Nuts & Volts - May 2007

SONIC REALISM

with Hi-Fi? Quite simply, virtually all the speaker systems on the market today destroy the proper phase relationships of the sounds that they reproduce. The fundamental reason for this is because of the practical limitations of the speakers (drivers) themselves. A single driver cannot reproduce the full range of frequencies that the ear can hear. In order to do so, multiple speakers are used. This is obvious. Tweeters are hopeless at providing booming base and woofers fade away at two or three thousand Hertz. The requirement of multiple speakers is the underlying problem.

Figure 1 illustrates a typical speaker setup. It consists of a tweeter and a woofer. For convenience, the speaker enclosure is placed on a support at ear level. In this way, the direct line distance from the tweeter and the woofer to the ear are the same. This means that the sounds from both speakers reach the ear at the same time. However, a typical ceiling is an excellent reflector of sound. There is a second acoustic path from the speaker to the ear that results from a reflection from the ceiling. So a reflected signal from the woofer, which is lower to the floor, will travel farther than a reflected signal from the tweeter, which is closer to the ceiling.

This changes the phase relationships and the wave-shape (see Photos

1 and 2). The more drivers that there are in the speaker system, the greater this phase distortion will be. There is also the reflection from the floor, as well as from other large objects in the room. These additional reflections

will not be considered in order to keep things as simple as possible.

It is important to remember that ordinary sounds are not sine waves but complicated combinations of sine waves that span many octaves. Speech is a good example. In order for speech to be intelligible, the frequencies of about 300 Hz to 3,000 Hz must be reproduced. However, few people think that a telephone produces high fidelity reproduction of someone talking. Natural speech contains sound components from about 100 Hz to over 10,000 Hz.

Let’s examine what happens to speech when it is reproduced by the speaker system in Figure 1. The speaker’s cross-over directs the high frequency components to the tweeter and the low frequency components to the woofer (if no cross-over is used, the natural response curves of the drivers perform the same function but less effectively). For the direct line path, the ear hears all of the components at the same time and their phase relationships are maintained. However, with a typical speaker system shown in Figure 1, the high frequency sounds reflected off the ceiling are heard first with the lower sounds bringing up the rear. The phase relationships of the reflected sounds are destroyed. In effect, the ear hears a different signal coming from the direction of the ceiling. Again refer to Photos 1 and 2.

The two photos show what happens when a signal consisting of

1,000 Hz and 3,000 Hz components is reproduced. The 1,000 Hz signal is routed to the woofer and the 3,000 Hz signal goes to the tweeter. The original signal is shown in Photo 1 and this is what is heard if both speakers are equidistant from the ear.

If there is a path length difference between the woofer and tweeter of only two inches (or multiple of two inches), the ear receives significantly distorted signal that is shown in Photo 2. Obviously, with signals consisting of many frequency components and many signal paths, the reflected wave shapes can be vastly different from the original. These distortions are the direct result of the loss of phase information.

Acoustic Peculiarities

One of the tell-tale indicators that this different signal is important is seen in the common problem of stereo setup. If I stand next to one speaker I will often have difficulty in determining if the other speaker is operating. But, this shouldn’t happen. If I’m talking to one person next to me and someone else 10 feet away starts talking, I certainly know that. I may not be able to understand that person because of the nearby person’s voice, but I surely hear that the far person is speaking. The same should be true of the far stereo speaker. The sounds coming out of that speaker are different from the close one (otherwise it wouldn’t be stereo).

Ceiling

Longer 1 kHz path

Shorter 3 kHz path

3000 Hz

FIGURE 1. The fundamental problem with most speaker systems is that multiple drivers are required. This separates the high and Ear low frequencies to different physical locations. The result is that reflected sounds have different acoustic path lengths for different frequencies. This is phase distortion.