Low freq extension equals driver area/numkber of drivers? ... (1 Viewer)

[Kevin C Brown]

Here's why I ask:

Let's say I have a 5.1 system. Let's just consider the surrounds.

Now, I upgrade to a 7.1 system. If I use the same speakers for the rears as the surrounds, I have effectively doubled the woofer area. Does that help low freq extension? (Any rules of thumb for that? 1/4 octave extension for doubling tha area, for example?)

Or does it become less clear, because if the pre/pro lowers the overall volume of *each* of the surrounds+rears speakers vs the original surrounds to maintain the same sound stage in the back of the room, I effectively get no or not much benefit? (Even though I still have double the woofer area?)

The real reason why I ask, is that I have full range towers for my surrounds now (for 5.1), but I'm considering going to 4 matching bookshelf speakers for 7.1.

With the 4, I get a better *timbre matching* back there, but should I worry about low freq? Maybe not anyway, because I crossover even now at 80 Hz, and the new bookshelves have a -3 dB point of 65 Hz.

Just curious what others think...

 

[Dustin B]

Adding more of the same driver will not effect extension. Will allow you to play louder, but won't affect extension. Will be exactly the same as before.

Also since you are crossing over at 80hz anyways, all the bass is going to your sub(s). So as long as the speaker makes it to below the crossover point (-3dB of 65hz will be fine for an 80hz crossover) you will be fine.

 

[Kevin C Brown]

Adding more of the same driver will not effect extension.

That can't be. My current surrounds, Def Tech BP-30's, have four 6.5" drivers each. Their -3 dB point is about 30 Hz. Most single 6.5" driver speakers go down to about 65 to 80 Hz. (Energy C-3's, B&W DM601 S3, Acoustic Energy Aesprit 300c, etc.)

Plus, a typical 8" sub won't hit 20 Hz, right? But my Vandersteen V2W has 3 x 8" drivers in it and can.

 

[Dustin B]

A 1" tweeter can play 20hz, just not very loudly.

The frequency response of the driver will not change when you add more. They will just be able to play louder. An 8" driver can be designed to play very well at 20hz. Just won't be able to move enough air to play it loudly unless you use many of them.

An 8" driver can also be designed to perform very poorly at 20hz. In this case it won't matter how many you use, the 20hz response will still suck.

 

[Mark Fitzsimmons]

By adding more of the same drivers you can get lower extention.

But you would have to use an equalizer.

Adding more drivers traditionally only allows you to play louder.

Lets say doubling a cone area gives a 3db increase (about right)

But we use the eq to remove that gain across the speakers bandwidth except in the lower notes. We now just gained more extention.

So in essence I am saying that regularly adding more speakers will not give more extention, however by using an eq we can use the natural gain in spl to create extention.

 

[Kevin C Brown]

Coolness, I learned something new!
Ahh, but to get back to my original question, then 4 vs 2 speakers with the same drivers *won't* help low freq extension because *everything* gets louder.
Unless... I wonder if there's any eq'ing that potentially happens in 7.1 setups with Logic 7 or THX Ultra2 that *would* help?
Mark & Dustin- Thanks!

 

[Murali Kalapala]

This brings up an interesting question.

If I have a Woofer that moves enough air to create a 40Hz wave at a particular Sound level in a room of particular volume, will this Woofer "theoritically" extend to 20Hz in a room 1/4 the volume and produce the same sound level?

Kris.

 

[Dustin B]

Will depend on how much of a change in room gain you get. A room a 1/4 of the size will have a higher room gain than the larger room. If the driver falls off below 30hz in a manner that matches the room gain curve of the smaller room then yes. If it's steeper than the room gain curve you'll get more low output but it still won't come up to flat.


And Kevin, the 3dB increase from cone area will only occur when the drivers are properly coupled. This requires them to be less than a driver diameter appart usually. If the drivers are farther appart than this you will get some increase less than 3dB. But you'll also have twice as much power going to them as well. So this will increase output a little as well.

No processor I'm aware of will be able to apply the type of EQ that would be required to do what Mark mentions.

 

[Mark Fitzsimmons]

No processor I'm aware of will be able to apply the type of EQ that would be required to do what Mark mentions.

Very true, I've never heard of anything that would be able to do this; and the practicality of such an eq is quite low.

 

[Brian Bunge]

Kevin,

I'm not familiar with your DefTechs, but are they sealed or ported? If ported, then it's quite possible that they are just tuned lower.

 

[Kevin C Brown]

Brian- Yuppers, they are ported on the lower back.

 

[Guy Usher]

DTs play games with their numbers, I love em but. . . They will as Dustin said play low but at what volume level, headphones will get down there and they have mighty small drivers.

 

[RichardH]

And Kevin, the 3dB increase from cone area will only occur when the drivers are properly coupled. This requires them to be less than a driver diameter appart usually.

Actually, the important aspect is that the two drivers are much closer together than the length of the wave they are both reproducing. So, the lower the bass note you're looking to produce and get that coupling benefit, the less close they actually have to be. But, you might as well put two subs right next to each other if you're going to have 2. Just as an example.

 

[Kevin C Brown]

I calculated once that 80 Hz is like 13.75 ft. (So lower Hz is a bigger wave.) So wow, they don't have to be on top of each other to get "reinforcement".

But if there isn't any eq, doesn't seem like it's going to help me much anyway.

(They would be about 3-4 ft from each other.) Unless I get additive effects from wavelengths larger than the distance each speaker is from it's neighbor, and not from speakers that are farther apart. (That could be a problem, right?)

And then there's phase stuff and constructive and destructive interference (which I do think I know is less of a problem at higher freqs anyway).

Man, simple on the surface, but not so underneath...

 

[Dustin B]

Isn't it something like a 1/3rd of the wavelength you want to couple?