Because listening to music in your home is an alternative to listening to music in a concert hall, the acoustical factors that make a concert hall satisfactory must be considered when planning room and loudspeaker configurations. These concert hall factors include direct sound, loudness, adequacy of bass, degree of source spreading, reverberation time, strength and quality of the reverberant field, envelopment, freedom from distortion, freedom from echoes, and quietness.

Wherever one sits in a concert hall, the direct sound should be clearly heard before the energy in early reflections and reverberation have risen to the level that starts masking that sound. Freedom from masking of the direct sound is particularly important in the frequency region above 700   Hz. If the direct sound is not clearly heard, the music lacks impact and the listener may tend to go to sleep. This problem is particularly important in shoebox-shaped halls with small seating capacities because the sidewalls that reflect early sound are nearer to the listener, which means that the reflections start masking the direct sound sooner. One possible design for a hall with limited seating is to make it fan shaped. This way, all listeners are nearer the stage, and the sidewalls do not reflect early sound toward the audience areas. In Boston Symphony Hall, which is shoebox-shaped, the statues and niches and edges of high-up windows reflect sound above 700   Hz back toward the stage, which decreases the energy that would otherwise reflect from the sidewalls and thus preserves audibility of the direct sound.

In a concert hall, the music must be loud enough as measured by the quantity Sound Strength G in dB. The magnitude of G is determined by the total overall sound-absorbing quantities of the audience, sidewalls, ceiling, and performers on stage. The greater their absorption, the lesser is G.

A common complaint in concert halls is inadequate loudness of the bass sounds. A measure of bass loudness is a quantity called Bass Index, which is the strength of the sound G in the 125   Hz octave frequency band minus the strength of the sound at midfrequencies (average of the strength G in the 500 and 1000   Hz bands). Generally, if the walls are adequately heavy and the absorption of the audience seats at low frequencies is not too high, the bass loudness will be satisfactory.

The sound in a concert hall is more pleasant if there are early reflections that come from lateral directions—the sidewalls in a shoebox-shaped hall. Early lateral reflections spread the source, which results in a fuller tone. Of course, these reflections must not come so soon as to mask the direct sound, but they must occur in the interval between 30 and 80   ms after arrival of the direct sound. The optimum shoebox-hall width for meeting this requirement is about 24   m. It is difficult to achieve sufficient early lateral reflections in halls where the audience surrounds the orchestra. In fan-shaped halls, early lateral reflections are nonexistent.

Reverberation is part of the music that one hears in a concert hall. No symphonic piece sounds good outdoors where there is no reverberation. Optimum reverberation time for the current symphonic repertoire is 1.8–2.1   s. For chamber music, the optimum time is 1.5–1.7   s. Reverberation does not contribute to loudness.

The quality of the reverberant field is enhanced if there are substantial irregularities on the sidewalls and ceiling. In some halls, this is accomplished by niches on the sidewalls above the top balcony and by coffers in the ceiling. These irregularities must be of depths of up to 0.5   m. Such great depths are not desirable below the upper balcony because lateral sound reflections of high quality are needed from there.

In the best halls, the audience feels enveloped by the reverberant sound. This is a matter of the strength G of the reverberant sound measured 80   ms after arrival of the direct sound—the higher the strength, the greater the envelopment. However, the design of concert halls of the future may be influenced by a trend in modern phonograph recordings. In these, some of the recording microphones are placed near the performers. The others record the reverberant sound. The relative strength of the two is then adjusted by the tonmeister and the tendency is to make the strength of the reverberant field relative to the direct sound lower than is experienced in the actual concert hall. The strength of the reverberant sound field may be reduced in a hall by adding appropriate sound absorption materials in the sidewalls and ceiling.

It almost goes without saying that these must be avoided in a concert hall. Distortion can come from repetitive small-scale irregularities on the sidewalls below the top balcony. This is sometimes called the “picket fence” effect. Noise generally comes from the ventilating system. Echoes are easy to identify in computer simulation of a hall during the planning stage.

The typical American living room has dimensions of, say, 7   ×   9   m with a ceiling height of 2.7   m. Window frames, draperies, and pictures on the walls provide diffusion of the sound field at high frequencies, but at frequencies below 1000   Hz, the sound field is generally not diffused. The reverberation times are often greatly different at different frequencies depending on the sound absorption qualities of furniture, wall hangings, and carpets.

There is general agreement that reverberation times at midfrequencies (500 and 1000   Hz octave frequency bands) should not exceed 0.5   s, and at low frequencies (62 and 125   Hz bands), it may rise somewhat, say, to 1.25 times that at midfrequencies. Control of the low-frequency sound is possible by selecting heavily upholstered furniture. In other words, the room should neither sound “dead” nor very reverberant, partly because the reverberation time of the recorded sound should come through in its pristine form.

Loudness is generally not a problem because the amplification of the playback system is easily adjustable.

Of course, the answer to this question depends on the system purchased. A five-loudspeaker system will be handled differently from a two- or three-speaker system. Let us first talk about the interaction of room resonances with loudspeaker location.

This category includes all loudspeakers surrounding the listener. Some listeners prefer the sound to come from loudspeakers that have relatively narrow directivity patterns at all of these frequencies. The reason is that they wish to hear only the sound that is recorded. If the patterns are broad, they will overlap in the room, and the recorded sounds from the different loudspeakers will arrive at different times, and a “smearing” effect results. Others feel that the smear is equivalent to increased “envelopment,” which they feel is a good thing.

There are electrical systems that are sold to “correct” the reproduced sound in the room. Olive et al. [1] made subjective tests of several such electrical systems. For the tests, they used a high-quality loudspeaker operating above a crossover frequency at 80   Hz and a high-quality subwoofer. The woofer was located in a left-front corner, and the other loudspeaker was located in a direction about halfway between the center of the room and the location of the woofer. Eight subjects were seated on chairs in the center. The reverberation time for the room was 0.4   s, and the levels for all of the correcting systems being compared were adjusted to be the same as determined by dBA measurements at the listening positions.

There is not much more to be said. When setting up a listening system, it is recommended that a suitable room-correction system be used to obtain the best response of the combined room/loudspeaker system.