Audio Signals and Analysis An examination of bandwidth, dynamic range and normal operating levels By John Eargle & Chris Foreman

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GAIN STRUCTURE IN AUDIO SYSTEMS
As we have seen, normal speech has a peak factor of about 12 dB. Music on the other hand can have peak factors that are in the range of 16 to 20 dB, depending on the nature of the material. Highly compressed music signals, such as are common in modern pop and rock music may have peak factors no greater than about 4 dB; however, classical music may present numerous operating levels, each requiring recalibration as the program progresses.

Many times during outdoor classical music events at summer festivals the sound reinforcement system is carefully adjusted manually, usually by an operating engineer working with an assistant producer with score in hand.

Figure 13 shows a typical example of how this is done. The engineer must be aware of how loud the orchestra will play and how these loudness peaks will translate through the music reinforcement system. The aim is to contain the peaks within an agreed upon level at selected positions in the large audience area. Such levels as these are often established so as not to produce any disturbance at monitoring points in nearby residential areas.

Figure 13: Example of shifting of operating levels in a musical program. A long-term classical music program progressing from an extended slow, soft section to a louder section (A); having raised the gain for the softer section, the mixing engineer must slowly reduce the gain by 8 dB as indicated (B).

At the same time, both engineer and producer know that low-level music passages may get lost in the ever-present noise level of large audiences, traffic, overflights and the like. Operating level shifts of the order of 12 dB are very common, and when smoothly executed may be barely noticeable as such.

RECOMMENDED GAIN STRUCTURE
System headroom and operating levels are normally defined at the line output stage of the operating console, while system noise floor is defined at the microphone input stage. The total dynamic range of the system is thus established and cannot be improved upon later in the audio chain. However, through careless down-stream gain structure it can be degraded.

As an absolutely safe procedure we recommend that a music or speech reinforcement system be setup to provide a nominal 20 dB of operating headroom over the normal “zero level” calibration. This should apply across the board, so to speak, to all electronic elements in the chain. Basically, once the headroom value in dB has been determined, the precise relationship between headroom and operating level should be maintained through all following line level electronics.

At the end of the chain the power amplifier-loudspeaker combination must be considered as a separate entity, and adjustments made so that a given signal level (e. g., 0 dBu) is assigned a given sound pressure level in the house. This process is shown in Figure 14 for a relatively simple reinforcement system.

Figure 14: Setting gain structure in a speech reinforcement system.

Our recommendation is that a VU meter reading of “zero” at the output of the operating console be assigned a nominal level mid-way in the seating space of about 72 dB SPL. You may wish to change this value slightly, depending on local requirements. This standard approach simplifies normal system operation; all the operator has to do is raise or lower the input fader of the console to attain a nominal zero dB reading in order to ensure consistent speech levels in the listening space.

(Excerpted from JBL Audio Engineering For Sound Reinforcement, ISBN 0-634-04034-0, used by permission of JBL Professional. This book is available at music and bookstores nationwide, or through Music Dispatch at www.musicdispatch.com or 800-637-2852.)

John Eargle is senior director of product and development for JBL, while Chris Foreman has more than 30 years experience in the professional audio industry as a sound contractor, consultant and manufacturer.