Bass or Treble? Measure the Frequency Response of a Speaker

Abstract

Whether you have already tried the Science Buddies Build a Paper Speaker activity or the How Loud Can Paper Speakers Get? project, or you just like music and are interested in exploring more about the science of sound, then this project is for you.

You probably know that sound waves can have different frequencies. If not, you can read more about that in the background section of the How Loud Can Paper Speakers Get? project. The range of human hearing is typically about 20 hertz (Hz) to 20,000 Hz. Frequencies at the lower end of this range are typically referred to as bass, the higher end is called treble, and those in the middle are called mid-range. Ideally, a single speaker would be able to play sounds at all frequencies "equally," meaning it would not make certain frequencies sound louder than others. However, this is not the case. Many times speakers are separated into tweeters for playing high frequencies and woofers for playing low frequencies. Why is this necessary?

The reason has to do with a concept called resonant frequency, sometimes called "natural frequency." Every mechanical system or object has a natural frequency at which it "likes" to vibrate. In general, smaller, lighter, and stiffer objects like to vibrate faster than bigger, heavier, and softer objects. For example, imagine "pinging" a small metal object (like a ruler or paperclip) by flicking it with your finger. It will vibrate back and forth very quickly—possibly faster than your eye can see. Now, think about much larger objects, like tree branches swaying in the wind; they rock back and forth very slowly. The same concept applies to speakers—smaller, lighter tweeters will vibrate at higher frequencies, while larger, heavier woofers will vibrate at lower frequencies.

A graph of the amplitude of a vibration versus the frequency of the vibration is called a system's frequency response. A good speaker should have a frequency response that is as "flat" as possible, meaning it reproduces frequencies at the same volume at which they were recorded. Otherwise, large peaks or valleys in the frequency response will result in some frequencies sounding much louder or quieter than others. Figure 1 shows an example frequency response for a set of computer speakers.

There are many science projects you can do involving the frequency response of speakers. Here are just a few ideas:

• Build your own paper speaker, like in the Science Buddies project How Loud Can Paper Speakers Get? Measure the frequency response by playing tones at different frequencies and recording the volume in decibels (dB) (the procedure for doing this at a single frequency is described in the project), and produce a graph like Figure 1. How flat is the speaker's frequency response? Would you characterize the speaker as a tweeter, a woofer, or a mid-range speaker?
• Investigate how different variables can change the frequency response of a paper speaker. Remember that natural frequency is controlled by the stiffness and mass of an object. What can you change about the basic design of a paper speaker to alter its frequency response? Can you design and build speakers with two distinct frequency responses, one tweeter and one woofer?
• If you have a home stereo system, see if you can look up the frequency response data for the speakers from the manufacturer. Do your own experiments to measure the frequency response. Do your results line up with the manufacturer's claims?

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Last edit date: 2020-11-20

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