How to Fill your room with sound?
I have lost count how many times someone has asked me, how do I get room filling sound from the speakers I am building? This is a more complicated question, that we should take some time on
The first thought, is I want bigger speakers. Or the drivers, I use should be bigger. If they are bigger, than they can cover more area and therefore fill up the room more. As the old saying goes, "there is no replacement for displacement." And although this is true, this alone is not enough. Let's show you why.
*For the purpose of this discussion, we are assuming you are only using two speakers and want to fill a decent sized room with sound. Having said this, what will be discussed in this article will be applicable to a multi speaker setup as well.
Beam me up Scotty
We have already figured out that a bigger driver can fill a room more, so why don't we see more large driver speaker systems? That is due to a simple phenomenon called beaming. Every driver will have a frequency associated with it, where it stops filling the room with sound and starts to really directionalize the sound. This means, that when you get off center of it, you are no longer hearing the same thing as you would be when standing directly in front of it. The sound is now projecting like a beam in front of you. This is problematic if you want to fill the whole room with sound. Anyone outside of the direct center will not hear the same sound as you. The easiest way to visualize this is to take a look at the off axis response of a 15" driver.
This graph shows the on axis and off axis response of this driver. As we can see, starting at 450hz, the off axis response starts to deviate from the on axis response. This means that after 450hz, people will start to hear something different than you.
How do you know where a driver will Beam?
Luckily where a driver will start to beam is directly related to the size of the cone. So almost all 15" drivers will start to beam around 450hz. This is known by a simple formula of f=2*c/π*D
- f= the frequency the driver will start to beam
- c= the speed of sound (343,000 mm/s)
- Diameter of the speaker in mm
By following this simple formula you can start to figure out where each speaker will beam. Which will be very important when implementing them into your design.
It's not the Size, it's how you Use it!
Since we now know that speakers will beam at a certain frequency based off the size of the driver, we can get a better idea that the size of the driver directly affects our design. If we want to use this 15" driver in a two way design, we will have to find a tweeter that will be able to cross over near 450hz! That is a tall order for any tweeter. This is also the reason that when you start seeing bigger woofers in systems you start seeing them become three way designs. If you want that room filling sound you will have to add a mid-range that can cover the range you want until it starts beaming.
So if we added a popular 8" driver like this Dayton, our new beaming would occur around 1500hz. Meaning you would want it to cover from at least 450hz to 1500hz. Then add a tweeter to take over from there or even another midrange. So we can start to see the trend. The bigger the driver, the sooner it will beam. Meaning that if you want to use large drivers to get room filling sound, you will want to implement them correctly based off the frequency in which they will start to beam.
Does a Tweeter beam?
Yes it does. In fact, the larger the tweeter the sooner it will beam. This is a reason some manufacturers really like 3/4" tweeters. They typically have a pretty wide dispersion that can cover off-axis really well. Unfortunately, the smaller the tweeter, typically the higher it has to crossover. This means the smaller tweeter won't typically work well in a two-way system with a larger woofer.
Where does this leave us?
In order to get this room filling sound, you will have to decide what sacrifices you are willing to make. If you want big drivers, you may need to make it a 3 or 4 way speaker. If you want the least amount of drivers, you may need to go with smaller drivers. Or another way to do it, is multiple drivers, like in the Uglies. But no matter what you do, always pay attention to the off axis response and beaming when trying to create a speaker with room filling sound.