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“We Need More Power, Captain!” But Just How Much Amplifier Power Is Needed?

There are two goals: Power the loudspeakers so they play as loudly as possible without burning out; achieve a certain loudness in a certain venue. Here's an exhaustive overview covering both topics and much more.

By Bruce Bartlett February 21, 2011

Determining which power amplifier is appropriate for a specific application is not as easy as one may think. To fully understand what needs to be considered, three specific case studies will be outlined:

• A client wants to play folk music in a coffee shop. How much amplifier power do they need?

• A rock group will be playing in a 2,000-seat concert hall. How many watts will you need to provide?

• A jazz-fusion group bought some PA loudspeakers. They want to play the loudspeakers as loud as they can get without blowing them up. Which amplifier should you recommend?

Questions such as these arise in any sound system design. To help you specify an optimum amount of amplifier power for a system, I’ll explain the relevant audio concepts here.

There are two goals:
1) Power the loudspeakers so they play as loudly as possible without burning out. In other words, determine the optimum powering for the chosen loudspeakers.
2) Achieve a certain loudness in a certain venue.

We’ll cover both topics.

Signal Levels
First let’s review the concept of average levels and peak levels.

As shown in Figure 1 (below), a musical signal changes in level (voltage) continuously as it plays. Imagine a musical passage with a low-level synth pad, but with high-level drum hits. The average level or volume of the passage is low, but the transient peak levels are high.

Peak levels may be up to 24 dB above average levels depending on the type of signal. Percussive sounds have much higher peaks than continuous sounds do (synth pads, organ, flute) – even if the two signals have similar average levels.

The peak-to-average ratio of the signal is called the crest factor or peak factor. In other words, crest factor is the difference in dB between the peak levels and the average level of the signal. Percussive sounds have a high crest factor.

Flutes, organs and violins have a low crest factor. The crest factor of speech is about 12 dB. Highly compressed rock music has a crest factor of about 6 dB: the peaks are about 6 dB higher than the average level.

See Figure 2 (below). It is a graph of signal level versus time when the amp is fed a typical musical signal. The average level corresponds to the signal’s loudness. The peak level is 6 to 24 dB above the average level, depending on the type of signal. In other words, the signal crest factor is 6 to 24 dB.

The bottom of Figure 2 shows an example of amplifier power output versus time when the amp is fed a musical signal. The amplifier is rated at 800 W continuous power. That’s the maximum power it can produce at rated distortion.

However, in this example the amplifier is putting out 50W on the average, so that occasional peaks of 12 dB don’t exceed the amp’s 800 W capability. Also, there is a little headroom so that the peaks don’t clip.

Headroom

In Figure 2, headroom is the difference in dB between the signal peak levels and the amplifier’s clipping level. Normally you want to allow at least 3 dB of headroom so that signal peaks don’t accidentally clip.

Peakroom
This term, coined by Syn-Aud-Con instructor Pat Brown, is crest factor plus headroom. Peakroom indicates how many decibels that peaks can be above the average level without clipping.


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About Bruce

Bruce Bartlett
Bruce Bartlett

Recording Engineer
   
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.”
http://www.bartlettaudio.com

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D J Pop says

Hello, I am new at sound reinforcement and I have a feedback destroyer and i tried to see if it works well and i am still getting the feed back.  Note I am inside of a garage with the equipment hooked up, i guess.  Should be in a larger place to see the full effect of the eliminator?

J. Seitz says

Did the definition of “headroom” change?  This article defines it as the space from above the peak level to the clipping level. In all of my sources, this is not the case. For example, in Barlett’s own textbook, he defines it as “the level difference in decibels between the normal signal level and the distortion level.”

Mistake or new definition?

Bruce Bartlett says

Wikipedia defines “headroom” as the difference between the nominal signal value and the maximum undistorted value. This article defined “headroom” as the difference between the instantaneous peak level and the maximum undistorted value.

Suppose a signal has a nominal or average long-term signal level of -15 dB below clipping. By the Wikipedia definition, you could say that the headroom is 15 dB. In other words, peaks can be as high as 15 dB above the nominal level without distorting.

We still need a term to describe the room between the instantaneous peak level and the maximum undistorted level. This article used the term “headroom”, but that’s not the original definition of “headroom”. Syn Aud Con instructor referred to that space between peak and clip as “headroom”, and defined “peakroom” as crest factor plus headroom. “Headroom” in this sense is a new definition.

This article proposed that new definition for “headroom” but I think it just confused the matter. We need a word to define the space between a peak’s level and the clipping level. If a peak starts to clip, you could say that peak has run out of headroom.

Regardless of how “headroom” is defined, a sound system needs some reserve power so that the instantaneous signal peaks do not exceed the headroom of the system - that is, they don’t clip (unless a little clipping is desired).

Bruce Bartlett says

Sorry to reply to the feedback comment so late… I just saw it today.

A feedback destroyer does not eliminate feedback. But sometimes it lets you get a little more volume before feedback occurs. The best feedback destroyers apply a narrow notch filter at the frequencies that are feeding back. Some units, though, apply a wide notch filter and so they change the tone quality and aren’t much help.

A garage is very feedback-prone because of all the hard reflecting surfaces. I think you’d have better results in an auditorium.

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