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 Subwoofers



Probably the most discussed aspect of car audio and the part that creates the most interest are subwoofer(s)

Subwoofers reproduce the lowest part of the frequency spectrum from 20htz to 80/100htz (sometimes slightly higher) All speakers create sound by moving air and the lower the sound the more air that need to be moved so in the case of subwoofers large amounts of air need to be moved quickly and accurately to properly reproduce low frequencies. Because of their large size and box requirement they are normally mounted in the boot with the aim of an accurate Sound Quality system being to create an integrated front stage and sub so that it appears all of your bass is coming from the front. This provides the most accurate and musical sound experience. (like when you go to a concert the sound comes from in front of you).


There are a number of aspects to consider in relation to subwoofers
- Sub selection
- Mounting
- Box design
- Box Construction

Important things that affect Sub Output
- Cone size
- X-Max (cone travel)
- Sensitivity
- Box design



Sub Selection
You need to consider the following
- What front stage you will be integrating the sub with. A good rule of thumb to follow is select a sub that is no more than 2 sizes larger than your largest front speaker.
i.e if your running 6" splits then ideally your sub should be 10"
If your largest front speaker is 8" then a 12" sub will integrate best

The reason behind this is if you have a gap of larger then 2 sub sizes it is very difficult to get your sub bass to sound integrated with your front speakers and your soundstage will be drawn backwards.





Of course there are tradeoffs with size.

Pro's of Larger Subs
- Greater cone area provides for greater output.
- Often (not always) larger subs have a higher sensitivity

Con's of Larger Subs
- Require larger enclosures.
- They are harder to fit in various locations around the car
- Often don't blend as well with front stage.
- Not as transient (tight and responsive)

In Summary
- Consider what size front stage you will be running and try to get a sub no more than 2 sizes larger
- If your running limited power then try to get a sub with a higher sensitivity or a larger sub as they move more air for the same amount of power… i.e. 15" instead of 10" of course the trade off may be lack of tight response…
- Figure out how much space you have to mount the sub and box and then select one that will work with an enclosure that size. (i.e. if you only have 1c/f then there is no point looking at 15" subs….


My Baby IDMAXD2V3

Voice Coil configuration
Voice coil configuration is another crucial aspect of sub selection and to help understand resistance and ohms you should read Tutorial 3 on Amplifier selection and resistance.

Subs come in a variety of configurations but the most common are
- Single 4ohm voice coil
- Dual 4ohm voice coils
- Dual 2ohm voice coils

The first misconception is that single voice coils subs are not as good. This is not true. The main reason for multiple voice coils is the flexibility in wiring options.

As mentioned in Tutorial 3 amplifiers produce more power with lower resistance. Monoblock amplifiers are designed to work with lower impedances and are often a better selection if you plan on running multiple subs. Class A/B amps can also produce sufficient power to drive subwoofers but usually this is done by bridging channels on an amplifier and you must be careful to only connect a speaker load that the amplifier can handle. Normally this is 4ohm's.

Generally class A/B amps are 2ohm stereo stable and 4ohm bridged stable. When you bridge an amplifier you connect a load across both channels and each channel “Sees" half of the resistance Typically class A/B amps will produce 50% more power bridged.

Example
Class A/B amp is 2 x 100rms @ 4ohms in stereo OR 1 x 300rms @4ohm bridged (which means each side of the amp sees a 2ohm load per channel. Most class A/B amps are not 2ohm bridged stable as this leave each channel with only 1ohm resistance which is too low, and the power supplies and other parts of the amp cannot cope with such a low resistance.

So having clarified that, what voice coil configuration should you select.

Single Sub Set-ups
If you're planning to run a single sub from a class A/B amp then a 4ohm single voice coil is fine. I would still recommended trying to get a dual 2ohm coil sub as you can connect the coils in series to get a 4ohm load (Ideal for your class A/B amp) OR later down the track if you wish to upgrade to a more powerful monoblock you can parallel the voice coils to get a 1ohm load and get maximum output from your Monoblock…




Dual Sub Set-ups
If running dual subs from a class A/B amp you will need to select 2 x dual 4ohm coils and run series - parallel so you get a 4 ohm load on a bridged amp.

For a monoblock you can get any configuration but for max power get dual 4ohm coils, parallel all the voice coils to get a 1ohm load.




Multiple Subs
Normally if you intend on running more than 2 subs you will need to go to a monoblock. There are some class A/B amps that will power 2ohm loads or produce sufficient power to run 3 or more subs but they are all expensive and if you have access to that sort of gear then you don't need my tutorial to help you.

3 x 4ohm voice coil subs in parrellel = 1.3ohm which is fine for most monoblock but not for class A/B amps.

4 x 4ohm voice coil subs in parrellel = 1ohm

Parallel = Connecting all positive terminals together then to positive of the amp, all negative terminals together then to the negative of the amp

Series= Positive from amp to positive terminal on Coil A then join to Negative of Coil B. Negative of amp to Negative of Coil A and then Link Negative of coil A with Positive of Coil B....... It gets complctaed


Summary
- If possible get a sub with 2 or more voice coils as it provides great flexibility for connection
- If you're running a class A/B amp bridged then remember to connect a single 4ohm load
- Multiple voice coils does not mean better. If the sub you like only has 1 x 4ohm voice coil that's fine just consider how you will get the most from your amps with that sub(s)


Mounting
I don't want to go into the details of box construction (separate tutorial) but suffice to say the following guidelines are good to follow
- Use at least 15mm MDF nothing smaller for sub box construction.
- Use really big screws, 14gauge or whatever you can fit. Subs hit with a lot of force and if they work loose they will not only damage themselves but you also loose significant output. Ideally use T-Screws which have a spiked captive nut inside the box that bites into the inside of the box and the screw threads into and pulls against.
- If you are making a sealed box ensure it really is sealed. Run silicone around all inside edges and screw the sub in tight.
- If you're using a ported enclosure make sure it is the correct size and the ports are the right size and length. It makes a HUGE difference in relation to output and also protection for your sub.
- If you're not sure try and find out all the specs or use the manufactures specs. If you still can't find out or you must use a prefab box then use a sealed box using the below table as a rough guide. Please note these are estimates and your best bet is to use the manufacturers recommended enclosure or have one graphed up using a box calculation program. such as Blaubox or Termpro.

10" sub = .6-.8c/f sealed or 1-1.2c/f ported
12" sub = 1 -1.5c/f sealed or 1.5-2c/f ported
15" sub = 2c/f sealed or 3.5c/f ported

Conversion for those who need it
1c/f = 28.31litres

Show Box Advice
For those who plan on making a box to show off their sub perspecs flexes and this absorbs energy and reduces output. It also scratches easily. I recommended using glass instead. 10mm or 12mm glass works very well and is a lot cheaper. I have seen 25mm perspecs flex and it is VERY expensive….

If you're making a very show'y box and your one of these people who likes to constantly upgrade to the newest and the best it might be a good idea to build a box that is pretty general in size and will accommodate other subs if you change later on. i.e. 1c/f sealed for a 12". Also think about how hard it will be to change the mounting face, if you get a different sub and the cutout is different (which most are)



Conclusion
- Decide how much space you are prepared to sacrifice for the sub box
- Decide how much bass you are going to want (SPL competition car / daily drive / better than your mates car etc…)
- Decide what amplifier you will be using to run your sub(s). How much power does it provide and what is the ideal resistance to get that power
- Buy a sub or subs that fit your budget, work in the required space and have a resistance that will allow you to get the maximum from your amp

Enjoy
 
 Document Summary
 
Document description: Part 4 of Pyroay's Car Audio Tutorials
Document written by: pyroay on 08 2005>, Copyright © 2005 pyroay, all rights reserved.
Document revised: 08 2005 - Revision 1.0
Document views: 14073 since 08 2005



 

 

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