Bose Panaray MA12 Technical Foundation & Discussion

Bose® Panaray MA12 Modular Array:

Technical Foundation & Discussion

Morten Jørgensen

Manager, Marketing and Product Planning

Kenneth Jacob

Chief Engineer

Bose Cor pora tion, Framing ham, MA, USA

Professional Systems Division

April 2002

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Bose® MA12™ Modular Array: Technical Foundation & Discussion

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Bose® MA12 Modular Array:

Technical Foundation & Discussion

Morten Jørgensen and Kenneth Jacob, Bose® Professional Systems Division

Summary

THE Bose MA12™ modular array takes advantage of the properties of cylindrical waves to meet customer requirements that until now could only be met with loudspeakers flown and aimed in more elaborate and expensive designs. With only two dimensions of dispersion rather than the three of the more common spherical waves, the sound from cylindrical waves diminishes much more gradually with distance from the source. As a consequence, listeners experience relatively little change in sound level from far away from the same gradual change in sound with distance makes the feedback from microphones in close proximity. The radiation pattern of the wedge-shaped: wide from side-to-side but sharply confined to the top and bottom of the array. The vertical radiation virtually shuts off above and below the speaker. As a result, much less reverberation is generated because almost no sound is radiated upwards to distant surfaces in the upper part of the room. The result is noticeably better clarity and intelligibility. The ultra-thin shape of the hide; it may be the most unobtrusive speaker yet developed given its exceptionally high output and full, balanced frequency response. The fact that the at ear level (so that listeners are confined within its wedge-shaped radiation pattern) means that it can usually be installed for a fraction of the cost of more elaborate ‘flown’ loudspeakers and loudspeaker clusters. Finally, it can be matched to a low frequency enclosure (Bose Panaray needed. Taken together, these features and advantages result in a product that represents an important new tool for satisfying the most basic and important customer requirements in a wide range of common applications.

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INTRODUCTION

CUSTOMER requirements for a sound system are diverse and cover the areas of acoustics, architecture, operation and service. Some of the most important requirements include the following:

- Customers value a system that has the right balance of low, mid and high frequencies – what is called ‘tonal balance’. Customers hear and complain about sound that is too ‘boomy’ or ‘shrill’ or ‘sibilant’, all examples of tonal balance problems.

-A system that plays at the right level is better than one that is too soft or too loud. Customers routinely complain about both excessive sound levels or when the desired impact can not be achieved because the system is unable to play loud enough.

-A system where the sound is perceived to come from the same direction as the action to which it corresponds is better in many applications. When, for example, a talker is on stage, a system whose sound is perceived to come from the stage is better than one where the sound comes from above. Lack of eye-ear correspondence is disconcerting and distracting.

-A system that delivers music with clarity, and speech with intelligibility, is better than one where instruments are garbled and speech is hard to understand. No other single customer requirement generates as many complaints as poor speech intelligibility. It often impacts the fundamental purpose of a venue – the sermon or lecture at a house of worship, or the announcement at the airport, for example.

- Customers are understandably concerned about the appearance of a sound system. They usually value a system that blends into its environment, and is out of the way. And when the system is visible, customers want it to be elegant yet unobtrusive.

- Finally, customers value a system that works reliably for long periods of time without degradation or the need for service. But should a problem occur, they want prompt, cost-effective service. No customer wants to shut down a facility in order to undertake repairs.

These customer requirements exist on any given project to one degree or another. For example, in a place of worship a customer might seek nearly ideal speech intelligibility. But in another situation, the required speech intelligibility might be set lower – to meet a government standard for an emergency announcement in a shopping mall, for example. Therefore, the intensity of need in each dimension on a specific project must be determined for each project.

Customer satisfaction occurs to the degree that the performance levels in these key areas of customer requirements are met at a competitive cost. The better system is always the one that meets customer requirements for the least cost.

The standard design approach for meeting these requirements is unofficially called the ‘hang-and-tilt’ approach. In this approach, speakers with controlled radiation patterns are

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hung in the air and tilted down. Hang-and-tilt has become the de facto standard for sound reinforcement in virtually every kind of venue, from retail spaces, to atriums, churches, schools, gymnasiums, auditoriums, city halls, airports, and sports facilities.

Manufacturers including Bose offer a wide range of speakers used in the hang-and-tilt approach, and similarly, offer a wide range of tools to help the designer of these systems. As a result, dealers, contractors, consulting engineers, and others have learned to deliver systems that perform well in satisfying the major customer requirements using this approach.

The purpose of this paper is to show that the Bose

MA12™ modular array represents a

significant and important extension to the hang-and-tilt approach. To do this, our strategy relies on an explanation of the speaker’s unique sound radiation pattern, and how that radiation pattern, and the thin line-shaped source necessary to produce it, often allows designers to meet customer requirements better and at a lower cost than before. The argument begins with a review of the fundamental assumption that first led the industry to the hangand-tilt approach and then moves on to explain the approach’s strengths and remaining weaknesses.

THE HANG-AND-TILT APPROACH

Original assumption

WHAT led the industry to embrace the hang-and-tilt method and dedicate decades of research, development and marketing effort to perfect it? Why do so many speakers end up in the air and tilted down?

The answer can be traced to a fundamental property of the speakers used – specifically, that the sound waves they radiate spread in all three dimensions: up and down, left and right, in and out. These are called spherical waves because the sound radiates in all directions, like a sphere. As a result, the sound intensity, or sound pressure level from spherical wave sources decreases by 6 dB whenever the distance is doubled, as shown in Figure 1. (To be exact, this is true beyond a certain distance from the speaker. At very close distances the behavior is different.)

For example, if a listener is four meters from the speaker and the level is 78 dB-SPL, then when the listener is eight meters away – twice the distance – the level is 6 dB less, or 72 dBSPL. And while a speaker often has different intensity levels at different angles (as is the case with any directional speaker), no matter what angle is chosen, when the distance is doubled along that same angle, the level decreases by 6 dB.

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Spherical-Wave Sources

72dB 8m

Figure 1. Sound from spherical waves radiates in all three dimensions: up and down, left and right, and front and back. As a result, the sound level decreases by 6 dB whenever the distance is doubled. The reason for a 6 dB drop (and not something else) is readily understood, and is contained in a footnote on page 16.

78dB 4m

84dB 2m

90dB 1m

If the 6 dB per doubling of distance behavior is ignored for the moment, and the only consideration were convenience, the easiest place to put a speaker in a typical room would be a position in the front of the room with the speaker aimed toward the audience, as shown in Figure 2a.

This placement, however, has a problem. In the example shown in the figure, the closest listener is one meter from the speaker, and the farthest listener is twenty meters, a ratio of twenty to one. This corresponds to a 26 dB difference in sound level, a very large level difference corresponding to a perception that the sound at the farthest location is perhaps four or more times softer than the front.

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Figure 2a. A traditional speaker producing spherical waves and mounted at ear height in the front of the room results in a 26dB difference in direct field level when the distance ratio of near to far is 1:20. A listener in the back would report the loudness to be four or more times softer than the front.

Thus while localization is good because the speaker is placed close to the visual activity, the system does poorly in creating the desired sound level in the audience area. No matter what volume setting is used, the sound is either too loud or too soft in most of the audience area – it is simply impossible to establish the correct level for the audience with such a big difference from front to back.

To achieve less variation in speaker-to-listener distances, and therefore less variation in sound level, the speaker can be hung in the air and tilted down at the audience as shown in Figure 2b. This is what is referred to unofficially as the ‘hang-and-tilt’ approach. The ratio in this example is 2:1, corresponding to a sound level variation of only 6 dB. The hang-and-tilt approach largely solves the level variation problem, which is why it was vigorously pursued as a way to satisfy customer requirements.

Figure 2b. When the same speaker is mounted in the air and tilted down, the level variation is reduced significantly – in this case to only 6 dB – which corresponds to a near-to-far distance ratio of 1:2.

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Of course, as with any engineering solution, the hang-and-tilt approach is not perfect. It has its strengths and weaknesses as they relate to the goal of cost effectively satisfying the major customer requirements. The details of these strengths and weaknesses are the subject of the next section.

Strengths of the hang-and-tilt approach

THE strengths of a good hang-and-tilt system are that with it, excellent tonal balance, consistent sound level, and speech intelligibility can be achieved. Moreover, because the speakers are located up and out of the way, they rarely interfere with sightlines.

Over the years, Bose and others have developed a number of technological solutions specifically designed to improve the quality of hang-and-tilt systems. For example, Panaray

LT speakers are designed with very narrow sound radiation patterns so that designers can carefully aim them only onto audience areas and avoid reflective walls and ceilings that can produce the excessive reverberation responsible for diminished speech intelligibility. These speakers also exhibit a very sharp rolloff of sound outside their primary radiation angles, making it easier to combine two or more in such a way that they exhibit a minimum of the inter-speaker interference that can lead to dropouts in sound.

Similarly, the Bose Panaray 502

A loudspeaker represents an important contribution to the field of hang-and-tilt speakers because it delivers consistent coverage over substantially a full range of frequencies using very natural sounding cone-type drivers in a very small package. This speaker is used in literally thousands of venues around the world where customers say it meets their needs elegantly and unobtrusively.

As a final example of the types of innovations that have led to better hang-and-tilt systems, until very recently it was thought to be difficult or impossible to include control of the lower frequencies in hang-and-tilt designs. This lack of control meant that bass sound waves were more or less allowed to go anywhere within a venue, causing a lack of clarity in music and some masking of speech (and therefore a reduction of speech intelligibility). Today solutions exist to control bass frequencies in hang-and-tilt designs with very nearly the same degree of precision as the higher frequencies, including a comprehensive technique developed by Bose. These solutions, which employ advanced array theory, have led to noticeable improvements in the sound quality of systems in which they have been used.

Weaknesses of the hang-and-tilt approach

THE hang-and-tilt approach also has some weaknesses. For example, the designer must ensure that the sound radiation pattern from the speaker being considered is appropriate for the purpose of covering the audience area. However, the choice of speakers is limited to only a few, which differ according to their radiation patterns. It is purely coincidence and therefore rare for the designer to find a perfect match between the available radiation patterns and the audience area. In general, the speaker being considered will have more or less coverage than what is needed.

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If the speaker’s radiation pattern is too wide for the audience, there is over-coverage as shown in Figure 3a. In these situations, sound radiates to areas other than the audience where it reflects off of surfaces and arrives at the ears of the listeners as reverberation, which causes degradation in clarity and intelligibility.

Figure 3a. The effect of choosing a speaker with a radiation pattern wider than the audience is shown. Sound striking surfaces other than the audience causes unwanted reverberation and reduced musical clarity and speech intelligibility.

If, on the other hand, the speaker’s radiation pattern is too narrow for the audience, as shown in Figure 3b, people outside the main beam will hear a serious degradation in tonal balance, level and clarity.

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Figure 3b. The effect of choosing a speaker with a radiation pattern narrower than the audience is shown. People outside the main beam get poor sound quality.

In situations where the radiation pattern from a single speaker is too narrow, another speaker is usually added. When that is done, however, the same set of challenges is repeated. Will the added speaker be able to just cover the area that was uncovered before? If it does, it is coincidental. In general, the added speaker will again have coverage that is too wide or too narrow.

Regardless, when two or more speakers are used to cover an audience area, their individual radiation patterns must be overlapped in order to avoid a coverage hole between their patterns. This interference zone, shown graphically in Figure 4, can result in significant and audible dropouts of sound at some frequencies. Without careful selection of speakers, their locations, and aiming angles within a cluster, there can be as much as 20-30 dB of energy missing in the middle of the frequency range crucial for speech. These dropouts caused by interference have a significant impact on clarity, intelligibility and tonal balance.

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Figure 4. The interference zone caused by the overlap of two speakers is shown. Inter-speaker interference can result in major sound dropouts – as severe as 20-30 dB – which harms tonal balance, clarity, and intelligibility.

For these reasons and others, hang-and-tilt systems require a significant investment in design time to achieve good coverage without excessive interference. To aid in this effort, the designs are usually created using computer modeling programs, where creating the room model, then selecting, positioning, and aiming speakers, and optimizing the design can take anywhere from a day to weeks, or even months in the case of large projects.

Once designed, sophisticated rigging is often needed to ensure that the speakers are properly and attractively installed. A professional engineer is often employed to implement the exact aiming angles dictated by the design and to ensure mechanical integrity and safety. Then the rigging hardware has to be purchased or fabricated and shipped to the site. The installation requires a lift or scaffolding to hang the speakers in the right place. And finally the installation often has to be reviewed by the local engineer to ensure that it meets code and safety requirements. Rigging and installation costs can climb into the thousands of dollars.

Once the system is installed, another significant investment in engineering time is required for system tuning and adjustment. Level matching the low to the mid and high frequencies and setting time delays in a cluster takes time and requires a skilled field engineer. So does setting time delays and matching levels from cluster to cluster and deciding on the overall room equalization.

Thus designing, installing, and tuning a hang-and tilt design is time consuming and requires a high level of skill in a variety of areas. These factors mean that hang-and-tilt systems are often expensive to create.

Hang-and-tilt systems also suffer from compromised performance in the area of sound localization. The visual activity is usually to the front of the listener, but the sound comes from above where the speakers are located. This lack of eye-ear correspondence is

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