Development and construction of passive wideband studio loudspeakers

V magistrskem delu sta opisana razvoj in izdelava pasivnih širokopasovnih studijskih zvočnikov. V Laboratoriju za metrologijo in kakovost Fakultete za elektrotehniko so se do sedaj za izvajanje demonstracij in akustičnih meritev uporabljali zvočniki Genelec 8040B, ki v svetu studijskih zvočnikov slovijo kot zelo dobri, njihova kvaliteta zvoka pa je dokazana z akustičnimi meritvami in podatkovnim listom proizvajalca. Zaradi pogostega prenašanja omenjenih zvočnikov med prostori fakultete se je pojavila potreba po dodatnem paru zvočnikov, ki bi s podobnimi karakteristikami služili podobnemu namenu. Z mentorjem vsa si zadala ključne lastnosti izdelanih zvočnikov, od tam naprej pa sem imel skoraj proste roke za izbiro komponent in uporabljenih tehnik izdelave. Sprva sem preučil lastnosti referenčnih zvočnikov Genelec 8040B, temu pa je sledila izbira nizkotonske ter visokotonske zvočniške enote. Izbira nizkotonskega zvočnika je bila preverjena s simulacijami ohišja z različnimi resonančnimi sistemi, iz česar je sledila še določitev pasivne membrane za efektivno znižanje spodnje frekvenčne meje zvočnika. Simulirano ohišje je bilo nato izdelano iz lesenih plošč, za visokotonsko zvočniško enoto pa je bil iz umetne mase ulit valovod, ki izenači smerni karakteristiki obeh zvočniških enot v frekvenčnem pasu od 1 kHz do 2 kHz, kar je ključnega pomena za lažjo izdelavo kretnice in končno smerno karakteristiko zvočnika. Kretnica je bila sprva simulirana na podlagi akustičnih meritev in frekvenčno odvisnih meritev impedance obeh zvočniških enot, nato pa še v različnih fizičnih izvedbah testirana z akustičnimi meritvami in sejami poslušanja glasbe. Končna verzija kretnice je bila vgrajena v ohišje zvočnika. V gluhi sobi Fakultete za elektrotehnike so bile izmerjene horizontalna in vertikalna smerna karakteristika zvočnika ter frekvenčni odziv na osi zvočnika s prikazanim harmonskim popačenjem. Na koncu je predstavljena uporabljena programska in strojna oprema, brez katere izvedba ne bi bila mogoča. Iz predstavljenih rezultatov je razvidno, da so izdelani zvočniki izpolnili zadane cilje, pri frekvencah pod 40 Hz pa so z nizkim nivojem harmonskega popačenja presegli pričakovanja.Subject of this masters thesis is development and creation of passive wideband studio loudspeakers. Genelec 8040B loudspeakers, which are well recognized in professional audio, have been used for test research purposes in Laboratory of metrology and quality on Faculty of electrical engineering for some time now, They perform well in practice as well as on paper. Due to only having one pair of loudspeakers, that need to be transported between different rooms, arised the need for a second pair of loudspeakers which will perform similar duty with similar characteristics. Me and my mentor created a list of technical requirements of newly developed loudspeakers and from then on, I was mostly on my own to choose components and create a final product. At first I studied the reference Genelec 8040B loudspeaker, followed by a choice of bass and treble loudspeaker units. The choice of low frequency driver was validated with low frequency enclosure simulations with different resonant systems. Passive radiator was chosen based on these simulations to achieve lower frequency response roll-off and reduce harmonic distortion at those low frequencies. Following simulations, two symmetrical enclosures were made out of 19 mm thick medium density fiber board (MDF). A tweeter waveguide was made by casting fiberglass reinforced epoxy resin. This waveguide is used to change directivity pattern of the tweeter so that it matches directivity of a woofer in frequency band from 1 kHz to 2 kHz. This is crucial for easier crossover design and smooth final directivity pattern. Design of crossover was based on simulations which used acoustic measurements and frequency dependent impedance characteristics as input parameters. Many versions of crossover were physically assembled and the process of improvement was repeated until satisfactory sound was achieved. The final version of the crossover was built in to the loudspeaker enclosure. On-axis frequency response with included harmonic distortion of the loudspeaker and its directivity measurements were performed in the anechoic chamber at the Faculty of electrical engineering. Measurements show that the loudspeakers fulfill all input requirements and they perform even better than expected at frequencies lower than 40 Hz