Factors Effecting Tonal Quality

Tonal quality is an elusive subject. This is in part due to differences of opinion between individuals as to what exactly is a pleasant or appealing tone. The tone of one flute may be preferred over another by one individual. Another person listening to the same two instruments may have a different preference. So, to a certain extent tonal quality is subjective.  I have learned not to be judgemental when it comes to the sound of my flutes. Someone else will love a flute whose tone I do not personally appreciate. I have seen this over and over at flute shows. And, when people come to my shop to audition instruments they make choices that I might not make.

It is difficult to discuss tonal quality because the words used to describe sound are themselves ambiguous. What, for example, is a sweet sound? Or, what is a mellow sound? When it comes to the word ‘breathy’ there is more agreement. But, again, is ‘breathy’ necessarily a negative quality? Not to the ears of some. Out of this fog of words and meanings I will attempt to draw some clarity. My opinions are certainly not the last word on this subject. I am not a professional musician or musicologist. But, I have been making Native American style flutes for many years. What I relate here is based on that experience.

A flute maker has only his ear to rely on when it comes to creating a sound-producing instrument. My ear is my muse. It is the guide that I have followed as I have slowly groped my way forward in perfecting my craft as an instrument maker. Each maker has his or her own ideal sound. My teacher, Clint Carlyle, makes beautiful flutes. They have a soft, melodic tone - almost a whisper. That, however, was not what I was called to produce. What I intuitively wanted was a strong, clear voiced flute. So, I took what I learned from Clint and began to make modifications in his flute design. I read, I asked questions, I listened and I experimented. I followed the muse of my ear.

With a flute the final sound is a consequence of many factors. Each of these variables has its individual impact of the sound that the flute produces. To make things trickier the variables are interrelated. I think that these variables can be loosely ranked in order depending on the degree of impact they have on the sound of the flute.

Flue Design.

The design element with the most impact would be the width and depth of the flue. The flue depth especially has a marked impact on sound. A flue that is overly deep will make for a weak airy sound. The flute will also have little backpressure and consequently not respond quickly to note changes. This makes for what I would call a mushy flute. Flue width is not as critical as depth. But, it is important. If the flue is too narrow you have a weak sound. Too wide and you lose clarity. Then there is the length of the flue to consider. I have found that a flue of about ½ to 3/8 inches in length to be the best for my type of flute. By length I mean the length of the dimensionally constant portion of the flue.

Flue Positioning.

The second factor would be the positioning of the flue. The flue can be either cut into the bottom of the bird or into the flute itself. In my experience a flue in the bird (sometimes called the Plains style) does not produce as much volume and clarity of tone as does a flue cut into the body of the flute itself (sometimes called the Woodlands style). In my opinion the flue in bird configuration does not allow the air stream to optimally impact the splitting edge. This is because the splitting edge is on the same plane as the bottom of the flue. For optimal sound quality the splitting edge should be at or near the center of the air stream.

True Sound Hole.

The dimensions of the True Sound Hole are the third most important element in creating sound. Too deep front to back and the flute becomes airy. Overly constricted front to back and the flute is sensitive to breaking into a higher octave and volume is compromised. Too wide side to side and clarity is compromised. Too narrow side to side makes for a weak sound.

The positioning and configuration of the splitting edge has a defining effect on backpressure (responsiveness), clarity and sensitivity. There are as many splitting edge designs as there are flute makers. Is it sharp or blunt? If blunt, how blunt? Where is the splitting edge positioned relative to the stream of air leaving the flue? What are the angles on the top and bottom of the edge? All these factors come into play. In even the most meticulously crafted instruments small variations creep in. Thousandths of an inch one way or the other make appreciable changes in tone. I like my splitting edge to be in the upper half of the air stream. But, its exact position is variable due to the vagaries of handcrafting. These subtle variations are what give a well-made flute its individual character.

Flue Surface.

The fifth variable affecting sound is the surface of the flue. The type of surface within the flue affects the laminar flow pattern of the air traveling through the flue. If the surfaces are rough the the laminar layer is thick and the flow is turbulent. Smooth surfaces allow the air to slide through the flue with minimal turbulence. The difference between an oil finished surface and a varnish finished surface although not detectable by the eye or hand is detectable by the ear.

Slow Air Chamber.

The dimensions of the Slow Air Chamber exit hole have a pronounced effect on tone. A large, smooth and properly angled exit hole contributes to good sound quality.

The importance of the way the air from your lungs enters, circulates through and exits the SAC cannot be stressed enough. This flow must be made as direct and smooth as possible. What we are concerned with is reducing turbulence. When the stream of air molecules leaves the flue and enters the TSH those little fellows should not be jostling each other for position. They should be friends with a common purpose. So, the blowhole or mouth hole diameter and length, the SAC size and finish, the ramp leading out of the SAC all come into play.

Bore Dimensions

I will put the diameter of the flutes’ bore as factor number six. The bore diameter is related to the key of the flute. Higher keys – smaller bore. Lower keys - larger bore. The ratio of the bore diameter to the length of the bore is open to question. But, most students of the subject agree that it should be in the neighborhood of one to eighteen. A lower ratio can lead to a hollow tone (some would say mellow). Higher ratios may lead to a thin or weak tone lacking in richness.

Wood Type.

I know you’ve been waiting for a discussion of the type of wood used to make the flute. Here it comes as factor number nine. Some might put it higher on the list. They may have good cause. But, I have let it slide down to number seven. I mean no disrespect to the wood. Perhaps I have been prejudiced by all the hype I have heard about wood type and mellowness.

With an acoustic guitar or violin the type and dimensions of the wood with which it is made have a defining effect on the sound it produces. This is because the wood acts as a resonator projecting the sound waves out into the surrounding environment. The wood of a guitar, violin or other similar instrument actually picks up the vibration of the string. It itself then vibrates in harmony with the string and magnifies its sound. It does this by beginning to vibrate in sympathy with the vibration of the strings. However, the vibration of the wood is much stronger than that of the string itself. Those of you who are familiar with the sound of an unamplified electric guitar understand what I am talking about. If you pluck the string of an electric guitar that is not turned on the sound is practically inaudible. The body of an electric guitar is solid wood. It does not resonate. The electric guitar depends on an electric pickup and amplifier to project sound.

With a flute the effects of this sympathetic vibration are much less pronounced. Yes, you can feel a good flute vibrate under your fingertips. But this vibration of the wood has only a secondary effect on the tone. The primary sound of a flute comes from pulses projecting out from both ends of the bore of the flute. Sound pulses or sound waves come out of the end or foot of the flute. Sound waves also come from the True Sound Hole. The flute acts more like a speaker box. The speaker box is called a cabinet resonator. It has a vibration of its own. Similarly, the wooden flute (if properly constructed) has a vibration of its own. It does not amplify the tonal vibration that the flute is producing. Instead it should add overtones that stem from its own characteristic tonal quality.

Now, the wood of which a flute is made affects tone in two ways. First, softer wood (more hollow space within the cell walls) tends to absorb or dampen sound vibration. Think of a car muffler. Second, hard or dense wood by not absorbing these same vibrations begins to vibrate in sympathy with the sound of the flute. You can get an idea of what I am talking about by rapping a piece of wood with the knuckle of your hand. If you do this to a piece of softer wood the resulting sound is rather dull. A harder wood will have a more pronounced and sharper quality. PVC plastic, which is used to make cheaper flutes and recorders, has practically no resonant quality. Try rapping on a piece of hard plastic. These instruments have a very sterile sound.


In order to fully appreciate what I am describing you need to know what overtones are. This requires a discussion of sound waves and notes or keys. The key of a flute in A above middle C on the piano is a sign wave that vibrates at 440 Htz. This wave has a certain fixed length. A pure sign wave is almost impossible to create. And, in fact for musical instruments we want a wave that is pure - composed predominantly of a particular wavelength. But, not too pure – or sound loses its character. Sound character, as in human character, comes from the pleasing mixture of inconsistencies. Mixed in with the 440 Htz sound waves of an A flute you hear other higher and lower waves of varying lengths. These are the overtones. Too many overtones and the A sound wave is obscured (approaching too close to chaos). Too few overtones and the tone sounds dry or sterile (like a personality that is too predictable).

So, when you take out/absorb too many of the higher overtones (think of these as the exciting overtones) the flute doesn’t have that bright edge. Some call this mellow; I call it dull.

Inside of the bore.

Number eleven in the list of factors affecting tonal quality is the way the inside of the bore of the flute is finished. Is it rough, smooth, does it have a hard finish etc. Finishing the inside of the bore with varnish or oil and sanding it smooth contributes to the sweetness and volume of the flute.

Number twelve is the size and placement of the tone holes. I like my tone holes to be on the large side rather than constricted. I feel that this gives greater fullness and volume to the notes. This is especially important for the first hole (hole #1) above the fundamental. If it is too small the note will sound constricted in comparison to the fundamental note.

Chimney dimensions.

Another factor that I will mention is the dimensions of the chimney. A chimney is created by having two wings on the end of the bird. A bird with a chimney flanks the True Sound hole on three sides.  Some flute makers don’t use a chimney. I feel that this is a mistake for two reasons. First, a properly dimensioned chimney seems to enhance volume. It also imparts richness to the tone. Perhaps this is my imagination. But, without a chimney the tone sounds a little raw. Second and more noticeable is the sheltering effect the chimney has on cross currents of air.  Without a chimney even slight currents of air can distort or interrupt the air stream leaving the flue. This has a very negative effect on the sound of the flute.

Direction holes.

One last thing – direction holes. Direction holes are those holes (usually four in number) on the top, bottom and sides of the foot end of a flute. Direction holes are the effective end of the flute. The area beyond these holes is not part of the vibrating column of air that is producing sound. Having this extra length makes a flute look larger and perhaps more imposing. But, this extra length of dead air space can have a dampening effect on tone. Some makers like this effect (more mellow etc). I like clarity and therefore I don’t use direction holes on my flutes.