Loud Speakers and Amplifiers - Sound Engineering Term 2 - Lesson 1

Topic: Loudspeakers and Power Amplifier

Define Loudspeaker

A loudspeaker is a device that converts electrical energy into acoustical energy.

Loudspeaker Parts:

 

Speaker Driver:

This is a single transducer that transforms electrical energy into acoustic energy.

Tweeter vs. Compression Drivers:

Tweeter

A tweeter radiates sound more or less directly into the air. There is usually little to control the dispersion of a tweeter other than the front surface of the cabinet in which tweeters are mounted. With careful design, tweeters can sound very nice and pretty, but their behavior and polar pattern can be inconsistent. It can also take a lot of power, which eventually translates to size, to make a tweeter loud enough to keep up with larger speakers. Where speakers can be redundant (i.e., more than one speaker can cover a frequency range), the anomalies in tweeter dispersion and frequency response can cause objectionable phase cancellation and comb filtering when multiple devices are used together.

Compression Driver

Compression drivers, on the other hand, can have very controllable polar patterns and frequency response characteristics depending upon the type of horn they are coupled to. In addition, the compression driver makes the device much more efficient, so it takes less power to reach a certain SPL. Horns can also be used in multiples due to their controllability. Because of these factors, horns and compression drivers are used in almost every high-power PA system. It’s just not practical to use tweeters, especially in the critical 1k to 6k range. Above that, tweeters are sometimes used. One issue designers of compression drivers must address is the fact that sound from all parts of the diaphragm must reach the throat of the horn at the same time (in order to be in phase with each other. A device known as a phasing plug, which has a series of different length passages through it, is often used to accomplish this, but it’s never an exact science. There are side effects to this as well. Some compression drivers develop so much pressure at the throat of the horn that nonlinearities in the compression of the air are introduced. These can cause significant harmonic distortion, which is partly to blame for some high-powered horn systems' unappealing sound.

 

Dispersion Horns

Active vs. Passive Speakers:

A passive speaker does not have a built-in amplifier, whereas an active speaker does. It should be noted that all speakers require an amplifier to work, be it active or passive.  

3 Way vs. 2 Way Speakers:

A two-way speaker has a low- and high-frequency driver, whereas a three-way speaker has a low-frequency driver (LFD), a mid-frequency driver (MFD), and a high-frequency driver (HFD).

 2-Way Speaker:

Three-way speaker:

Bass Bins and Subs:

Bass Bins are low frequency drivers with one, two, or four drivers of the same size.

Single Bass Bin:

Double Bass Bin: 

 

Note:

The drivers of speakers are frequently measured in inches. For example, a 14-inch bass driver or a 2.5-inch tweeter.

Full Range Speakers:

A full-range loudspeaker drive unit is one that reproduces as much of the audible frequency spectrum as possible.

Point Source vs. Line Array Speakers:

Point Source:

Sound waves propagate in the shape of a sphere, spreading out horizontally and vertically. With every doubling of distance, the surface area of the propagation front increases by a factor of four. In other words, the sound intensity per unit area drops by a factor of four, and the sound pressure level attenuates by 6 dB.

These are literary directional speakers.

Line Array:

Sound waves propagate in the shape of a cylinder, spreading out horizontally but not vertically. With every doubling of distance, the surface area of the propagation front increases by a factor of two. In other words, the sound intensity per unit area drops by a factor of two, and the sound pressure level attenuates by 3 dB.

Line Array Speakers:

This is a stack of loudspeakers that cover the same frequency range. The key to a line array is that the speakers face slightly different vertical angles, allowing them to consistently cover a greater depth of field than a single PA speaker can.

Common Speaker Brands:

Below are some of the most common brands for live sound reinforcement loudspeakers.

  1. JBL
  2. Peavey
  3. Martin Audio
  4. Yamaha
  5. Nexo
  6. Bose
  7. QSC
  8. Dynaudio

Speaker Impedance and Wattage:

Speaker impedance is a measure of the amount of opposition to current flow in an electric circuit. This is measured in ohms, and it's important for amplifiers, AV receivers, and speakers that are connected to each other because mismatched impedance causes distortion.

Peak, RMS, Program and Continuous Power:

Note:

Speakers don’t generate power; amplifiers do. The amplifier supplies power to the speaker.

Therefore, the power specification that you will see for speakers – measured in watts – refers to how much power the speaker is designed to safely receive from an amplifier before it starts to distort.

Speakers input power and amplifiers output power.

500 Watts Continuous

1000 Watts Program

2000 Watts Peak

What do these mean?

Peak Power Rating:

A speaker’s peak power rating is the maximum wattage that its voice coil can handle in very short, occasional bursts — the power required to hit the speaker’s maximum output for a millisecond or two. While this peak rating may be a big, impressive number — like 1000, 1500, In the real world, 2000 watts isn't the most meaningful number. Remember, peak wattage is like the number at the top of a car’s speedometer. Your daily driver may be capable of 140 MPH, but if you sustain that speed for too long, you’ll do some mechanical or thermal damage to your car. And under constant peak power, the wires in a speaker’s voice coil would quickly melt. What we need is a way to determine how much "average" power a speaker can handle over time, which leads us to…

RMS and Continuous Power Rating:

In the real world, the most useful spec for a speaker’s power handling is its continuous power rating. This is the wattage that a speaker can comfortably handle for an extended period of time, like a show or rehearsal. Imagine speaker manufacturers running pink noise through the speakers for hours on end at varying levels, making sure they don’t burn out. While speaker testing methods differ from one manufacturer to the next, the continuous wattage rating usually falls between 25 and 50% of the peak wattage. To continue our analogy, if peak power is the top of your car's speedometer at 140 MPH, continuous power is around 60 or 70 MPH, which is where your car is most fuel-efficient.

As we exit the real world and head over into Nerd Territory, we confront the conundrum of “RMS power.” RMS stands for Root Mean Square, a mathematically defined statistical measure used by engineers to calculate the average of values over time. All this squaring and square-root-deriving is necessary to calculate the average of signals that alternate above and below zero (like audio, or voltage). The “negative” values on the bottom half of the waveform are squared, so they become positive values.

While we’re here in Nerd Territory, it’s worth pointing out that “RMS power” and “watts RMS” are actually misnomers. However, “RMS power” is a commonly accepted shorthand that’s become synonymous with continuous power, and most manufacturers get away with using the terms interchangeably.

Program Power Ratings

A speaker’s “program” power rating, sometimes known as the “music” power rating, has come to mean a doubling of its continuous wattage rating. A speaker rated at 400 watts continuous power, for example, would be rated at 800 watts program power. It's not always an exact doubling; for example, a speaker with a continuous power rating of 300 watts and a program power rating of 750 watts may be seen.

This “program power” rating came about because manufacturers generally recommend a power amp that can deliver about twice the continuous power rating of a passive speaker. This extra “headroom” helps ensure that the amplifier isn’t driven into clipping distortion. An amp rated at 800 watts continuous power is usually a good match for a passive speaker rated at 800 watts program for live sound reinforcement.

Define Power Amplifier:

A power amplifier is a device that boosts a signal from line level to speaker level.

Amplifier Power Classes:

Class D

Class AB

Class H

Class B

Class A

Power Amplifier Operating Modes:

As discussed below, most modern amplifiers have three operating modes:

  1. Stereo Mode
  2. Parallel Mono Mode
  3. Bridge Mono Mode

Stereo Mode:

This is more like having two amplifiers. Here the two channels operate independently.

Usage:

This is the most common amplifier mode.

Parallel Mono Mode: (Parallel Mode)

One input channel is connected to both amplifier output channels. It is important to note that the connection is made within the amplifier. After the input, the amp acts exactly like a stereo mode amp.

Usage:

This would often be used when you want to power one side of a stereo sound system with one amplifier in an event where you have multiple speakers.

Bridge Mono Mode: (Bridge Mode)

Bridge mono mode combines two amp channels into one mono, much more powerful amp channel. The input routing is similar to that of parallel mono mode.

The difference between bridge mode and parallel mode is that the polarity output of channel B in bridge mode is inverted. When channel one pushes, channel two pulls.

The advantage of bridge mono is that you have a much higher power rating; the disadvantage is that you have only one amp channel.

Usage:

This is often but not always used for subwoofers.