Frequency Response

Frequency response describes the ability of a voice, instrument, acoustic or electronic device to produce or respond to any specific tone or frequency of sound. Frequency response has several basic characteristics, including bandwidth, uniformity and balance. These characteristics are illustrated using a frequency response curve. Figure 1 shows the frequency response curve of a microphone. The curve shows relative level of response in decibels (dB) on the vertical axis of the graph.

Figure 1

Most of us have a good idea of our singing-voice range. Whether attempting to imitate Frank Sinatra in the morning shower or singing the “Star Spangled Banner” at a ball game, we have all realized the limits of our vocal chords. At some notes, our voice just quits. We have just described the frequency response of our voices in their low-frequency and high-frequency limits. This range of response is called the bandwidth of the frequency response. Most manufacturers of audio components will describe this bandwidth with two numbers, such as: 20 Hz - 20,000 Hz or 20 Hz-20 kHz. The k in kHz is an abbreviation for 1000. It is important to know the bandwidth of each piece of equipment in use. You also need to know the frequency content (bandwidth) of the instrument or voice you are reproducing. For example, the low frequency limit of the grand piano is about 30 Hz. Therefore, the microphone whose response is shown in Figure 1 would not be entirely suitable to record the piano. That microphone will not pick up the lowest notes of the piano.

The overall balance shown in a frequency response curve is usually a good indicator of the usefulness of any microphone or other device for a particular application. For example, the response of the microphone in Figure 1 was tailored for use as a lavalier (neck) microphone. The lower frequency response has been reduced (rolled off) to lower clothing and handling noise. It also will help decrease low frequency feedback. The midrange frequencies centered near 700 Hz were reduced by 5 dB. This will reduce the “hollow” sound created when the microphone is worn on the chest. The high frequencies about 3 kHz are gradually boosted to increase the “presence” of the voice. Figure 2 shows the response of a different microphone. Notice the equal balance in the overall response. This microphone is designed for recording. Since the balance of this microphone is relatively flat, it will faithfully reproduce whatever source it is recording without much “coloration” in the sound quality. The resulting recording will sound much more natural than with the microphone from Figure 1.

Figure 2

It is worthwhile to mention that when using frequency response to compare microphones or loudspeakers, one response is not necessarily better than another. It may only be better for a particular application. It’s much like using lenses and filters in photography. A clear lens by itself will have a “balanced” or “flat” response to different frequencies of light. This will result in a true replica of the photo subject or source. However, if a color filter is used, the accuracy of the photo is sacrificed in order to create a result that has a certain impact or effect. Neither is incorrect, merely different.

Another characteristic of frequency response that usually indicates the quality of sound is the uniformity or smoothness of response. It is difficult, however, to compare response curves from different manufacturers. The smoothness of any one curve is dependent on the testing equipment and the number of averaged samples of that piece of equipment.

This information will not provide enough guidelines for purchase decision-making; however, it can amplify your audio vocabulary for better understanding of future articles.

by Travis Ludwig © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

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