Now that you know how Frequency Bands and Drivers affect speaker selection you can think about enclosure design. Enclosure design affects the bass response of the speaker, but not the other bands. The woofer (or subwoofer) uses the enclosure as an “acoustic spring” to improve the response of the driver, and some enclosure designs increase the efficiency of the driver at low frequencies. The 3 main enclosure factors that affect the sound are the size, the construction, and the configuration.
Size
The size of an enclosure dictates what size of drivers can work in it. As we discussed in part 3, a larger woofer will generally be able to reproduce lower bass (this is good!). The size of the enclosure also dictates how much air fits in the enclosure. This is important because the woofer requires a certain volume of air to make good use of the “acoustic spring” effect. If the enclosure is small then the speaker will likely not be optimized for as low of bass as a larger enclosure would be. This optimization is called tuning. On DIY audio sites you will routinely see people mentioning a frequency that their enclosure is tuned to, for example my subwoofer is tuned to 23Hz. This tuning means that the “air spring” effect of the enclosure is most helpful at that frequency. In general, larger enclosures can be tuned to lower frequencies, which will support deeper bass reproduction.
There are three performance factors that have to be balanced when designing a speaker enclosure: Volume, Frequency Extension, and Size. If you want a loud subwoofer that plays really low then it will probably need to be pretty big. If you want a really small subwoofer that’s loud then it will not play as low as a larger (or quieter) one. Just remember that if you buy a small enclosure you will be compromising in one or both of the other 2 areas.
Construction
When it comes to enclosures, the enemy of good sound reproduction is vibration. If the box vibrates, or resonates, it will “color” the sound. When a speaker is reproducing sound you just want to hear what was on the media, nothing more. If the enclosure vibrates it adds additional sound waves to what you hear (this is bad).
Good enclosures are built with thick wood or stiff composites (fiberglass, etc.). They should have lots of internal bracing, so they may look like a box but they should actually be much stronger than a simple box would be.
To figure out if a speaker enclosure will color sound you can do a couple of things:
- Pick it up - Is it heavy? If it’s made of plastic that’s probably bad. If it seems light for it’s size then that’s probably bad. If you pick it up and think you might need to go to the chiropractor afterward then you’ve probably found a good, solid enclosure. It should feel dense, kind of like a block of concrete.
- Rap your knuckles on it. Does it sound hollow and resonate like a drum, or does it have a high-pitched “knock knock” sound like a thick wood front-door? A solidly-built enclosure should have a very high-pitched resonance, or none at all. If the “knock” test fails then the material is probably too thin or there is not enough bracing.
Configuration
There are quite a few enclosure configurations out there in the world, but there are 3 common designs that you are most likely to encounter. Each configuration has it’s pros and cons, here they are:
Infinite Baffle
Pros
- Simple Design – The Infinite Baffle design isn’t really an enclosure at all. This design is what’s in use when you mount a subwoofer in a wall with the back of the driver exposed into a large room such as your attic. The “enclosure” doesn’t really affect the response of the subwoofer at all, so whatever the driver provides by itself is what you get.
- Accurate Response – IB subwoofers aren’t affected by an enclosures, so in general, what goes in is what comes out.
- Low cost – No enclosure = no enclosure costs
- Good WAF (Wife Approval Factor) – If your room is set up right you can hide the sub in the wall behind the couch, or some other inconspicuous place.
Cons
- Lower SPL capability – Since the driver has no real enclosure it has no “acoustic spring” to work with. The driver is more likely to run out of excursion capability (how far it can move in and out) quicker than in other enclosure configurations. It will not be as loud at low frequencies as the other 2 designs.
- Specific Driver Required – You have to have a driver that’s made to work without the “acoustic spring”, otherwise it won’t work well at all.
- Requires a hole in the wall – Often a sub-in-a-box is a neater, tidier solution. An IB sub is also not portable.
Sealed (also called Acoustic Suspension)
Pros
- Simple Design – Sealed enclosures are also very simple; just a driver and a box.
- Accurate Response – A sealed enclosure is similar to the IB configuration, except the enclosure is providing “acoustic spring” support to increase efficiency in the lowest frequency ranges. This configuration is actually more accurate than an IB config at high SPLs because there is less distortion (since the driver is not moving as far). Sealed enclosures have better phase linearity and transient response than ported designs, so bass hits will often seem tighter and less boomy.
- Better roll-off below the tuning frequency – Sealed enclosures will have better bass extension than ported enclosures below the frequency they are tuned for. If you want the most musical sound out of your speakers then sealed enclosures may help you achieve that by reproducing more of the audible frequency range.
Cons
- Less efficient than ported – Sealed enclosures don’t use the sound waves that come from the back of the driver for anything useful, so there is some wasted energy there. To get the same output level at the same frequency you will need to go larger than with a ported enclosure.
Ported (also called Bass Reflex)
Pros
- Efficiency – The port is the hole you see in the box. It can be any shape and on any side of the box, but if you see a hole then it’s probably a ported design. The port funnels the sound waves that come from the back of the driver out into the room. Ports are tuned to a certain frequency, so they funnel most effectively at their tuned frequency. At the tuned frequency the waves from the port and the waves from the front of the driver combine optimally to significantly increase the output of the speaker. Manufacturers use the ported design to either increase bass output, or make up for small enclosures
- Cool factor – What’s cooler than a funky-looking hole in the enclosure, and the term “bass-reflex” on the box?
Cons
- Worse accuracy – Since the “air cushion” is moving back and forth in the enclosure (through the port) it’s not as springy as in a sealed enclosure. The “air spring” doesn’t have as much control over the driver, so it flaps around a bit more. This results in bass that can be a little boomier and seem less “tight”
- Poor control below tuning frequency – Below the frequency that the port is tuned to the “air spring” is almost useless. This causes the driver to move much more when playing really low, easily running out of excursion capability if you exceed certain volumes at really low frequencies. Because of that lack of control, physical damage is very possible when using a ported enclosure. When I first built my subwoofer I played some cannon sound-effects through it at very high levels. On one of the cannon shots I heard a loud “flap”, and then nothing else. I had played audio through it below the 23Hz tuning frequency and I caused the speaker voice-coil to shoot out of the magnet gap. That driver went into the garbage and I had to order a new one. I now have a filter on the amplifier to drastically cut frequencies below 20Hz (and I’m also more careful).
- More complex – To properly tune a ported enclosure the speaker parameters must be known, the box sizing must be exact, and the port volume must be calculated properly. It’s not too hard to do (there are free software programs to do it), but the implementation of the design must be accurate or the tuning will be off.
There are a few more configurations such as transmission line, bandpass, passive-radiator, and others, but this covers 99% of what you’ll run into.
What Matters
When it comes down to picking speakers for a frugal system you shouldn’t worry too much about the configuration. Focus on size and construction, and the end product will probably sound very nice regardless of the configuration. Ignore the marketing terms and the fancy paint. Rap on the enclosure, pick it up, have a listen, and then think before you buy.
On Saturday I’ll talk about Arrays in part 5 of this Speaker series. Join in the discussion if you have any questions or comments!
Related posts:
Pingback: Speakers, Part 9: Subwoofer Recommendations | Frugal Home A/V
Pingback: Audio Tweaking And Tuning - Speaker Settings | Frugal Home A/V