Frequency At any point in the air near the source of sound, the molecules are moving backwards and forwards, and the air pressure varies up and down by very small amounts. The number of vibrations per second is called the frequency f. It is measured in cycles per second or Hertz Hz. The pitch of a note is almost entirely determined by the frequency: high frequency for high pitch and low for low.
A contrabassoon can play Bb0 at 29 Hz. When this note is played loudly, you may be able to hear the individual pulses of high pressure emitted as the reed opens and closes 29 times per second. Human ears are most sensitive to sounds between 1 and 4 kHz - about two to four octaves above middle C. That is why piccolo players don't have to work as hard as tuba players in order to be heard.
This link converts notes, frequencies and MIDI numbers. The woodwind family of instruments Some of the woodwinds are shown in the picture at right. Click on the piecture for an enlarged version. A metre rule at left gives the scale. From left to right are bassoon, clarinet, alto saxophone, cor anglais, oboe and flute. The picture is not complete: to the flute could be added piccolo one octave higher , alto flute a fourth lower and bass flute one octave lower.
Similarly there are the soprano, alto and bass clarinet; the musette, oboe d'amore and bass oboe, the contrabassoon and several saxophones: sopranino, soprano, alto, tenor, baritone, bass and contrabass. With the instruments positioned as shown, the bassoon crook would normally protrude towards the viewer, and the cor anglais crook would bend away. The air column determines the pitch A sound wave can travel down the tube, reflect at one end and come back.
It can then reflect at the other end and start over again. For a note in the lowest register of the flute, the round trip constitutes one cycle of the vibration. In the lowest register of clarinets, two round trips are required: see Flutes vs clarinets.
The longer the tube, the longer the time taken for the round trip, and so the lower the frequency. In woodwind instruments, the effective length is changed by opening and closing finger- holes or keyholes along the side. This is the way pitch is changed within the same register of the instrument: all holes closed gives the lowest note, and opening the holes successively from the bottom end gives a chromatic scale.
The use of simple and cross-fingerings to change the length of the standing wave is discussed in much more detail and with specific examples in Flute acoustics , and the principles are the same for all woodwinds. Changing the effective length of the pipe is not the only way of changing pitch, however: on any wind instrument, you can usually play more than one note with the same fingering.
The harmonic series The sound waves going up and down the instrument add up to give a standing wave, a vibration pattern of the air in the instrument. Several different such patterns are possible. On a flute, with all keys down, you can play about seven or eight different notes. Their pitches approximate are given below. The frequencies of these sounds are whole number multiples of the frequency of the lowest f 1. We call them the harmonic series.
Try playing the series on any instrument, without changing the fingering. You will notice the half-sharp on the 7th. For more detail, see Flute acoustics and Clarinet acoustics Eight harmonics of the lowest note on a flute. Harmonics and the different instrument bores Why can the air in the flute vibrate in these different ways? Well, the tube is open to the air at both ends, so the pressure is pretty close to atmospheric, but the air is free to move in and out.
Inside the tube the pressure can be higher or lower, but the air is less free to move. The diagram on the left shows the different vibration patterns or modes that satisfy the condition of the flute: zero pressure and maximum vibration at both ends.
The frequency is the speed of sound divided by this wavelength, and that gives the harmonic series f 1 , 2f 1 , 3f 1 etc. This is a slight simplification: the pressure node is a little distance outside the pipe, and so L, the effective length of the tube that should be used in such calculations, is a little longer than the physical length of the tube.
The end effect is about 0. Flutes vs reed Instruments Reed instruments are different: the end in the player's mouth is not open to the outside air, so the air is not maximally free to move in and out.
The pressure is not fixed at atmospheric - in fact it can have its maximum value at this closed end. Consider the clarinet: it is mainly cylindrical and is open to the outside air at the bell end, but closed at the end in the mouth. Conical bores: oboes, bassoons and saxophones What about oboes, bassoons and saxophones?
Like the clarinet, they are closed at one end and open at the other, but the difference is that their air columns are in the shape of a cone. The resulting pressure and air motion vibrations are shown in the right hand diagram. Although woodwind instruments have traditionally been made of wood and brass instruments were originally made of brass, the material used to make the body of the instrument is not always a reliable guide to its family type, and it is better to examine how the player produces sound.
The instruments in this family used to be made of wood, which gives them their name. Today, they are made of wood, metal, plastic or some combination.
You play them by blowing air through the mouthpiece that's the ' wind ' in woodwind and opening or closing the holes with your fingers to change the pitch. Metal caps called keys cover the holes of most woodwind instruments.
Woodwind instruments produce sound when the player blows air against a sharp edge [1] or through a thin piece of wood called reed [2] , causing a column of air to vibrate. The air blown into the woodwind causes the instrument to vibrate and the sound to be heard.
Blowing more or less air into a woodwind changes the amplitude of the sound waves, and changes the volume of the sound. Clarinets use a reed in order to cause only one end of the instrument to be open to the air which is the end that expels the air.
The player has to force air into the instrument through the reed to cause vibrations. This causes the pressure to be different than those instruments that have both endings open to the air, and the instrument to play an octave lower than it would if both ends were open 1.
The reed also causes the clarinet to have a less cutting sound than the flute. The clarinet has holes along the length of the tube, and depending on which ones are open affects the pitch of the sound, because the air has to travel more or less far along the tube.
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