Saxophone acoustics: an introduction

This introduction to the physics of the saxophone requires no mathematics beyond multiplication and division, nor any technical knowledge of acoustics. For some preliminary information about sounds and vibration, read the introduction to How do woodwind instruments work?. For background on topics in acoustics (waves, frequencies, resonances, decibels etc) click on "Basics" in the navigation bar at left.

News: A new paper about the science behind some advanced performance techniques on the sax, published in JASA. This and an earlier paper, published in Science, are introduced in this page.

News: Our new data base of saxophone acoustics showing the response and sound spectra for each note.

close up of saxophone

      To set the mood, listen to a movement of a quartet for saxophone, flute, bassoon and cello.

Overview

photo of soprano and tenor sax with metre rule The saxophone player provides a flow of air at a pressure above that of the atmosphere (technically, a few kPa or a few percent of an atmosphere: applied to a water manometer, this pressure would support about a 30 cm height difference). This is the source of power input to the instrument, but it is a source of continuous rather than vibratory power. In a useful analogy with electricity, it is like DC electrical power. Sound is produced by an oscillating motion or air flow (like AC electricity). In the saxophone, the reed acts like an oscillating valve (technically, a control oscillator). The reed, in cooperation with the resonances in the air in the instrument, produces an oscillating component of both flow and pressure. Once the air in the saxophone is vibrating, some of the energy is radiated as sound out of the bell and any open holes. A much greater amount of energy is lost as a sort of friction (viscous loss) with the wall. In a sustained note, this energy is replaced by energy put in by the player. The column of air in the saxophone vibrates much more easily at some frequencies than at others (i.e. it resonates at certain frequencies). These resonances largely determine the playing frequency and thus the pitch, and the player in effect chooses the desired resonances by suitable combinations of keys. Let us now look at these components in turn and in detail.

The reed controls the air flow

The reed is springy and can bend. In fact it can oscillate like a spring on its own—for a saxophonist this is bad news: it's called a squeak. Normally, the reed's vibration is controlled by resonances of the air in the saxophone, as we shall see. But it's also true that the reed vibration controls the air flow into the saxophone: the two are interconnected.

Let's imagine steady flow with no vibration, and how it depends on the difference in pressure between the player's mouth and the mouthpiece. If you increase this pressure difference, more air should flow through the narrow gap left between the tip of the reed and the tip of the mouthpiece. So a graph of flow vs pressure difference rises quickly: it has positive slope. However, as the pressure gets large enough to bend the reed, it acts on the thin end of the reed and tends to push it upwards so as to close the aperture through which the air is entering (the arrow in the sketch at left). Indeed, if you blow hard enough, it closes completely, and the flow goes to zero. So the flow-pressure diagram looks like that in the graph sketched below.

sketch of mouthpiece and diagram of flow vs pressure

The reed (as any saxophonist will tell you) is the key to making a sound. The player does work to provide a flow of air at pressure above atmospheric: this is the source of energy, but it is (more or less) steady. What converts steady power (DC) into acoustic power (AC) is the reed. The first part of the graph is something like a resistance: flow increases with increasing pressure difference. Just like an electrical resistance, an acoustic resistor loses power. So in this regime, the sax will not play, though there is some breathy noise as air flows turbulently through gap between reed and mouthpiece. The operating regime is the downward sloping part of the curve. This is why there is both a minimum and maximum pressure (for any given reed) that will play a note. Blow too softly and you get air noise (left side of the graph), blow too hard and it closes up (where the graph meets the axis on the right). (In the diagram above, the upper curve could represent a stiffer reed or a more open mouthpiece, or less lip force: in call cases, more pressure is required to close the reed.)

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Readers with a background in electricity, seeing the region of the curve in which flow decreases with increasing pressure, will recognise this as a negative (AC) resistance. Whereas a positive resistance takes energy out of a circuit, a negative resistance puts energy into the circuit (as happens in eg. a tunnel diode oscillator). In the saxophone, it is indeed this negative AC resistance that provides the energy lost in the rest of the instrument. Most of the energy is lost inside the bore, in viscous and thermal losses to the walls, and a relatively small fraction is emitted as radiated sound.

Playing softly and loudly

This diagram allows us to explain something about how the timbre changes when we go from playing softly to loudly. For small variation in pressure and small acoustic flow, the relation between the two is approximately linear, as shown in the diagram below at left. A nearly linear relation gives rise to nearly sinusoidal vibration (i.e. one shaped like a sine wave), which means that the fundamental in the sound spectrum is strong, but that the higher harmonics are weak. This gives rise to a mellow timbre.

sketches of playing regimes for p and ff

As we play more loudly, we increase the pressure (which moves the operating point to the right) and we also increase the range of pressure. This means that the (larger) section of the curve we use is no longer approximately linear. This produces an asymmetric oscillation. It is no longer a sine wave, so its spectrum has more higher harmonics. (Centre diagram.) Have a look at the spectra for different dynamic levels on the note A#3. Observe that the increase in the level of higher harmonics is much greater than that in the fundamental.

When we blow even harder, the valve closes for part of the part of the cycle when the pressure in the mouthpiece is low due to the standing wave inside the instrument. So the flow is zero for part of the cycle. The resultant waveform is 'clipped' on one side (diagram at right), and contains even more high harmonics. As well as making the timbre brighter, add more harmonics makes the sound louder as well, because the higher harmonics fall in the frequency range where our hearing is most sensitive (See What is a decibel? for details).

While talking about decibels, we should mention that spectra, including those on our saxophone site are usually shown on a decibel scale. This means that one notices easily on the spectrum a harmonic that is say 20 dB weaker than the fundamental, even though it has 10 times less pressure and 100 times less power. What is important is that your ear notices it too, because of the frequency dependence referred to above. However, it is much more difficult to notice the presence of harmonics if you look at the waveform.

Soon after this page went up, saxophonists wanted to know a lot more about playing more loudly. Usually, blowing harder makes it louder: more pressure times more flow gives more power. Further, blowing harder eventually takes you into the non-linear and clipping ranges that produce stronger high harmonics, and therefore a sound that is both louder and brighter. However, if you blow too hard, you close the reed completely and it stays closed. A hard reed requires greater pressure differences to bend, and so allows you to blow harder without entering the non-linear or clipping ranges. On the other hand, the smaller nonlinearity makes the sound more mellow (and therefore less loud, all else equal). This is complicated by the embouchure: because of the curvature of the lay, the position where you place your lower lip and how hard you bite makes a difference to the effective length and therefore stiffness of the reed. It's like a lever whose fulcrum is moved. Mouthpiece design also influences the amplitude at which non-linear or clipping ranges begin: a mouthpiece with an obstruction near the reed tip becomes less linear at lower pressures and so produces the brighter (harsher?) and louder sound beloved of some saxophonists. Alright, I admit it, I have such a mouthpiece in the case, too.

Many examples of sound spectra are given on the data base. To compare spectra at different loudness, go here.

Control parametes for playing

For any given fingering, reed and mouthpiece set-up, the player can make a range of sounds, varying the pitch, loudness and spectrum. Various control parameters are available to the player: pressure in the mouth can be varied, so can the bite force, the position at which the lip presses on the reed, and sometimes the configuration of the vocal tract. We investigated this on the clarinet, rather than the sax. The reason is that we have a robot clarinet player in which we can vary all these parameters in a controlled way. We report these results in more detail on the clarinet robot site, along with links to a scientific report on that project.

The saxophone is a 'closed' pipe

The saxophone is open at the far end or bell. But it is (almost) closed at the other end. For a sound wave, the tiny aperture between reed and mouthpiece – a much smaller cross section than the bore of the instrument – is enough to cause a reflection almost like that from a completely closed end. The rest of the saxophone is approximately conical. Of course most saxophones are bent, and these sections are not conical. There's also the bell, some of whose effects we shall discuss below.

For the purposes of this simple introduction to saxophone acoustics, we shall now make some serious approximations. First, we shall pretend that it is a simple conical pipe – in other words we shall assume that all holes are closed (down to a certain point, at least), that the bore is conical, and that the mouthpiece end is completely closed. This is a crude approximation, but it preserves much of the essential physics, and it is easier to discuss. Of course the saxophone doesn't come to a sharp point: it has a mouthpiece. The mouthpiece is shorter and fatter than the cone it replaces, and it has approximately the same volume.

sketch of saxophone    photo of saxophone

At left is a schematic of a soprano saxophone, an idealised conical bore and a truncated cone. At right is a photo of a Yamaha soprano saxophone. The larger saxophones are bent to bring the keys within comfortable reach of the hands. The bends make only modest differences to the sound, so we shall picture straight saxophones in the diagrams here. This link shows modern Yamaha soprano and tenor saxes, with a metre rule. This link shows three experienced saxophones (sop, alto and tenor) from the author's collection.

The natural vibrations of the air in the saxophone, the ones that cause it to play notes, are due to standing waves. (If you need an introduction to this important concept, see standing waves.) What are the standing waves that are possible in such a tube?

To answer the question, we must take into account the fact that the saxophone is approximately conical. This means that sound waves 'spread out' as they travel down the bell. This means that the amplitude of the waves gets smaller as we go from mouthpiece to bell. The fact that the saxophone is open to the air at the far end means that the total pressure at that end of the pipe must be approximately atmospheric pressure. In other words, the acoustic pressure (the variation in pressure due to sound waves) is zero. The mouthpiece end, on the other hand, can have a maximum variation in pressure: it is an antinode in pressure. If we were dealing with a cylindrical pipe (such as a flute or clarinet), where the standing waves are sinusoidal, we would expect the maximum and the zero of a wave to be one quarter wavelength apart. But the variation in the amplitude of the wave due to the variation in cross sectional area complicates the story.

impedance spectra of a soprano and tenor saxophone

So we have devoted a whole page to comparing cylindrical and conical pipes and, if you want the details, you should read that page now. However, the result is this: the standing waves in a cone of length L have wavelengths of 2L, L, 2L/3, L/2, 2L/5... in other words 2L/n, where n is a whole number. The wave with wavelength 2L is called the fundamental, that with 2L/2 is called the second harmonic, and that with 2L/n the nth harmonic.

The frequency equals the wave speed divided by the wavelength, so this longest wave corresponds to the lowest note on the instrument: Ab on a Bb saxophone, Db on an Eb saxophone. (See standard note names, and remember that saxophones are transposing instruments, so that the written low Bb3 is actually Ab2 for a tenor saxophone in Bb, Db3 for an alto in Eb, and Ab3 for a soprano in Bb. Hereafter we refer only to the written pitch.) You might want to measure the length of your instrument, take v = 345 m/s for sound in warm, moist air, and calculate the expected frequency. Do you get a better answer if you use the real length, or if you use the length of the cone made by extrapolating back to a point? Then check the answer in the note table.

The acoustic response of wind instruments is often quantified using the acoustic impedance spectrum, which we discuss below. This specifies how hard it is to make air vibrate at a given frequency, or what acoustic pressure is produced by air vibration at a given frequency. Saxophones operate at peaks in the impedance. The graphs above right show the impedance spectra for low Bb on a soprano and a tenor. The instrument will play several of these peaks – called bugling.

So, with all of the holes closed, you can play the lowest note: (written) Bb3, with a wavelength somewhat more than twice the length of the instrument. With this fingering, however, you can also play other notes by overblowing – by changing your embouchure and changing the blowing pressure. The harmonic series on Bb3 is shown below.

harmonics on Bb

How the reed and pipe work together

To sum up the preceding sections: the bore of the saxophone has several resonances. For the fingerings of the low notes, the frequencies of these resonances are approximately in the ratios of the simple harmonics, 1:2:3, but successively more approximate with increasing frequency–we'll see why below under frequency response. The reed has its own resonance--which is approximately what you hear when you produce a squeak. One good way to produce a squeak is to put your teeth on the reed. In normal playing, with your lower lip touching the reed, you damp (ie reduce the strength of) the reed's resonances considerably. This allows the resonances of the bore to 'take control'. To oversimplify somewhat, the saxophone usually plays at a strong bore resonance whose frequency is lower than that of the reed. (We shall see below how register holes are used to weaken the lower resonance or resonances and thus make one of the higher resonances the strongest.)

When the saxophone is playing, the reed is vibrating at one particular frequency. But, especially if the vibration is large, as it is when playing loudly, it generates harmonics (see What is a sound spectrum?). These set up, and are in turn reinforced by, standing waves. Consequently, the sound spectrum has strong components at harmonics of the fundamental being played. For notes in the low range of the instrument, there are usually resonances at the frequencies of some of the harmonics. These result in greater radiation of the sound at those frequencies. In the high range of the instrument, the higher resonances do not systematically fall near harmonics of the note being played. See the data base of saxophone acoustics for examples.

Register holes are discussed in more detail on the page about clarinet acoustics. See more about register holes on the clarinet page.

Spectrum of the saxophone

In the section on standing waves, each wave is considered as a pure sine wave. The sound of the saxophone is a little like a sine wave when played softly, but successively less like it as it is played louder. To make a repeated or periodic wave that is not a simple sine wave, one can add sine waves from the harmonic series. So C4 on the saxophone contains some vibration at C4 (fo), some at C5 (2fo), some at G5 (3fo) etc. The 'recipe' of the sound in terms of its component frequencies is called its sound spectrum. Many examples of sound spectra are given on the data base. One fairly consistent feature is a change in the slope of the spectral envelope, which is due to the cut-off frequency.

Opening tone holes

If you open the tone holes, starting from the far end, you make the pressure node move up the pipe, closer to the mouthpiece---it is very much like making the pipe shorter. Starting near the bell, each opened tone hole raises the pitch by a semitone, which requires a pipe that is about 6% shorter. After you open all of the right hand finger holes, as shown below, you have the fingering for G4, which is shown below. (Black circles mean hole closed, white circles mean hole open.)

For the moment, we can say the an open tone hole is almost like a 'short circuit' to the outside air, so the first open tone hole acts approximately as though the saxophone were 'sawn off' near the location of the tone hole. This approximation is crude, and in practice the wave extends somewhat beyond the first open tone hole: an end effect.

(For the technically minded, we could continue the electrical analogy by saying that the air in the open tone hole has inertia and is therefore actually more like a low value inductance. The impedance of an inductor in electricity, or an inertance in acoustics, is proportional to frequency. So the tone hole behaves more like a short circuit at low frequencies than at high. This leads to the possibility of cross fingering, which we have studied in more detail in classical and baroque flutes.)

The frequency dependence of this end effect means that the higher note played with a particular fingering has a larger end effect than does the corresponding note in the lower register. If the saxophone really were a perfect cone with tone holes, then the registers would be out of tune: the intervals would be too narrow. This effect is removed primarily by the the shape and volume of the mouthpiece.

Register holes

A small hole can serve as a register hole. For instance, if you play E4 (call this frequency fo) and then push the octave key, you are opening a hole part way down the (closed part of the) instrument. This hole disturbs the resonance that supports the fundamental of E4, but has little effect on the higher harmonics, so the saxophone 'jumps up' to E5 (2fo).

Where to put it? The acoustically obvious place to put a register hole is at a pressure node of the upper note which is also a region of large pressure variation for the lower note. Opening the bore to atmospheric pressure at a pressure node makes no difference to that note. The trouble is that each note in the upper register has its pressure node at a different position. One can imagine a saxophone that had a separate register hole for each note, but that would be a lot of keys. In fact, only two register holes are used for the second register from D5 to F#6. This is of course a compromise: the register hole is never at the pressure node of the standing wave of the required upper note. This is not a big problem in practice. The register hole is small, so it is not really a 'short circuit', except at low frequencies. So it does not too much affect the higher frequency standing waves. (We explain how the mass of the air seals the hole at high frequencies below.) It does however disrupt the fundamental, and that is its purpose: to stop the instrument dropping down to its bottom register.

Effect on the impedance spectrum of opening an octave key (A4-A5)

This figure shows the effect of the register key on the acoustic impedance spectrum of a soprano sax. The first peak in the continuous curve is the resonance that controls the standing wave that produces the note A4. Opening the register hole both weakens and detunes that resonance, so the second resonance dominates. For more detail, see the paper by Chen, Smith and Wolfe.

Just in case you haven't noticed on your own saxophone, the octave key is automated: one key opens one or other of two register holes, according to whether or not the third finger of the left hand is depressed. So the upper register hole (right at the top near the mouthpiece) opens for notes above G#5, whereas from D5 to G#5 the lower hole is used. This is an example of a mechanical logic gate, which is well worth examining closely. The octave keys on oboes are only partly automated and on bassoons---trust me, you don't want to know.

The octave key register holes are used from D5 to F#6. In higher registers, other register holes are used. (Some players use the key for F6 as a register hole. This hole is designed primarily as a tone hole, so it is bigger than it need or should be for an ideal register hole. Some players adjust the mechanism for the alternate F6 fingering so that it opens that hole only a little way. This improves its performance as a register hole, but compromises the intonation of the alternate high F fingering.)

Cross fingering

On the saxophone, successive semitones are usually played by opening a tone hole dedicated to that purpose. Being a closed, conical pipe the saxophone overblows an octave, and so one would need eleven tone holes to cover the keys in one octave before repeating fingerings using the octave key for the register holes. Because players don't have this many fingers, the keys and clutch mechanisms are supplied, so that one finger can close or open two or more holes. This fingering system is very similar to that developed by Theobald Boehm for the flute. Where cross fingering is used, it is usually used to control other keys. For instance, one of the fingerings for the note F#4 or F#5 is a simple fingering: the right hand index finger closes its hole, and the right ring finger opens a tone hole with a key (upper figure below). The other is a cross fingering: all three fingers on the left hand, plus the middle finger on the right close their holes. In the cross fingering, one tone hole is open, and the next closed. In the simple fingering, three successive tone holes are open.

pictures of simple and cross fingerings

An open tone hole connects the bore to the air outside, whose acoustic pressure is approximately zero. But the connection is not a 'short circuit': the air in and near the tone hole has mass and requires a force to be moved. So the pressure inside the bore under a tone hole is not at zero acoustic pressure, and so the standing wave in the instrument extends a little way past the first open tone hole. (There's more about this effect under Cut-off frequencies.) Closing a downstream hole extends the standing wave even further and so increases the effective length of the instrument for that fingering, which makes the resonant frequencies lower and the pitch flatter.

The effect of cross fingerings is frequency dependent. The extent of the standing wave beyond an open hole increases with the frequency, especially for small holes, because it takes more force to move the air in the tone hole at high frequencies. This has the effect of making the effective length of the bore increase with increasing frequency. As a result, the resonances at higher frequencies tend to become flatter than strict harmonic ratios. Because the saxophone's tone holes are large, cross fingerings have relatively little effect on the pitch in the first two registers. Nevertheless, comparing the two fingerings discussed above, one sees that the first open tone hole in the simple fingering is both smaller and further down the bore than the first open tone hole in the cross fingering.

A further effect of the disturbed harmonic ratios of the maxima in impedance is that the harmonics that sound when a low note is played will not 'receive much help' from resonances in the instrument. (Technically, the bore does not provide feedback for the reed at that frequency, and nor does it provide impedance matching, so less of the high harmonics are present in the reed motion and they are also less efficiently radiated as sound. See Frequency response and acoustic impedance. To be technical, there is also less of the mode locking that occurs due to the non-linear vibration of the reed.) As a result, cross fingerings in general are less loud and have darker or more mellow timbre than do the notes on either side. You will also see that the impedance spectrum is more complicated for cross fingerings than for simple fingerings.

We have studied cross fingerings more extensively on flutes than on saxophones, by comparing baroque, classical and modern instruments. (There is of course no baroque saxophone.) See cross fingering on flutes or download a scientific paper about cross fingering.

Other effects of the reed

As well as controlling the flow of air, the reed has a passive role in saxophone acoustics. When the pressure inside the mouthpiece rises, the reed is pushed outwards. Conversely, suction draws the reed in towards the bore. Thus the reed increases and decreases the mouthpiece volume with high or low pressure. (Techncially, we say it is a mechanical compliance in parallel with the bore.) Indeed, it behaves a bit like an extra volume of air, which could also be compressed and expanded by changing pressure in the mouthpiece. It has the effect of lowering the frequency of each resonance a little. However, soft reeds move more than hard reeds, so soft reeds lower the frequency more than do hard reeds. Further, this effect is greater on high notes than on low, so soft reeds make intervals narrower and hard reeds make them wider. This is useful to know if you have intonation problems. (See also tuning.)

Of course, the mouthpiece itself also contains a volume of air (an acoustic compliance) and so it has an effect on the tuning of the resonances. The combined compliance of the mouthpiece and reed is approximately equivalent to the volume of the cone that would be required to extend the body of the saxophone to a point.

Effect on the impedance spectrum of mouthpiece and reed

This figure shows the effect of the mouthpiece and the reed on on the acoustic impedance spectrum. One measurement is made with the mouthiece replaced by a conical section, with just a short cylinder matching it to the measurement head. The next has a mouthpiece, but no reed compliance. Measurement was on A#3 for a soprano sax. For more detail, see the paper by Chen, Smith and Wolfe.

Cut-off frequencies

When we first discussed tone holes, we said that, because a tone hole opens the bore up to the outside air, it shortened the effective length of the tube. For low frequencies, this is true: the wave is reflected at or near this point because the hole provides a low impedance 'short circuit' to the outside air. For high frequencies, however, it is more complicated. The air in and near the tone hole has mass. For a sound wave to pass through the tone hole it has to accelerate this mass, and the required acceleration (all else equal) increases as the square of the frequency: for a high frequency wave there is little time in half a cycle to get it moving.

sketch: inertance of air in open tone holes

So high frequency waves are impeded by the air in the tone hole: it doesn't 'look so open' to them as it does to the waves of low frequency. Low frequency waves are reflected at the first open tone hole, higher frequency waves travel further (which can allow cross fingering) and sufficiently high frequency waves travel down the tube past the open holes. Thus an array of open tone holes acts as a high pass filter: some thing that lets high frequencies pass but rejects low frequencies. (See filter examples.) The cutoff frequency is about 800 Hz for the tenor and 1300 Hz for the soprano. In the sound spectra of low notes, you will notice that the harmonics fall off above this frequency. You will also notice that the impedance spectra become irregular above this frequency. See the data base of saxophone acoustics.

The cutoff frequency and the conical shape together limit the ability to play high notes on the saxophone, unless the player uses his/her vocal tract as a resonator instead. The stiffness of the reed is another: a saxophone will only play notes with frequencies lower than the natural frequency of the reed. (There is a more detailed explanation of cut-off frequencies and their effects here.)

impedance spectra of a soprano and tenor saxophone

Frequency response and acoustic impedance of the saxophone

The way in which the reed opens and closes to control the air flow into the instrument depends upon the acoustic impedance at the position of the reed, which is why we measure this quantity. The acoustic impedance is simply the ratio of the sound pressure at the measurement point divided by the acoustic volume flow (which is just the area multiplied by the particle velocity). If the impedance is high, the pressure variation is large and so it can control the reed and the flow of air past it. In fact, the resonances, which are the frequencies for which the acoustic impedance is high, are so important that they 'control' the vibration of the reed, and the instrument will play only at a frequency close to a resonance. (There is further explanation on What is acoustic impedance and why is it important?). The section below shows how the major features of the saxophone's shape give rise to its acoustic impedance spectrum, and thus to how it operates.

We repeat the graph of the impedance spectra of the lowest notes of soprano and tenor. We also have a data base of saxophone acoustics that shows saxophone impedances and sound files for many notes and some multiphonics.

(We also have such a data base of clarinet acoustics. It's worth contrasting them: First, the soprano saxophone is actually a little longer than a clarinet – about 700 vs 670 mm. However, the first resonance of the clarinet (call it fo) occurs at a lower frequency than that of the saxophone (call it go). Further, the peaks of the saxophone curve occur at all the harmonics (go, 2go, 3go etc), whereas those of the clarinet curve occur only at the odd harmonics (fo, 3fo, 5 fo etc. The differences among open cylindrical pipes (flutes), closed cylindrical pipes (clarinets) and closed conical pipes (saxophones, oboes, bassoons) are explained in Pipes and harmonics. This difference gives the clarinets a big advantage: a clarinet of a given length can play lower notes than can conical instruments of the same length. (Baritone sax players envy the small case that bass clarinettists lug around.) Clarinets pay for this advantage: the odd harmonic series means that they overblow a twelfth instead of an octave, which makes the fingering more awkward, especially around the 'break' between registers. They are also less loud than a comparable conical instrument. (The baritone sax player can blow the bass clarinettist away on that score.)

For frequencies above about several hundred hertz, the resonances become weaker with increasing frequency. This is due to the 'friction' of the moving air against the inside of the instrument (technically, the viscous and to a smaller extent thermal losses in the boundary layer). This affects higher frequencies more than low.

However, a problem faced by the conical instruments but not by cylindrical instruments is that the first resonance is weak. We saw above under cut-off frequency that, at low frequencies, it is easier to move the air backwards and forwards, because less acceleration is involved. This means that the impedance of the 'air at the downstream end of the instrument' (technically the radiation impedance at the end) is low for low frequencies. Further, the cone is good at matching the low impedance of the large end to the high impedance at the small end. Together, these effects make the lowest resonance of conical instruments weaker than those of their cylindrical cousins. That is why the whole curve turns down to lower values at low frequency. (Compare with curves for the clarinet.)

Effect of the bell. The bell 'helps' the sound waves in the bore to radiate out into the air. (Incidentally, the presence of a large, effective bell is what makes brass instruments loud: try playing a trombone with the tuning slide taken off.) More sound radiated means less sound reflected, so the standing waves are weaker. However, this effect is only strong for high frequency: as the frequency increases over this range, the resonances are more weakened by the bell at high frequency than at low. This is because the bell is much smaller than the wavelengths of the low frequency waves, and so is less effective at radiating these waves. The bell cuts in at 1.8 kHz for the tenor and 2.6 kHz for the soprano. Above these frequencies the instruments have hardly any resonances – but they do behave as good megaphones!

Effect on the impedance spectrum of the bell

This figure shows the effect of the bell on the acoustic impedance spectrum of A#3 on a soprano sax. For the dotted curve, the bell was replaced by a cone having the same length. For more detail, see the paper by Chen, Smith and Wolfe.

The effect of the bell is that of a high pass filter. We could say that this is rather like the cut-off frequency effect of a series of open tone holes. In fact, one purpose of the saxophone's bell chiefly is to provide a high pass filter for the lowest few notes, so that they have a cut-off frequency and so behave more similarly to the notes produced with several tone holes open. As far as fundamental frequencies go, the bell is really only important to the lowest notes. However, it is important to the high harmonics of most notes.

Saxophone and oboe players will know the acoustic effect of this weak first resonance discussed above: it makes it difficult to play notes at the very bottom of the range. It is especially difficult to play these notes softly. We saw in How the reed and pipe work together that the higher harmonics of the reed motion could be 'helped' by resonances of the bore. When you play loudly, you generate proportionally more power in the higher harmonics (this is due to increasingly non-linear motion of the reed). So the lowest notes are not so difficult to play loudly, where the strong, high harmonics of the reed are supported by resonances of the bore, compared with soft notes, where you are relying on the the weak fundamental resonance.

We have not mentioned the effect of the mouthpiece. First, we note that the bore is not a simple cone: it is a cone truncated at a comfortable diameter to take a mouthpiece. This truncation affects the tuning: informally, we can think of it as making the pipe slightly like a cylinder, which stretches the frequency gap between resonances. This means that, unless compensated, it would stretch the interval between registers to over an octave. Now the geometry of the mouthpiece is a little complicated, but one contribution to the acoustic response is that it compensates for the 'missing volume' of the cone. Indeed, its volume (when added to the effective volume of the reed) is comparable with that of the missing cone. The compliance of the mouthpiece reduces the acoustic impedance spectrum over a range near about 1 kHz. See the data base for examples.

To the effect of the mouthpiece volume, we may add the compliance of the reed, discussed above. This acts in parallel with the bore, and its impedance decreases at high frequency, so its effect is to reduce the rise in impedance with frequency: softer reeds give lower overall impedance at high frequency. Further, the very high resonances are weaker and occur at lower frequency when you use a soft reed. This is discussed in more detail under the "effect of hardness" section on the clarinet page.

Vocal tract effects

In the simplest model, the mouth is considered as a source of high pressure air, and the reed is loaded acoustically only by the acoustic impedance of the bore of the saxophone, which is downstream. The acoustical effects of the vocal tract (upstream) are often small. They can, however, be important and even dominant if the player produces a resonance in the vocal tract whose acoustic impedance is comparable with that of the bore. This is easier to do in the higher registers, where the instrument's resonances are weaker. Vocal tract resonances are important in various effects including pitch bending and the famous glissando from Rhapsody in Blue. We have recently published a scientific paper on vocal tract effects in the saxophone. A popular version is given in this link.

More to come

We shall continue to add to this site as we find time and as our research results are published. One FAQ is 'Why don't we provide a Virtual Saxophone service like the Virtual Flute?' The answer is that we are working on the version for clarinet and hope to have one soon. The sax will probably be next.

Experimental measurements on this page were made by Jer Ming Chen as part of his doctoral research, and are described in the paper below.

Scientific papers with more detail:

For further reading, we recommend
  • A technical reference: The Physics of Musical Instruments by N.H. Fletcher and T.D. Rossing (New York: Springer-Verlag, 1998).
  • Other references, some less technical, are listed here.
  • For background on topics in acoustics (waves, frequencies, resonances etc) see Basics.
  • Our data base of saxophone acoustics.

Also in this series:

Research and scholarship possibilities.

close up of saxophone

Yamaha logo

Acknowledgments

Our research work on saxophones is supported by the Australian Research Council and by the Yamaha Corporation.

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Joe Wolfe / J.Wolfe@unsw.edu.au
phone 61-2-9385 4954 (UT + 10, +11 Oct-Mar)
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Happy birthday, theory of relativity!

As of June 2005, relativity is 100 years old. Our contribution is Einstein Light: relativity in brief... or in detail. It explains the key ideas in a short multimedia presentation, which is supported by links to broader and deeper explanations.
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