Woofers, Tweeters and Crossovers - High, mid and low frequency speakers and crossover networks

Speakers are wonderful things. You connect them to a piece of electronic equipment and they produce sound. It doesn’t matter if you’re talking about a computer, a home entertainment center or a smart phone; as long as there is sound to produce, they will do it.

 

 

 

The humble speaker of today actually started out in the 1920s, almost a century ago. While efforts to replace electromagnetic speakers with some newer technology have been made, nothing has touched it for faithfulness in sound production and price. Other than minor changes in materials and designs, the speaker of today is essentially the same as the first one developed so long ago.

 

Speakers convert a complex AC (alternating current) signal into mechanical energy in the form of sound waves. Sound waves are nothing more than moving air. This air hits our eardrums and is converted back into neurological signals that our brain interprets as sounds.

 

To create sound waves, a speaker has to move. The electrical connection is attached to a coil, making it an electromagnet. This electromagnet is housed in a fixed magnetic field, created by a permanent magnet. Since the signal to the electromagnetic coil is an AC signal, the positive and negative poles of the electromagnet are constantly changing. Obeying the laws of physics, this causes the coil to move within the fixed magnetic field. The speaker cone, attached to the end of the coil, moves back and forth, creating the sound waves.

 

If a perfect sine wave was put into the speaker, it would produce a humming sound. The tone of that sound would depend upon the frequency of the sine wave. A higher frequency would produce a higher tone. The volume of that hum would depend upon the amplitude of the sine wave, or how high the sine wave’s voltage was.

 

However, most sound is much more complex than a simple sine wave. If you consider an orchestra playing, there will be several frequencies being played, all at the same time. Several different volume levels will be present. Not only that, but each type of instrument interjects its own unique characteristics into the sound wave, modifying it so that we can identify that instrument. The same speaker has to be able to play all of that at the same time.

 

Enter the Crossover Network

The more complex a sound wave, the harder it is for a speaker to faithfully reproduce the sound. Even so, they normally do a remarkable job of doing so. The true challenge comes when the speaker is required to produce frequencies or tones that are drastically different at the same time. That’s like asking it to vibrate 100 times a second and 10,000 times a second at the same time.

 

The human ear can hear a range of tones, generally stated as being from 20 Hz to 20,000 Hz (Hz, or “hertz” means cycles per second). To give you an idea of what that means, the lowest note on a piano resonates at 27.5 HZ, while the highest note on a piano resonates at 4186.01 Hz. So, we can hear a range of notes that goes from lower than the lowest note of a piano up to much higher than any musical instrument can reach.

 

To faithfully reproduce such a wide range of tones, it is normal to use more than one speaker at a time, creating a “speaker system.” Such a system can produce more accurate sound by splitting the full frequency spectrum into different parts, with each speaker only receiving the signal for a certain frequency range. That reduces the workload for each speaker, helping it to produce the tones more accurately, with less total distortion.

 

The job of splitting the frequencies and sending them off to each individual speaker in the system is done by a Crossover Network. This is essentially a series of filters, which filter out the frequencies that should not go to each speaker. The most common sort of crossover network is what is known as a passive 2-way crossover. This type of crossover is normally built into the speaker cabinet, with the outputs going to each of the speakers mounted in the cabinet.

 

In a 2-way crossover, there is a low-pass filter and a high-pass filter. The “pass” in those names means that it is allowing a certain part of the signal which is allowed to pass through the filter. So, the output of the low-pass filter goes to the speaker which is responsible to reproduce low frequency sounds; and the output of the high-pass filter goes to the speaker that is responsible for reproducing high frequency sounds.

 

We can add as many different speakers to a speaker system as we want, splitting the signal into smaller and smaller frequency bands. These additional filters in the crossover are referred to as “band-pass filters.” A band-pass filter is essentially a combination of a low-pass and a high-pass filter together, both of which are designed to leave a certain frequency band that is not affected.

 

Not all crossovers are created equally. In addition to the number of bands that the filter creates, there are also differences in the amount of filtering they provide. This is defined as the “order” of the filter. There are four main orders used for filters:

 

First order

20 dB/decade

6 dB/octave

Second order

40 dB/decade

12 dB/octave

Third order

60 dB/decade

18 dB/octave

Fourth order

80 dB/decade

24 dB/octave


First order filters are the most common, although all of those types are used in speaker systems. What the numbers in the table are showing is how much filtering the crossover provides per every 10,000 Hz or octave. When you consider that every 3 dB (decibels) is roughly the equivalent of halving the signal, we see that a first order filter still allows 1/4 of the unwanted frequency to pass through, while a fourth order filter allows only 1/32 of the signal to pass through.

 

When designing and building a speaker system, it is important that the crossover selected not only match the number and type speakers which are installed in the cabinet, but that it can handle the total amount of power (measured in watts) that is to be sent to the speaker system.

 

Active Crossovers

For larger systems, such as those that are used for performances by rock bands, active crossovers are used instead of passive crossovers. An active crossover splits the signal before the final stage of amplification, essentially working with the signal that comes out of the sound mixer board, before it is sent to the amplifier.

 

Some active crossovers have their own final amplifiers built in; however, it is most common to use them in conjunction with an external amplifier. Even for those cases where they use an external amplifier. The crossover does have some minimal amplification, to compensate for signal loss through the filters.

 

Most active crossovers offer some level of control, so that the sound engineer can select the exact crossover point for the filters, the number of bands he wants to create and whether he is creating a stereo or mono sound.

 

The outputs of the active crossover are then sent to individual amplifiers; one for each frequency band being created. In this sort of an arrangement, all the speakers within a speaker cabinet are receiving and reproducing the same signal. The speaker stacks which are commonly used for such performances will have several speakers dedicated to each of the bands.

 

On to the Woofers and Tweeters

Since the audio signal is broken up into different frequency ranges to be sent to different speakers, it only makes sense that the speakers be designed to handle those frequency ranges. That’s where woofers and tweeters come in. A woofer is a speaker designed for low-frequency sounds and a tweeter is a speaker designed for high-frequency sounds.

 

Woofers

At a glance, the main difference between woofers and tweeters is that the woofers are a lot larger than the tweeters. A good woofer might be 12 inches in diameter or more. There are a couple of reasons for that. First of all, the speaker has to move slower and the diaphragm (the speaker cone) has to move farther to create the sound wave. Secondly, the speaker must produce a higher volume of sound, as low frequency sound waves don’t travel as well as high frequency ones do and are much more likely to dissipate and be absorbed by surfaces they come into contact with.

 

The speaker enclosure and the woofer interact with each other; so the speaker enclosure is usually designed specifically to match the woofer. There are several types of designs, but the two basic categories are a sealed enclosure and a ported enclosure. Sealed enclosures try to trap the sound coming off the back side of the speaker, providing the cleanest, crispest bass sound. However, the sound volume is lower.

 

Ported speakers are designed to allow that sound to escape, adding to the volume. However, the sound coming off the back of the speaker is 180 degrees out of phase with that coming off the front of the speaker. That can cause the sound waves to cancel each other out. However, the extra distance that the sound waves coming off the back of the speaker have to travel prevents that. Instead, the sound becomes less distinct and “muddy” due to the phase shift between the two sets of sound waves.

 

Tuned ports are used on some speaker enclosures. These ports are created to a specific size, so that they will cause the sound to reach the area in front of the speaker exactly one cycle later than the sound coming off the front of the speaker. While this still creates distortion, it is less than that caused by an unturned port.

 

Tweeters

Tweeters do not interact with their cabinets at all, and at times are used without a cabinet. While the construction is similar to a standard electromagnetic speaker, they usually use a dome-shaped diaphragm in place of a speaker cone. These are referred to as “dome tweeters.” This diaphragm can either be made of plastic, plastic impregnated silk, aluminum or titanium. Each material type produces its own unique sound characteristics.

 

Since tweeters are extremely small, they don’t produce a lot of volume. To help this, many are attached to a horn. This horn resonates or vibrates with the tweeter, mechanically amplifying the sound that it produces, in much the same way that a trumpet or other brass instrument amplifies the buzzing of the musician’s lips.

 

Midrange

 

As previously mentioned, some speaker systems use three or more speakers. In those cases, midrange speakers are attached to each of the band-filters. A midrange speaker is essentially the same in appearance as a full-range speaker or woofer. The major difference is that s midrange speaker will not be as big as a woofer, but only about 5 to 8 inches in diameter.  

Rich Murphy