Loudspeakers, Part 1
Basic Principles and Types
A loudspeaker’s job is to transform an electrical signal into an acoustic pressure wave. It is the
functional inverse of a microphone. As such, much of what can be said about microphones also
applies to loudspeakers. There are three main types of transducers used for loudspeakers:
dynamic moving-coil, electrostatic (condenser type), and piezo-electric. There are also a few
other, more exotic, types but we shall not discuss them here.
The moving coil, or dynamic, loudspeaker is by far the most widespread in use. Sizes range from
very small high frequency devices of less than 1 inch diameter to low frequency drivers of 18
inches. Commercial drivers have been produced in the 30 inch range, but are uncommon. No
matter their size, all moving coil loudspeakers feature a permanent magnet. Within the magnetic
gap a voice coil is hung, just as in the case of the moving coil microphone. The coil is typically
wound on either a phenolic or aluminum cylinder and may consist of several layers of wire. More
expensive units may use edge-wound ribbon wire to minimize air gaps and increase efficiency.
The voice coil is centered in the magnetic gap with a corrugated cloth ring called a spider. The
coil is also attached to a diaphragm. The diaphragm is usually paper, but may also be made of
various plastics, or even metallic sandwiches1. The voice coil is hidden from view with a dust
cap. It is a common misconception that the dust cap is the same diameter as the voice coil. This
does not have to be true. The dust cap diameter may be many times larger than the voice coil. The
diaphragm is attached to the loudspeaker’s frame at the mounting end with a surround. The
surround may be corrugated treated paper or a corrugated or half-round of various plastics. Some
high frequency drivers do not have a surround, and instead terminate the diaphragm directly to
the mounting section. Although loudspeaker diaphragms are usually round, they do not have to be
so, and various elliptical shapes are sometimes used to maximize cone area in confined spaces. A
good example is the once ubiquitous 6x9 loudspeaker used in the rear shelf area of cars.
Dynamic loudspeakers are given a nominal impedance rating (typically 4 or 8 Ohms, although
many other values are possible). The actual impedance is in fact complex, and varies from
inductive to capacitive with magnitudes that may be less than half the nominal value to several
times the nominal value. It is not uncommon to see a peak impedance in the 30 Ohm range for an
8 Ohm loudspeaker near its resonant frequency. Moving coil loudspeakers are also given a power
rating, although the measurement means vary widely, and thus are not always comparable across
models or brands. Small, inexpensive units may dissipate less than 1 Watt, while large
professional units may handle several hundred Watts. Power handling by itself though, is no
measure of sonic quality. Pricing varies from the $1 range for small hobbyist units to several
hundred dollars for professional PA drivers.
Electrostatic loudspeakers, like the microphone counterparts, are basically a large capacitor with
one plate serving as the diaphragm. Electrostatics are notoriously difficult to drive due to their
negative (capacitive) impedance angle. Also, they tend not to be very efficient. Their strength in
comes from good transient response coupled with good phase and delay characteristics.
Electrostatics are not nearly as common as moving coil types, nor as rugged, and are generally
found in only in audiophiles circles. Some of the more expensive headphones on the market use
electrostatic drivers.
1
Metallic sandwiches usually have a foam core or other treatment that reduces resonance effects.
ET163 Audio Technology Lecture Notes: Loudspeakers, Part 1
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Piezo-electric transducers are typically used for high frequency reproduction. The process is the
reverse of the piezo-electric microphone. A voltage is used to cause a crystal to flex, which in
turn is connected to a diaphragm. The crystal cannot flex a great deal, so low frequency
reproduction is very limited (why this is so shall be seen later). Piezo-based high frequency
drivers can be had for as little as one dollar and can go well beyond the upper limit of human
hearing.
Frequency Response
Ideally, a loudspeaker would cover the entire human hearing range, from 20 Hz to 20 kHz, with
sufficient amplitude, low distortion, and flat frequency response. As has already been noted, this
is not possible with piezo-based transducers. It is possible to produce full-range dynamic or
electrostatic loudspeakers, although dynamic loudspeakers are usually designed to cover only a
limited range of spectrum. Several different drivers are then combined to cover the human
hearing range. Low frequency drivers are referred to as woofers, high frequency drivers as
tweeters, and mid-frequency drivers as squawkers (a term that has fallen out of favor, and which
largely has been replaced by the less colorful midrange). Drivers that are designed for the very
lowest octave are referred to as subwoofers. If the spectrum is split into two parts using a woofer
and a tweeter, the system is referred to as a two-way system. If a midrange is added, it is referred
to as a three-way system, and so on. Commercial systems can be as high as five-way, but for cost
considerations, most home systems top out at three-way. It is important to note that the frequency
spectrum will not be divided linearly among the drivers. This is due to the fact that humans
perceive ratios of change in frequency, not absolute Hertz, and that the output spectrum of human
voice and musical instruments is not spectrally uniform (more on this subsequently).
In order to combine the response of several limited frequency range drivers, some form of
frequency selection needs to be added. Also, as the efficiency of each driver may be different,
some form of level control is required as well. These two requirements are achieved through a
crossover network. A crossover can be as simple as a capacitor in series with a tweeter along with
an inductor in series with a woofer in a two-way system (i.e., a first order network). Second and
third order networks are popular choices, using L, T, or Pi configurations. The choice of
crossover frequencies will depend greatly on the capabilities of the drivers used as well as the
application. A small home loudspeaker system might be crossed at 2 or 3 kHz, while a two-way
PA system may be crossed at only 800 Hz. In higher power systems, the crossover network will
be separate from the loudspeakers and will precede the power amplifiers. This is known as an
actively crossed system, and has numerous advantages including greater power efficiency and
driver matching, and lower distortion. It is generally more expensive as several power amplifiers
are required, and thus, better suited for high-power and critical monitoring applications.
Polar Response (Directivity)
Practical loudspeakers do not radiate in an omnidirectional pattern at all frequencies. Again, like
their microphone counterparts, loudspeakers can be made to radiate in various patterns. Much of
this has to do with the design of the enclosure and/or the clustering of several devices. Enclosures
will be examined in Part II. For simple boxes, wavelength becomes a determining factor in
directivity. As the wavelength shrinks above the diaphragm and box dimensions, a narrowing of
response occurs. At low frequencies, these boxes tend to radiate omnidirectionally, but tend to
beam high frequencies straight forward (i.e., along the major axis). Consequently, sitting off-axis
from a typical loudspeaker will produce a reduction in apparent high frequency content.
ET163 Audio Technology Lecture Notes: Loudspeakers, Part 1
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Efficiency and Sensitivity
Efficiency is the measure of how much acoustic power is produced for a given electrical power
input. Typical values for moving coil loudspeakers run in the .5% to 3% range, although
professional drivers in the 7% to 8% are available. There are ways of increasing this through
proper enclosure and loading design. Similar to efficiency is sensitivity. This is usually specified
as the SPL in dB at one meter on-axis for a one Watt input. Other values may be also be used.
Typical numbers for home systems would be in the mid 80 to low 90 dB/1W/1m range. Note that
as sensitivity is measured only on-axis, a very “beamy” loudspeaker could have a higher
sensitivity than a more omnidirectional design that has higher efficiency.
Example Problems
1. Q: A moving coil loudspeaker has an efficiency of 3%. What is the acoustic output power for
a 100 Watt electrical input? A:  = Pout/Pin so, Pout = Pin*, Pout = 100W*.03 = 3 acoustic
Watts.
2. Q: A loudspeaker has a sensitivity of 90 dB-SPL at 1 Watt, 1 meter. What is the level for a 2
Watt input measured a 2 meters (assuming free-field measurements and a linear
loudspeaker)? A: A doubling of power is a 3 dB rise, however, sound pressure level falls off
as the square of distance change. Thus, a doubling of distance produces one quarter of the
SPL, or a 6 dB drop. The combination of the two changes results in an output level of 87 dBSPL.
ET163 Audio Technology Lecture Notes: Loudspeakers, Part 1
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