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

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