Loudspeakers & Monitoring

The quality of any sound is based on what you hear from the loudspeaker interacting with the room acoustics. But choosing the "best" loudspeaker depends on many factors, both for the loudspeaker and also the location it will be in. Key requirements though are that it must be able to handle a range of frequencies from 30-16kHz and reach 120db-SPL.

A loudspeaker is a transducer that converts electric energy to acoustic energy - essentially the opposite of a microphone. Like a mic, speakers come with different elements. The moving coil is the most common and is used for a wide range of monitors because it is sturdier and handles a high SPL. Ribbon speakers are used for high frequencies because they have smooth high frequency response. Electrostatic (capacitor) is rarely used because they are expensive and have a low dB rating.

There are two ways of powering speakers. Passive power uses external amplifiers, while active power uses a built in amplifier to power the speaker internally. Speakers can't linearly output all frequencies - a large speaker that generates low frequency sound waves can't efficiently create high frequencies and vice versa. So multiple speaker drivers are needed to create the whole frequency range. Using a crossover network to divide the frequency spectrum into high/low sections, a large driver creates the low frequencies (woofer for the bass) and a small driver creates the high frequencies (tweeter for the treble). The point where the divide occurs is called the crossover frequency. One division is a 2-way system loudspeaker, with the crossover at 1500-2000 Hz. Two crossover frequencies is a 3-way system loudspeaker, with divides at 400-500 Hz, and 3500-4000 Hz.

A passive crossover network has an external amplifier before the crossover frequency divide, which may cause distortion. An active crossover network has the crossover divide before the two internal amplifiers power it, also called Biamped. This is less expensive, causes less distortion, requires less power, and gives a better transient response.

A speakers specications re based on tests in an anechoic chamber (room with no reflections), so there will be different performance in every room because of the different acoustics. Some things to look for include the following.
Wide frequency response - at least 40-20kHz needed for professional audio. Studios use 3 sets of speakers: low quality with midrange response only like in car radios, average quality ones with more high/low, and good high powered wide speakers.
Linearity - where the input loudness is reproduced at the same level iwht no more than +/- 3 db variance.
Amp power - sufficient power to create loud sound levels without distortion. 30 Watt tweeters and 100 Watt woofers.
Dynamic range - pro audio needs 40dB-SPL for soft sounds and 120dB for loud for an 80 dB range. Most consumer speakers only have 50dB to 105dB capabilities for a 55 dB range.
Sensitivity - tells you the overall efficiency of the speaker. Should be 93dB or higher.
Polar Response - how a speaker focuses sound at the monitoring position. You want to monitor with the fewest number of reflections - no affect of the room. Use bass traps for the low frequencies because the longer wavelengths can't be controlled by room dimensions.
Arrival Times - reproduced sound must reach the listener within 1 ms of one another.
Polarity - if the driver cone motions are opposite (compression versus rarefaction) then they are out of phase and must be rewired.

Distortion is the appearance of a signal in the reproduced sound that was not in the original sound. It can be created anywhere in the sound chain. There are several types of distortion: Intermodulation (IM) - 2 or more frequencies occur at the same time and create combination tones and dissonances unrelated to the orginal sounds. Rating of 0.5% IM or lower is best.
Harmonic - introduced harmonic not in the original signals, occurs when input and output of sound system are nonliner.
Transient - inability of audio component to respond quikly to a rapidly changing signal.
Loudness - overload - signal recorded or played back at level greater than the system can handle.

Monitor placement also affects the sound quality, dispersal and arrival time. Flush mounting the speakers keeps the walls and protusions from affecting the sound. When monitoring stereo, there are 2 dimensions, so 2 symmetric speakers placed in an equilateral triangle with the listener is best for monitoring. Far field monitors are large high quality speakers flush with the wall, while near field are on or near the console closer to the listener for just direct sound. Surround sound expands the depth by placing the listener in the middle of the aural image instead of in front. 5.1 surround has 6 discrete channels - 5 full range and one low frequency subwoofer.

To calibrate the speaker system you need an obective measure of the correlation between the monitor sound and the room sound. A spectrum analyzer will display the frequency response over time or in real time. An SPL meter will give approximate levels in real time (RTA). Fix the problems at the source by modifying the monitor position, amplifier, acoustics instead of attempting to use signal processing/EQ to fix it.

Reference levels for film are 85dB, TV is 79dB, and music is 79-82dB.

Headphones give a wide, flat uncolored frequency response, that is consistant across different studios and allow you to hear sutle changes. However you don't get the same sound quality as from monitors, and it is an unaturally wide aural image.

The 60%/60 min rule states that you should not listen to your headphones for more than 1 hour per day at 60% of the maximum volume.


Make your own loudspeakers