Do Headphones Have Subwoofers & How Do HPs Produce Bass?


Whether we’re listening through loudspeakers, studio monitors, headphones or acoustically, the bass and sub-bass frequency ranges are important, giving give weight and power to what we are hearing. Subwoofers are regularly used to create the low-end frequencies and often built into loudspeakers or build as their separate units in studio monitoring and theatre sound setups.

Do headphones have subwoofers? Headphones, by the nature of their small design, do not have subwoofers to push large amounts of bass frequencies. Rather, they sound bassy due to their close proximity to the ear; the enclosure they make with the ear, and via the natural resonances of bone conduction.

In this article, we’ll define subwoofers in greater detail and discuss the different ways in which headphones produce bass without a subwoofer.


What Is A Subwoofer?

A subwoofer is a loudspeaker designed to reproduce low-frequency audio signals in the form of sound waves in the bass and sub-bass ranges.

Consumer-grade subwoofers, like those found in automobiles, typically reproduce 20 Hz – 200 Hz while professional live sound reinforcement subwoofers are designed to produce sound under 100 Hz. THX-approved systems are designed to produce under 80 Hz.

Because of their specialized but limited range, subwoofers are always used in conjunction with other speakers in a system that is ideally capable of producing the entire audible range.

2.1 stereo, for example, has a left channel speaker, a right channel speaker and a subwoofer.
5.1 surround has speakers for the left, centre, right, left surround and right surround channels along with a subwoofer.

The low-end of human hearing is felt more than it is heard and the subwoofer’s job is to propagate those low-frequency sound waves through the air so that the listener can hear/feel them.

Subwoofer drivers are nearly all moving-coil designs with a large durable diaphragm. The robust nature of the moving-coil driver allows subwoofers to push the great amount of air required to produce low-end sound with relative ease.

How Do Subwoofers Work?

Subwoofers are transducers that convert audio signals into sound waves via electromagnetic induction. They actually work in the exact same way as dynamic headphone drivers, only they are much bigger and have a much more limited frequency response.

The following simplified diagram could represent either a subwoofer, a regular loudspeaker, or a headphone driver:

Note that subwoofers are often used in tandem with audio cross-over which essentially split an audio signal into different bands of frequencies. Professional set up will oftentimes only send the low-end frequencies to the subwoofer to avoid sending any extra energy.

The audio signal is sent to the voice coil via two electrical lead wires (one connected to each end of the coil). The audio source creates a circuit with the voice coil. As the low-end audio signal is sent to the subwoofer, it causes an alternating current to flow through the conductive coil or wire.

This alternating current causes a varying magnetic field in the coil. The coil is suspended within a separate permanent magnetic field (supplied by the magnetic structure). Magnets attract and repel each other and, therefore, the coil will move relative to the audio signal that passes through it.

The voice coil is attached to and suspended on a large diaphragm. As the coil and diaphragm move back and forth, the diaphragm pushes and pulls air and creates low-end sound waves that mimic the audio signal applied to the driver.

It’s important to note that bass frequencies naturally require more acoustic energy than higher frequencies. Subwoofers, therefore, require high-power audio signals and are designed to accept and utilize these signals.

The cross-over system mentioned briefly above sends the low-end high-energy part of the audio signal to the subwoofer and effectively “relieves” the other speakers from having to deal with the low-end at all.


How Do Bass Frequencies Move Through Air?

To really understand how subwoofers work, we should learn about how sound moves through the air and, in particular, how bass frequencies move through the air.

The Mechanics Of Sound Through Air

Let’s start with how sound moves through the air. I’ll try my best to be brief here. A complete study of sound mechanics is far too complex for a quick article on headphone bass response!

Sound is made of mechanical waves. Mechanical waves are created by vibrating objects and can only propagate through a medium (gas, liquid or solid). In this section, we’ll focus on the medium of air.

That is why there is no sound in outer space. Humans can hear sounds in solids and in liquids but, for the most part, we hear sound (and particularly sound from a subwoofer) in air, a gas.

As a sound source vibrates (like the diaphragm of a subwoofer or a headphone driver), it causes coinciding vibrations in the air molecules around it.

For instance, when the diaphragm moves outward, it pushes against the air molecules around it. Those air particles then push against other adjacent air particles and so on.
Conversely, when the diaphragm moves inward, it pulls the surrounding air molecules toward it. Those air particles now pull against other adjacent air particles, and so on.

This particle movement is ultimately caused by variations in localized pressure caused by sound waves. Sound waves do not carry air molecules along their paths. Rather, they cause the air molecules they pass through to vibrate locally.

The rate at which the air molecules vibrate is very complex to calculate because sound is very complex. Audible sound has a wide range of frequencies spanning from 20 Hz – 20,000 Hz (cycles/second).

To better understand, let’s imagine a single-frequency 100 Hz sine wave travelling through the air. The molecules it effects, then, would vibrate 100 cycles per second about their resting position. Now take a sound that has a complex arrangement of frequencies at varying amplitudes and you’ll understand how complex these vibrations truly are.

As you can imagine, there would significant loss in a sound wave’s ability to vibrate air as the sound wave travels farther from the source. This is due to in part to the inherent friction between air molecules along the path of the sound wave and also because sound waves tend to propagate in all directions at once.

Standard atmospheric temperature and pressure (20°C – 101,325 Pa) causes sound waves to travel through the air at 343 m/s (1125 ft/s, 1235 kph or 767 mph).

This is important to note because it allows us to calculate the wavelengths. The faster the sound can travel, the longer its wavelength will be at a given frequency.

λ = v/f

Where:
λ is the wavelength of sound (in meters)
v is the velocity of sound (in meters/second)
f is the frequency of sound (in cycles/second or Hertz)

I have created tables that relate sound frequencies to their wavelengths. You can find them in My New Microphone’s article titled Fundamental Frequencies Of Musical Notes In A=432 & A=440 Hz.

Bass frequencies make up the lower end of the audible range of human hearing. The sub-bass range is often described as 20 Hz – 60 Hz and the bass range is often defined as 60 Hz – 250 Hz.

If we are to plug those numbers into the above formula for wavelength, we’d have the following values at standard atmospheric temperature and pressure:

Frequency Range NameFrequency RangeWavelength Range
Entire Audible Range20 Hz
-
20,000 Hz
17.15m
-
0.01715m (17.15mm)
Sub-Bass20 Hz
-
60 Hz
17.15m
-
5.72m
Bass60 Hz
-
250 Hz
5.72m
-
1.37m

So lower frequencies have longer wavelengths.

How Subwoofers Produce Bass

If we take a common subwoofer that only produces sound under 100 Hz, the shortest wavelength it will produce is 3.43 meters (at 100 Hz).

Producing these long wavelengths and low frequencies is difficult.

First, it requires a lot more energy to produce low-frequency sound waves than high-frequency sound waves. Second, human hearing is naturally poor at lower frequencies which means low-end sound waves need to be even stronger in amplitude if we are to hear them.

The diaphragm of a subwoofer (or any driver) must move slowly to produce lower frequency sounds. This means air has time to escape around the sides of the cone. This inefficiency means that smaller cones/diaphragms will have a more difficult time producing low-end.

Smaller diaphragms would also require greater displacement relative to their radius to push enough air. A larger speaker cone is much more practical to move the diaphragm back and forth with enough force to create a low-frequency pressure wave with enough amplitude to move our eardrums.

Bass Frequencies Through Air

Now to answer the initial question of how bass frequencies travel through the air.

In general, low-frequency waves travel further than high-frequency waves because there is less energy transferred to the medium. Low-frequency waves are also more omnidirectional and are less prone to scatter when reflected off of a surface.

A low-frequency sound wave can effectively move around objects and cause larger objects to resonate and produce sounds themselves.

So bass frequencies do have some advantages when travelling through air. However, they must have big loudspeakers such as subwoofers to effectively reproduce them and propagate them through the air.

Of course, this is only to do with playing back audio. Low-end frequencies happen in nature without the use of a subwoofer all the time.

Headphones are not designed to project sound waves across long distances and so they do not have subwoofers. They produce bass in an entirely different way.


How Do Headphones Produce Sub-Bass & Bass Frequencies?

First, let’s repeat a statement I made previously. Low-end frequencies are felt more than they are heard. We can feel the low-end of a kick drum at a live event or the deep bass of a subwoofer on the dancefloor.

Of course, we can sometimes feel the vibrations of bass-heavy music in our headphones but it is faint and localized around our ears.

That is to say that the ways in which headphones produce bass are much less visceral than the ways in which subwoofers produce bass.

Headphones do not have to propel bass frequencies across long distances in order for the listener to hear them. The drivers/speakers of headphones are practically attached to the listeners’ ears.

Proximity

The close proximity of the headphone driver to the eardrum means the driver (headphone speaker) doesn’t have to move as much air to produce a decent bass response when the headphones are worn properly.

The inverse-square law state that the intensity of a sound wave is quartered for every doubling of distance. In other words, the sound pressure level drops by 6 dB for every doubling of distance.

Try moving your earbuds or headphones just an inch from your ears and you’ll hear the distance. Since the low-end requires a lot of energy, more energy will be lost for every doubling of distance. Put another way, a quartering of the intensity of a bass frequency is perceived as a greater drop in loudness.

Closed Space/Tunnel

Many headphone/earbud form factors create a sealed (or at least a semi-sealed) enclosure with the speaker on one end and the eardrum on the other. This coupling of diaphragms allows the bass frequencies to have more of an effect on the eardrum and a greater perceived loudness.

This helps to explain why closed-back headphones often have more perceived bass than open-back headphones and why pushing earbuds further into your ear canal will also boost the perceived bass response.

Bone Conduction

We’ve been discussing the eardrum quite a bit in this section. However, the eardrums themselves aren’t overly reactive to frequencies below 80 Hz. Remember how we feel these low-end frequencies more than we hear them?

Well, our brains still process these low vibrations as sound but we rely more on the resonances of our bodies to sense these sound waves.

With headphones, this is where the ear structure and bone conduction come into play.

Basically, the sound vibrations in the headphones physically vibrate our skull and the tiny bones in our ears, which sends signals to our brain that help us perceive the sound frequencies. This is particularly true of bass frequencies.

Over-the-ear headphones press against our skulls and can so their bass frequencies are more easily perceived.


Which Headphones Have The Best Bass Response?

I remember when Beats by Dre first came out in my high school years and all my friends were obsessed with the exaggerated bass response (okay I was, too). Listening to them now, I enjoy some rap and electronic music but I know that the bass response is not typically what most mixing engineers had in mind.

The Beats are but one example of a bass-heavy set of headphones. Of course, the term “best” is highly subjective and there are far too many headphone models to go through to determine which specific pair has the “best bass response.”

However, there are some driver types and headphone form factors that are generally better designed to produce bass frequencies.

Bass Response Of Various Headphone Drivers

The moving-coil dynamic driver, which we’ve discussed in this article, is great at producing bass frequencies. This is one of the many reasons why the moving-coil driver is used in the vast majority of headphone, loudspeaker and subwoofer designs.

The rugged design of the moving-coil dynamic driver allows for larger diaphragms and greater displacement of those diaphragms which leads to better low-end frequency response.

Planar magnetic headphone drivers have a very clean and accurate bass response and are capable of moving lots of air.

High-quality electrostatic headphones are among the best-sound headphones in existence. Not only are they incredibly life-like but their bass response is superb so long as they are matched with an appropriate headphone amplifier.

Balanced armature drivers typically have very limited frequency response and poor bass response. However, specially designed BA drivers can produce excellent low-end results with even better clarity than the aforementioned drivers. These complex drivers are very expensive, though.

Bass Response Of Various Headphone Form Factors

Closed-back headphones, in general, have more bass than open-back headphones. This is because the closed-back headphones “seal in” the sound waves and are more directly-coupled to the eardrum. Allowing the air to escape from the headphone cup reduces the effectiveness of the bass.

In-ear monitors and earbuds that completely seal your ear canal will become directly coupled with your eardrum. This means the bass response will not be reliant on the movement of air and will likely yield more perceived bass. However, when not sealed, the small drivers may struggle to produce enough bass.


Related Questions

What is headphone frequency response? The headphone frequency response specification refers to the range of frequencies a pair of headphones is capable of producing. The frequency response graph provides a detailed chart of the frequency-specific output of the headphones.

Can too much bass in an audio signal damage headphones? Too much audio signal, in general, has the potential to damage a headphone driver. However, because bass frequencies require more energy, boosting the bass may overload the driver before boosting higher frequencies. It’s important to note that the headphones will distort well before they would even become damaged.

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