REL T/9i Subwoofer Test

REL T/9i Subwoofer Test: Crossover Blending and Frequency Sweep Guide

REL's T/9i is a 10-inch down-firing subwoofer designed specifically for integration with two-channel audiophile and home theatre systems.¹ Unlike AV-oriented subwoofers built to maximise bass extension and impact, REL's design philosophy prioritises seamless blending with main speakers through its high-level input (a Neutrik Speakon connector that taps the speaker terminals of your power amplifier directly), giving the subwoofer the same amplifier characteristics and timing as your main speakers. The T/9i covers 28 Hz to 120 Hz at -6 dB.

The blending test is the most diagnostic sweep you can run on the T/9i. Set the subwoofer's crossover control to your main speakers' approximate -3 dB point, then sweep slowly from 40 Hz upward through 200 Hz. The goal is to hear a smooth, uninterrupted increase in frequency without any hump or notch at the handoff between the T/9i and your main speakers.² A hump indicates the sub and mains overlap too broadly; a notch means they do not overlap enough, leaving a gap in the response. Fine-adjust the crossover frequency and phase control while sweeping until the transition is as smooth as possible.

Below 40 Hz, the T/9i's down-firing configuration interacts heavily with the floor material and proximity to room boundaries. Sweeping from 40 Hz downward while moving the subwoofer slightly will reveal how much the floor coupling changes the bass level at each frequency. Hardwood and concrete floors produce more coupling than carpet. At 28–30 Hz, the T/9i is near its -6 dB point; audible output is present but begins to roll off noticeably. Keep volume at a comfortable level during deep testing.

Specifications¹

Testing the T/9i crossover blend with your main speakers

Testing the crossover integration between the T/9i and your main speakers requires both active simultaneously during the sweep. Set your receiver or amplifier to output full-range signal to the main speakers with no high-pass filter on the main outputs, engage the T/9i via its high-level Neutrik input, and set the T/9i's crossover control to approximately match your main speaker's rated -3 dB rolloff point. Sweep from 200 Hz downward to 20 Hz in Manual mode. Through the crossover region (typically 60–120 Hz depending on your main speaker size), the transition should produce a smooth, continuous increase in bass presence without a hump or gap.

Adjusting the T/9i crossover control while the sweep plays

Position yourself at your normal listening seat and run the sweep in a repeating loop near the crossover frequency between 60 Hz and 120 Hz. Turn the T/9i's front-panel crossover control slowly while the sweep plays. When the crossover frequency matches the main speakers' rolloff point, the handoff sounds seamless. Too low a crossover setting creates a gap in the response; too high a setting causes both to contribute at the same frequency, producing an audible bass hump.

Switching phase to find the optimal integration point

After matching the crossover frequency, switch the T/9i's rear-panel phase control between 0° and 180° while the sweep plays at 80 Hz. One phase position produces a fuller tone; the other produces a thinner one. The fuller phase position confirms the subwoofer is adding to the main speakers' output rather than partially cancelling it at the crossover frequency. Use this phase test before finalising the crossover position, because the optimal crossover frequency and phase are interdependent.

Measuring floor coupling effects on T/9i bass output

Below 50 Hz, the T/9i's down-firing driver couples directly with the floor material beneath the subwoofer, and this coupling is a significant variable in its actual in-room performance.³ Set the sweep to Manual mode and hold at 30 Hz. Then move the T/9i laterally in 10 cm increments while the tone plays and listen at your seat. The perceived level changes with each position because floor coupling varies with proximity to room boundaries and with the floor material's acoustic impedance.

How floor material changes the measured rolloff curve

Hardwood and tile floors transmit bass energy into the room through the floor structure, producing more coupling than carpet, which absorbs a portion of the energy before it can radiate back into the room. Run the rolloff sweep from 80 Hz down to 25 Hz with the T/9i on its intended surface and note the frequency at which output becomes difficult to hear at your listening seat. If possible, place a thin rigid board under the T/9i temporarily and repeat the sweep: a hard surface under the subwoofer typically raises perceived bass output by 1–3 dB at the deepest frequencies.

Using the coupling comparison to choose the final placement

The comparison between the original floor coupling result and the board-enhanced result reveals how much of the T/9i's low-frequency performance depends on the floor interface. If the improvement with the board is consistent and audible, a permanently placed floor-coupling board improves in-room bass output without repositioning the subwoofer. If the improvement is minimal, the floor surface is already transmitting bass energy effectively and the placement is optimised for its current floor type.

Run the board comparison before committing to a permanent position, because the result predicts whether placement alone will settle the low end. If the board lifts the deepest frequencies and you would rather not add hardware, moving the T/9i toward a boundary instead can achieve a similar gain through natural room coupling. The aim is a placement where the rolloff reads evenly at your seat without extra aids, so measure floor coupling under a down-firing sub before committing to a fixed spot.