N o is the reference efficiency of a driver. It is used as a guide to determine whether a speaker will work better in a ported or sealed enclosure. Vb is the net internal volume of the speaker enclosureĮBP is the efficiency bandwidth product. Violet = Qtc: 0.9 Green = Qtc: 0.8 Red = Qtc: 0.7 If you don't know what Qtc you need, start with a Qtc of. A lower Qtc will start to roll off earlier and will roll off at a slower rate. Higher values of Qtc will give a peak in the output with a sharper rolloff. 707 is the most common and generally produces the flattest frequency response with approximately a 12dB/octave rolloff. Qtc is the total Q of the speaker in an enclosure including all system resistances. Qms is the mechanical Q of the speaker and only takes the speaker's mechanical properties into consideration. Qes is the electrical Q of the speaker and only takes the electrical properties into consideration. Speakers with loose suspension have a higher Vas and use larger enclosures. Stiff speakers have lower Vas and tend to use small enclosures. Vas is the volume of air that has the same compliance as the speaker's suspension. You should notice that, unlike sealed enclosures, the impedance is lowest at the resonant frequency. This image (below) shows the impedance curve for a ported enclosure. Enclosures of different sizes will produce different resonant frequencies. You can see how the resonant frequency shifts when the driver is mounted in an enclosure. The image below shows the impedance curve with a woofer in 1 ft^3 and in an infinitely large enclosure. The fc is higher than the fs with a given driver. The voltage applied to the speaker is constant for all frequencies.ġ5Hz 25Hz 35Hz 45Hz 55Hz 65Hz 75Hz 85Hz 95Hzįc is the resonant frequency of the driver in a sealed enclosure. You can see by the bar graph that the current flow through the voice coil is the least at resonance. Put your mouse/cursor over the different frequencies below the following impedance curve. The image below shows how a speaker's impedance changes across the audio spectrum. The speaker's impedance may go from 4 ohms to more than 20 ohms at resonance. At resonance, the speaker's impedance increases dramatically. I would strongly recommend reading it before you continue with this page.įs is the resonant frequency of a speaker in free air (not in an enclosure). If you haven't already read the speakers page of this site. Understanding the Thiele/Small Parameters
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