1 Introduction 2 Theoretical aspects - acoustics 2009

Proceedings of the ACOUSTICS High Tatras 2009 “34th International Acoustical Conference - EAA Symposium”
DESIGN OF OMNI DIRECTIONAL SOUND SOURCE
Anna Danihelová a), b), Rastislav Klačanský a), b), Stanislav Košúth a), c), Miroslav Němec a), b)
a) Technical university in Zvolen
b) Department of Physical and Musical Acoustics
c) Department of Woodworking, Machines and Equipment
T. G. Masaryka 24, 960 53 Zvolen, Slovakia
www.acoustics.sk
Abstract: This paper deals with designing of omni directional sound source. Concretely it is
interested in loudspeaker box consist of twelve loudspeakers and baffle board. To the construction
it will be used direct radiating loudspeakers located in one sealed baffle board. Enclosure volume
was calculated by simulating software on the loudspeaker parameters base. Frequency response
was deliberate too. This sound source will be used for experimental measuring of closed and open
areas acoustical parameters.
1 Introduction
By detection of acoustic parameters of any areas it is necessary to use accurate software and
hardware. To the hardware belongs the computer with suitable sound card, measuring
microphone with straight characteristic, amplifier of necessary power, radio system and
cabling. The radio system is usually the weak segment, considering the demanding
conditions. It is necessary to activate the area with sufficient acoustic signal [1]. That means
that the source of sound must be able to achieve high level of acoustic pressure in all
directions [5]. The compensation of omnidirectional sound source with simple radio system
can cause distortion of results. This is the reason to propose, build and later in the praxis
verify the special radio system – omnidirectional source of sound. This paper is focused on
the proposition of design and baffle-board volume for this source.
2 Theoretical aspects
2.1 The propose of radio system
For the construction of omnidirectional source was the encased baffle-board suggested. The
reason is the simplicity of the proposal and necessity of moderate volume considering
quantity of speakers. For achievement of good directional characteristics also at the higher
frequencies was the shape of dodecahedron selected. The speakers will be stepped in every
side of baffle-board. The critical is the frequency by which the speaker start to direct. It
depends on the average of membrane [3]. This insufficiency is partially compensated with the
central cone at the suggested speaker´s type membrane ARX 165-22/4 (Fig 1). Used speaker
ARX 165-22/4 is an electrodynamic speaker (Fig .2), whose membrane is directly connected
with external environment without any auxiliary elements – auditory canal.
By installation of the speaker into embosomed case is the air volume behind the membrane
acting like an air spring, which is with its toughness moving the resonant frequency of the
system in higher frequencies direction.
It appears from this that to the malleability of radio system is the malleability of baffle-board
embosomed environment adding.
(1)
Proceedings of the ACOUSTICS High Tatras 2009 “34th International Acoustical Conference - EAA Symposium”
where, Sa aperture of membrane,
co velocity of sound propagation,
ρ specific weight of air.
By the option of baffle-board volume is it possible to go informative by the relation
(2)
where, V is volume of baffle-board,
cr malleability of radio oscilation system,
f0 terminal frequency of baffle-board,
fres resonant radio frequency,
c0 speed of sound in the air,
ρ density of the air,
S membrane surface [2].
2.2 Proposal verification by means of computer simulation
Primarily was the baffle-board with volume of cca 35dm3 suggested on the source of
exactness and compatibility of entire equipment (Fig 1). There are planed measuring in the
terrain. By verification of the computer simulation was an insufficient transfer of low
frequencies under cca 115Hz detected.
Figure 1. Drawing documentation: Operating drawing of baffle-board parts in perspective, drawings of
subassemblies and general 3D perspective at omnidirectional sound source – first proposal
The baffle-board proposal was changed and the volume was expanded to cca 100 dm3 (Fig 5).
Transfer characteristic (Fig 2) is favourable from under cca 80 Hz, what is adequate for the
measuring.
Proceedings of the ACOUSTICS High Tatras 2009 “34th International Acoustical Conference - EAA Symposium”
Figure 2. Line response software simulation of acoustic zone lower part of twelve speakers located in
embosomed baffle-board with volume of 100 liters.
In the figure (Fig 3, Fig 4) are additional parameters obtained by simulation: performance
power handling capacity curve important for excitation of harmonic signal and speaker
membrane deviation by performance 300W (max permitted source performance).
Figure 3. Software simulation – curve of maximal performance power handling capacity of speakers system
concerning allowing permanent membrane deviation (frequency dependence)
Figure 4. Software simulation – membrane deviation of speakers by max permanent incoming power of
speaker system 300 W (frequency dependence)
Proceedings of the ACOUSTICS High Tatras 2009 “34th International Acoustical Conference - EAA Symposium”
Figure 5. Drawing documentation: Operating drawing of baffle-board parts in perspective, drawings of
subassemblies and general 3D perspective at omnidirectional sound source – second proposal
3 Conclusion
Suggested speaker system as an omnidirectional sound source will be used for measuring of
environment acoustic parameters by help of measuring system Dirac. Its exploitation is
hypothesized in embosomed also semi-closed environments. There is a plan to compare the
results achieved by measuring, where the simple speaker system was used.
Acknowledgements
This research was supported by Slovak Grant Agency, Ministry of Education, Slovak
Republic, project No 1/0841/08.
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