Background Noise

Have you ever noticed that your radio seems awfully loud when you stop your car after listening on the highway? Or have you ever felt that palpable relief when the air conditioner shuts off? We don't notice background noise---but background noise determines what we can hear and understand in the foreground.

Some times we don't want to hear everything. Imagine if you could hear and understand every conversation at your office. It would be terribly distracting. But when we do want to hear every little thing---at a religious service, in an important meeting, at a play, or at a concert---background noise is critical.

During a lecture or sermon, any audible sound not made by the speaker is noise; during a performance, any audible sound not created by a performer is noise. There are, of course, many aspects to excellent acoustical design. However, in any space intended for listening, strict control of noise is fundamental. I call these spaces "critical-listening space."

Sources of noise include traffic, airplanes, machinery, plumbing, lights, and people in other spaces. In a space for listening, the worst offender is usually the heating, ventilating, and air-conditioning system (HVAC).

The (undamaged) human ear is so sensitive that we can detect sounds that displace the eardrum by roughly the diameter of a hydrogen molecule.¹ This means that background noise determines the softest sound that a performer or speaker can effectively utilize. Even in spaces that most people would consider quiet, the background noise level can be twenty to thirty decibels above the threshold of hearing. You don't notice this, but the performer has lost twenty to thirty decibels of dynamic range!

This is illustrated in the graph below, with HVAC noise criteria curves for comparison.²

Speech Intelligibility

When a theatre is truly quiet, an actor can use his entire dynamic range, from a shout to a whisper, and still be clearly understood. Since the quiet moments in a drama are often the most electrifying, strict control of background noise is essential.

The ancient Greek theatres are known for their almost magical acoustics for speech. One can hear a drachma drop from the farthest seats in the theatre at Epidaurus. How can this be so? The answer is that no audible sound covers up the sound of the coin striking stone--and so it is heard.

Music

Similarly for music, a silent background allows a performer to exploit his entire dynamic range. The loud climaxes of a musical performance can be wonderfully stirring, but the quiet moments set off these climactic moments and give them their power. Without the quiet moments, music is all on the same dull level. Furthermore, some of the most intense, magical moments in music are the softest. These moments are only possible when the hall in which they occur is truly quiet.

Recording engineers understand this. They commonly turn off all mechanical systems and most lights while recording.

Speech and Music in the Same Space

Many spaces are used for both speech and music, the best example being church sanctuaries. For such mixed use, a silent background is particularly important. This is because reverberation (the persistence of sound in space) is necessary for music. Without reverberation, music sounds flat and dull. But reverberation can interfere with speech intelligibility by prolonging the sounds of speech, smearing them in time.

A good speaker corrects for this effect by speaking slowly and clearly, working with reverberation to enhance the sound of his voice. However, reverberation amplifies background noise, creating a double difficulty for speech. Contrary to common belief, speech can work quite well in a properly designed reverberant space, but only if background noise is minimized.

Audience Noise

Audience noise is beyond the direct control of the architect or acoustical consultant. However, research shows that audience members are significantly quieter when background noise levels are very low. In the City of Birmingham Symphony Hall, Birmingham, England---a hall with exceptionally low background noise---audience members are so attentive during quiet music passages that they hold their breath to listen.

Examples of Spaces with Low Background Noise

• Meyerson Symphony Hall, Dallas
• Domain Forget, Charlevoix Quebec
• New Jersey Performing Arts Center, Newark, NJ
• Clemens Theatre, Christopher Dock Mennonite School, Lansdale, PA

Gymnasiums, Restaurants, Cafeterias

Listening may not be the primary function in your space. However, people are always hearing, and the acoustics of these spaces can almost always be improved by lowering background noise (as well as other measures). I was recently in a gymnasium, for instance, with horrible screeching noise from the lights. The poor gym teacher who has to work in that space must have a perpetual headache.

The Common Condition

In the vast majority of places where I listen---churches, theatres, lecture halls, recital halls, concert halls---background noise imposes a haze in front of the sound. This noise itself goes unnoticed by most people. Instead, they notice that their experience is diminished: the tone color of the violins is dull; the sound lacks clarity; they can't quite understand the words.

The usual response to the ubiquitous blanketing of desired sound by background noise is to turn up the amplification. Amplification often adds fifteen to twenty decibels above the background noise to the level of the speaker. Wouldn't it be better to reveal the speaker by removing twenty to thirty decibels of background noise?

Furthermore, lower background noise makes the job of the sound system vastly easier.

See and Hear for Yourself

The effect of background noise can be compared to looking through a dirty window; one doesn't notice the dirt on the window, one simply can't make out the view. Clean the window for a striking improvement in clarity. And so it is when background noise is controlled. You can hear this for your self by the following simple experiment. You need two other people to help.

• Two people stand on opposite ends of the room.
• The third person is on hand to manage the noisemakers: fans, ventilation systems, lights, dimmer racks, air-conditioning, etc.
• Turn on all noisemakers. In actual practice not all these devices may be on at the same time, but for the sake of demonstration turn them on to create the greatest contrast.
• Converse with the person across the room. Note the effort necessary to make yourself understood. Note the effort necessary to understand.
• Turn off all the noisemakers at once.
• Listen.
• Proceed with your conversation.
• Note the change in effort necessary to understand and be understood.

Even in rooms that have other acoustical difficulties, the improvement should be clear and palpable.

What Is To Be Done?

When designing a new critical-listening space, consider the location of noise producing machinery such as air-handlers, and the design of a silent mechanical ventilation system from the very beginning. Few people realize that it is possible to supply cool air to a room without creating any noise. It is. However, since this is not the usual practice, such a silent system must be considered from the beginning of design.

An architect once called me to help with the design of a high school auditorium. I discovered that it was too late in the design to move two large air-handlers from their location on the roof of the auditorium. They might just as well have been put on stage! I did my best to help out, but nothing I could recommend for the inside of the room will cancel out the deleterious effect of those two huge noisemakers.

In the case of historic renovation, noise control may be the only option available for improving the acoustics of a space. In some cases, merely quieting a noisy mechanical ventilation system will affect a drastic improvement in the acoustics.

Conversely, many a perfectly lovely space has been ruined acoustically by loud new ventilation systems. When planning the renovation of a worship or performance space, make noise control the first consideration.

Noise control is fundamentally important to the success of any building for listening. It can make the difference between excellent sound and the usual mediocrity. This is especially true on a tight budget. Better to save money by leaving out the seats! After all, these buildings are often meant to last for more than a hundred years.

Conclusion

Since the level of background noise determines what we can hear in a space, it determines the level of acoustical excellence. This is truly the secret of great acoustics. Awareness of this secret in the early stages of design brings excellent acoustics for your new critical-listening space within your grasp.

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¹Stanley A. Gelfand, Hearing, Marcel Dekker, 1990, p.325.

²Leo Beranek, Noise and Vibration Control, revised edition, (Institute of Noise Control Engineering, 1988), p.569.

³1995 ASHRAE Applications Handbook, p.43.5.

⁴Calculated from William A. Yost and Mead C. Killian, "Hearing Thresholds," Encyclopedia of Acoustics, ed. Malcom J. Crocker, (John Wiley & Sons, Inc, 1997), p.1549.

⁵J.P. Newton & A. W. James, 'Audience noise-how low can you get?,' Proceedings of the Institute of Acoustics, Vol. 14 Part 2, pp. 65 -70 (1992).