Last week, we started the Audio 101 series by explaining frequency response and how it relates to the music that we hear. This week, we’ll build on that and discuss on-axis and off-axis response. It will be important to understand the nature of axis response if we are to design a car or home audio system that will sound good to us. If you have not yet read the article on frequency response, it is recommended that you start there.
What is Axis Response?
When we discussed frequency response in our first article, we did so without the added complexity of axis response. It is important to understand that when speakers are measured by manufacturers, they are measured completely on-axis. In other words, the microphone is placed directly in front of the speaker a set distance away (typically 1 meter). The microphone is at the same vertical height as the speaker, and is facing the speaker directly, not from an angle. The speaker itself is also facing the microphone. This is done with the assumption that when one listens to a given speaker, the speakers will be positioned such that they will be facing the listener directly.
The reason for this is due to the effect that a change in axis or listening position has on the frequency response of a given speaker. When listening to a loudspeaker, you may notice that, if you walk over to the side of the speaker, the sound changes. While you still hear the music, it may lose some of its “sparkle” or “sizzle,” and may even become muffled. The effect of this is a result of a loss of output at specific frequencies.
To illustrate this, I’ve modeled a few frequency response charts. We’ll be paying attention to the frequency response above 1000hz. The following chart shows a theoretical frequency response for a speaker, simulated at 0 degrees (on-axis).
The following chart shows the same simulated speaker at 15 degrees off-axis. Note how we started to drop off in frequency response after 10KHz.
The following chart shows the same simulated speaker at 30 degrees off-axis. This time, the effects have amplified, and we’ve begun to lose magnitude above 5KHz.
The following charts show the same simulated speaker at 45, 60, and 75 degrees off-axis.
As you can tell by now, the farther we go off-axis, the more of the frequency response we lose. By the time we get 45 degrees off-axis, we’ve started to lose a significant amount of output in the top end of the frequency range. We have also discovered that our off-axis listening position primarily affects higher frequencies; typically those played by a tweeter (and in some cases, a midrange).
Actual On/Off Axis Response
While simulations are all well and good, what will concern us more heavily is the actual on/off axis response of a real loudspeaker. To begin, I’ve taken the frequency response plot of a Morel MDT-44 tweeter, provided by Morel. You will note here that Morel took the liberty of measuring this tweeter on-axis, 30 degrees off-axis, and 60 degrees off-axis.
Here, we can see trends that are consistent with what we simulated earlier; the farther off-axis we go, the more our frequency response is affected. It is worth noting that the above frequency response was taken on a large baffle in an anechoic chamber for raw accuracy.
Let’s take another example. The following is a frequency response chart of a Morel TiW Titanium 6″ Woofer, also showing a 30 and 45 degree off-axis response:
Here, we see a similar behavior, although the effects are somewhat different as this is a woofer, not a tweeter.
How does this help me?
If you recall in our last article, we discussed frequency response and what you might find in a given frequency. That information can be applied here to get an estimate regarding what parts of our music may be affected when we move off-axis. A reduction in magnitude from 5KHz on up will mean that our cymbals may become muffled, our triangles may not be heard at all anymore, and parts of our female vocals will begin to sound faint. In essence, we won’t be getting the full range of music anymore. As we lose more and more of our top end frequency response, we will find that our music begins to sound less “bright.”
The exact effect in decibels for a given frequency will vary from speaker to speaker based on size, design, and a number of other properties. The Morel MDT-44 tweeter provided above is an example of a tweeter that does fairly well off-axis; and with a price tag of over $100 EA, I would hope so. It maintains a reasonably flat frequency response as far as 45 degrees off-axis. While we won’t typically be paying attention to the off-axis response of a subwoofer or woofer, we might pay attention to that of a midrange, and we will certainly be paying attention to that of a tweeter.
To provide some examples, say you are setting up a home theater install and you are only going to have one couch as seating for that theater. In such an environment, you are unlikely to run into any issues off-axis, as you will be no more than 5 degrees off-axis. However, if you are setting up a theater in a large livingroom where your audience may be 45 degrees or more off-axis, your choice of tweeter will be more important, and if you have purchased retail speakers, you may want to reconsider where your speakers are placed and the direction in which they are facing.
In car audio, this brings a very important aspect to your install. Here’s a picture of the tweeter pods in my Chevy Cruze, modified to fit the above Morel MDT-44. The factory tweeter pods can be found in this same location, facing the same direction.
As you can see, depending on your listening position, you will be at least 15-30 degrees off-axis of one of the tweeters, and as much as 90 degrees off-axis of the other. In such an environment, your choice of a tweeter will have a great effect in what your music will sound like, and you’ll want to find a tweeter that has as good of an off-axis response as you possibly can. While there are ways to create custom fiberglass pods, such installs add a significant level of complexity that goes beyond the scope of this article.
In addition to helping you choose a tweeter for a given home or car audio install, an understanding of on/off axis response will also assist you in tuning a sound system and assist in the placement of your speakers. If you know you’ll have a roll-off above 10,000Hz and your speakers are required to be played off-axis, then you’ll be able to use an equalizer to boost your frequencies above 10,000Hz to make up for it.
While this will help you better understand the fundamentals, take this as an introduction to on/off axis response, as there is more to it that I’ll get into in future articles. I’ve asked my friend Mike Z. to share some of that and explain why.
Looks like tweeters will be less than delightful unless pointed right at your ears right? Wrong! Luckily there are many phenomena that “average” the off axis response once you get playing a speaker in an normal room environment. The short wavelengths of a tweeter reflect and refract off anything from the speaker baffle itself to walls, ceiling, and even the coffee table. When they do, they create their own modes that sum and your ears pick up on these reflections even though they arrive a split second after the direct sound. There is no microphone that can recreate how we hear, but since we understand it we can overcome the challenge by creating ways to “see” in-room responses. These are called spatially averaged measurements and can change the picture of a speaker dramatically. Usually, a speaker is set up, and a mic is placed in a designated position, measurement taken, then moved to another location in the grid and measurement repeated, until all points are averaged. If all goes well, you should see a speaker that has a much more gentile roll off in the upper end. These rules apply for all speakers, however there are things like horn loaded tweeters (waveguides), ribbons, and Di-pole/Bi-pole speakers which need special consideration. As you can see, this is one small aspect a designer has to consider but very important as the room can dictate a majority of what you hear. What you see on paper vs. what actually is can sometimes differ so it is important to make sure you know exactly what you are looking at and its usefulness. A tweeter datasheet can tell you a lot about the tweeter’s behavior, however that is in a scientific test application, not as a part of an integrated loudspeaker system. [Michael Z.].
I hope this has helped you gain a better understanding of audio. If you liked this article, feel free to promote it by hitting the “share” or “like” buttons below.