Speakers have been around for quite some time and originally they have featured a single driver and hence a single driver type.

Over time, many changes have been made to make loudspeakers wide, tall, loud, bassy and so forth. However, when the audio quality has become a mission for sound engineers, they have realized that sound separation is a must.

To achieve that, different speakers (aka. electroacoustic transducers) need to focus on producing different sounds. In other words, to improve sound quality, each speaker must only produce sounds within the limited frequency range.

This allows the driver to specialize within the frequency band so that it does not need to reproduce too many sounds at the same time causing sound distortion.

As a result, the most common setup to deliver the balanced sound is to have three separate speakers where one is responsible for high-frequency sound delivery and the other two for mid-range and bass respectively.

To achieve an overall balanced sound, the three speakers are usually placed in a single enclosure often referred to as a loudspeaker system. With all types of speakers built in, the loudspeaker is usually designed to cover the full spectrum of human hearing that ranges from 20 to 20,000 Hz.

Tweeter

A tweeter is the smallest type of loudspeakers that is also known as the treble speaker. The speaker is designed to reproduce the upper limit of the audible frequency range. It varies between tweeters, but typically the sound frequency it delivers ranges from 2,000 Hz to 20,000 Hz.

There are special types of tweeters that can generate sounds up to  100,000 Hz, yet those sound waves remain outside of the human hearing capacity.

The term ‘tweeter’ comes from the word ‘tweet’ which represent high pitch sounds made by some birds.

different loudspeaker enclosures

To produce high-frequency sounds, the diaphragm has to vibrate faster, which makes the smaller speaker size essential. In fact, to produce higher frequency sounds, the speaker diaphragm must move faster making the size of the diaphragm a critical factor. Larger drivers are not able to move at a fast and as a result – not able to generate quality high frequency sounds.

Modern tweeter diaphragms are made of silk, polyester film or fabric, aluminium and other special alloys or even titanium. The diaphragm is attached to the surround’s (aka suspension) wide end which represents a metal rim that supports cone movement. It is connected to the basket using a flexible metal ring that allows coil movement and keeps it securely in place.

Most of the time the treble speakers sit within the main enclosure together with the other speakers, however there are setups where tweeters are designed to sit in their own semi-independent unit. 

types of loudspeaker enclosure

Mid-Range Driver

A mid-range speaker is a driver that is also known as a squawker. It is designed to deliver sound from 250 to 2000 Hz frequency range.

Most of the time the driver has a cone shape, however, you can also come across dome type drivers, but those are less common.

The design is simple, most of the time where the diaphragm has a voice coil attached to the neck and the cone surround attached to the outer side of the diaphragm.

The most common material used for cones of mid-range drivers is paper. At the same time, resins and polymers are being used more often now as the provide better vibrational damping than paper. Hi-end drivers can also be made using fiberglass, carbon fiber light metal alloys or even Kevlar.

Across the entire sound spectrum, mid-range speakers handle the most important part of it. First of all, mid-range is typically the busiest of all frequency ranges as most of the sounds we hear day-to-day fall within that range. In addition, due to receiving mid-frequency range sounds on a constant basis, human ears have been trained to recognize discrepancies and distortions with ease.

Secondly, most musical instruments sit, and most importantly – human voice sit within that range, which is critical to the overall sound quality of the song or sound track being played. In fact, the majority of basic sound reproduction devices such as radio, TV sets and many others, only have one driver which is a mid-range driver.

It is important to understand that a reasonable sound quality can be achieved by only reproducing middle frequencies. The same cannot be said about high or low frequencies as even when being reproduced together, the overall sound experience delivered won’t be anywhere practical or listenable.

Low Range or Bass

The low range frequency gets reproduced by woofers and sub-woofers. The word gets derived from the dog’s barking or a ‘woof’, which uses lower frequency waves, compared to birds ‘tweeting’ that occupy the top of the audio spectrum.

The difference between woofers and sub-woofers is the frequency range they are designed to reproduced with former typically working within the 40 Hz to 500 Hz range and latter occupying sub 100 Hz frequencies.

Most consumer grade loudspeakers combine woofers and sub-woofers into a single speakers, yet as you move up in the sound fidelity, those two get separated for purer, cleaner and more refined low frequency sound. 

Woofer

A woofer, also called a bass speaker is a term for loudspeaker or a driver tasked with reproducing low frequency sounds.

Most of the time, it features a electrodynamic driver made of strong paper or various polymers.

With the lowest end of human hearing being around 20 Hz, woofers don’t typically exhaust human hearing capabilities working in 40 Hz and upwards range.

three different types of sound radiation systems

With some loudspeaker systems, the range covered by a woofer extends out to 3000 Hz or even 5000 Hz. Those speakers are called ‘mid-woofers’ and are typically found in home theatre systems where sub-woofer comes in it’s own enclosure. In practical sense, they leave the lower reproduction to sub-woofers and take on the woofer and mid-range speaker frequency delivery.

Woofers are typically bigger, heavier and have significantly larger magnets than other speakers in the enclosure. To reproduce the lower-end efficiently, the speaker has to have greater surface and more raw electric power, hence the bigger diaphragm and magnet.

The amplifier low-frequency signal engages the magnet that pushes a relatively flat surface of the diaphragm to create a mechanical air movement. This makes the vibration less frequent, but a lot more intense compared to other drivers in the enclosure.

moving coil loudspeaker

Sub-woofer

Sub-woofers are typically complete loudspeakers. They are designed to be hosted in separate enclosure that is physically positioned away from the main loudspeaker hosting high, mid and low frequency range drivers.

The objective of the sub-woofer is to reproduce the lowest two or three octaves of the human hearing which are known as bass or sub-bass.

Most of humans can register 20 Hz and above frequencies, which is where sub-woofers operate, staying below 200 Hz for consumer grade products and 100 Hz for professional, studio grade applications.

Most subwoofers are made of a single speaker mounted in a special loudspeaker enclosure. Two speaker subwoofer designs are less common, where one of the speakers would work in the reverse order of the other one. This ensures when the first speaker pushes the air out of the enclosure, the second one is pulling it in and vice-versa.

The enclosure is typically made of real wood to both make it robust and assist low-end sound reproduction. The design of the enclosure can vary and can have a port (i.e. bass reflex), passive radiator, sealed enclosure, bandpass, horn or infinite baffle.

Full-range Driver

A full-range speaker, as the name suggest, is designed to cover the entire audio spectrum from low to high frequencies.

As you’ve learned above, low, mid and high frequency drivers feature different designs to be able to reproduce sound within their respective bands efficiently.

The challenge of a full-range speaker are physical limitations and constrains that are sometimes are mutually exclusive when efficient low or high frequency range reproduction is considered.

electrostatic speakers for middle range frequency band

To deliver as much of the audible frequency range as possible, there are a number of compromises that had to be made. One of the key things to get right is the size, as it cannot be too little, since you need to generate physical air movement to obtain low frequencies. At the same time it cannot be to large, as then the diaphragm would be too bulky to generate enough frequency for the high end of the audio spectrum. As a result, the most typical size of full range drivers is between 3″ and 8″.

The core design is similar to other drivers and features a single driver, voice coil to power the diaphragm. In addition, to mitigate some of the physical limitations described above, additional engineering decisions are applied. One of such examples is the horn or whizzer cone (see picture above) placed at the narrow end of the diaphragm to improve high-frequency reproduction. Other solutions include the use of the radiating dome instead of a traditional dust cap and many others.

Overall, full-range speakers are rather a compromise between sound quality, simplicity and physical size. They do not tend to perform exceptionally well withing any of the frequency ranges, are able to deliver sufficient amplitude and result in a slightly inaccurate sound reproduction at times. They are masters of none, but rather of a compromise that is justified in countless audio applications where practicality is more important than sound fidelity.

Wrap up

Despite many years of development and ingenuity, the concept of a speaker has not changed much. Whether it is a Bluetooth bathroom speaker or a concert hall speaker, the principle behind it is exactly the same. There are different speakers because there are different purposes to solve and price points to match, yet the underlying technology still revolves around creating sound vibrations by moving air at a particular frequency.