Simply put, a subwoofer is a specially designed speaker dedicated solely to reproducing lower frequencies, typically in the 20 to 400 hertz range. No matter what kind of music you like, or how softly or loudly you like to listen, a quality subwoofer can make a night and day difference in terms of sound quality. However, the secret to great bass is making sure your subs and amp are evenly matched and will properly work together.
For that you need to consider a number of things including “impedance”, which measures the resistance of a speaker’s voice coil to the audio current supplied by the amplifier. Impedance is measured in Ohms and it’s symbolized by the Greek letter Omega: (Ω) for shorthand.
There are several different impedance and voice coil setups, and we’re often asked the question “what is better, an 8 ohm or 4 ohm subwoofer?” To answer that question, we’ll need to demystify the difference between them, but before that, you’ll need a good understanding of the following things:
- How impedance works and why does it matters
- How to manage the overall impedance load of your subs
- How to match subwoofers and amplifiers
- Understand different subs wiring option
- Have an idea about how to manage the overall impedance load
So, without any further ado, let’s dig into the often misunderstood and sometimes puzzling world of impedance.
What is speaker impedance?
At the heart of every speaker is a voice coil. Simply put, a voice coil is what makes a speaker a speaker. It puts up the electrical resistance and performs the work. The resisting property of a voice coil is what is referred to as impedance and is measured in ohms. The lower a speaker’s impedance, the easier it is for an amp to supply power to it (and the more current can flow).
Another way of looking at it is to say the lower the impedance, the higher the load on the amp (and the harder it has to work).
To sum it up, these general relationships can be summarized as follows:
- Low impedance → more current → greater load → more power
- High impedance → less current → smaller load → less power
Looking at the relationships above, it appears that the lower the speaker impedance, the greater the power the amp delivers through that speaker. This is true to the point where the amp is overloaded and can not push out anymore current and power.
At this point, either the amp fuse will blow, or its protection circuit will kick in and turn the amp off. Therefore, avoid running an amp with an impedance load that’s less than the stated minimum (normally 4 ohms in the car audio industry).
To give you a visual representation of impedance, we’ll use the analogy of water flowing through a water pipe.
So, think of your amplifier as a pump, your subwoofer as a water pipe, and the audio signal — your music — as water flowing through the pipe.
The diameter of the water pipe represents the impedance of your subwoofer. The larger the diameter, the less impedance there is for the water to flow through it.
A speaker with a lower impedance is like a bigger pipe in that it lets more electrical signal through and allows it to flow more easily. For that reason, a 4 ohm speaker is considered more “power hungry” and will relatively tax your amplifier more than an 8 or 6 ohm speaker.
As a result, a car amplifier rated at 100 watts at 8 ohms impedance load can deliver up to twice that at 4 ohm impedance load. The lower the impedance, the more easily electricity (the audio signal) flows through the speaker.
Why does speaker impedance matter?
First and foremost, it’s worth mentioning that a speaker’s impedance is not a constant value. It’s changing as the sound frequencies go up and down. For example, at 30Hz to 60Hz, the impedance of a speaker might be 2.7 ohms, while at 200 hertz, the impedance might be 4 ohms, depending on the speaker in question. Consequently, the overall impedance of a speaker can be represented as a curve that changes as the input frequency changes.
A Speaker’s impedance is constantly changing because of two main reasons:
The resonant frequency f0 (pronounced: F-naught) is the point at which the weight of the moving parts of the speaker becomes balanced with the force of the speaker suspension when in motion. Below this point, the loudspeaker becomes increasingly unable to generate sound output for a given input signal.
Every driver has a resonance frequency at which it moves freely once it is tapped or otherwise engaged. At that frequency the free movement of the driver generates what is known as “back electromotive force” which opposes the flow of current and creates a large spike in impedance.
Voice coil inductance
Voice coil inductance changes a speaker’s impedance because the voice coil acts as an inductor. It’s measured in millihenries (mH). The industry standard is to measure inductance at 1,000 Hz.
The impedance specification seen on a speaker is just a rough average (aka nominal impedance). And although 4 ohms is the standard impedance in car audio, this standard is more of an average impedance for speakers and amps when driven within their performance limits.
So, why do you need to know about speaker impedance?
Well, you need to make sure the impedance of any speaker (or speakers) connected to an amp is within the capabilities of the latter.
Using mismatched speakers and amplifiers can cause problems when the amplifier is not up to the task. Thereby, the capability of an amplifier has to be considered before applying a load to it (hooking up a speaker).
You don’t want to spend a ton of cash on a couple of subs and an amp, to find out that your amp can’t run the subs because wired together, their impedance would be too low for the amp to run without being overloaded.
Almost all car amplifiers on the market can drive a 4-ohm load. Most amps can work with 2-ohm loads on each channel, but not when the channels are bridged together. However, very few amps can handle a load as low as one ohm.
Problems arise when an amplifier is overloaded, which is a result of low speaker’s impedance. In other words, this happens when you connect a couple of subs in parallel to the same amplifier (more on this down below). With this wiring arrangement, the amplifier sees an overall impedance load (due to Ohm’s law), and must supply proportionally more current as a result.
For instance, if you wire two 4 ohms subs in parallel, their overall impedance load would be 2 ohm, and pairing them to inadequate amplifier, will put a lot of strain on the latter and make it overheat trying to push out more power than it was designed to produce.
When an amp overheats, either the amp fuse blows, or its protection circuit kicks in and shuts the amp down to prevent it from burning up.
Impedance matching matters
Using mismatched speakers and amplifiers can cause problems when the amplifier is not up to the task. So, before buying a 4-ohm speaker, make sure your amplifier can handle it. Most amplifiers in the market can safely handle 4 and 8 ohms.
As we’ve mentioned above, the secret to great bass is making sure your subs and amp are evenly matched and will properly work together. However, if you think that you can only pair a 4 ohm sub to a 4 ohm amp, or an 8 ohm sub to an 8 ohm amp, you’re wrong.
In theory an 8 ohm speaker can be paired up to a 4 ohm amplifier as well. Only then the speaker wouldn’t be able to fully handle the amp’s full power output.
It’s the other way around, too, however, one should be very cautious with this, since hooking up a 4-ohm speaker to an 8-ohm amplifier will put a lot of strain and overtax the amplifier. The latter will be overloaded and it might even blow out.
Wiring options change a sub’s impedance
Impedance world is confusing, and to add insult to injury, in multiple-sub systems, the total impedance changes depending how the subwoofers and their voice coils are wired together — in parallel or in series.
Parallel wiring means that the terminals of each subwoofer are connected to the same things — plus to plus, and minus to minus.
To figure out the total impedance of multiple subs wired in parallel (assuming the impedances of all the drivers are the same), take the impedance value of one of the subwoofer and divide it by the number of devices.
For example: four 4-ohm speakers wired in parallel have a total impedance of 1 ohm.
Series wiring means that the subwoofers are wired one after the other — a plus of one to a minus of another.
To figure out the total impedance of multiple subs or voice coils wired in series, add their impedances together. For example two 2-ohm speakers wired in series have a total impedance of 4 ohms.
4 ohm Vs. 8 ohm speakers — What’s the difference?
Our brains are wired to always think that more is better which is not always the case. This is especially true when it comes to speakers’ impedance. So, what’s the difference between an 8 ohm Vs.4 ohm subwoofer?
Well, technically speaking, the only difference between them is the level of impedance. In other words, the amount of resistance a speaker’s voice coil applies to the audio current supplied by the amplifier.
A lower impedance speaker will accept much more power. A 4 ohm speaker, for instance, will extract about twice as much power from your amplifier than an 8 ohm speaker.
That said, a subwoofer rated at 4-ohm impedance load will hit harder and produce more output than an identical 8-ohm subwoofer, given similar input wattage, because the resistance is much lower.