible for voice intelligibility. However, millions of reflective sound waves are bounced back and forth off surrounding surfaces in the horizontal and lateral domain; first against the loudspeaker baffle, then off the floor, sidewalls and ceiling, the front wall, and finally the back wall. These reflections contribute to higher reverberation times (RT) that would influence the loudspeaker's ‘voice’.
First and second-order reflections
First-order reflections, as they are so-called, occur when the sound waves from the loudspeakers reflect off the nearest object. The reflection will then bounce off other objects in its path and be reflected yet again. These second and third-order reflections would continue if the surfaces and energy of the sound waves enable them to. Second and third-order reflections are not as detrimental to sound reproduction as first-order reflections. Sounds reflected from walls take more time to reach, so it arrives delayed about 10 to 20 milliseconds later, which is desirable because these later reflections do provide a sense of spaciousness.
A reflection at 5ms will affect all frequencies below 1.5Khz, but above 5ms the ear has already processed the signal and the reflection does not interfere with the direct sound. Depending on the frequency and the distances involved, the delay may cause the positive peaks of the direct sound to arrive in your ears at or near the same time as the negative peaks of the reflected sound. This results in a partial or total cancellation of sound for that particular frequency. Moreover, other frequencies may arrive and add, thereby causing peaks in the response and affecting the desired flat frequency response. It is said that the critical range would be from 700Hz to 7Khz. The direct sound at this range should be relatively flat however, some roll-off at extreme highs is inevitable due to air absorption. Below 700Hz is not as important. Therefore, we need to achieve at least a 10ms delay for a “reflection-free zone” at the listening area, otherwise, early reflections will negatively affect imaging and transient response.
Reverberation (sound decay) is audible in larger rooms because of its longer distances which increases the reverberation time (after a 30ms delay) and interferes with the listening process. The situation is compounded if you are seated further away from the loudspeakers, the more of the ‘room’ you will hear rather than the loudspeakers. This is also evident if you were to increase the normal listening levels (above 85db) which will excite room modes and boost the peaks. Therefore, a shorter reverberation time of around 0.5 to 1 second is ideal for home listening. Heavy home furnishings and strategic placements of absorption and diffusion devices will help you get there. The acoustic industry standard of acceptability is RT60 when testing rooms i.e., the time (seconds) it takes for an initial impulse recorded during a test signal in a given room to decay to 60db.
Standing Waves
Standing waves is a phenomenon that occurs when low-frequency sound waves are continuously reflected back and forth between boundary surfaces where the wave pattern is stationary and exhibits itself as high and low acoustic pressure zones. The standing-wave patterns in a room are determined by the room's geometry (height, width, and length) and the relative position of the loudspeakers in the room. The frequencies at which standing waves occur are determined by the distance between opposing surfaces, and these are called the room modes. That means at certain locations in the room you have a pattern of dips and peaks at the lower frequencies.
Low-frequency describes energy below 250Hz whose wavelengths consist of height and width. A wavelength of 200Hz is 1.7 meters, 50Hz is 7 meters and 20Hz is 17 meters. Therefore, lower frequencies produced larger waves (longer and taller) that may not fit in your listening room and would therefore bounce right back causing standing waves to materialize. Low frequency below 250Hz will wrap around the loudspeakers and spread out evenly in an omnidirectional pattern exciting the air between the adjacent walls and producing room modes or unwanted excess pressure areas between wall surfaces. This sustained energy of high and low pressure in the room will either provide sound reinforcement (bass boom) or conversely, null the low end (bass cancelation) at certain frequencies which are not desirable.
Listening-room geometry - There is no perfect room
The listening room structure i.e., materials, size, dimensions of height, width, length, and shape would be the biggest influence on music reproduction. You may reckon the listening room to be the final element in music reproduction. A preferred room geometry would be rectangular, having concrete instead of wood surface structures. They should not have low ceilings and no windows, or room cavities that would break symmetry contributing to acoustic distortion. The strategic advantage of an ideal dimensional and structured room will better integrate with the loudspeakers and eventually help mitigate room modes.
Perhaps the best ratio for a practical listening room if you can afford to build from scratch or find such dimensions could be a ratio of 1:2:3. In a small-size room example, it would have a ceiling height of 10 feet, a width of 20 feet, and a length of 30 feet. However, the relatively low ceiling of 10 feet may not be ideal for taller loudspeakers. Therefore, a medium-sized room would have a ceiling height of 12 feet, a width of 24 feet, and a length of 36 feet. Tower loudspeakers (loudspeakers taller than 6 feet) may not work in a medium-sized room but may work well in a large-size room with a ceiling height of at least 16 feet, a width of 32 feet, and a length of 48 feet. All measurements are considered only from hard room boundary surfaces. Another room configuration that works is the Golden Ratio in which a ratio between two numbers equals approximately 1.618. which is also strongly associated with the Fibonacci principle where each number is the sum of the preceding two numbers: 0, 1, 2, 3, 5, 8, 13, 21… e.g., if the room height is 13 feet, the width should be 21 feet and the length is 34 feet. The Audio Engineering Society (AES) standard listening room may be another example but you would have to be an AES member to get the dimensions. AES is a professional body for engineers, scientists, and other individuals with an interest or involvement in the professional audio industry.
Room treatment strategies
The objective of room treatment is to reduce the reverberation time to below 1 second, absorb and or scatter first-order reflections and absorb standing waves generated by loudspeakers that were influenced by the room geometry. The following mitigation strategies would help flatten the curve:
Preventive strategies
Room geometry - Designate the best possible room in your home as the listening room whose geometry would have dimensions in progressive ratios. A square room or a room size that is 3,000 cubic feet or less should be avoided, in this regard bigger is better.
Loudspeaker placement - The primary method to mitigate standing waves is to place the loudspeakers and listening position in the best possible location where it has the least room interference to achieve quality bass in terms of definition. Loudspeakers with a good off-axis response may help alleviate negative early wall reflections, whereas a poor off-axis response loudspeaker in this instance will do more harm than good.
Use multiple subwoofers – This strategy was first studied by Dr. Floyd E. Toole from Harman International. The hypothesis suggests subwoofers placed in strategic areas because of their interaction will lower the variation of the response around the room by averaging over the room modes. The Harman room mode calculator may be downloaded on their website to explore this prospect. Otherwise, place one sub in the front corner and the other at the back of the room on the diagonal. Try experimenting with another along one side of the room. All subwoofers should however be the same make and model and gain matched with the loudspeakers.
Natural strategies
Adopt a near-field listening position or utilize a smaller listening room that would mitigate early boundary reflections. However, smaller rooms will have bass problems, especially with full-range loudspeakers.
Decreasing the normal listening volume (below 80db) will mitigate standing waves from forming, however, this would also dull the listening experience.
Install glass sliding doors or windows or open doors or windows to allow the bass energy to exit easily without worrying about standing waves. However, they will cause noise pollution to your neighbors and would also create isolation problems by introducing noise from outside coming into the listening room and interfering with the listening experience. You would also lose sound pressure levels that are not contained but would leak.
False ceilings can act as bass traps that would help absorb some low frequencies.
Furnishings - We say listening rooms are 'live' (longer reverberation times) if they are partially furnished i.e., they have no carpets, drapes, cushioned furniture, or bookshelves to damp or diffuse reflections. 'Dead’ rooms on the other hand are preferred as they are well furnished and treated which makes voice intelligible and music sound better. More damping is always better, at the expense of lower SPL levels. Some audiophiles prefer a hybrid setting (live-end, dead-end) where the live-end is at the listening space and a dead-end at the loudspeakers’ area. This is said to retain an ‘ambient field’ much like a recording space. However, the listener’s area should be a 'reflection-free zone' to maintain image integrity.
Technological strategies
The use of a high-quality 1/3 graphic equalizer is often recommended by pro-audio professionals and is used in recording studios. Pink noise is generated in the home at the normal SPL listening levels. The sound is picked up by a calibrated measurement microphone placed at the listening seat at ear level. The sound is fed into a computer program or a standalone spectrum analyzer that captures and plots a Real-Time Analysis (RTA) chart i.e., frequency response (20hz to 20khz) reading of the room. The result is analyzed to identify the problem frequencies (dips and peaks above 6db) where a one-third octave equalizer is incorporated between the preamp and the power amp to compensate for a flatter frequency curve. Nevertheless, most sound engineers would address only the glaring peaks and dips to smoothen out the frequency response curve. This is the last option only after moving the loudspeakers and listening position for the best possible curve. This method is usually not recommended for audiophiles as it would require another component to be added to the audio chain which violates the audiophile principles of “less is more”. Moreover, it is argued that the amplifiers would have to correct that frequency boost and dips as set by the equalizer causing phase and amplifier current delivery anomalies. This may be a case where the cure may be worse than the disease.
Another area those audio engineers would address is Reverberation Time. They would pop a balloon or clap their hands to measure the impulse and the gradual decay in the room. Another graph is analyzed to find the RT60 tolerance standard which is the time (in seconds) it takes for the impulse to decay to 60db (hence RT60). They would normally take 3 measurements at 3 different locations to get a sense of the decay rate. If the rate of decay is too long, they will then use more room treatments to dampen and bring decay to a tolerable level normally at a standard of 0.3 seconds.
Today you will find numerous software programs, receivers, and active loudspeakers with built-in digital room equalizers they claim can ‘fix’ the room modes which are driven mostly by the home theatre industry. Aftermarket DSP modules are also available. The manual EQ method mentioned earlier is perhaps a better option because there is a greater degree of control whereas you don’t know how the processors handle the dips (area of concern) and peaks (not so much of a worry) in the room especially in the area of attenuation. In some cases, extreme dips up to 9db or more give rise to amplifier problems such as headroom which leads to amplifier clipping and woofer overhang at certain frequencies. The room correction DSP may address some frequency-related problems but they are not capable of solving all of them. They are further limited by their ability to address time-related problems with reverb and resonance within the room.
Intrusive strategies
Although mitigation through strategic loudspeaker placement is the primary method (is said to provide a positive contribution factor of up to 70%). However, to achieve the best results, acoustical room treatment is a secondary method that is said to provide a positive contribution factor of up to 30%. Room treatments if used strategically will help smoothen the existing frequency curve and influence how music is being perceived i.e., improve the tonal balance. However, if the primary method of loudspeaker placement is not properly applied, the secondary method even if applied judiciously will not reap the stated benefits.
Application of acoustic treatments
Achieving unadulterated sound reproduction in a given room requires devices to absorb standing waves and to diffuse or scatter early wall reflections traveling toward the sweet spot. Aftermarket room treatment devices are available in various sizes, shapes, materials, and textures. The characteristics of all materials when exposed to acoustic pressure waves will either simultaneously absorb, reflect and or allow sound pressure to pass through. There are 3 categories of treatment options; absorption, diffusion, or both designed to work in unison.
Absorption techniques utilizing specialized “Bass Traps”
The most common acoustical problems are peaks and nulls, comb filtering, echo, and ringing at bass frequencies. All of these could be mitigated by using bass traps. All frequencies propagated below 100Hz are considered low-frequency energy. This range is perhaps the most problematic to cure. If this energy is pumped into the room (i.e., volume beyond the normal listening SPL) more low-end absorption is required. Similarly, smaller rooms that use large loudspeakers with deep bass will suffer the same fate. The correct application of any sound-absorbing device is to first figure out what frequency you need to absorb and how much of that frequency should you absorb. Using the rates and levels of absorption that matched their usage is a more efficient way to choose the appropriate treatment to deal with the problem. Using a Real-Time Analyser (RTA) to measure the frequency response at the sweet spot would reveal this.
The following acoustic bass traps incorporate several techniques that are designed to absorb low-frequency energy to attain a smoother low-frequency response by reducing room modes.
Helmholtz Resonator
The Helmholtz resonator may be customized to absorb low-frequency energy up to 80Hz. It uses a tube that has a certain length based on the design frequency they are trying to absorb. A slot is designed on the tube top whose width and dimensions work with the body to allow air to go in and “vibrate” absorbing the narrow frequency range it had been designed for.
Diaphragmatic Absorber
The diaphragmatic absorber is a pressure-based technology that covers a range of frequencies designed by Acoustics Fields. It is a sealed box similar to a loudspeaker cabinet in design and construction that may weigh over 100 lbs. The front wall or diaphragm has a calculated mass that “slowly enters inside the cabinet”, yes it moves, when low-frequency energy strikes it. The cabinet is filled with a proprietary material that creates a low-pressure area for the unwanted pressure wave to collapse within. The depth and density of the cabinet may be customized to absorb frequencies up to 150Hz.
Membrane
A membrane absorber is a pressure-based technology that uses a membrane with a certain density attached to a cabinet (body) with a certain depth and density. The membrane density, cabinet depth, and mass will determine the low-frequency rate at which the unit will absorb at a particular frequency. A membrane absorber designed by RPG will absorb frequency ranges of 40, 63, and 80Hz that require less surface coverage area with fewer units.
Bass panels
Bass panels are commonly used as low-frequency absorbers and more recently have been commercialized for home theatre use. They are available in custom shapes, sizes, floor standers, ceiling hung, and wall mounts that may be available with a wide selection of fabric-colored covers. They are friction devices that turn sound energy into heat through friction from the layers of proprietary fillers inside their panels. The pressure generated by the low-frequency wave is slowed by the cabinet structure. The wave energy then enters inside the cabinet and is absorbed based on the cabinet depth and the type of cabinet filler material. They absorb mostly mid and high frequencies with less absorption at the lower frequencies. How low the absorption extends depends on the thickness of the panel. For example, a 1-inch-thick panel will absorb 1Khz, 2-inch will absorb 500Hz, a 4-inch 250Hz, and 16-inch 63Hz. They are usually filled with thick industrial fiberglass, foam, or rock wool. When placed away (gap) from the wall a 4-inch panel will absorb even lower up to 150Hz.
Diffusion techniques
Diffusion scatters sound waves in different directions rather than absorbing them. This avoids the problems caused by nearby surfaces without reducing the energy in the room as absorption does. Where absorption is efficient down at a given low-frequency roll-off, diffusion is more effective at midrange frequencies that seek to minimize the impact of early or larger reflections thereby reducing our brain’s ability to localize reflected sound which distorts imaging. If diffusers/absorbers are used appropriately, the relevant room boundaries will appear (sound) much larger because they delay early reflections which is the primary objective. However, all outstanding issues must first be managed before diffusers can be used to their full effect, such as managing low-frequency energy propagating within the rooms. Not all diffusion techniques are equal and the selection of middle and high-frequency management should take into consideration the correct rate and levels of absorption/diffusion, especially for music and voice. Conducting an impulse test for the Reverberation Time (RT60) of the room at the sweet spot would help in this effort.
Open-celled foam
Most middle and high-frequency issues can be dealt with using open-celled foams to absorb and scatter reflections. The type (stiff foam preferred) and design shapes used (pyramid or mix preferred), thickness (at least 3-inch), and several foams (covering at least 20% of the surface area) and their positions are key to successful dispersion. The foams may be positioned in an alternate vertical and horizontal array to create two dimensions of diffusion. The open-celled foam is lightweight and available in many colors to be mounted on any wall surface. There may be a variety of cotton and synthetic materials that can be grouped to achieve certain rates and levels of absorption for middle and high frequencies. These lightweight foams may be used to treat side-wall and ceiling reflections, with absorption to improve imaging.
Quadratic Diffuser
A quadratic diffuser uses a series of wells or troughs that have a variety of depths. The depths are said to be designed to diffuse frequencies from 100 Hz to 4 kHz or even higher. Energy enters the wells which are then directed back and forth between the width and depth. This process is then redistributed in a 180-degree hemispherical array back into the room. The array of energy redistribution is dependent on the positioning of the diffuser within the room. You will need to choose the frequency response that will match their purpose. The depth of its well determines how low the diffusion extends and the width determines its highest effective frequency.
Multi-dimensional diffusers (3D)
The choice between the use of multi-dimensional diffusion is dependent on the distances and orientation of the walls to the listening position. This may be an effective way to scatter reflections making the surface boundaries seem to ‘disappear’. Application on the front wall directly facing the listener and the rear wall directly behind the listening position is the preferred areas of coverage.
Planar types loudspeakers
Electrostatic and ribbon designs (dipole) generate a ‘figure of 8 radiation pattern’ due to the driver’s front and back waveforms. The back wave is not absorbed in a cabinet and as such will bounce off the front wall early and back to the loudspeakers and the listening position. This may cause reinforcement or cancellation at certain frequencies with time smearing. Absorption and diffusion techniques such as bass traps or quadratic diffusers should be placed on the wall directly behind these loudspeakers to arrest these problems. Toe-in is strongly recommended to mitigate the issues further by manipulating the deflection angles.
A dozen recommended tips and tweaks
Addressing the low frequencies first should be the priority if time and budget are an issue because it’s foundational to good sound. Uncontrolled pressure in the lower frequencies will negatively color the middle and high frequencies. Bass control opens up the midrange and facilitates voice intelligibility which is why there is a market for mini-monitors and bookshelf loudspeakers.
Use suitable bass traps in the area behind the loudspeakers where the energy is the greatest and at the room corners where low energy tends to consolidate would be the likely places to experiment with. If unresolved, attack the sidewalls in the middle of the room but don't over-treat i.e., no more than 15% of the total room boundary area, otherwise, you lose much of the bass energy required for a dynamic presentation.
Bass in small rooms is generally lacking, therefore a bass boost from around 200Hz and below should be welcome in a small room situation.
If you must use DSP correction devices, apply them in a very narrow bandwidth to smoothen out the bass region below 500hz and none for the top end.
Treat the sidewalls against early reflections with absorption that incorporates diffusion characteristics. The idea is to delay reflections for more than 10ms to maintain the integrity of the middle frequencies. Simply instruct a friend or family member to stand with a mirror at the listening level and move along the sidewall. Seated at the listening position, you will look at the mirror and if you can see the loudspeaker edge, mark the spot on the wall and mark the other as the reflection ends. From the first marker to the second marker would be the area that requires treatment. Do the same exercise for the opposite sidewall. The treatment height would be around a meter across your head position which is a narrow band. I believe total diffusion should not be more than 20% of the room surface being treated otherwise you may lose a sense of spaciousness.
Diffuse the front wall with a 2D diffuser and the rear wall behind the listening seat with a 3D diffuser.
Use absorption/diffusion at the ceiling between the loudspeakers and the listening seat to scatter and absorbed early reflections. This would reduce the strength of the indirect sound resulting in less severe cancellations. The floor area in between the loudspeakers and the listening seat would be best served with a thick carpet or throw rug. We only want to diffuse frequencies between 500hz to around 2khz. These treatment methods will improve the feeling of spaciousness much like the live-end dead-end concept discussed earlier.
Use bass panels (2’ x 4’ x 1’) filled with Rockwool or similar type material placed directly behind the loudspeakers and 1 foot away from the front wall to smoothen out the bass response. A rule of thumb suggests allowing for a foot of distance for every inch of diffuser thickness away from the wall.
Ensure that the loudspeakers are at least 3 feet away from the nearest boundary surfaces to mitigate early wall reflections.
Seating against the back wall is not recommended as reflections will bounce off the wall behind you and severely affect imaging unless you have serious abortion coupled with 3D diffusion directly behind you.
The loudspeaker height should not be more than 50% of the ceiling height (hard surface area).
A ‘dead’ listening room is not an ideal solution.
Conclusion
The series of reflections or standing waves if not minimized or controlled will rob the loudspeaker of its generic voice. These room modes will either add to the sound, subtract or do both at given frequencies. Therefore a dedicated listening space must be created because we cannot overcome the room but must make the room work for you. We do this by using room acoustic treatments based on the frequency and amplitude of energy we are trying to manage. When dealing with sound, we must treat the excess energy with strategies that work for those frequency ranges. We must first identify the problem frequency range (Spectrum analyzer and impulse test) and quantify how much of it we have to manage so that our treatments may be effective and efficient and thereby cost-effective. This is achieved by deploying strategically adequate quantities of purpose-design devices with proven materials for the frequencies desired. This will make the difference and help your room to translate music properly with a wider soundstage, better images, clear dynamic contrast, and a natural tonal balance. No amount of audio components will solve the problem even state-of-the-art components, the acoustic environment can still radically impact your loudspeaker's voice. This is why we say the room is an essential component and is considered another link in the audio chain that must be dialed in.