Concert Hall AcousticsIntroductionConcert halls around the world are designed in a vast variety of different shapes and styles, ranging from designs such as the ‘Shoebox’ shape to far more arena style designs. The architectural design process is key to creating a suitable auditorium  for performances of all varieties, musical performances such as classical concerts and opera to live theatre performances on stage, the various elements of acoustical design needs to be applied In order to ensure speech clarity, sound absorption and prevention of reflections along with spaciousness and reverberation. arena format with platforms of seats in radial arrangement towards the stage, to the vineyard shape, to the full surround concert hall with the stage moved towards the middle. Acoustic design and analysis tools allow a much better understanding of room acoustics in the design stages, it is essential that the auditorium is crafted with the acoustics in mind, clarity, spaciousness or reverberance are influenced through the architectural design. Taking into consideration all of these elements, acousticians have more information to interpret the implications of particular architectures on acoustic performances for the design of new halls and the realization of a range of diverse project’s ambitions. This should lead to the emergence of a greater diversity in performance space architectural design. It should also be noted that the majority of new concert halls must be able to accommodate a range of event types from symphonic and chamber music, through jazz, rock, pop world music and conferences. (PARAPHRASE) History of Concert hall design     Concert halls that are highly ranked by classical music listeners tend to have the following characteristics:A mid-frequency decay time of 1.6 to 1.8 seconds for music from the “Classical” period (1750 to 1820), and around 1.9 to 2.1 seconds for music from the “Romantic” period.An initial time-delay gap at or below 25 msec. This is the time between the direct sound and the first reflection, and produces a sense of “intimacy” for shorter settings. This is why the “shoebox” shaped halls tend to be preferred by conductors and audiences – the initial lateral reflections start rather quickly, due to the relatively short distance that sound has to travel from the orchestra to the side walls to the listeners.”Warmth,” in that the bass tones are strong. This translates to a low-frequency decay time that is somewhat longer than the mid-frequency decay time.Spaciousness, in that the sound seems to come from a space wider than the instrument making the sound. This tends to be tied into phase differences in the signals hitting the left and right ears (which is often referred to as the IACC, interaural cross-correlation).Envelopment, in that the reverberation appears to come from all directions, rather than from limited directions. In practice, this means that an ideal hall will mix the reflections together rather quickly, and won’t have any strong discrete echos from any one location, or a part of the hall where the reverb hangs around too long (as can happen in cathedrals with high naves).(REFERNECE)Direct sounds and ReflectionsA good concert hall amplifies sound so that in most of the seats music is clear, intimate, and engaging. Even in the back of the seating area the listener can distinguish different instruments and hear their individual contribution to the musical ensemble, even if the reflections are the dominant energy contribution of the sound field. The reflections contribute to the loudness of the sound and they envelope the listener, giving an impression of reverberation. The perception of loudness and reverberation can be predicted well with the standardized room acoustics parameters  (Paraphrase)Human hearing cannot distinguish reflections arriving shortly after the direct sound.1 Instead, these reflections are fused with the direct sound and they do not change the perception of the sound source directionClarityThe Clarity measurement (C50) compares the sound energy in early sound reflexes with those that arrive later. It is expressed in dB. A high value is positive for speech clarity.https://www.classical-scene.com/2010/03/07/explanation-of-clarity-versus-reverberation-in-concert-acoustics/T60 Reverberation time  The reverberant sound in an auditorium dies away with time as the sound energy is absorbedby multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the sound to die away and the room is said to be ‘live’. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically ‘dead’. But the time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer. In order to provide a reproducible parameter, a standard reverberation time has been defined as the time for the sound to die away to a level 60 decibels below its original level. The reverberation time can be modeled to permit an approximate calculation. .  (Paraphrase)Reflector: Sound reflects off a hard wall like a snooker ball bouncing off the cushion. Flat, large hard surfaces are a common surface in concert hallsAbsorber. If a surface is soft the sound is absorbed. Absorption is usually avoided in concert halls because it removes valuable sound energy. The audience and seating are the most absorbing areas in most halls.Diffuser. A rough surface, a diffuser, disperses sound in all directions. Diffusers have many uses.  For example they can remove detrimental echoes caused by surfaces such as the rear wall behind the audience.  The reverberation time of a hall is the oldest and one of the most subjectively important measures of a good concert hall. Reverberation is the decay of sound after a note has stopped being played. It is mostnoticeable in a large space with hard surfaces, such as a cathedral wherethe sound echoes around long after a note has finished. In small spaceswith plenty of soft materials, such as a bedroom, the sound is quicklyabsorbed by the soft furnishings, and dies away rapidly.When people talk about rooms being live or dead this is largely their perception of the reverberation in a room. The musicologist, Thurston Dart, summarises the influence that reverberation has on composers:  BACK GROUND NOISE   – Symphony Hall case studyAn acoustic consultant will usually specify a low level of background noise for an auditorium.  One possible sources of background noise is the noise from the ventilation system.  To reduce background noise, all the building services elements are mounted on springs and any connections are made flexible by using elements such as rubber hoses.Symphony Hall Birmingham has a very low level of background noise, so any noise made by the audience, such as coughing or rusting of sweet papers, is very noticeable.  It is said that since Symphony Hall opened, audiences in Birmingham have become quieter!    Symphony Hall is built to a traditional ‘shoebox’ shape, a design which dates back to the great halls of the late 19th century (acoustically-acclaimed halls such as Vienna’s Musikverein, and the Concertgebouw in Amsterdam are constructed to this shape). Russell Johnson had used this shape before with great success and for Symphony Hall he refined this still further.  It has a reverberation chamber behind the stage and extending high along the sides, adding 50% to the hall’s volume, the doors to which can be remotely opened or closed. The U-shaped reverberation chamber area has a volume of 12,700 cubic metres (450,000 cu ft).1 There is an acoustic canopy which can be raised or lowered above the stage. Dampening panels can be extended or retracted to ensure that the ‘sound’ of the space is perfectly matched to the scale and style of the music to be performed.5 There are also reverse fan walls at the rear of the hall which provide further reflections of sound. All the walls and the ceiling are 200 millimetres (8 in) thick and are made of concrete.1The hall is built only 30 metres (100 ft) from a covered railway line. To prevent the transmission of vibrations, the hall is mounted on rubber cushions, as is the railway track. The hall is also shielded from heavy traffic on Broad Street by double skins of concrete. Large, low-speed air ducting cuts the ventilation noise.5 One design innovation is the reverberation chamber – a 12,700 cubic metre void which is equivalent to about 50% of the volume of the Hall itself. It envelops the platform end of Hall in a U shape and links with additional chambers that run along the sides of the Hall at high level. A series of huge, concrete doors each weighing one tonne, opens from the Hall and can be adjusted to create the required degree of ‘echo’.Evolution of Concert Hall Design