One aspect of sound performance that is often overlooked is the effect directionality has on sound performance for people sitting outside of the “sweet spot”. At your desk, you can make sure your speakers are always pointing at you. In a room with one seat, you can obviously point all of the speakers at the listener. But most of us listening to audio in our homes have rooms with multiple listeners, and that makes directional effects important to understand. Also, directional effects determine the strength of reflections, always a problem for speakers indoors. The topic of directional effects in the propagation of waves is something that has been both an academic and professional interest of mine, I hope this article sheds a bit of light on how this theory applies to home theater.
Horizontal Directional Effects
The first thing to consider is how a single driver’s directionality is affected by frequency. The animation below shows an ideal 6-inch driver in free space (no ground, no walls) which is sitting 10 feet away from the 3 listeners, spaced 5 feet apart. As you can see, as the frequency increases, the range of bright yellow gets to be a smaller and smaller wedge. At high enough frequencies nulls or dead zones (dark blue wedges) appear. What this means is that a listener who is off-center has to deal with the speaker becoming colored in the mid range.
This next plot shows how different drivers are colored for a listener sitting five feet off-axis. Deep nulls at higher frequencies are avoided in practice because most speakers use multiple drivers. The importance of using multiple drivers is evidenced below. Smaller drivers do not have naturally tight beams so the sound is more evenly distributed in the room at high frequencies.
This pretty much covers the horizontal directional effects seen in bookshelf speakers or tower speakers, but in a home theater, you’re likely to come across a center channel with two woofers and dipole/bipolar surround speakers.
Starting by looking at the conventional horizontal woofer-tweeter-woofer design for center channels, its obvious the tweeter will behave like the case above. However, with two woofers, there is interference to worry about. This is the biggest drawback to this type of speaker. It’s low profile and goes nicely under your TV or screen, but its directional effects are huge drawbacks for its sound performance for a whole room. The animation below shows more nulls, and at lower frequencies, in the listening area than previously.
When looking at the effect this has on the frequency response of the speaker, its obvious the lower midrange nulling is a huge problem. This is ultimately a big consideration when choosing a center channel.
The other common type of speaker in a home theater is a dipole speaker. Ideally, a dipole speaker sends out the negative signal out of the back in addition to a positive out the front. This creates a null when the speaker is to the side of the listener, and focuses its energy onto walls that reflect sound back to the listener. This creates a very large sound field from the speaker, making it truly feel “surround”. This type of speaker creates a very even sound throughout the listening area as well.
Vertical Directional Effects and the Impact of Reflections
The directional effects in height of a speaker are important for one very important reason, reflections off of the floor and ceiling are a major problem in room acoustics. With that in mind, we consider a 6 inch driver, 3 feet off the ground, and observe the impact of a 30% strength ground reflection. The animation shows a multitude of ridges that move with frequency. This directly impacts the observed frequency spectrum and hurts the clarity of the sound.
With three drivers, there are three ground reflections, which tend to smooth each other out. In addition, three drivers arranged vertically means a tighter beam and less energy to reflect. The following animation shows the improvement of a tower speaker over a bookshelf speaker in this respect.
The final plot shows the difference three drivers makes in terms of the frequency response observed 10 feet away from the speaker. This clearly shows that using a multiple driver speaker is a very good idea for preserving the speaker quality the designer created.
This article has shown you some of the effects of spatial properties of a driver. There is always a careful balancing act to be played between speakers which are focused to prevent reflections and speakers which don’t sound good for most listeners. There are many more things to worry about when designing speakers and setting up home audio, but this ought to be on your mind when you are putting together a room with an ear towards quality sound.