Home Theater Audio and Acoustics Part III ESD303 2018 CEDIA Expo San Diego Instructor Introduction Todd Welti Harman International Senior Principal Engineer Acoustics M.Sc. Acoustics, Cedia SMA Acoustics, Designer Level II Cert. todd.welti@harman.com 818-895-8124 FPV pilot and crasher of quadcopters Learning Objectives Learn how speakers and rooms interact. Learn how to differentiate room issues and speaker issues. Learn which measurements are relevant and which are not. Learn How to optimize bass response across the seating area. Learn general approach to equalization. Learn best approach to room treatment. Page 1
Administrative Details Please turn off cell phones. Please complete the course evaluation at the end of class. The code for this course is ESD303. Content property of Floyd Toole and Focal Press used by permission Course Overview This course was created by Floyd E. Toole, PhD. It contains illustrations and ideas from the book Sound Reproduction by Floyd Toole, Focal Press, 2008. soundnwine@sbcglobal.net Room acoustics demystified The loudspeakers and the room operate as a system. The effect of the listening room is most noticeable when listening to a single channel which we do a lot of the time: the center channel. The influence of the room diminishes as the number of active channels increases. Page 2
Room acoustics demystified Most of the sound we hear in a room is reflected. Therefore, the off-axis performance of loudspeakers is especially important. Acoustical materials and devices must perform uniformly at all frequencies above about 200-300 Hz. If not, they can degrade the sound quality of well-designed loudspeakers. Room acoustics demystified A well-furnished living space (carpet, sofa, chairs, drapes, bookcases and cabinets) can be an excellent listening environment. Custom home theaters need to be artificially furnished. Because it is a designed space, customers expect excellent performance. Room acoustics demystified Excess reverberation time (over 0.5 s) interferes with speech intelligibility, but one that is too low (below about 0.2 s) results in an oppressively dead room. Most normally furnished rooms are within this range. Some custom home theaters end up being too dead. Humans are well adapted to listening in reflective spaces and prefer a certain amount of natural reflected sound around them. Page 3
Room acoustics demystified Room equalization is a name that raises unrealistic expectations. There are a few things it can do and many things that it cannot do. It is important to know the differences. Common equalization practices may improve the sound from bad loudspeakers, but can degrade the sound from truly good ones. Caution is advised. More on this later. Room acoustics demystified All rooms have resonances that cause some bass notes to be too loud, others to disappear, and bass transients to boom. Most annoyingly, bass changes from seat to seat. There are no ideal room dimensions that eliminate these problems. However, some multiple subwoofer solutions deliver more uniform and better bass for more listeners. In the low-frequency range, equalization not only works, but may be necessary. More on this later. Delivering great sound at all frequencies In ESD301 we learned that sound in rooms behaved differently at low frequencies and at middle and high frequencies. Different solutions are required. Here we address each one in turn, explaining what needs to be done to deliver great sound to happy customers. Page 4
Two kinds of problems need solutions 30 20 db 10 0-10 Room Transition Loudspeaker 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Above the transition the speaker has considerable control. Below the transition the room will have its way no matter what the speaker does. Different forms of acoustics at work 30 20 db 10 0 Wave Acoustics Modal Region Transition Geometrical Acoustics Statistical Region -10 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Let s look at this region The rule of simple reflection: Angle of incidence = Angle of reflection Angle of incidence Angle of reflection Page 5
Reflection vs. scattering (diffusion) If the reflecting surface is not flat and smooth, sounds will be redirected differently depending on their wavelengths compared to the depth of the surface irregularities. depth Loudspeaker directivity All loudspeakers must deliver a strong, high quality direct sound to all listeners: o To provide clearly localized sound for steered special effects in movies o To provide clear localization and high quality sound for multichannel music o To provide the basis for convincing envelopment and immersion in both movies and music For aimed speakers This is a very demanding room layout: large audience. Well-designed forward-firing loudspeakers have sufficient coverage to do the job. The same or very similar loudspeakers can be used in all locations, for perfect timbre matching. This is good. Do this if the situation permits! Page 6
For on- or in-wall speakers In this example, the wide dispersion of bidirectional in-phase a.k.a. bipole loudspeakers is needed. Acoustical treatment of walls is now a more important factor because of unnecessary sound spillage. Note that these wide dispersion loudspeakers are used to achieve good audience coverage, not to create the mythical diffuse sound field. All horizontal center channels are not equal! Look for a midrange driver. Problems with surrounds The center seats are the best seats. Moving from the center of the room towards the sides does two things: o The sense of envelopment diminishes and then is gone o Listeners become progressively more aware of the nearest side surround loudspeaker as a source of sound. It is distracting. It exists in both cinemas and home theaters. It has nothing to do with a lack of diffusion! Page 7
Unbalanced sounds caused by propagation loss from left and right sides cause problems for off-center listeners in small rooms. Floor-to-ceiling line sources and CBT (Constant Beamwidth Transmission) loudspeakers have much less propagation loss. In the meantime Sitting in the center of rows has advantages. Try to design layouts with seats in these locations. Tiered seating will be necessary if every row is to have such a seat. Origin of the dipole surround Conceived in the early days of four-channel surround with a monophonic, 7 khz low-pass filtered, surround channel. The goal was to create more reflected sounds to improve the surround effect, but the reflections came from the front and back of the rooms the wrong directions. The situation is totally different now, and we have real solutions. Page 8
It s not really a dipole Not a true dipole: the frontand back-firing units are not coincident. To preserve bass, the bidirectional out-of-phase null exists only above ~500 Hz and it works for only a single row of listeners. Below ~500 Hz it is an ordinary loudspeaker. Sound quality is compromised. Multidirectional on/in-wall options More dipole variations On their own they don t sound good. Timbre matching to the front speakers is not possible. Why would we do this? Page 9
FACT! Surround loudspeakers should sound similar to the front loudspeakers timbre matching. This is best achieved if all horizontal-plane loudspeakers (L,C,R fronts, sides and rears) are identical, or are simple variations on the same design. The improvement is audible a seamless soundscape. This is the 7-channel cinema paradigm All loudspeakers are high quality forward firing direct radiators. All loudspeakers are in phase. Those in groups are connected in parallel A fact of life: all seats are not equally good. Choosing loudspeakers Speaker/room interaction is a critical factor How a speaker radiates its sound is a major determinant of how it sounds in a room. This information should be in the product specifications. Suitably interpreted, it should enable us to predict some of what happens in real rooms. Room acoustical treatment must integrate with the needs of loudspeakers, and vice versa. Now, a reminder of a topic introduced in ESD301 Page 10
Measuring Frequency Response: the Spinorama 70 frequency-response measurements on horizontal and vertical axes Measurements made at 6 ft (2 m) in an anechoic chamber Direct Sound Direct Sound Listening Window ±30º Horizontal ±10º Vertical Listening Window Page 11
Early Reflections Early Reflections Sound Power Sound Power DI: A Measure of Sound Dispersion 10 db Directivity Index 5 db Directivity Index 0 db Early Reflections Only Page 12
A Comprehensive Picture of Performance This is an extremely good loudspeaker 10 db 5 db 0 db DI Interpreting Spinorama data: The inverted DI is an estimate of sound power for the loudspeaker if it had a perfectly flat listening window (or on axis) response DI (using Listening Window as reference) (normalized spectrum) An idealized DI for a cinema speaker A lesson in the importance of understanding acoustical wavelength. Page 13
A big cinema/dubbing -stage system DI data from manufacturer s spec sheet Medium cinema systems, dubbing theaters, etc. DI data from manufacturer s spec sheet Large home theater systems, control rooms, dubbing stages, etc. DI data from manufacturer s spec sheet Page 14
Cone & dome systems for smaller production facilities, stereo systems and home theaters. DI data was available from manufacturer on request Loudpeaker/room interactions What is the nature of the sound field in a room? DIRECT SOUND DIRECT & EARLY-REFLECTED SOUND REFLECTED SOUND Obviously all sound components contribute something at all frequencies in all rooms. These patterns will change with different loudspeakers in different rooms. Movie display venues have a volume range of about 100:1. Can they be similar? Let s look at a small domestic room first. 273 cu ft 80 cu m Page 15
From ESD301: Predictions from anechoic loudspeaker measurements 20 10 db 0-10 PREDICTED ROOM CURVE DIRECT SOUND EARLY REFLECTIONS LATE REFLECTIONS 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Real vs. predicted room curves 30 20 db 10 Each speaker location yields a different curve at low frequencies Above about 300-400 Hz the curves are similar to each other and to the prediction 0-10 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Regions of dominance 30 20 db 10 0-10 Room Transition Loudspeaker 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Below the transition the room will have its way no matter what the speaker does. Above the transition the speaker has considerable control. From: Toole, JAES 1986 This is not new! Page 16
Transition frequencies for different rooms Small domestic room: 70 m 3, RT= 0.4s, f c = 150 Hz Room Transition Loudspeaker Smallest room ST 202: 125 m 3, RT=0.2s f c= 80 Hz Loudspeaker 350 seat cinema: 1900 m 3, RT=0.5s f c = 32 Hz Loudspeaker 800 seat cinema: 4200 m 3, RT=0.65s f c = 24 Hz Loudspeaker 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Measurements in a 4414 ft 3 (125 m 3) room Anechoic spinorama 6-seat average room curve The inverted DI is an estimate of sound power for the loudspeaker if it had a perfectly flat listening window (or on-axis) response Sound power underestimates high frequencies. The direct sound is dominant. Measurements in a 4414 ft 3 (125 m 3) room Anechoic spinorama This loudspeaker, is flawed, and it receives mediocre scores in double-blind listening tests. 6-seat average room curve Page 17
A speaker with two problems: Frequency Response Directivity Non-flat frequency response, which EQ can fix, and irregular directivity, which EQ cannot fix. What have we learned? From low through to middle frequencies, sound power is a good predictor of room curves. If the on-axis frequency response of a loudspeaker is flattish an estimate of a steady-state room curve can be obtained from the inverted DI of the loudspeaker. A change in room volume over a ratio of 150:1 made no consequential difference in this relationship, except in small rooms, where room modes/standing waves corrupt the bass. Page 18
The good and bad about equalization The increasingly popular automatic equalizers, especially those that attempt to correct room curves at middle and high frequencies, can make mistakes. Lacking any data on the speakers, they assume that all problems can be solved with EQ. While this may result in improved sound from truly awful speakers, it may also result in degraded sound from good speakers. However, equalization at low frequencies, below about 200 Hz, is not only safer, but in most installations, necessary! When considering the audible effects of direct and reflected sounds what we measure with these simple systems in a normally reflective room is not what we hear! So... Anechoic measurements on loudspeakers allow us to anticipate how they might measure in a room. AND As we learned in ESD301, they allow us to anticipate how they might sound in that room Page 19
No More Guessing From anechoic data, we have predicted in-room subjective ratings. Correlation coefficient 0.86 No Excuses for Flawed Speakers Like this: $8,000/PR Like this: $470/PR Possibilities $1,800/PR There is superb performance in a plain vinyl-wrap box. $200/PR Add a subwoofer for a very pleasant surprise. Page 20
The intelligent choice of loudspeakers...... requires measurement data. Except for a few professional monitors it basically is not advertised. Many (most?) manufacturers do not, or can not, make measurements of this kind. But some can and do. So ask for it!! A sad statement about a mature industry There is more useful and reliable information on the side of a tire than in most loudspeaker specifications! The future is here. It s just not widely distributed yet. William Gibson But this new standard incorporating the spinorama method should help... Page 21
After selecting loudspeakers They need to be installed in the correct locations, some of which present mounting challenges. Nearby room boundaries and cabinetry impact sound quality, choose a mounting option that does not compromise speaker performance Here are some tips on avoiding mistakes Example: a good 6-Inch, 2-Way Speaker Free standing Used as intended, 3 ft. or more from walls and floor, it measures well and sounds good Top curve: on-axis Middle curve: total sound power Bottom curve: directivity index Installed Flush in a Wall The bass is boosted sounds tubby. Low frequencies need to be attenuated with a bass tone control or an equalizer. This done, it sounds superb. Page 22
A Beautiful Wall Unit, but the Sound? Floor-standing speakers in the cavities beside the screen are often not equalized. Pity. Result: lots of bass! Too much of a good thing? Designed for In-Wall/Ceiling Installation On-Wall Installation Too much low bass, and some upper bass coloration Stand-off brackets move notch down in frequency Excessive bass needs attenuation This done, it sounds good. Page 23
On-Wall Designs should need no EQ Bookshelf speakers in bookshelves What is wrong with this? Nothing if the bookshelf is filled with books. If it is empty, you have just put the speaker in a resonant cavity. Speakers in Cavities Still too much bass Upper-bass and midrange resonant colorations Page 24
Damping Cavities Filling cavities will improve things, but it is still flawed Closing the Cavity Is Best Fill the vacant space with books or close it with a wood baffle. Too much bass, the only remaining problem, is easily cured. Summary (a) (b) (c) Winners Speakers sound best in simple environments: o Away from room boundaries o In a room boundary o On a room boundary o In each case, either designed for the location or bass equalized (d) (e) Page 25
Subwoofers In the subwoofer frequency range, wavelengths are long. Any location in, on or close to a room boundary works equally well. Now, let s focus on the low frequencies 30 20 db10 0 Wave Acoustics Modal Region Transition Geometrical Acoustics Statistical Region When wavelengths are comparable with room dimensions we are in a region where very different rules apply. -10 20 50 100 500 1K 5K 10K 20K FREQUENCY (Hz) Let s look at this region Bass in really important! We care about: o Bass extension how low it goes o Bass balance proportion of bass compared to mids and highs o Bass quality are all notes appropriately loud o Bass quality are bass transients tight, not boomy. And, we want all listeners in the room to have similar, and similarly good bass. Page 26
What about equalization? It is possible to attenuate the most objectionable room resonances with high resolution measurements and a parametric equalizer matched to the Q of the resonances. PROBLEM: it only works at one location in the room. All other seats will sound different, and probably worse. We need to force the sound to be similar in several seating locations at the same time reduce the seat-toseat variations. Low-frequency absorbers/bass traps These are always a good idea if you have the locations, budget, and a compatible interior décor. They reduce the energy in room resonances and attenuate the accompanying standing waves. However, there are other, newer, ways to address the problems In rectangular rooms use multiple subwoofers in arrays or In any room, use multiple subwoofers with signal processing. How is this possible? Mode manipulation: using location, or multiple subwoofers to determine the amplitude of specific room resonances. A natural first-order - + + axial mode + + Cancellation with two subwoofers + Drive the null: A pressure source at a velocity maximum / pressure minimum. Page 27
And it works for multiple modes + The first three axial modes + + + + These two woofers attenuate odd-order modes and amplify the even-order mode In these locations they attenuate the first three modes This leads to some new approaches: In rectangular rooms, in the subwoofer frequency range below 80 Hz, the number of modes is small enough that optimum arrangements of subwoofers and listeners are possible. Therefore, the process of optimizing bass reproduction in rooms must begin with: o the number of subwoofers, o the subwoofer locations, o the locations of the listeners. NOTE: In the following rectangular-room solutions involving multiple subwoofers it is assumed that the subwoofers are: Identical to each other Supplied with the same signal Operated in the same polarity (in phase) Adjusted to the same output (gain setting). Page 28
The number of subwoofers and where they are placed determines which room modes are energized and by how much. Minimizing seat-to-seat variations: PRO: High efficiency CON: Huge seat-to-seat variations PRO: High efficiency CON: Huge seat-to-seat variations Minimizing seat-to-seat variations: PRO: Small seat-to-seat variations, moderate efficiency PRO: Small seat-to-seat variations CON: Very low efficiency Page 29
Minimizing seat-to-seat variations: PRO: Very small seat-to-seat variations CON: Very low efficiency Minimizing seat-to-seat variations: Three CEDIA recommended four-subwoofer arrangements with similar sound output capability. PRO: moderate efficiency Small seat-to-seat variations Three recommended woofer layouts Any of these arrangements of two or four woofers result in simple patterns of standing waves. This makes it possible to arrange seating or to adjust room dimensions to achieve good bass for multiple listeners. Page 30
These are the null locations If the room is rectangular If the four walls are very similar in construction The object of the exercise is to avoid having listeners heads on, or within about 18 inches (0.5m) of these lines. Vary the picture size For a given seating arrangement, minimize the number of poor seats by adjusting the viewing distance Vary the room shape and size Large rooms can sound superb, but they use a lot of expensive real estate Page 31
But avoid this, if possible In a narrow room listeners on the ends of the rows are too close to the side surround loudspeakers. These are the rooms that force difficult decisions... and compromises. Change the seats and/or spacing Find a seat/recliner design and size that creates a desirable seating arrangement What if... The room is not rectangular? The room is rectangular but: o it has asymmetrical acoustics: different walls have different construction masonry, drywall, plaster, large windows? o all of the optimum subwoofer locations are not available? o seats are not in optimum locations? Page 32
Sound Field Management (SFM) Multiple subwoofers Multiple measurements A computer optimization routine Electronic processing in the signal path to each subwoofer: o gain, o delay, o one parametric filter. A typical example: front left subwoofer only Four subwoofers with Sound Field Management No aggressive EQ is needed, and all listeners get to hear very similar and very good bass. Page 33
A Direct Comparison No strong resonances, no audible ringing = tight bass. All of this is achieved without low-frequency absorbers! My own difficult entertainment room Note the acoustically absorptive Bichon Frise with toy positioned to absorb the floor bounce A family room entertainment system 100% 100% Total power: 200% As it was. Large (up to 28 db!) seat-to-seat variations using the original front pair of subwoofers. Nobody heard good bass. Page 34
4 subs + SFM optimization = better bass for all! Power output 100% 25% 6.3% 6.3% Total power: 137.6% Less total power is used, yet sound levels have gone up by almost 10 db. More, but smaller, subwoofers can do the job! Getting good bass in rooms Equalization is necessary in almost every case. It guarantees good bass for one listener Good bass is possible for multiple listeners but multiple subwoofers are needed. o In rectangular rooms it is straightforward o In non-rectangular or difficult rooms it is also possible, but more complicated and expensive. Quick Review: A better subwoofer guarantees better sounding bass. True or false? False, because the quality of bass sound is dominated by the room. Never put a subwoofer in a corner. True or false? False. For a single subwoofer, the corner is a good starting point for finding out what might sound good, although it may not turn out to be the best location. With multiple subwoofers, corners are the required locations for one of the preferred arrangements. Page 35
Quick Review: In-room equalization always results in better sound. True or false? False. At low frequencies, equalization almost always can improve things. However, at frequencies above about 300 Hz, there is a possibility that sound quality can be made worse. Reflections in rooms A certain amount of reflected sound in a room is a good thing. Not enough reflected sound makes a room sound dead and oppressive. Not even pleasant for conversation. Too much reflected sound causes a room to sound empty, and interferes with speech intelligibility. Well-furnished small rooms tend to be about right, but custom rooms will need treatment. Reverberation time: RT60 RT 60 is one of the most important measurements in large concert halls and auditoriums. Values of 1.5 to 3 seconds are common. In small listening rooms mid frequency values in the range from 0.2 to 0.5 seconds are acceptable. o Above 0.5 s speech intelligibility is impaired o Below 0.2 s the room sounds uncomfortably dead Page 36
Confirming that the room is suitable Subjectively: with one person talking at the location of the center loudspeaker, have another person move from seat to seat in the room. If easy conversation is possible at normal voice levels, the reverberation in the room is not a problem. By measurement: ensure that the mid-frequency (around 500 Hz) reverberation time is between about 0.2 and 0.5 seconds. Use the center channel loudspeaker. Predicting RT60 A simple approximate method: o Cover the floor wall-to-wall with clipped-pile carpet on felt underlay o Cover about 25% of the wall area with absorbers. If possible, absorbers should be at least 3 inches (75 mm) thick and should be well distributed around the room. o This should get you into the good range of RT60. Predicting RT60 A better approximation, allowing for different materials. This uses the Sabine equation: RT=0.049 V/A (metric version: RT=0.161 V/A) where V = room volume in cu.ft. and A is the total absorption in the room in Sabins. A= (S 1 α 1 + S 2 α 2 + S 3 α 3 +... where S = area in sq.ft. and α is the absorption coefficient for the material covering that area. Page 37
Example: confirming the simple approximation 24 x 20 x 9 ft room. V=4320 cu.ft. 480 sq.ft. carpet with absorption coefficient of 0.6 at 500 Hz. S 1 α 1 = 480 x 0.6 = 288 Sabins Wall area = 792 sq.ft. Absorber area = S 2 =.25 x 792=198 sq.ft. Using 3-4 inch fiberglass, α 2 = 1.0 which gives S 2 α 2 = 198 Sabins A= (S 1 α 1 + S 2 α 2 ) = 288 + 198 = 486 Sabins RT = 0.049 V/A = 0.049 x 4320 / 486 = 0.44 s Why a center channel is a good idea A phantom image is created by identical sounds radiated by both speakers (mono) This results in two sounds at each ear instead of one Why a center channel is a good idea Page 38
Phantom images have problems! This is the frequency response error in stereo center images! Phantom images have problems: The dip in the frequency response is audible as a coloration in all sounds. (Note: side wall and other room reflections help to fill the dip a good thing.) Speech intelligibility is significantly reduced. The location of the image is correct only for listeners seated on the center line of the room. Lesson: use a center channel Some specific reflections are useful If the loudspeakers are well behaved off axis, sidewall reflections can add breadth to the soundstage, and add richness to timbre. Some of the antipathy towards these reflections may have to do with the typically poor off-axis performance of loudspeakers over the years. Recording engineers claim to prefer them to be attenuated while they work. However, recent research indicates that they adapt to reflections. Page 39
Side-wall bounces The greatest effect is on a solo loudspeaker, such as the workhorse center channel. As more channels become operational, the effect of the room diminishes. Note: adjacent wall reflections from LF & RF are radiated at almost 90º off axis, i.e. much reduced output at mid/high frequencies. Direct and reflected lateral sound The ears receive direct and first reflected sounds from beneficial lateral directions, and from less good, frontback directions. The precedence effect will keep any discretely panned sounds correctly localized to the speaker. Lateral scattering adds desirable complexity Page 40
Flutter / slap echoes / zings As the room treatment is developing, it is necessary to test for unwanted flutter echoes between parallel reflecting surfaces. Adding or moving patches of absorbers or diffusers can easily eliminate problems. Because it is primarily a high-frequency effect, acoustical treatment need not be elaborate. Flutter echo test what is it? Stand somewhere, clap hands and listen. Find a place where you hear the characteristic zing of flutter. This does not mean that there is a problem with the theater, but it is a useful way to demonstrate the effect. This is a special case because the sound source and your ears are at the same location. Flutter echo test the real thing This is the real test of how the theater performs. Have someone clap at each loudspeaker in turn, while you listen at each seat. There should be no audible flutter. These are the sound source and listening locations that matter most. Testing at other clap locations is a prudent safety measure. Page 41
Room EQ is a misnomer We can only change the frequency response of the signals supplied to loudspeakers in the room. Reflections cannot be added or removed Reverberation time cannot be changed Seat-to-seat variations in bass cannot be reduced We cannot change the frequency-dependent directivity (DI) of loudspeakers, or the frequency-dependent absorption of acoustical materials and furniture. We must not try to EQ ripples caused by non-minimum-phase phenomena they are not audible problems and the EQ degrades the loudspeakers. A real world attempt to improve a flawed loudspeaker with equalization...... including double-blind subjective evaluations A flawed loudspeaker RESONANCES The first order problems are with resonances. The directivity is reasonably well behaved. Page 42
Subjective Rating SPL (db) Different approaches to fixing the problem: 1. Assume that the problem is with the loudspeaker and EQ the anechoic on-axis response to be flattish and smooth 2. Assume that the problem is with the sound delivered to the listening area and EQ the steady-state in-room curve to be flattish and smooth Average room curves 80 No EQ Flattish anechoic on axis Flattish in room 75 70 EQ RANGE Listener preferences: listening at 10 ft (3 m) on axis Flattish Flattish on axis in room No EQ Now, let us look at what these solutions did to our loudspeaker: Page 43
Spinorama: No EQ, the original speaker RESONANCES Spinorama: Flattish on axis RESONANCES Spinorama: Flattish in room RESONANCES Page 44
Flattish on axis Flattish in room This off-axis resonance in the loudspeaker was added by an automatic algorithm which was trying to equalize a non-minimum-phase crossover dip equalization cannot No EQ correct this kind of problem! Sometimes it is necessary to have human intervention including an understanding of how loudspeakers work. Top to bottom in order of listener preference. Steady-state in-room measurements did not reveal medium-q resonances for proper correction. It is still a loudspeaker with some flaws and listeners could hear it. What have we learned from this example? Both forms of EQ improved the perceived sound quality. Improving the loudspeaker itself seemed to have been the better solution. Steady-state in-room measurements and EQ were not able to detect and correct some of the resonances in the flawed loudspeaker. If so, a well designed loudspeaker should be able to deliver good sound to a listening position in a room, at least above the transition frequency. The risk: in-room EQ might make it worse. This is a superb loudspeaker Would you expose it to Room EQ above the transition frequency? If so, why? Page 45
Subjective Rating From home theaters to screening rooms and cinemas a truly good loudspeaker shows its excellence. NO ROOM EQ!!! These cinemas seat: 24, 60, 114, 161, 211 & 516 people ROOM LOUDSPEAKER Always remember... So don t expect a perfect looking room curve to sound perfect. And there is more: The number of playback channels determines how fussy we are about sound quality. Flattish Flattish on axis in room Mono Stereo Surround No EQ The lesson: Impress people in stereo or multichannel. Listen in mono to determine if you have good loudspeakers or not. Page 46
Summary: Use 7.1 channels, or more. 5.1 only if necessary. Conventional cone/dome/horn systems can perform all roles: L, C, R, and surround in most rooms. Some room layouts require very wide dispersion from the surround loudspeakers use bipoles. The prime listening location is the best place to be put as many listeners as possible on the center line. Summary: equalization At frequencies below about 300 Hz EQ is a necessity in most rooms. Reduce peaks. Do not attempt to fill narrow dips. In most rooms, a single subwoofer, however good, can be equalized to provide excellent sound only at a single seat. With multiple subwoofers, correctly located in rectangular rooms, or electronically optimized in any room, similarly good bass can be delivered to several listeners. Summary: equalization At frequencies above about 300 Hz, it is a risky activity if it is based exclusively on in-room measurements. If you are confident that the front loudspeakers are good, be very cautious about equalization above about 500 Hz. It may do more harm than good. Page 47
Summary: side-wall reflections... First reflections from the front speakers: For stereo music lateral reflections add a pleasant spatial effect, but only if the loudspeakers are well-behaved off axis. With lesser loudspeakers they can be distracting. For movies and television, the added image broadening might assist in softening the monotonously dominant center channel. However, at most, it is a small effect so it can be a decision you and your customer can make. Summary: side-wall reflections... Opposite wall reflections from the side surrounds: Lateral, side-to-side, reflections can be an aid to envelopment and immersion small reflective areas are good. Locate them asymmetrically to avoid flutter. The judicious addition of engineered or geometric diffusing elements may be a further enhancement, but remember that shallow units only scatter high frequencies. Summary: absorbers Absorbers should be at least 3 to 4 inches thick. Thinner materials simply attenuate high frequencies, degrading sound quality. Use the lowest density and the greatest depth of fibrous material that is practical. Cover it with an acoustically transparent fabric easy to blow through. The ends of the room, front and back, are excellent locations for concentrations of absorbing material, with several small patches on the side walls. Page 48
Summary: diffusers Diffusers are useful to attenuate reflections without losing the energy; it is redirected. Engineered diffusing surfaces should be at least 8 inches thick in order to scatter all frequencies above the transition frequency (about 300 Hz). For the same reason, geometric scattering/diffusing devices (hemi-cylinders, etc.) should be at least 12 inches deep. This is not practical in all rooms so do what you can. Conclusion Learned how speakers and rooms interact. Learned how to differentiate room issues and speaker issues Learned which measurements are relevant and which are not Learned How to optimize bass response across the seating area Learned general approach to equalization Learned best approach to room treatment Questions? Thank You! Please complete and submit course evaluations. Page 49