Some Conclusions . . .

Sound reproduction in domestic size living spaces has been a life-long interest to me. I have learned much from observation and from my own experimentation. Mostly they confirmed what others already had written about, such as in the many publications of the AES, or the classic texts of Olson and Beranek. But in sound reproduction there are still areas that have not been studied to the point where clear prescriptions or limits can be given as, for example, to sound distribution in acoustically small spaces, or to audibility thresholds for non-linear distortion. I have always been fascinated by the multi-disciplinary approach that has to be taken to the design of loudspeakers, if the goal is higher than another typical consumer product. That approach must include old and new understandings in the fields of mechanics, electronics, acoustics, and psycho-acoustics, as well as extensive experience with test and measurement.

So here are some conclusions that I have come to. I will write them in the form of statements. The background for many of them can be found on the pages of this website. Other conclusions were drawn from observations at CES, dealer show rooms and private listening venues. Many of them are obvious, but I state them since their sonic consequences seem to be underestimated. There are probably more and I might add them as I think of them.

Loudspeakers

• The best one can hope for with 2-channel sound reproduction is the illusion of listening into the recording venue. Physics does not allow the accurate reproduction of the original sound field with only two speakers.
  
  
• Since sound reproduction is about creating an illusion it becomes very important to avoid or minimize any clues that would detract from the illusion. Such clues come from linear-distortions, such as frequency and polar response, and from non-linear distortions with their generation of tones and sounds that were not in the original.
  
  
• Linear distortion - frequency response, polar response, resonance - affects primarily the timbre and clarity of a loudspeaker.
  
  
• Non-linear distortion - intermodulation, harmonic, clipping - affects primarily the maximum tolerable sound pressure level.
  
  
• There is a level of non-linear distortion that is "good enough" relative to other flaws in the loudspeaker. Further reduction of this distortion brings no audible improvement. 
  
  
• For accuracy it is necessary to reproduce sound at near realistic SPL so that the ear generates the correct timbre due to its own distortion. Loudness control or response shaping gives a poor approximation to the Fletcher-Munson curves. 
  
  
• I have heard at the AES Convention in 2000 a 6-channel (2 front, 2 elevated side/front, 2 side speakers) Ambisonic microphone recorded (by Chesky) surround demonstration which totally excited me. My listening experiences since then with SACD and DVD-A commercial recordings have left me cold and I happily return to two channels with the ORION. 
  
  
• There are many different loudspeaker designs available commercially. They all change electrical signals into acoustic signals. But if the goal is to reproduce sounds accurately, then a speaker must be either an acoustic point source (monopole, omni-directional) or an acoustically small bi-directional source (dipole). "Small" means that the physical dimensions are small compared to the wavelength being radiated or that the shapes do not interfere with the polar response of the point source.  
  
  
• I have not come to conclusions about a line source that extends floor-to-ceiling, is infinitely long acoustically, and thus generates a cylindrical wave. It seems that this could be an alternate approach to illuminating a room uniformly at all frequencies.
  
  
• All accurate speakers will essentially sound the same when listened to in a setup that is appropriate to their specific design.
  
  
• Since loudspeakers are listened to in closed spaces there are fundamentally only two ways in which they should illuminate the room sound-wise: Either omni-directionally or uniformly directional over the whole range from low to high frequencies. This allows the delayed, reflected sounds from the room boundaries to have the same spectral signature as the direct sound.
  
  
• The transition in polar radiation from 4p to 2p (baffle step) that is typical for the majority of loudspeakers guarantees non-uniform illumination of the room.
  
  
• Reflected sounds are perceptually masked if their initial delay is >6 ms and if the reflections are full spectrum copies of the direct sound. This requires omni-directional or dipole loudspeakers that are free standing in the room.
  
  
• Omni and dipole loudspeakers can sound nearly identical in any given room when properly set up. The room must approach frequency independent reflection-diffusion-absorption behavior above 100 Hz.
  
  
• Conventional box speakers are always omni-directional at low frequencies and increasingly forward directional at high frequencies and thus the room reflections color the sound.
  
  
• When designing a loudspeaker it is essential to perform free-space measurements to see the effects of driver directivity and baffle shape on the important polar response. This requires that any reflecting surfaces and objects are at least 10' (3 m) away from the source, that the distance to the microphone is greater than the largest dimension of the baffle, and that the source is rotated around its acoustic center axis. This setup can provide a reflection free 10 ms time record and frequency response data down to 100 Hz. At lower frequencies it is more practical to use boundary measurements, but their integration with the free-space data requires thought and experimental verification.
  
  
• The 3D free-space response of tweeters and very small loudspeakers can be measured in typical domestic rooms, if the required microphone distance is small compared to the reflection path distances. 
  
  
• In an active loudspeaker system each driver has its own power amplifier. This gives maximum control over the mechanical motion of each driver and most efficient use of amplifier power. Drivers of different sensitivities (SPL/W/m) are easily combined, while with passive crossovers the driver of lowest sensitivity determines the loudspeaker's overall sensitivity. Amplifier power has to be wasted in the process. 
  
  
• The power amplifiers of an active loudspeaker system see a benign load (resistive, slightly inductive) over their assigned frequency range, unless it includes the mechanical resonance of the driver (highly capacitive and inductive). The single and much larger power amplifier that is required for a passive crossover loudspeaker has to drive a complex load, which places more stringent requirements on its dynamic stability and overall performance. Different amplifiers may sound different.
  
  

Open baffle loudspeakers

• Open baffle speakers are inefficient in terms of the mechanical movement that is required to create a given level of sound. This not only applies to speaker cones but also to panel vibrations.
  
  
• Open baffle loudspeakers reproduce bass with less room interaction. It is more articulate than from box speakers.
  
  
• If dipole behavior covers the full frequency range, then the room response becomes perceptually masked by the direct sound.
  
  
• The radiation from the rear of the cone must not be absorbed, but the distance to the nearest reflecting/diffusing surface should be at least 3' (1 m).
  
  
• An open baffle circumvents the box problems of delayed radiation through cone and enclosure panels. They occur typically in the mid-frequency range and are difficult to suppress.
  
  
• Large panel radiators or long line radiators suffer from severe lobing at higher frequencies. It manifests in critical room and listener placement. 
  
  
• Even though a dipole requires a 6 dB/oct boost towards low frequencies, it takes little power to drive it to maximum excursion at its lowest bass frequencies. Amplifier power could be an issue as frequency increases, where it requires higher cone acceleration to reach Xmax. Thus SPL is limited by driver volume displacement at the very lowest frequencies and becomes amplifier limited as frequency increases.
  
  
• Realistic bass levels can be obtained from dynamic drivers in open baffles, not from panels. For extreme SPL requirements the number of drivers could get very large and, therefore, below 50 Hz they are more economically replaced by sealed box subwoofers.
  
  
• At frequencies where a 8" driver would become directional it has wider frontal dispersion for an open baffle than if the baffle were closed in the back.
  
  
• Open baffle speakers reach deeper into the room and are less subject to the room response if their polar response is well behaved.
  
  
• ORION exemplifies open baffle loudspeaker design in terms of polar response control and dynamic range. It circumvents the limitations of large panel radiators and yields a small package.
  
  
• The low masses of the moving parts in an ESL, a planar magnetic, or a ribbon driver are necessary to generate useful sound pressure levels. The force generated by an electrostatic or planar magnetic motor is weak. Since SPL is proportional to air volume acceleration, and moving parts Acceleration is Force divided by Mass, the mass has to be lower if the force is too weak to generate sufficient acceleration. Furthermore, since excursion is limited with these drivers the radiating area has to be large to move a sufficient air volume.. These relationships seem to be difficult to grasp by audiophiles. Marketing departments and even some designers like to tout low mass as an inherent benefit giving greater "speed" or frequency response to their speaker, when it is only affecting sensitivity in SPL/W.
  
  
• It is difficult to screw up an open baffle speaker design to where it sounds worse than your typical box speaker.

Box loudspeakers

• Small boxes have fewer problems with panel resonances, cone re-radiation, and polar response than large boxes.
  
  
• Box panels can radiate more sound at certain frequencies than coming from the cone.
  
  
• When building boxes from 3/4 inch thick wood, then the un-braced areas should be less than 4 inch squares to obtain high stiffness and to push panel resonances into the kHz region and where they can be decoupled from the driver's structure and airborne vibration.
  
  
• The sound behind the driver cone should not come back out through the cone.
  
  
• Typical box speakers have a generic sound due to their polar response, panel resonances, re-radiation through the cone and vented bass.
  
  
• Bass from box speakers has more "punch" than from open baffle speakers, but is less airy.
  
  
• Vented bass speakers are resonant structures and store energy which is released over time. For accuracy, bass must be reproduced from sealed or open baffle speakers that are non-resonant.
  
  
• Closed box speakers are best listened to from very close distance to minimize masking from an uneven room response. 
  
  
• PLUTO is not merely another 2-way box speaker. Secondary radiation from enclosure panels and through the driver membrane from inside of the enclosure were eliminated. Being an active speaker, two drivers of very different sensitivities could be combined in order to obtain omni-directional radiation. Bass response was extended by equalization and not by a resonant vent.
  
  

Listening rooms

• The room is rarely at fault. If it is comfortable for conversation and living in it, then it is also suited for sound reproduction. The problem is usually the inadequate polar response of the loudspeakers and their placement in the room.
  
  
• Loudspeakers should be positioned out in the room, at least 3' (1 m) away from reflecting surfaces. The further the better. 
  
  
• Speaker placement to the inch based on some room acoustic calculation is nonsense.
  
  
• Rooms should have lots of diffusive elements and not sound like a stuffed pillow if open baffle or omni speakers are used.
  
  
• Placing absorbers at reflection points is the wrong approach. It only absorbs high frequencies and increases the difference between the direct sound and the delayed room response. It works against perceptually masking the room response as merely a copy of the direct sound.
  
  
• Equalization for a certain response at the listening position is fraught with serious problems. DSP can do many things, but which acoustic inputs to take, and how to process them, is still very much at a research stage. It will change the sound you hear.
  
  
• When I hear an unfamiliar loudspeaker in an unfamiliar room and it does not sound right, then I look for faults in the loudspeaker's design and placement long before I blame the room.
  
  

Listeners

• People listen differently. Performing musicians and members of the audience are used to different perspectives and focus on different aspects of the sound. Both are valuable for analyzing a loudspeaker. People who only listen to loudspeakers and thus always compare loudspeakers are poor judges of accuracy.
  
  
• Very few sales people of "high end audio" ever listen to unamplified life sounds. They are highly susceptible to marketing department suggestions.
  
  
• Unbiased listeners have no difficulty recognizing accurate sound reproduction, even with hearing damage or with hearing aids.
  
  
• Unfortunately, marketing departments and dealers think that bass and high frequencies need to be emphasized for products to sell. 
  
  
• Some listeners prefer euphonic loudspeakers. Accurate, and thus neutral, loudspeakers are not that exciting unless the source material is.
  
  
• I find it disappointing when loudspeaker manufacturers run extensive double-blind listening tests with trained and untrained listeners where they only compare loudspeakers to each other, but not to any live source. These are strictly preference tests within a given paradigm.
  
  

Source material

• A loudspeaker can never do better than to accurately convert electrical signals into acoustic signals. Thus the source material determines ultimately how well an illusion can be created.
  
  
• Recording is still an art, not a science. Two loudspeakers in a room cannot reproduce the original sound field. Surround sound could be science based, but today is far from it and mostly pan-potted mono.
  
  
• Lossy compressed recordings (e.g. MP3) lose too much inner detail when encoded at less than 128 kbps to be perceptually accurate. This shows up most easily on applause and the least on voice.
  
  

Associated equipment

• The loudspeaker is by far the weakest link in the reproduction chain. Unless you have really poorly designed associated equipment you cannot get significant improvement in accuracy by going to very expensive equipment. Marketing departments like to tell you otherwise. Hearing a change is not an indication of greater accuracy. Some products are designed to make an audible change so the customer will notice it. Other products rely on the power of suggestion which works the better, the higher the price tag. 
  
  
• In Class A/B power amplifiers the time variant and amplitude dependent crossover distortion is more harmful than harmonic distortion, because of its impulsive and thus wideband nature. It does not register in the typical high signal level harmonic distortion specification. Crossover distortion changes with bias conditions and is thus a function of the thermal control loop of the amplifier. It must be tested dynamically as output power switches from high to low levels and device temperatures change.

This is It