Test Bench: The SEAS ML15RCY/TP Dedicated Midrange

The device to be explicated in this Test Bench is the 5.25” ML15RCY/TP dedicated midrange driver from SEAS Titan Prestige line of high-end transducers incorporating titanium voice coil formers. Titanium has become a popular voice coil former material over the last several years as it is not only more rigid than aluminum and less subject to deforming at high excursions, but is a paramagnetic material, and in its pure form, is not magnetically conductive. (The compound of titanium and oxygen was discovered in 1791 by the English chemist and mineralogist William Gregor and was independently rediscovered in 1795 and named by the German chemist Martin Heinrich Klaproth after the Titans of Greek mythology.)

In terms of features, the 5.25” SEAS ML15RCY/TP is built on a proprietary six-spoke cast-aluminum frame, comprised of narrow 6.75mm to 8mm tapered spokes, completely open below the spider (damper) mounting shelf for cooling (Photo 1 and Photo 2). The shelf is the primary cooling apparatus for the ML15RCY/TP driver. Without a pole vent, all the air movement in the gap area is channeled out over the front plate.
  The cone assembly consists of a mostly flat-shaped black anodized aluminum cone and is designed to push the breakup modes to outside the normal pass-band of the driver (above a suitable crossover frequency designed to transition the driver’s directivity appropriately to a tweeter). Compliance is provided by an inverted NBR surround, with the remaining compliance coming from a 2.6” diameter flat black cloth spider (damper).

The motor is an FEA-optimized ferrite magnet type with the T-yoke pole piece fitted copper sleeve shorting ring (Faraday shield). Driving the cone assembly is a 25mm (1”) diameter short (typical of midrange drivers) voice coil wound with round copper on the non-magnetically conducting titanium former. Last, the IEC 268-5 long-term power handling is rated at 100W (obviously also dependent upon whatever high-pass filter frequency is incorporated) and the voice coil tinsel lead wires are terminated to gold-plated solderable terminals located on one side of the frame.

Since this midrange is likely to be crossed over at a minimum of 200Hz, I chose not to use the multi-voltage LEAP 5 LTD Thiele-Small parameters (TSP) testing protocol and instead used established parameters with a single 1V TSL admittance (current)/voltage sweep using the Physical Labs IMP Box (the same type of fixture as a LinearX VI Box). The goal here was to establish an enclosure volume to ascertain the impedance resonance and Q. This information is useful if a passive high-pass network is employed, and of course, not relevant if using active filters.

Following my established protocol for Test Bench testing, I no longer use a single added mass measurement and instead use the company supplied Mmd data (8.23 grams for the ML15RCY/TP). The single impedance sweep for each driver, along with the Mmd data, was used to produce the TSL parameters for the two SEAS ML15RCY/TP midrange samples. Figure 1 shows the free-air impedance curve. Table 1 compares the single sweep TSL data and factory parameters for both SEAS samples.
    LEAP parameter calculation results for the two samples correlated fairly well with the SEAS factory published TSP, except for the 2.83V/1m sensitivity, which was higher than the TSP calculated numbers and likely taken as an average SPL. However, I did run a box simulation with the factory data, and the result was within less than 1dB of my measured data, so obviously the fs/Qt ratios are rather close.

As usual in this column, I followed my established protocol and proceeded to set up computer enclosure simulations using the LEAP TSL parameters for Sample 1. Since this driver will likely have a high-pass filter of at least 200Hz to 250Hz, I only want to establish the F0 and Q for a sealed box (a sealed box is recommended by SEAS) volume as reference for a passive network design. Using the LinearX LEAP 5 legacy software’s Quick Design utility, I came up with a sealed box volume of 203in3.

Figure 2 displays the frequency response result for the ML15RCY/TP midrange in the single simulated sealed enclosure at 2.83V. The closed box alignment seen in Figure 2 produced a 3dB down frequency of 97Hz (F6=77Hz) and a Qtc=0.69, while Figure 3 gives the box impedance. Here the F0 and Q data could be used to design a LCR resonance conjugate filter to facilitate a 200Hz to 400Hz passive high-pass filter. For the purposes of this explication, Klippel analysis was not included.
  I next mounted the SEAS ML15RCY/TP midrange in a foam-filled enclosure that had a 12”×7” baffle and then measured the device under test (DUT) using the Loudsoft FINE R+D analyzer and the GRAS 46BE microphone (courtesy of Loudsoft and GRAS Sound & Vibration). I measured both on- and off-axis from 200Hz to 20kHz at 2.0V/0.5m, normalized to 2.83V/1m (one of the really outstanding tricks FINE R+D can do) using the cosine windowed Fast Fourier Transform (FFT) method. All these SPL measurements also included a 1/6 octave smoothing (this is done to match the resolution of the 100 point to 200 point LMS gated sine wave curves used in this column for a number of years).

Figure 4 gives the ML15RCY/TP on-axis response, indicating a moderately smooth rising response that is ±2dB from 200Hz to 5.7kHz with the breakup mode centered on 8.2kHz and a response that extends out to 20kHz. Figure 5 displays the on- and off-axis frequency response at 0, 15, 30, and 45 degrees. The -3dB at 30 degrees with respect to the on-axis curve occurs at 3kHz, so a low-pass cross point in that frequency or somewhat lower should work well to achieve a good power response. Figure 6 gives the normalized version of Figure 5, while Figure 7 displays the CLIO horizontal polar plot (in 10° increments with 1/3 octave smoothing). And finally, Figure 8 gives the two-sample SPL comparisons for the 5” SEAS ML15RCY/TP driver, showing a close match between 0.5dB to 1dB throughout the operating range of the transducer.
  For the remaining series of tests on the SEAS ML15RCY/TP midrange transducer, I initialized the Listen SoundCheck AudioConnect analyzer and ¼” SCM microphone (graciously supplied to Voice Coil magazine by the kind folks at Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I rigidly mounted the 5” driver in free-air and set the SPL to 94dB at 1m (3.87V), using a pink noise stimulus. Then, I measured the distortion with the Listen microphone placed 10cm from the driver. This produced the distortion curves shown in Figure 9. Note the very low third-harmonic distortion in the operating range of the driver.
 
I then employed the SoundCheck software (V22.02) to get a 2.83V/1m impulse response for this driver and imported the data into Listen’s SoundMap Time/Frequency software. Figure 10 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 11 shows the Short-Time Fourier Transform (STFT) plot.
  Looking at all the TSP and response data I collected for the new SEAS ML15RCY/TP Titan series midrange, the performance looks very good overall. For more information, visit the SEAS website at www.seas.no. VC

This article was originally published in Voice Coil, July 2025