The sound of an audio system is most heavily influenced by the devices that generate the sound - the speakers.

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From signal to sound

Speaker Parameters

Audio Resistance

Mechanical stuff

Control the bass

Multiple speaker systems

Actual devices in use

Links to information

The Final Transducer

A basic speaker functional diagram

A three-way speaker diagram

The Speaker is a Dictator

The final transducer in an audio reproduction chain or system is the speaker, often referred to as a loudspeaker, to provide some context. For a well-operated system, this component is the one that dictates the overall quality of the sound. One author has suggested that it is best to start with the speakers when designing a new system. More often than not, when people complain about the sound from a system, it is often the speakers and the room acoustics that lie at the heart of the problem.

A Type of Motor

A speaker is a type of motor that moves a cone or plate to produce sound. The audio signal passes through a coil of wire, connected to the cone, suspended in a magnetic field. The alternating wave of the signal produces a magnetic field that interacts with the surrounding magnetic field and causes the coil to move in accordance with the audio signal. In theory, a speaker is the opposite of a microphone. In fact, small speakers are sometimes used as microphones. The picture to the left shows a simplified diagram of a speaker driver circuit.

Frequency-dependent Design

The design of a speaker depends on the range of frequencies it reproduces. Because of the different demands of high and low-frequency reproduction, most speakers that need to cover a wide frequency range have separate drivers for high and low frequencies. In some cases, and third speaker may be used to cover mid-frequencies. These are called two-way and three-way speakers. An electronic circuit called a Crossover must be placed in the enclosure to separate the incoming signal into the frequency bands needed by each separate driver.

The Enclosure

The driver is always placed in some type of enclosure designed to match the properties of the driver.

Together, the driver and the enclosure make up a typical loudspeaker. In free space, the moving cone produces sound waves from the front and the back that are of opposite polarity. At low-frequencies, these waves are omni-directional, so the front and rear waves tend to cancel each other, with a subsequent loss of bass frequencies. The speaker is put into an enclosure that either completely captures the rear wave (acoustic suspension) or redirects it in a way that reinforces the front wave (Bass reflex design). Some enclosures use a more elaborate construction to control the radiation from the rear of the driver. See below and Additional Resources below for more details.

Basic Speaker Characteristics

Response curves for three Klipsch speakers and a subwoofer.

Directivity curves of a B&C 6FH51 loudspeaker at several frequencies

Comb Filter Effect

Comb filter effect due to interference between direct

and reflected sound waves.

Frequency Response Curve

As stated above, a speaker is designed to cover a certain range of frequencies. However, no speaker is capable of reproducing then entire range evenly. A line which shows the relative response vs frequency is called its frequency Response Curve.

Directivity

Every speaker has a distinct directionality that varies with frequency. The pictures to the left show such curves for a particular device. Notice how the lower frequencies are more omni-directional while the beams narrow at higher frequencies. The size of the emitting diaphragm or cone affects the dispersion pattern, as does the baffle on which it is mounted. In January 2022 Bruce Main published a helpful article that has some contour diagrams that help to visualize the effect of frequency.

Sensitivity

Sensitivity measures the loudness of a speaker for a given signal voltage. The typical measurement is Sound Pressure level (SPL) at one meter for a 1-volt input.

Power Capability

The power capability is usually expressed as the maximum SPL it can generate or as a sustained level measurement. Its very helpful to have both.

Impedance

Impedance is a measurement in Ohms of the resistance of a speaker to an audio signal. The impedance of a speaker varies with frequency. The value may be given as the minimum impedance, which is very important to know, or as the value at a given frequency. It is most helpful to see the entire impedance vs frequency curve. The picture to the left shows both the frequency response (red) and impedance curve (brown) of a particular speaker.

For more details, see the Wikipedia article on

Electrical Characteristics of Dynamic Loudspeakers.

Ken DeLoria has published an article that describes 8 Prime Factors that Can Determine Sonic properties of Loudspeakers.

In Jan 2023, Joe Begin updated his 2019 article on The Keys of Loudspeaker Electroacoustic Measurements. This gives examples of the problems incurred when trying to get accurate measurements of loudspeaker performance. (Part 2) (Part 3)

Speaker Impedance

Capacitance and Inductance as a Function of Frequency

Details

Speaker Polarity

When a positive DC voltage is applied to one terminal of a speaker, if the cone moves forward, it is marked as the Positive terminal. The other terminal is marked negative. When speakers are connected together, the polarity must be taken into account. They must be "in phase" with each other, If two speakers are connected with opposing polarity (out of phase) and face the same direction, their sound will tend to cancel out. This consideration also applies with a multi-way speaker. However, there are certain crossover designs where the tweeter is supposed to be wired out of phase with the woofer.

when I mentioned the need for speakers to be in phase with one another, I used the terms most often used. The correct statement is to use the term polarity. When the polarity of two speakers is opposite, they are 180 degrees out of phase. It is when two speakers are connected with the same polarity, but one is further forward than the other, that they become out of phase, When the same sine wave is applied to them the sound from one will lag the other. A delay must be applied to one to get them in phase. You will hear the term "coherent design" when multiple speakers are used together so that they are positioned as to be in phase with one another. It's funny how simple concepts can be so complicated!

It's Audio Resistance

The impedance of s speaker is the resistance that it offers to an audio signal in the rage of 20 Hz to about 20 kHz. It is measured in Ohms. For home systems, the impedance is usually from 4 ohms to 16 ohms. For professional systems, it is not too unusual to see impedance get as low as 2 ohms. The power amplifier used to drive the speaker must be capable of operating at the speaker's rated impedance through the entire frequency range.

It's Got Three Electronic Components

As was shown above, a speaker uses a coil of wire to generate the energy needed to move its cone or diaphragm. The wire itself has a certain basic resistance measured as Direct Current (DC) resistance. This coil and its associated mechanical structure also offers some Capacitive Reactance (Capacitance) that decreases with frequency. Because the coil consists of several turns of wire close together, it creates an Inductive Reactance (Inductance) that increases with frequency. These three electronic components combine with the properties of any crossover and the enclosure to give the final impedance of the speaker.

It Varies With Frequency

Because of the way a speaker is constructed, its impedance is a function of the frequency of the audio signal. The interaction between these electrical components is complex. Its helpful to remember that Inductance increases with Frequency while Capacitance decreases with Frequency' The result is its Frequency Response Curve. This curve results from the nature of the driver (or drivers) itself, any crossover circuits, and the enclosure design. Because the impedance of a speaker varies with frequency, the value is usually stated as "nominal" impedance and is typically slightly higher than its lowest impedance.. Thus one should try to obtain the actual response curve to be sure the amplifier can drive it safely. This is especially important when connecting speakers in parallel. For more detail, see Additional Resources below and the the section below on the Enclosure.

It Depends on the Total Number of Speakers

One or more speakers may the connected to a single amplifier, either to increase sound level, area coverage, or both. Multiple speakers may be connected in parallel or in series. The diagram to the left shows how four 8-ohm speakers can be connected so as to create an 8-ohm load to the amp. Note that the polarity of the connections must be observed. An excellent video that addresses loudspeaker connections can be found at BiAmp.

Additional Resources


Speaker Driver Types

A Dynamic Speaker Driver

Planar Magnetic Driver

A Dynamic Ribbon Driver

An Electrostatic Driver

THE DYNAMIC SPEAKER DRIVER

The vast majority of speakers are designed with dynamic driver elements. The voice coil is suspended in a magnetic field and connected to a solid material that can move when an audio signal is applied to the speaker. Some speakers consist of two separate drivers for high and low frequencies. The are called Coaxial speakers. Some type of crossover is used to send the correct frequencies to the two drivers. The Audioholics website has details about dynamic drivers.

MAGNETOSTATIC PLANAR DRIVERS

Magnetostatic-planar magnetic speaker drivers, like the dynamic drivers mentioned previously, work on electromagnetic principles. However, rather than having a voice coil attached to a cone-like diaphragm, the planar magnetic driver has a thin planar (often rectangular) diaphragm with an embedded conductive wire (also planar) placed in a planar magnetic array. Magnepan is the industry standard producer of planar magnetic loudspeakers.

RIBBON DYNAMIC DRIVER

With a ribbon driver, a thin conductive material, usually corrugated, is suspended between the two poles of a magnet. An audio signal passed through the ribbon causes it to vibrate and produce the signal. Most often, this type of driver is used for high-frequency drivers.

THE ELECTROSTATIC TRANSDUCER

An electrically-charged diaphragm is effectively sandwiched between two large perforated stator plates and acts as a parallel-plate capacitor. The device is generally charged via a high-level DC biasing voltage or a strong electret material. This type of speaker presents a highly capacitative load to the amplifier and not all amps can drive them satisfactorily. Martin Logan is well-known for their high-performance electrostatic speakers. These speakers usually require a separate woofer for very low frequencies.

OTHER DRIVER TYPES

A number of other designs are used for special purposes, Piezoelectric drivers work very well for higher frequencies. The Heil air motion transformer (or simply “air motion transformer” or AMT) is a special type of speaker transducer that is usually used for higher frequency drivers also. Several other less-used designs such as Tactile Transducers can be found that are used in certain situations. Details about of these driver types can be found at MyNewMicrophone.com and at Gear Patrol. Also see the Dayton Audio site,

Constant Voltage Systems

ONE AMP. MANY SPEAKERS

When it is deemed necessary to use a large number of speakers, a Constant Voltage System is usually needed. Commonly called a 70-volt system, these systems are also used with 25 and 100 volts or even higher as well. Actually, these systems are more properly called Constant or High Impedance Systems, or High-voltage Audio Distribution Systems. but the older term is more widely used.

TRANSFORMERS ARE THE KEY

These systems use transformers to control the power to each speaker. The diagram to the left shows how this is done. While this shows a transformer at the amp, amplifiers can be designed to operat6e a constant voltage system directly. In most cases, the transformer at each speakers has taps for different power levels. As the diagram shows, each speaker can be set for a different power draw, but the total power needed to drive the speakers must not exceed the power capability of the amp driving the system.

ADVANTAGES OF C-V SYSTEMS

  • Much longer distances allowed for speaker runs
  • Speakers don't affect each other's levels
  • Smaller wire sizes can be used for distribution lines
  • No complex series-parallel wiring
  • Stereo pairs can use common ground line.

ADDITIONAL INFORMATION ON THIS TOPIC

Speaker Enclosures

A Bose radio utilizing its waveguide design for improved bass response.

^ A 3-way Passive speaker Crossover Network ^

WHY IS AN ENCLOSURE NEEDEED?

The radiation at low frequencies from a standard dynamic driver is omni-directional. The radiation from the rear of the speaker is obviously out of phase with the front radiation, so at low frequencies, the front and rear radiation tend to cancel one another. Thus, some type of enclosure is required yo trap or somehow use the rear waves for good low frequency response.

THIELE-SMALL PARAMETERS

Thiele/Small parameters (commonly abbreviated T/S parameters, or TSP) are a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published by driver manufacturers and can be used to design proper enclosures for a given driver.

For example. one important TS parameter is Vas – Equivalent Compliance Volume, i.e. the volume of air which, when acted upon by a piston of area Sd, has the same compliance as the driver's suspension:

where ρ is the density of air (1.184 kg/m3 at 25 °C), and c is the speed of sound (346.1 m/s at 25 °C). Using SI units, the result will be in cubic meters. To get Vas in liters, multiply by 1000. Cms is the Compliance of the driver's suspension, in meters per newton (the reciprocal of its 'stiffness'). This illustrates how the enclosure must be matched to the characteristics of the driver. Consult Wikipedia for more details on TS parameters. An online calculator for TS parameters is here. More that 130 calculators for various design purposes can be found at HiFi Audio Design.

One thing Thiele found when developing his equations is that there is an inherent trade-off between box size, low-frequency response, and efficiency. You can design for any two but not all three at the same time. At the time, efficiency was very important due to amplifier power limitations, Now, with things like class D amps, efficiency is no longer as critical.

ACOUSTIC SUSPENSION

Here the entire rear radiation is captured in the sealed enclosure. For adequate low-frequency response, the speaker must have a very low resonance frequency, This means a very flexible cone surround - high compliance.

BASS REFLEX ENCLOSURES

This design usually matches the resonance frequency of the enclosure to that of the bare speaker. Some type of opening or port is used to radiate the low frequencies, thus increasing the output level and lower frequencies. The enclosure is treated inside in some way to minimize radiation of higher frequencies. Because the low frequency energy below the enclosure resonance point "unloads" the driver, it is possible to damage the driver with these low frequencies. To deal with this, it is helpful to insert a high-pass filter with a steep slope somewhere in the signal chain.

TRANSMISSION LINE ENCLOSURE

In this design, often used for subwoofers, the tuning port is a long passage inside the enclosure to tune the low frequency response and radiate the rear wave to reinforce the front wave from the speaker. It is very effective in reinforcing the bass response, but it also means a somewhat larger enclosure. Some of these designs can be very elaborate.

HORN-LOADED DESIGNS

A driver can be attached to a horn which causes more of its radiation to be directed to the front. This increases the efficiency of the driver and makes it more directional. The bass reflex port or transmission line can also be designed to terminate as a horn. Expanding geometry and mouth size determine the acoustic behavior of back loaded horn enclosures and separate them from standard transmission line design. Martin King has published a detailed article on designing a back-loaded horn speaker design. John Sheerin in 2006 published a detailed article on horn design.

BAND-PASS DESIGNS

the bandpass box is a combination of a sealed and a ported design. In a single reflex bandpass box, the sealed chamber is found at the rear, while the ported chamber, through which the sound comes from, is found at the front. In a double reflex bandpass box, the chambers of the front and rear areas are connected to the listening area itself, with the help of a port. For higher power operation, two drivers are often used,

SELF-POWERED SPEAKERS

These days, with the advent of high-quality class D amplifiers, it is quite common to use self-powered speakers. The enclosure then houses one or more amplifiers, crossover elements, and some type of DSP that is used to tailor the frequency response. The crossover components can be much smaller and less expensive since they don't have to have the low impedances required if driven by the amplifier. This usually means that the enclosure, crossover, amps, and DSP are carefully matched to the measured frequency response of the drivers by a design engineer for optimum performance.

Things you didn't realize about bass

Alpha Sound produced a video in July 2022 that shows some interesting experiments with bass speakers. He demonstrates physically the effect of the bass reflex port. He also demonstrates the use of multiple speakers and digital delays to give a bass speaker some directionality and how two speakers can be set up to provide a cardioid directional pattern. An example is the QSC cardioid sub shown below.

Some Examples

^The ElectroVoice Sentry III Monitor Speaker^

^ The Martin Audio TORUS Speaker ^

v The AirBlade Tweeter v

^The Decwave D032 Dual Horn-loaded Subwoofer^

THE BEHRINGER B2092A BANDPASS SUBWOOFER

This is a sub that I have owned for many years. It is no longer made, unfortunately, having been replaced by new designs such as the NEKKST 10S. It was intended to be used with Behringer B2031A monitor speakers, which are excellent low-price monitors. A detailed review of the latter is here. The subwoofer uses an innovative bandpass design that gives excellent dynamic response and linearity, using two high-compliance drivers driven by a 360-watt amplifier. An active filter at 80 Hz limits the high-end response and a 30-Hz filter controls the low-frequencies. The frequency response is given as 32 - 80 Hz (-3dB) and 28 - 100 Hz (-10 dB). A review of the sub can be found here. While some like to denigrate Behringer products like this which are made in China, these are made according to Behringer specifications, Mine are still operational after 30+ years.

The Klipsch Klipschorn . This speaker was designed to be placed in the corner of a room, where the two room walls become the outer walls of the horn-loaded bass section. First designed in the 30's and manufactured in the late 40's, and it has been in production since 1946. It is a fully horn-loaded three-way speaker using a 15-inch bass driver. This classic speaker has undergone many changes over the years as better drivers became available, but the basic horn-loaded bass concept remained as a unique feature of this speaker, giving it its acclaimed bass response and efficiency. The latest models do not require corner placement. It has a frequency response of 33 Hz to 20 kHz (4 dB) and a max SPL of 121 dB. Unfortunately, not everyone can afford these as they sell for about $15,000 for a pair.

THE ELECTROVOICE SENTRY III MONITOR

The EV Sentry III was introduced in 1972 based on a design by Dick Small using the recently published Thiele-Small design equations. It features a special 15-inch woofer in a ported enclosure crossed over at 500 Hz to a 28-inch sectoral horn, pictured in the header image, in turn crossed over at 10 kHz to a horn-loaded tweeter. The cabinet had two tunings, one at 40 Hz for maximum efficiency and one at 29 Hz that required a special filter peaked at 30 Hz for flat response down to 22 Hz. It had a rather smooth, neutral sound for use as a control room/recording monitor. Audiophiles such as I used these in our homes, and mine are still functioning since 1972 after a woofer re-cone in 2002. The picture to the left shows the Sentry III without its removable grille cloth frame.

A MODERN PROFESSIONAL SPEAKER

An example of a modern professional speaker is the Martin Audio TORUS. This speaker is used in their Constant Curvature Array systems for large venues. Typically, these are stacked to form a Line Array, which may include a subwoofer. For applications that typically require a throw between 15-30 meters, a full-blown line array is not always practical, optimal or affordable. Conversely, a point source solution may not be sufficient in coverage and SPL. TORUS is designed to fill that gap perfectly, combining optimized coverage, SPL profile and cost efficiency.


Each TORUS cabinet is designed for a flexible horizontal dispersion pattern and this can be manually adjusted between 90°, 60° or 75° (Asymmetrical) via the unique Dynamic Horn Flare™. This not only moves and locks the waveguide but also adjusts the horn mouth, including the low diffraction termination into the baffle, to ensure correct geometry and optimal performance in each of those settings.

More Details

THE SOUNDPRISM AIRBLADE TWEETER

Probably the most striking component of the esoteric SoundPrism Excellent model is an AirBlade tweeter, a design by American inventor Eugene J. Christensen, assembled under licence by hand in the UK at Arya Audio. Due to its complexity only available in small numbers. it is an extended development of the Oskar Heil Air Motion Transformer, whereby the high frequencies do not get bundled in the AirBlade, but radiated over a horizontal plane of 180 degrees. Both at the front and at the back of the harmonica-shaped membranes. This technology allows the tweeter to react at a speed that is four times faster than a conventional dome, is at least as transparent as a pure electrostatic driver, but does not have the detrimental beaming that characterizes all electrostatics.

A HORN-LOADED SUBWOOFER

The Decwave W032 subwoofer is an example of a horn-loaded subwoofer. It is a Dual Folded Horn using two ten inch subwoofers. Designed for use in a car,