THE SOUND QUALITY SYSTEM (CONTINUED) by Anthony Forgey

In the previous issue we established that an the idea SQ system is to reproduce music signals in such a way as to present a holographic image of a soundstage in a manner that is as faithful to the original performance as possible.

I also stated that in my opinion, the speakers are the most important link in the audio chain. In this issue we will look more closely at this.

There are many factors which influence the ability of a speaker (or driver) to accurately reproduce music the way the designers of the speaker intended. Among these are the speaker (driver) specifications, size, mounting location, and enclosure type. We could go into much detail on the anatomy of a driver, its Thiele Small Parameters, cone makeup and so on. Such research will be left to the reader’s own study. I would like to deal with the solid basics, the fundamentals which generally apply to all drivers.

Since it is desirable to have the sound stage in front of the listening position, it stands to reason that the best location for the drivers is the front of the car. For the purposes of this study we will be looking only at the typical 2 way split system and a discussion on subwoofers will follow in future issues.

The Midbass Driver

The midbass driver will generally be playing frequencies from 55 hertz (Hz) to 2.2 kilohertz (kHz). This means that while the driver is moving to reproduce the thump of a kick drum, it also has to play the deep notes of a bass guitar at the same time as playing the warm mid range tones of a saxophone at the same time as playing the highest notes of a flute. Incorrect mounting, enclosure size and panel resonance don’t make the task any easier either. So, what can we do to maximize the performance of midbass drivers? At the risk of sounding over simplified, I will name the big 2. Let’s assume that we are mounting the driver in the stock location in the front doors with the door cavity acting as the enclosure.

The Baffle

The baffle is the panel in which the driver is to be mounted. The more rigid and solid the baffle, the better will be the performance of the driver. Even a baffle of only one piece of supawood bolted to the metal of the car door will improve the performance of the driver. It holds the metal still allowing the driver to do its work.

This most basic addition cannot be over stressed. The improvement will not only be felt in the thump of the kick drum, it will improve all of the music produced. So, rule 1 is a solid baffle.

Try this, tap on the middle of your front door panel. Notice anything? The panel has a resonant sound. It has movement. So, while your speaker is producing waves, your car’s door panel (inner and outer) is reacting to those waves, making waves of its own. The waves of the speaker and the panels interact and produce sounds that are not part of the recorded music. How can this be solved? One of the most cost effective ways is to line the panels with something which acts to reduce panel vibration and resonance. There are a number of products available which have been specially designed for this purpose. Skeptics may consider this lining of the panels to be of no use, but ask anyone who has done it, it makes a significant audible difference (certainly in every car I have heard) this is rule 2, reduce panel resonance and vibration.

The Tweeter

The tweeter typically plays the range of frequencies from 2.2 kHz to 20 kHz.

The cone movement for high frequencies is much less than a midbass driver, so the solid baffle requirement is not nearly as critical as with the midbass driver. The biggest factors in tweeter mounting is placement. Where you mount the tweeter and where you aim it are the big 2 for tweeters. Here I must introduce two terms: on-axis and off-axis. On-axis mounting of a tweeter is when the tweeter is directly facing the listener. Off-axis is when the tweeter is turned so that it aims 90 degrees away from the listener.

It is best to experiment with tweeter position before final mounting unless you plan to use the factory located positions in the car. Experimenting is as easy as a blob of prestik and some patience. Some tweeters perform best mounted low down in the doors or even the kick panel position. Other tweeters perform best above the dash in the A-pillars either firing off-axis (across the windscreen) or on-axis, or somewhere in between. Stick the tweeters down with prestik, put some music on and listen. Move them to an on-axis position, listen again. Keep on until you find the optimum location and angle. Experimenting with these positions will aid in correct mounting of the tweeters in order to optimize the soundstage depth and width, whilst still maintaining tonal accuracy.

Happy experimenting!