A low budget woofer test setup

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Starting a new audio project often begins with building or completing the measurement equipment. In case of amplifiers we need power supplies, dummy loads, signal generators and signal analyzers. So some DIY power supply, power resistors mounted on heatsinks, voltage attenuator and a cheap USB audio-interface linked to a laptop running ARTA is my way to go at a low budget. Now that I dedided to build some bass-box additional measurements are required: Speaker electrical impedance, acoustical frequency response and acoustical distortion.

I resisted the temptation to buy a calibrated microphone and started with my TASCAM DR-40X pocket recorder normally used for live recordings. Being stuffed with a stereo mic and an internal soundcard this comes in handy to be used as an USB-microphone. Connect it to your PC and run some audio-analyzing software and you are done. The built-in mics a directional ones working as pressure gradient receivers, as most common mics do. For low frequency measurements a true pressure capsule (omni-directional) is recommended like the specific measurement mics are. Nevertheless this is no dealbreaker here - just seal the rear openings of the capsules with some tape - and the capsules become omni-directional with extended LF-response.

Digging deeper I wondered wheather my USB-recording micro can play sound as well. And yes, it turned out this is a true full-duplex device that can record and play sound simultaneously. This makes it suitable for all kinds of audio response measurements. Next step was measuring the measuring equipment, i.e. overall response in a loopback configuration. With a best THD of 0.0025% @1kHz and a frequency range from 20Hz ~ 20kHz +0.1/-1.5 it is beaten by my EMU-Tracker pre. But for all measurements other than extreme low THD like SPL these differences are marginal.

To keep things as simple as possible I did not care for calibration of the mics nor for their individual frequency response. Somewhere on the net somebody measured these and they did not look bad with a slight low-frequency rolloff that I hope is compensated by my mod. As long as absolute sound level measurment is not your goal, measuring speaker impedance, T/S-parameters and frequency response are all relative measurements that do not require calibration. And the slight deviations in ultra-low frequency area are nothing compared to the lf-rolloff that woofers exhibit. With a fixed test setup, i.e. fixed audio levels an mic position, comparisions between different speakers can be done as well.

Besides images of test setup I append measurement plots of a visaton W170S8 in a sealed 10L box and a B&C 10HPL51 in a 50L ported box, captured at identical test conditions. This contribution is intended as an opener for nice solutions around DIY low-budget measuring equipment. Just a trigger for more ideas from other members...
 

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  • setup_SPL_H1H2H3_10L_closedbox.jpg
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  • setup_SPL_H1H2H3_50L_Reflexbox.jpg
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  • SPL_H1H2H3_10V_25cm_BC_10HPL51_50L.png
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  • SPL_H1H2H3_10V_25cm_Visaton_W170S8_10L.png
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  • Limp_Bassreflex_BC_10HPL51.png
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