What can you tell me about the driver used in the PM-1?
The planar magnetic driver in the OPPO PM-1 has a diaphragm size of 85 x 69 mm in an oval shape. The total diaphragm area is about 46.1 square centimetres, just slightly larger than a 3-inch diameter round driver. Its driven area is a large circle that reaches the edges of the oval across its minor axis while leaving some un-driven areas along the periphery of the oval's major axis. It has two layers of conductors on both sides, and it is progressively corrugated to provide stability and structural dampening, as well as allowing for lower resonance, considering the degree of diaphragm tensioning. The combination of tensioning, corrugation, and diaphragm parameters was optimised for best performance.
We chose a circular design because we wanted to create a headphone that would be smaller, lighter, more efficient, and usable on the road while also sounding exceptionally revealing and natural for those who appreciate the best in sound quality. The circular concentric driver is much more efficient in terms of weight and sensitivity than a rectangular design since it does not have static conductor areas that are not aligned with the magnetic field and are therefore passive. The PM-1's concentric magnet system is also more efficient in terms of magnetic energy generated per the amount of magnet material and metal.
Several very proprietary design solutions and manufacturing processes are used to optimise its performance and achieve consistency. An additional magnetic gap is created that makes use of the magnetic flux fringing effect for even distribution of the driving force over the diaphragm area.
The sound radiating area and its mandarin-shaped opening were also highly optimised. One of its functions is to act like a phase plug to counteract and disperse very high frequency modes from the diaphragm and smooth a few peaks. The rear plate of the driver also has openings that are oriented radially for the primary purpose of acoustically unloading the diaphragm and achieving maximum transparency of sound.
Together, the PM-1's magnetic system and diaphragm allow for ultimate efficiency of used materials and conductors.
What can you tell me about the tuning of the headphones?
We wanted to ensure that the headphones sounded equally exceptional with all types of music. In order to accomplish this, we knew that we needed to have a neutral sound signature, and we wanted music to have a natural, life-like quality to it. The headphones have a neutral sound that has a great deal of emotion and drama due to its full dynamic scale in the lower mids and bass. The bass is well controlled and has a fine balance of being dramatic and dynamic. We did not want the bass to feel anaemic and dry, as is sometimes the case with electrostatic and other top tier headphones. We also did not want to exaggerate the high frequencies, because when treble emphasis is present, we tend to experience listening fatigue very quickly. Headphones are often tuned with a boost at both frequency extremes. This can result in an exciting sound that provides an immediate "wow" effect during a short demo, and the effect may be particularly striking during a side by side comparison, but ultimately this is not a characteristic we associate with accurate, high quality sound. Furthermore, this kind of sound signature can become very fatiguing and psychologically stressful during long term listening sessions. With the PM-1's neutral frequency response and excellent resolution, every detail of your favourite recordings is present and balanced throughout with regard to bass, midrange, and treble.
Why doesn't the sound change dramatically when I place my hands over the headphone grilles like it does with some other open-back headphones?
We designed the PM-1 so that you could listen with them in practically any environment without adversely affecting the sound. With the PM-1, you can lean back on a chair, lie on a pillow, or even have a hat or thick hair over the headphones, and the sound will not be affected as much compared with other totally open cans. This is achieved by a combination of dampening felt behind the driver so that there are no reflections from the headphone cup structure back through the diaphragm, and also by the arrangement of holes on the rear and front openings of the driver that make the driver less sensitive to external obstructions. Some level of noise isolation will even allow you to listen to the headphones outdoors, on a bus, train, or even an airplane (with some noise limitations and sound leakage considering its open-back nature).
In addition to the tuning for the open-back design, the PM-1 is designed to have low acoustic impedance. The frontal cavity between the diaphragm and ear drum is vented. Not only does this allow for additional cooling and prevention of a vacuum effect, it also makes the headphones less sensitive to the acoustic load that listeners' ear and head shapes present to the headphones. This also ensures that the PM-1's performance is consistent regardless of head shape variations and fit issues related to the anatomical differences in listeners' ear cartilage and ear canals (which can be very significant) or whether eye glasses are worn or not.
Ultimately, the PM-1 provides very consistent performance regardless of listeners' anatomy, wearing preferences, and listening environment.
Since the PM-1 has such high sensitivity, why do they require a higher volume level from my headphone amplifier than other, less sensitive headphones?
Sensitivity is only one factor where an amplifier's ability to drive a pair of headphones is concerned, and perceived loudness is also related to the headphones' tonal balance. The PM-1 is very balanced across the audible spectrum and has a neutral sound signature. It is easy to crank up the volume without feeling that it is too loud or too harsh. Another factor is the output impedance of the headphone amplifier. If a headphone amplifier has high output impedance, low impedance headphones will get less power than high impedance headphones because of the power division between the amplifier's output impedance and the headphone's impedance. Some headphone amplifiers, and this is especially true with certain tube amplifier designs, output more power into high impedance headphones. With this type of headphone amplifier, you may notice that you don't have to turn the volume knob quite so high for 300 and 600 ohm headphones versus the 32 ohm PM-1, even though the PM-1 has a higher sensitivity rating than the aforementioned headphones.
Modern mobile phones and portable audio players generally have low output impedance and are designed to drive headphones with high sensitivity and low impedance. The PM-1 can be comfortably used with smart phones and portable audio players.
In addition, there is also a psychoacoustic effect where listeners can equate higher distortion with increased loudness. A harsh, distorted sound will always appear to sound louder. Due in part to its planar magnetic design, the PM-1 has very low levels of distortion, particularly at the mid and high frequencies where other headphone designs may exhibit audible distortion. One of the most critical aspects of accurate sound reproduction is achieving a realistic dynamic range. These vanishing levels of distortion allow the PM-1 to get closer to this realistic dynamic range and the sound remains clear and non-fatiguing despite the higher volume levels.
How long are the cables for the PM-1, and what are they made of?
The 6.35 mm terminated standard cable is 3 metres long and the portable cable with a 3.5 mm plug is 1 metre long, intended to be used with portable devices such as digital audio players and smart phones. The length of the portable cable was chosen to minimise cable management issues when used with a device such as a smart phone while in your pocket.
The standard cable is made of OCC (Ohno Continuous Casting) pure copper material. Each wire in the cable contains 150 strands of very fine 0.05 mm diameter conductors. The total conductor diameter is 0.76 mm, roughly between the size of AWG21 and AWG20 wires. There are four such wires in the cable, providing signal and return paths for each driver. The OCC manufacturing process forms a single copper crystal for the entire conductor. Research has shown that this process results in better signal conductivity due to the elimination of copper crystal grain boundaries in the conductor. The conductors go through a cryo-treatment process to relieve the strain after being twisted into wire used for the headphone cable. The portable cable is made of OFC (Oxygen-Free Copper) which has better conductivity than regular copper.