Briefly discussed is the change in architectural design of concert halls from early in the 20th century to the present time. The growth of sound in a concert hall is reviewed with emphasis on hearing the direct sound clearly. Next comes the desirability of hearing early lateral reflections, that is, those from side walls. The strength (loudness) of the orchestral sound follows with emphasis on the degree of seat upholstering and the cubic volume of the hall. It is shown that in a large number of successful concert halls, the midfrequency, fully occupied, reverberation time (RT) and the ratios of the volume to the total surface areas are nearly the same even though the volume per seat varies over a wide range. The bass index for a number of halls is discussed in relation to the weight of side walls and added sound absorbing materials. The need for surface irregularities is emphasized. Finally, maximum and minimum seating capacities for shoebox-shaped, surround-shaped, and fan-shaped concert halls are suggested.Brief History (Beranek, 2004)The highest-rated halls acoustically were built before 1901. They are Grosser Musikvereinssaal in Vienna, Austria; Symphony Hall in Boston, Massachusetts; and Concertgebouw in Amsterdam, The Netherlands. All three are shoebox in shape and have lightly upholstered seats. To listeners, the sound in them is beautiful, almost luxurious, because of the rich reverberation, the quantity and quality of early reflections that give breadth to the music, the balance of tone among the orchestral sections, and the loudness (sound strength) that brings listeners to their feet following a fortissimo conclusion. Also, the sound is nearly uniform in about 90% of the seating areas and the players clearly hear each other, certainly in Boston and Vienna.Since the advent of the Berlin Philharmonie Hall in 1963, architects and owners have often placed beauty and novelty of architecture above acoustics. In Berlin, Germany, nearly half of the audience is seated behind and to the sides of the stage. That hall has been a success even though the orchestral balance differs considerably from one seat location to another because the sounds of the various instruments are radiated in different directions and because the listener is much closer to one part of the orchestra than to others. Nevertheless, many listeners enjoy being to the rear or sides of the orchestra so they can see the gestures and facial expressions of the conductor. Some say that at these locations they feel as though they were part of the performing group. A negative factor is that with the piano lid up, listeners seated at the stage rear hear only the low string notes. This is also true for soprano voices, where the high tones are only projected forward.Clarity of the Direct SoundWhen a musical note is suddenly sounded on the stage, say by a violin, the sound radiates outward from the instrument and then strikes walls, ceiling, and audience. Each surface then reflects a sound wave that subsequently bounces around the room from one surface to another. At a listening position in Boston Symphony Hall located just off the center line and two thirds the distance from the stage to the main-floor rear wall, that is, 26 m (85 ft) from the stage, the rise in the energy density following the arrival of the first seven strongest reflections at a central seat is shown by the heavy line in Figure 1. The zero point on the bottom scale is the time at which the direct sound arrives. The direct sound of a played note will usually continue throughout the 70-ms span that follows. The dashed curve D(t) shows the rise in energy density that would occur if the equivalent energy were to be fed gradually into the hall instead of arriving in separated reflections.The first reflection is from the lowest sidewalls and the undersurfaces of the first balconies. The second is from the undersurfaces of the second balconies and their fronts. The third is from the back of the stage. …