![]() The group delay times in the low-end of the sealed box are significantly shorter. ![]() The vented en- closure has a total harmonic distortion of 3 %, while the sealed one shows 3.34 %. In av- erage, the sealed construction produces a slightly higher SPL output. The sealed cabinet has a more natural low-frequency roll off, how- ever, the vented enclosure produces a higher total SPL output in the bass range. Results indicate that the sealed box breaks up at lower frequencies than the vented enclosure. The differences are analysed by using frequency and impulse response measurements in an anechoic environment. The subjects of the research are the Genelec 1031A speaker’s vented cabinet and custom designed and built sealed enclosures. Within the confines of an experimental and comparative re- search, this study analyses the effect of vented and sealed designs on the sound out- put. The author of this paper focuses on presenting the sound quality differences of vented and sealed speaker enclosures and aims to gain further understanding of the principals of each design. On-axis frequency response and distortion. The time domain response to a step input (Figure 4) shows a considerable amount of time smearing with the peak in mid-frequency output occurring about 1.5ms after the high frequencies, and the low frequencies about 8ms later. The vertical off-axis response shows the interference notch in the crossover region between the midrange and the tweeter at around 3kHz. In the horizontal plane, there is some evidence of mid-range narrowing in the crossover region between the woofer and midrange, due to interference between the driver outputs, and again in the upper range of the midrange around 2kHz but otherwise the directivity is well controlled. The off-axis responses are shown in Figures 2 and 3 for the horizontal and vertical planes respectively. Harmonic distortion performance is very good, especially considering the low-frequency extension, with the 2nd harmonic remaining below-40dB (1%) at all frequencies above 60Hz and the 3rd harmonic above 45Hz at 30Hz, the harmonics are both lower than-26dB (5%). The effect is subtle, but it does allow the user to choose between the phase accuracy of a near-sealed cabinet alignment or the slightly higher power handling and lower distortion that a more 'open' port provides. The dashed line shows the effect of adding the optional foam bung to the port the response is reduced from 40Hz to 110Hz but is increased below 40Hz due to a slightly lower order roll-off. Low-frequency extension is very good with a 3rd-order roll-off with-10dB at around 30Hz. The response lies between +/-3dB limits from 50Hz to 18kHz except for a narrow dip at 7kHz. Figure 1 shows the on-axis frequency response and harmonic distortion performance for the SCM25A Pro. The vertical heat sink fins on the rear panel, along with the logo, indicate that this speaker is intended to be used in the horizontal (landscape) orientation with the woofer alongside the midrange and tweeter. Signal input is via an XLR-type input on the rear. Instead, ATC supplies a plastic foam 'bung' to optionally place in the port to change the low-frequency alignment. Unlike most active monitors in this class, there are no electronic equalisation switches with this model to correct for such things as room boundary proximity. The cabinet has external dimensions of 264mm x 430mm x 408mm and one speaker weighs 30kg. These amplifiers endow the SCM25A Pro with a claimed maximum continuous SPL output at 1m distance of 109dB. ATC specifies the internal Mosfet amplifiers as having 150W output for the woofer, 60W for the midrange and 25W for the tweeter, with crossover frequencies, using 4th-order filters, at 380Hz and 3.5kHz. It is a 3-way active consisting of a 7-inch carbon/paper coned woofer, ATC's familiar 3-inch soft-dome midrange and a 1-inch soft-dome tweeter housed in a ported rectangular cabinet with in-built electronics. ![]() T he ATC SCM25A Pro is the mid-sized offering from ATC's range of loudspeakers for professional use. ![]()
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