Architecture for Acoustics

The architecture of the enclosure should contribute as much as possible to overcoming the inverse square law and the bass loss problem. Some of the general characteristics for a good general purpose auditorium are indicated. .

The above comparison of general description and measurable parameter was inspired by a review article about the opening of the Minneapolis Orchestra Hall published in Time magazine on November 4, 1974.

The article gave this general description: "A capacity crowd of 2573 discovered that the new $10 million Orchestra Hall is a winner, with truly superior sound. The term for the way in which a stage projects sound into an auditorium is 'throw'. Orchestra Hall has a throw that even Tom Seaver might envy. ... the new hall also has remarkable even dispersion of sound,... admirable balance and clarity, a striding bass and an exciting musical presence unsurpassed perhaps by any concert hall in the world. ..At times the volume of the orchestra approached the painful - clearly the result of the conductor's understandable desire to show off the hall's dynamic range."

Projection of Sound

A good auditorium will accomplish effective projection of the sound to the rear of the auditorium so that those distant listeners will not experience the extreme loss of sound level caused by the inverse square law. That projection is normally achieved by having a sufficiently long reverberation time. Another significant contributer will be a high, reflective ceiling to reflect sound to the back of the auditorium.

Clarity

While the richness and fullness added by auditorium reverberation is desirable, such reverberation decreases clarity of articulation. So fullness and richness work against clarity, and a reasonable reverberation time must be reached by an appropriate compromise of clarity vs fullness.

Clarity can also be diminished by undesirable echoes. At specific locations in auditoriums, clarity can be diminished by anything which blocks part of the direct sound and therefore increases the fraction of reverberant sound reaching a person.

Balance of Highs and Lows

Achieving a proper balance of the high and low frequencies of sound in the back of an auditorium is usually a matter of boosting the bass. If an auditorium has a short enough reverberation time that it approaches the inverse square law, then the bass loss problem is encountered. In most auditoriums, this bass loss problem is partially overcome by having a reverberation time for low frequencies which is longer than that of the high frequencies.

Even Dispersion

Sound is more pleasing if it is evenly dispersed, with no prominent echoes, no significant "dead spots" or "live spots" in the auditorium. This even dispersion is usually achieved by avoiding any focusing surfaces and avoiding large flat areas which reflect sound into the listing area. Sometimes it is desirable to add some anti-focusing surfaces.

Musical Presence

After all the main characteristics of auditorium acoustics are accounted for: proper reverberation time, longer reverberation time for low frequencies, even dispersion, etc., there still remain nuances which cause persons to prefer some apparently comparable auditoriums over others. Words used to describe the reasons for such preferences are "intimacy", "musical presence", or other words to imply that the listener felt more a part of the performance and not isolated from it. One physical factor which has been correlated with such preferences is the time between the direct sound and the first reflected sound which reaches a listener.

Quantitative studies indicate that the first reflected sound should arrive within about 30 msec of the direct sound, and that time lags of 50 msec or more draw negative reactions from listeners.