LEAR BD4.2: The next-generation hybrid

Continued from Aesthetics

Exclusively designed by LEAR of Hong Kong,  BD4.2 is claimed to feature ground-breaking innovations:

1. Quad-bore configuration

2. Metallic acoustic transfer tubing

3. Adjustable bass control

4. Acoustic low-pass filter

First, separate acoustic transfer tubes, or bores, help minimize the possible interference with sound waves coming from each drivers, while shaping their linear characteristics in specific manner to tailor-fit them to the manufacturer's requirement. As far as I know, no IEM manufacturer has ever tried quad bore, especially when even a 12-driver JH Audio Roxanne has only triple bores.

Second, metallic bores have previously been implemented by Suyama of Japan, and such tubes tend to have lower resonance frequency compared to the ones made of materials with lower density. Moreover, they also yield stable high frequency response due to the tube's constant geometry. The same logic may apply here with BD4.2 as well.

Third, although making the low frequency response fully adjustable may sound tempting, adding an extra adjustable module always increases the cost of development and production. Moreover, most of the bass control implemented in conventional IEMs are poorly designed, and lacks accuracy.

Fourth, other than Ultimate Ears UE900 and Shure SE846, and I've never witnessed such filter placed at the output of an IEM. It usually utilizes a long acoustic transfer tubing, and requires a good amount of calculation, not to mention precise engineering to be practically beneficial.

Consequently, the ultimate question is: Are all of the innovations claimed by LEAR true, and if so, is it possible that they work to the overall benefit of users?

PRO: Well-damped time domain characteristic especially in the low frequency region, where the effect of the acoustic low-pass filter dominates. Channel matching is superbly good too. The IEM's high modifiability is a huge plus.

CON: Inverted polarity, which can easily be corrected by swapping the cable's two-pin plug inversely.

ON SECOND THOUGHT #1: Custom version(LCM) users should be aware that the insertion depth and the projected frequency response of their units will be somewhere between those of the reference plane and 3 mm. Here's a good news though: Unless inserted extremely shallow, BD4.2's treble linearity is not affected by the insertion depth.

ON SECOND THOUGHT #2: Source impedance must remain below 1 Ω in order to retain the IEM's linear characteristic unless very specific tonal balance is desired. Otherwise, the output follows its own impedance characteristic as the source impedance increases, and the treble is attenuated accordingly. With added impedance of 33 Ω, the sensitivity at 10 kHz drops down about 8 dB relative to 100 Hz.

ON SECOND THOUGHT #3: As claimed by the manufacturer, all four bores are working independently covering their own frequency ranges. While upper metallic bores are dedicated to mid-range and treble, the bottom ones are woofers, working together to cover the frequency range below 400 Hz.

The quad-bore shown on the right with the bass control on the left

And this is where BD4.2 shines the brightest: The top two bores can actually be damped according to users' personal tastes by using a piece of foam only, and enable users to easily experiment with various tonal balances at their disposal.

ON SECOND THOUGHT #4: On top of highly modifiable bores, the bass can also be tuned with the adjustable bass control along with the rear housing vent. Surprisingly, the bass control is quite accurate, and boosts the frequency range below 1 kHz at the scale of whooping 24 dB(minimum to maximum) with extremely low Q-value.

ON SECOND THOUGHT #5: BD4.2 comes with five different types of ear sleeves, and each type has its own advantage. Please be advised that the stock sleeve used in this analysis is the single flange sleeve.

ON SECOND THOUGHT #6: Here are my recommendations. While the first one is strictly diffuse-field oriented with the treble extension limited to mimic the sound of Suyama's Fitear 334, the second recommendation sounds very close to Sennheiser IE800 with the mid-high response enhanced.

Recommendation 1: First of all, equip a pair of Sony MH1 sleeves(Sony hybrids, Phonak stock, or Shure EA510) to finely shape the treble response above 10 kHz. Second, minimize the bass control. Third, insert a piece of earbud foam(0.5 mm x 0.5 mm x 0.5 mm) to the right upper metal bore. Fourth, couple the IEM with a 33 Ω resistor in serial. This setting is recommended when a source device with high output impedance is used to drive the IEM.

Recommendation 2:  First of all equip a pair of Sony MH1 sleeves(Sony hybrids, Phonak stock, or Shure EA510) to finely shape the treble response above 10 kHz. Second, leave the bass control at stock position. Third, insert a piece of earbud foam(0.5 mm x 0.5 mm x 0.5 mm) to the right upper metal bore. The source impedance shall remain below 1 Ω.

ON SECOND THOUGHT #7: The IEM in stock configuration is slightly bright in the mid-high range when compensated with Harman International's reference target.

IN CONLUSION: LEAR BD4.2 is unlike anything I've ever measured so far. Yes, the claims made by the manufacturer turn out to be all true, and the accumulated effects from the innovations add up to the benefit of users.

The stock configuration presented by LEAR is superb too, but the fact that the sound signature can be finely adjusted without having to modify the IEM's internal components shall be praised & glorified the most. There are simply endless possibilities for users to discover by means of both simple acoustical & electrical tweaks. And because of that, I acknowledge BD4.2 as one of the IEMs with highest quality of all, and one of my most favorite headphones as well.

More information on this product: 
http://lear.hk/products/All/LUF_BD4_2/52