Reverberation and Ambiophony
October 1964 Radio-Electronics

October 1964 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Here is a new word to add to your technical lexicon: Ambiophony, compliments of Mr. Gilbert A. Briggs, in a 1964 issue of Radio-Electronics magazine. The word "ambiophony" comes from the Greek words "amphi," meaning "around" or "on both sides," and "phonos," meaning "sound." Thus, ambiophony refers to a sound reproduction technique that creates an immersive, three-dimensional auditory experience by using speakers placed around the listener, thereby reducing unwanted echo and/or adding reverberation. It explains how the Haas effect influences perceived sound direction and how strategically placed and delayed speakers can create a desired acoustic environment, improving sound intelligibility in large venues like cathedrals and enhancing musical performances in acoustically "dead" recording studios by simulating natural reverberation for the musicians and listeners. The article also highlights specific implementations of ambiophony in places like St. Paul's Cathedral and BBC television studios.

Reverberation and Ambiophony

By G. A. Briggs

A famous British audioman explains some recent methods in commercial sound.

Those two words may have a good deal to do with the quality - technical and musical - of the next record you buy. They describe methods of electronically changing the acoustics of a hall - for getting rid of excess echo, or for adding it where sound is too flat and dull. Ambiophony is being used in Britain to let musicians in a treated recording studio hear themselves more as the broadcast listener hears them - and therefore help improve their performance.

Haas has shown that, due to precedence, when two or more speakers are in use, the listener appears to hear only the one nearest him, provided volume levels are similar, because sound from it is the first to reach his ears. This applies until the second speaker is more than about 50 feet farther from the listener than the first. Then the sound from the second speaker appears as an echo. This reduces the intelligibility. (56 feet = 50 milliseconds time lag.)

It follows that in assembly halls more than 50 feet long, we must avoid blurring the reproduction for certain listeners when several speakers are in use, say under balconies and in remote corners difficult to reach by the main speakers. This can be helped by controlling directional effects with column speakers, and by carefully adjusting individual volume controls connected to speakers judiciously placed. The desired effect is that the audience have the impression of listening to only one source of sound.

But in big places like cathedrals and railway stations with a high reverberation time, and with open-air activities over a large area, a time-delay system related to speed of sound and distance becomes necessary.

I think the 1952 Pamphonic installation in St. Paul's Cathedral must have been one of the first to be adopted.¹

The difficulties were enormous, due mainly to a reverberation time of 6 seconds at mid-frequencies with full congregation (Fig. 1), large concave surfaces and a dome, producing strong echoes, and a time lag up to about 0.25 sec due to long distances.

Briefly, the acoustic problems were solved by column speakers with controlled directional effects, restricted frequency range of 250 to 4,000 cycles, and time-delay circuits for three pairs of speakers to correspond with distance (plus a few milliseconds).

It is interesting to note that delay times can be chosen to obscure the presence of additional speakers. This is done by allowing the supplementary sound to reach listeners between 10 and 14 msec after the original sound, bringing the Haas effect into play. (Very necessary, for instance, in open-air theatres.)

Delay Equipment

The Philips EL6911 is a system for artificial reverberation as well as sound delay. [In this kind of delay system, the original sound is recorded on a continuous loop of magnetic tape, which then passes a row of playback heads, one after another. The recorded sound can be picked off any or all the heads, and each successive head gives a longer delay between the original and the reproduced signals. In some models, delay can be adjusted precisely by sliding the heads along the tape. The tape loop is erased just before it reaches the recording head again. -Editor] Each of four delay circuits in the Philips has its own main amplifier to which appropriately placed speakers can be connected.

An interesting application of this equipment is to be seen - and heard - in the Central Station in Amsterdam. With its typical curved glass roof, this terminus has a delay time of more than 10 seconds. Column speakers were arranged in pairs down each platform, the delay being increased progressively toward the end, in step with the distance, thus eliminating echo effects.

Ambiophony

Time-delay systems are used to reduce echo and reverberation effects.

Artificial reverberation of many types is of course regularly added to recordings and broadcasts and is a day-to-day practice like adding butter to bread. (On some pop records they seem to use lard!)

A rather novel application has been adopted in Studio 4 in the BBC Television Center. This is known as ambiophony and is mainly for use on broadcasts of orchestral concerts, the basic equipment being the Philips EL6911 described just now.

I am indebted to Mr. M. G. Foster of the Engineering Information Department of BBC for details of how the system works.

To reduce the inevitable background noise in a television studio during a broadcast, caused by moving cameras, microphones, staff, artists and often of scenery and properties, the acoustic treatment must be absorbent. This is satisfactory for most programs, especially drama and light entertainment, as in any case the acoustic climate of an individual scene is always modified a little by the scenery.

When musical programs, particularly with large orchestras, are produced, absorbent acoustic treatment immediately makes itself felt. The reverberation time is much less than it would be in a normal concert hall or sound studio of comparable volume specifically designed for music.

Since the acoustic treatment is absorbent, there is little or no reflection of the sound of the orchestra from walls and ceiling, as there would be in a concert hall. This has a twofold effect. First, as far as the players and conductor are concerned the music they are playing appears to have a sound quality different from what they would normally expect, and that can affect their performance. Second, the listener - whether in the studio or over the air - is conscious of a general flatness of tone and lack of fully blended musical effect, which could be likened to the difference between listening to an orchestra playing in a good concert hall and then in the open air.

Studio 4 at the BBC Television Center has a reverberation time of approximately 1 second. According to accepted modern standards, a good concert hall or sound broadcasting studio of similar volume (357,000 cubic feet) should have a reverberation time of between 1.6 and 2 seconds.

In the past it has been common to attempt to overcome this in television studio acoustics by adding reverberation artificially, with an echo chamber or a reverberation plate. While this certainly improved the musical effect for the listener, it did nothing for the artists. Musicians playing in an acoustically "dead" studio are inclined to force their tone - especially the string section - with detrimental results to the performance.

In an attempt to produce controllable acoustic conditions in the studio itself, the BBC has been experimenting with the electronic system known as arnbiophony.² This has involved installing of 62 speakers around the studio walls and in the roof space. Some of these are shown in the photo.

The speakers are fed with the orchestral sound through the time-delay device. They reradiate the sound much the same as the walls and ceiling of the studio would do if they were reverberant instead of absorbent.

Time delays are introduced into the speaker system so that each one "speaks" at the approximate time that the sound of the orchestra would take to reach the speaker's position in the studio, the first reproducing head being normally set to give a delay of 30 msec.

To imitate the randomness of true reverberation, the speakers are not connected strictly according to the delay appropriate to their position, although the correct general trend is maintained. The apparent reverberation time can be increased further by introducing artificial echo into the output of the microphone feeding the system.

The circuit provides some natural regeneration because a portion of the speaker output is picked up by the microphone supplying its input. It takes care to adjust the system for enough output from the speakers, but without acoustic feedback. This is usually achieved by using two completely separate microphone placements, one for the broadcast and another to feed the ambiophony equipment.

The ambiophony mikes are generally much closer and are arranged to favor the string and woodwind sections of the orchestra, which are usually those affected most by deficiencies in studio acoustics.

Vocalist's microphones, particularly those that move about on booms, complicate the situation, since they will pick up the speaker output. As the ambiophony delay is designed to create a natural effect at the position of the orchestra, roving mikes might take up positions in the studio where the effect would sound most unnatural. Six of the speakers were therefore transferred to a separate circuit having no delay to enable players in the orchestra to hear each other more easily.

Though it needs more experimentation, ambiophony has already provided a significant improvement in the sound quality of serious musical programs on television."

Ambiophony really relates to the technique of ambient sound, and it is made clear by Philips that there are many directions in which it can be applied. For instance "dead spots" in older theaters and public buildings can be filled effectively by speakers which-if used without suitable time delays - would merely make things worse. Artificial echo effects can easily be added to outdoor sound reinforcement at open-air theaters, "Son et Lumière" (Sound and Light") plays, concerts, etc.

To prolong the reverberation time to the order of seconds, the signal from the last of the eight playback heads is returned to the recording amplifier via an attenuator, so that the closely spaced pattern of reflections is repeated at a lower level. The attenuator control determines the decay rate.

The initial delay - largely responsible for the impression of auditorium size - depends on the position of the recording head relative to the playback heads. It can be changed in a few minutes.

Thus, with this type of equipment, ambient conditions can be enlarged, enlivened or subdued to suit the type of program and the discriminating faculties of the human ear.

¹ Parkin & Taylor, "Speech Reinforcement in St. Paul's Cathedral." Wireless World, Feb.-Mar. 1952

² R. Vermeulen, "Stereo Reverberation," Philips Technical Review, Vol. 17, No.2, March 1956, p. 258.

3 R. F. A. Pottinger, L. Salter, E. G. M. Alkin, and C. L. S. Gilford, "The Broadcasting of Music in Television." BBC Engineering Division, Monograph No. 40, Feb. 1962.