Low Stage Part 01 - Two Speakers

Tweeters on "A" pillars

Is your car audio's sound-stage low? Are you being told to place tweeters on the "A" pillars? Do high highs fix height?
If you have height problems, you are not alone. Most systems have height problems. Unfortunately, just as many are patched with tweeters up high. The argument is that you need speakers at eye level to fix height problems. The pundits further argue that high frequencies are responsible for height. Wow!

Hello Kitty Band-aid

This reminds me of a time when, standing behind a shameless retail salesperson, I heard him tell a consumer that a woofer needed a port to breath or else die. Can you imagine a woofer being slowly chocked to death every minute of every day, whether being played or not? Clueless!
Needless to say, tweeters placed high don't address the fundamental problem. Instead, such attempts equate to no more than a Hello Kitty Band-aid. They are cute. Let me expand.
I once built an award winning truck that had waveguides underneath the front seats. The horn mouths were located right behind the front passengers' Achilles-heels with the horns firing forward towards the front of the car; where the pedals are located. In other words, they were right over the carpet's floor and aimed away from the listener.

Waveguides under the seat

But even under this very unusual scenario, there was no problem with either stage height or high frequency loudness. The only problem was that for a few early shows, judges struggled to score the vehicle because there were no visible speakers anywhere. And let me tell you, you never want to baffle judges. You want to impress them instead. And once the word got out that the speakers were horns and that they were being used in such an incredible manner, the scores jumped immediately.
This does not mean that placing a speaker in any one location is a bad thing. It is just that placing them there does not solve the fundamental problems behind the psycho acoustical effect of a lower stage.
To understand the issues at hand and the potential solutions, I will have to cover several aspects of acoustics; more than I would like to do in a single article. I will therefore break it into parts. My goal is to be able to clearly explain one concept at a time.

Not a Point Source

Two Way Second Order Crossover

The fact is that (1) all non-coincident systems deploying multiple drivers exhibit radiation lobes in the vertical plane around the crossover frequency. (2) But these lobes are not aligned with the design's central axis. Instead, a Polar Axis tilt results. This tilt is measured in degrees.

In simpler terms, this means that every time you use two speakers to play the same frequency their combined output will create areas of cancellation (destructive interference) next to areas of cooperation (constructive interference). Moreover, the resulting patterns of interaction are not symmetrical; they tilt more towards one of the speakers.
To visualize these concepts, look at the following illustration.

Sound wave measurement for a typical two way system

Two speakers, a tweeter and woofer, are surface mounted above one another as it's commonly done in home speakers. To channel high frequencies to the tweeter and low frequencies to the woofer, a second-order or 12dB/Oct crossover set at 2.2 KHz is used between the two speakers. Measurements of the acoustic pressure waves are then taken at 2.2 KHz. The resulting measurements clearly show two sets of waves; one for the tweeter and one for the woofer. Dark areas represent low pressures while white areas show high pressures.
Now direct your attention to the image below. A listener with his ear placed at the tip of the red arrow would hear most of the 2.2 KHz sound as coming mainly from the tweeter. The sound would be loud and relatively clear depending mostly of the quality of the tweeter. At this point, any attributes of the woofer are less relevant. Again, the white portions of the wave represent high pressure portions of the wave.

Listener focused on high pressure area

Next, look at the tip of the red arrow below. There, the listener would hear most of the 2.2 KHz sound as coming from the woofer. The dark area represents the low pressure of the wave as it radiates from the speaker. It is important to note that low pressure does not mean that loudness is low. The perceived sound would be just as loud as with the example above. This is because sound is made from two halves; low and high pressure halves. The change between these periodically repeated halves is what we hear. In this case, the quality of the sound would be mostly dependent of the quality of the woofer.

Listener focused on low pressure area

 Finally, look at the next arrow pointing at the middle area where the two waves interact. A close analysis shows that the tweeter's wave above is at a low pressure stage while the woofer's wave below is at a high pressure stage. Subsequently, the arrow's head is placed over an area of cancellation. In this case, gray means no sound.

Listener focused on cancellation area

The area between the two waves actually beams outwardly from the speaker and angles down from the center axis. This is a cancellation beam and is highlighted below. A listener on this axis would hear the effect of two waves trying to cancel each other at 2.2 KHz. 

Acoustic Pressure and Lobing Graph

This now takes us to the typical dispersion pattern graph as is commonly used by acoustical engineers:

The fundamental problem that all these graphs and images are attempting to illustrate is that the cancellation effects that result from using two speakers distort and destroy any illusion of a realistic sound. As we will see later, this distortion is also behind the height problems experienced by many car guys. 

I will take us deeper on the subject on a future post. Until then, happy listening!

A video showing all components of a two way system: