Continued from part 1 & part 2.
SONY MDR-MA900 is geared up with two vital technologies, which were originally developed for MDR-F1 nearly 15 years ago by the headphone maestro, 投野耕治-san who developed the king of headphones, MDR-R10. The first one is an acoustic bass lens, and the second one is an impedance compensator.
1. Acoustic bass lens
2. Impedance compensator
The effects these technologies inflicting on MDR-MA900's sound signature will be discussed.
1. An economical, yet effective technique
On the user manual of MDR-F1, there is a brief description on this bass-enhancing technology:
"On the front side of the driver unit of the MDR-F1 is an "Acoustic bass lens", a box shaped chamber covered by an acoustical resistor material. The acoustical resistor material allows high frequency sound to pass through and filters the low frequency sound. Consequently, the low frequency sounds are concentrated to the center part of the driver unit, and sufficient bass sound is delivered to the listener. At the same time, frequency response is controlled by the "Acoustic bass lens" to provide mild yet extending high frequency sound."
In short, this bass lens is an acoustic low-pass filter, accompanied with partial damping of the sound using a porous material.
Still doesn't ring a bell?
This is a well-known acoustic tuning technique, and is actually quite primitive to be called as a 'technology'. As this 'lens' is easily applicable due to its lower manufacturing cost compared to other techniques, it can be easily seen on headphones from other manufacturers as well. Yet, it does not mean the technique is just a commercial gimmick; they are very effective in evening out peaks.
D: far from the ear / N: close to the ear / E: N with acoustic bass lens
Going back to the patent, there is no need for identifying each curves; It is obvious which one is the result of the lens.
The developer of MDR-MA900, 松尾伴大-san, describes acoustic bass lens this way:
"By utilizing an acoustic damping material, the low-frequency response is first concentrated, and then radiated to the pinna directly. This acoustic focusing technique has been used previously with MDR-F1, and consequently, the low frequency response should be perceived more or less the same by any listeners."
2. A resistive voltage divider within
Although there is a mention about the impedance compensator on the user manual of MDR-F1, the description is just too vague to correctly identify what it is.
"In full open air type headphones, impedance around f0 fluctuates, thus the output impedance of the audio player affects the sound reproduced by the headphones...Conventional full open air type headphones need to be connected to speaker output terminals or to be driven by a special amplifier in order to reproduce intended quality sound. The MDR-F1 does not need special connections because they have impedance compensator which eliminate the variation of sound quality created by amplifier output impedance."
So the compensator's role is to help maintain linearity in the low frequency, regardless of the source impedance of which the headphone is connected to. Basically, SONY is claiming that MDR-F1 and MDR-MA900 operate free from damping-related response errors.
In order to further investigate the technology, with a friendly advice from my dear friend, ソノベ-san, the very patent that deals with this compensator has been tracked down: 特開平09-307980.
According to SONY, the impedance compensator is simply a resistive voltage divider with a pair of 22 Ω resistors installed in parallel on each channel. With a simple formula, a consequential source impedance can be calculated:
Zo = (R1 * R2) / (R1 + R2)
And this is why MDR-F1 & MDR-MA900's rated impedances are both 12 Ω.
So how does it work? With this voltage divider, a source impedance of 470 Ω becomes 21 Ω, and the consequential damping error on MDR-F1's output decreases almost in half, from 7 dB to 3.8 dB. While this seems like a brilliant way to minimize the effect of damping, the filter still adds 11 Ω to even a 0 Ω source. That means an expensive, dedicated pure voltage source will be completely wasted: Sound quality compromised for the sake of circumstantial variables. Having an unnecessary resistive voltage divider in an acoustic signal transmission is definitely not a good news for audio enthusiasts, IMO.
Furthermore, this filter totally screwed up my impedance measurement data; the reference load of 100 Ω on my impedance measurement jig has been compensated too! It was measured 17 Ω, which is not correct.
In order to correctly figure out MDR-MA900's true impedance value, a frequency-dependent attenuation calculation shall be applied. By comparing differences between the level of attenuation with a serial resistor loaded/unloaded, MDR-MA900's original impedance comes out to be 8 Ω on each channel.
response error of 2 dB @ f0
SONY MDR-MA900 is like an old-innovation revisited. Even with all the sleek looks, the technology the manufacturer utilized is quite classical. Although these core technologies have been passed down and revamped on later models many times, it seems the designer, 松尾伴大-san, truly wanted to revitalize his master, 投野耕治-san's underrated work, MDR-F1.
So did 松尾-san get what he wanted? Maybe. MDR-MA900's acoustic uniqueness shines off among other open-air headphones for sure. I was not impressed with the original MDR-F1, but MA900 is such a pleasure to listen to. However, he should have considered the fact that most of us audiophiles actually own a dedicated headphone amplifier with an output impedance less than 1 Ω. Considering the MSRP of $299, who would most likely be potential buyers: Audiophiles? or normal users?