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Musical instrument sound radiation

Follow AKUTEK research in this ‘live’ article: Dynamic Directivity of musical instruments

What is direct sound?

In room acoustics, sound at any point is usually considered a sum of two major components, the direct sound component and the reflected (i.e. reverberant) sound component. While the direct sound component in an unobstructed sound field in theory is quite simple compared to the complexity of reflected sound, it is in many important practical cases not simple to deal with. Floor, music stands, co-players and other elements common in the musician’s environment will interfere with the direct sound component.

The fact that reverberant sound is an important component of the sound from most musical instruments themselves makes it even harder to distinguish between the direct sound and the reflected sound. Whenever direct sound is treated as a component that is separate from reflected sound, the initial time delay gap Dt of the impulse response of the room will determine the low frequency region F< 1/Dt  where direct sound and reflected sound are inseparable.  Given that the proximity of bodies, music stands or floor, Dt could well be less than half a millisecond, thus the inseparable frequency region extends to 2kHz. Consequently, when playing an a’ tone (pitch=440Hz),  the first four harmonics will be affected by the natural environment of a musical performance.  Moreover, if the direct sound path is obstructed by a body, the direct sound will be increasingly attenuated towards higher harmonics due to diffraction.

On the other hand, one should not forget that even if “direct sound” is obstructed and its frequency response is distorted, it can still make a significant contribution to perceived sound in a reverberant sound field. An example of the latter is sound from instruments in the orchestra pit of an opera hall, diffracting over the edge of the pit or even over the edge of a screen. In cases of listening from the shadow zone, the edge around which sound diffracts appears itself to become a source of sound. Add to this the common cases of phantom sources (image sources) associated with strong early reflections to the shadow zone, and it becomes evident that direct sound can be impossible to distinguish by blind detection from something that is not direct sound. 

At least, we require that observed direct sound carries information about the origin of the sound. For a stationary source it should be possible to direction in which the source can be found.

In theory, direct sound is a special case of diffracted sound. Generally, sound transmission can be considered being diffracted sound received either in the highlight zone, in the shadow zone or in the transition zone. Highlight zones and shadow zones are of varying degree of perfection, perfect highlight zone being the ideal direct sound case.

Above 0.5kHz, most musical instruments tend to get increasingly directive towards higher frequencies. This means that the frequency spectra of free field sound are different in different directions, and the spectra vary both in magnitude and quality. Moreover, the spectra vary from one pitch (note) to another, so that the specter in each direction is generally different for e.g. a” and a#”.  Movement by players of hand-held instruments will result in changing spectra at a given receiver point, even if pitch is held constant. Thus the spectra vary generally with time.

If the received frequency spectrum of received sound varies in free field anyway, can we observe a difference if the direct path is partly obstructed? This question is yet to be answered. If the answer is negative, the concept of direct sound will be even more challenged.

So, direct sound in the meaning often assumed in theory is hardly found in practice. Rather, one will find an initial phase of the sound onset which is of special significance, and which approaches theoretical direct sound as the initial time gap increases. Anechoic conditions represent the practical limit for the concept of direct sound.

Significance of  initial sound

Human hearing has evolved in environments where the following abilities where crucial to survival:

· Detection of periodic sound

· Coherence of initial sound may be significant

· Separation of sources (food or predator, friend or enemy)

· by source character

· by pitch detection

· by spotting of source direction  

Initial sound as a carrier of pitch– information. Pitch detection methods reviewPitch detection– by Auto-Correlation or Cepstrum. Enhanced Autocorrelation in Real World Emotion Recognition.

Griesinger: Phase Coherence as a Measure of Acoustic Quality:

1 The Neural Mechanism
2 Perceiving Engagement
3 Hall Design

 

Directivity of musical instruments - how to deal with it?

Directivity of musical instruments and its significance to sound quality as perceived by the audience has been investigated since late 60-s. Paper (0.4MB pdf)

Click on link to research on this topic by Filipe Otondo.

AKUTEK research: Dynamic Directivity of musical instruments

How dynamic room diffusivity and instrument directivity tie together: Diffusivity and directivity (paper) and presentation by M Skålevik

 

Direct sound from a musical instrument


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