Musical Instruments - University of Michigan



MUSICAL INSTRUMENTS

ANALYSIS

Sound

Sound is the transmission of pressure waves of a certain frequency through a medium.

Musical instruments are characterized by two main features:

1) Vibration

Vibrating objects (such as guitar strings and human voice boxes) initiate the pressure waves that propagate sound.

2) Amplification

The shape and size of an instrument contributes to how it amplifies and shapes a sound.

Frequency

One characteristic that makes a sound unique is the pitch or frequency of the note. The frequency is the number of oscillations a pressure wave makes in a second. The human ear can hear between 20 and 20,000 oscillations per second, also called Hertz (Hz). However, human’s hearing diminishes with age and most adults can’t hear above 15,000 Hz.

For reference, dogs can hear up to 45,000 Hz and bats can hear up to 100,000 Hz.

Musical notes represent specific frequencies, for example concert A is 440 Hz. If the note is “flat” then the frequency is lower than the ideal frequency. If the note is “sharp” then the frequency is higher than the ideal frequency.

If a singer is flat when recorded digitally, the producer can tighten up the track and shift the song into the correct pitch.

Musical Instruments

Musical instruments are tools that create vibrations that humans find pleasing. The vibrations depend on the shape and size of the instrument as well as the technique used to start the vibration.

The cavity of the instrument (such as the tubes of the flute and oboe you built) helps define what note will be produced.

With the oboe you may have noticed that your embouchure (mouth positioning) would produce either a high or low frequency note. What you heard was the same note in a different octave. That is one way in which how you start the vibration (how you blow on the reed) contributes to the final note.

Why is music pleasing?

There is no definitive answer to this question, but Physicists have proffered a few observations about pleasing sounds. Oftentimes instruments and voices with a pleasing quality are able to create overtones in their cavities. This means that not only do they achieve the pure tone of the note (such as 440 Hz for a concert A), but they generate overtones which are multiples of the pure note (such as 880, 1320, and 1760 Hz for A).

In these cases the cavity is resonating at multiple frequencies, which can be observed with a spectrum analyzer. From an aesthetic point of view, people find notes with overtones more pleasing than pure notes (notes that only produce the fundamental frequency).

It is not simply overtones on a note that produce pleasing sounds. Musicians have an unquantifiable technique for navigating between notes in a pleasing fashion that is not achieved by current electronic music generators.

APPLICATION

Materials

1 pre-assembled Petri microphone

1 amplifier

1 1/8th inch-double bare ends wire

1 alligator lead card

1 guitar apparatus

1.

[pic]

Figure 4: Electric Guitar

Pluck your “guitar.” What causes the sound you hear?

Press down on the guitar string firmly with a pencil and pluck again. This is called “forcing a node.” How did the sound change and why? Explain. Is this similar to the flute and oboe?

2. Assemble your Petri device so it is a microphone attached to the amplifier as shown below.

[pic]

Figure 5: Petri microphone

Place the Petri microphone under the guitar string, with the magnet-side nearest to the string. Get some cellophane tape and tape the microphone to the base (this will prevent the magnet from being attracted to the string).

Turn on the amplifier and pluck the guitar again. How does adding the microphone change the sound of the guitar?

Challenge Work:

1. What is the general relationship between the size of an instrument and the pitch of the sound that it produces?

2. How do you make instruments start vibrating? List at least four ways.

Summary:

Final Clean-up

Please disconnect all alligator leads and reattach them to the clip card. You are finished with the microphone and loudspeaker, but some parts are re-used so please return the device to the cart. Return all equipment to the carts.

Bibliography and recommendations for further reading:

Wikipedia contributors, "Pitch (music)," Wikipedia, The Free Encyclopedia, (accessed July 7, 2006).

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