With the constant downward price pressure on consumer electronics devices, one area that typically gets compromised is audio quality. This compromise is often reflected by the manufacturers' choice of speakers. The speakers chosen by these manufacturers are constrained by the physical size of their equipment, the manufacturing process in mounting those speakers, and of course, cost.

Attention is generally not given to the speaker response over the audible frequency range (speaker transfer function) because it is thought that the equalizers in the audio processors will compensate for flaws in these lower cost speakers. This rarely happens. Listen to the soundtrack of a DVD played through a $3000 HDTV and the same soundtrack played through a good quality sound system. The difference is very noticeable. With some minor changes, however, the TV can approach the audio quality of that sound system with very little incremental cost.

To achieve high-quality audio, the speaker response needs to be flat across the audible spectrum. Manufacturers of high-end stereo equipment have recognized and achieved this and in many cases their equipment ships with no bass or treble adjustments. So how do high volume manufacturers of consumer electronics do this without adding significantly to the cost?

How flat is flat?
An ideal speaker would be capable of reproducing all the audible frequencies at the same volume at which they were recorded. Rarely in life are things perfect, so the generally accepted standard is a variation of ±3dB around a centerline.

The 3dB point is considered to be the point at which volume changes are barely perceptible. Even if a speaker transducer were to possess this characteristic, the process of mounting that speaker into a cabinet will change its response characteristics, so compensation will still need to be applied after the speaker is mounted in its final enclosure.

Measuring the speaker
The first step in the compensation process is to understand exactly what the speaker transfer function looks like. This can be done in a number of ways. Companies such as Real Sound Lab and Dirac Research have developed sophisticated measurement techniques that do not require the use of sound chambers and their measurements take into account the effects of the listening room within the measurement.

The more traditional approach uses equipment from Audio Precision and a sound chamber to deaden the effects of reflections and concentrates the measurement on the speaker itself.

The various measurement techniques will yield similar but not exactly the same results. In the end, what matters is that the measurement is repeatable and believable. If the test methodology is flawed, then the end result (the speaker compensation) will also be flawed. The objective is to generate a speaker transfer function that is not subject to the GIGO (garbage in, garbage out) effect.

The graph shown in Figure 1 was taken with an Audio Precision 2700. It measures the frequency response of a name-brand 40-inch LCD HDTV. You can see that this is clearly not a flat response. The variation around the centerline (of -18) varies from 6 to 8dB. Also, notice the significant drop in mid-range frequencies. The midrange drop will affect the intelligibility of the dialog (particularly female speech). The goal is to bring response back so that it is within ±3dB of centerline.

Figure 1 - Audio Precision measurement of Speaker Transfer Function