Hey guys, due to my recent trip to the Sennheiser plant in Ireland and the great people organizing the event and connecting us, I had the opportunity to write with and talk to some of their leading people working in the Audiophile Department. I asked if I could share these conversations as I thought this might be interesting to some of you here, it certainly was for me. 
My parts are in blue.
My parts are in blue.The first question I can think of is: Why only dynamic drivers (HE-1 doesn't count)? Is it an economical decision to stick with what you know and what production line you have set up? Is there any real interest in exploring different driver types, both for headphones and IEMs, maybe even hybrids? I have asked myself that for a long time.
JK: We constantly explore the cutting edge of technology of all transducer types. For in-ears, the answer is quite straight-forward: We believe that our dynamic driver technology in the True Response driver is on the technical level superior to all other implementations. During the start of the development of that driver in the late 2000s, the R&D team carefully benchmarked all available technologies and we arrived at the 7mm transducer we have today, which combines extremely low distortion across the entire frequency band, consistent frequency response between 20 Hz and 20kHz, small size, headroom up to 126 dB and is at the edge of what is consistently manufacturable. All of these factors have trade-offs with each other, and we believe this implementation is the best solution without handing over any compromises to the customer.
What was initially found over 15 years ago is still true today. Physics haven’t changed dramatically, and all other transducer types have significant draw-backs. Even the most recent innovations like MEMS speakers are severely limited by their small size and stiffness, which limits the output they can generate with low distortion. For instance, we tested some MEMS transducers which already started distorting at 2-3kHz due to the limited excursion. We do not believe that this technology is useful for either audiophile products or even noise cancelling devices, which require even more bass performance.
Diving into the question whether we plan to build hybrid earphones: There is simply no reason for us to do it. Our 7mm transducer does everything we believe is conducive to good sound, and there is nothing we would add. We understand the novelty and exciting story behind it, but we try to stay true to our commitment of best sound, and not flashy stories to sell product.
For large headphones, the answer is a bit more nuanced. There are some obvious advantages to large diaphragms, and we have extensively researched and pre-developed all commonly known technologies. The problem lies in the manufacturing maturity and required investment for other transducer types.
For the time being, we only see potential in those technologies for very high price points. Sennheiser is many times larger than almost all competitors in the audiophile space, and we require a certain pay-off for investments we do. At the same time, our scale also demands that we do not launch “prototype” products which are just the first thing you can build. There is always the expectation of serial production with high yield, excellent reliability and designed to sound the best we can achieve. This makes entering a new technology a lot harder. That being said, you won’t find bigger supporters for a new large-diaphragm headphone from Sennheiser than in this company. We’ll do our best to make it happen.
Second question: I love the sound of the HD-600 and prefer it to the 650. However I have heard and read the claim many times, that they are basically withing margin of error in their sound signature. Is that true in your opinion? They did not sound that much alike to me when I compared them.
If they were within margin of error, we would be happy with a single product. The original product was the HD 580, with a relatively open baffle and a different grille on the outside, which was a lot more closed. This headphone represented a diffuse field loudness equalized response, i.e. as scientifically neutral as you can get. With the HD 600, the very closed outside grille was exchanged with the metal grille we know today, which was previously reserved for our electrostatic products. As a result, the headphone was very open, with excellent detail retrieval, but slightly brighter than the initial HD 580.
With the HD 650, we readjusted the openness of the headphone by increasing the acoustic impedance of the baffle and in the transducer damping to get closer to the HD 580 again and smoothen the response.
Just looking at the tuning, the HD 650 is closer to the ideal. However, I personally prefer the increased openness and detail retrieval the HD 600 offers. Even within Sennheiser, you won’t find a consensus on the topic HD 600 vs HD 650
Also: The sub-bass roll-off, that the 600 and 650 possess, is that a technical limitation or is it part of the sound signature? Would raising the sub-bass change the overall sound too much and if so, is there a way to keep the experienced sound signature the same while raising the sub-bass?
I am asking that, because the only "real flaw" the HD-600 has for me is the sudden roll-off in the bass. Have you guys experimented with that?
In theory, it’s a technical limitation of the design. For headphones with an acoustically open front volume, the behavior is analogue to a speaker in a room: the amplitude will decrease below the resonance frequency. This behavior can be manipulated with 3 general approaches:
All approaches have trade-offs which will compromise the current performance of the headphones in some ways. For instance, one could lower the resonance frequency by increasing coil weight, but this would also reduce the brilliance of the treble. One could increase the compliance of the surround (this was implemented in the HD 660S2), but this may increase tumbling/distortion, and create more partial vibrations in the high frequencies.
You could also decrease the openness of the front volume, this is something you can do experiments on as a user: you could put tape on the stainless steel mesh on the baffle or install earpads with isolating fabric or dense foam. However, the increased occlusion has significant psychoacoustic drawbacks on the perception of sound and measurably increases the load of the transducer by increasing the weight/resistance of the air in the front volume, effectively increasing the weight of the transducer, so you’re losing treble again.
Bass tubes reduce the acoustic short circuit between front and back volume and can help polish the frequency response and reduce THD (like in the HD 560S), but the bass loses perceived speed and impact compared to the HD 660S2 without bass tube.
The HD 600 has been carefully developed to balance all these parameters with the primary goal of practically zero distortion and a smooth frequency response without audible resonances. All band aids for the bass response would take away from the these qualities, and I much rather have a draw-back that can easily be compensated by EQ than a weakness which you can not compensate, like increased THD.
How did you guys came to the conclusion of what sound fits into what price category? I am aware of all the other factors that increase or decrease manufacturing cost, but there had to be some conscious effort to create different sound signatures to sort them into the different price tiers of the products, right?
For us, there is no clear correlation between sound signature and price. We can create warm, analytical and neutral sound signatures at any price point. We demand a higher price for those products which incur a higher cost for us, primarily through the increased transducer cost. Our entry offerings have a copper coil, while the audiophile headphones exclusively use aluminium coils, which take much longer to bond and take double the machine time. Some transducers like the HD 800 S and of the HD 600 are hand-made, which is obviously more expensive, and they use higher quality materials. For instance, the HD 800 S uses a stainless steel mesh for higher consistency and longevity compared to paper on the 500 series. The HD 600 and HD 650 use silk as transducer damping, which is the finest mesh you can use and reduces distortion by air turbulence to a minimum. These are some examples of how we go the extra mile in the product to produce superior sound, but it has nothing to do with the tuning by itself.
One thing we’ve seen is that customers in the highest price segments prefer a slightly bright presentation, so that would be a general customer demand we usually follow.
How has your stance or philosophy on "good audio quality" in general and developing signatures and designs for new products changed over the years, working in the industry? Or is it still the same?
I started out with HD 600 before joining Sennheiser, and my highest priority used to be simply a balanced, natural sound signature. I then bought an HD 800, and while the extra space was nice, but what I love most about it that it had a tendency to “pull me into the music”. So instead of just being in kitchen radio mode, I would stop what I’m doing and start just intently listening to the music. When I joined the product management team and got insights into what makes headphones sound good, I started spending more attention to different attributes that help with creating that experience. So I know spend a lot of attention on the quality of openness (low occlusion), angling of transducers, “speed” of the transducer (combined low weight and low damping) and excuse slight gimmicks in the tonality. The tonality you can always change with EQ, the detail retrieval or bass impact is much harder to adjust.
Some other personal questions: What is your favorite headphone/IEM, what gear are you listening to in your own time and what kind of music?
It’s between the old Orpheus, the HD 600, the HD 560S and a forthcoming headphone to be announced in May. I think the old Orpheus is flat out the best sounding headphone, with a natural timbre and extraordinary brilliance, but obviously impractical. I still love the HD 600 because it’s basically a baby Orpheus: natural timbre and excellent detail retrieval. The HD 560S holds a special place in my heart because it was the first audiophile headphone I helped developing and I did the acoustics myself. I’m also a big fan of cost-conscious design, and the value for money is extraordinary. For the last one you’ll have to be a bit patient.
What are you most proud of during your time working with Sennheiser?
Visiting the manufacturing lines Ireland makes you aware that the decisions you take during development will have to be carried out by dozens of people for years or even decades to come. Beyond that, the commercial success of your decisions has a direct impact on how many people are employed, so you have a very direct responsibility for other humans and their families.
As you may know, there has been some back and forth for our Ireland factory, where headphone production was outsourced for a while. I’m probably the proudest of the fact that over my tenure, Ireland has been steadily rebuilt into the headphone and transducer manufacturing temple that it should be, and I’m thankful for the trust the organization placed on me.
What do you think of wireless audio and aptx lossless? Have you tried this new codec, is there even a noticable difference?
I’m personally not too involved in that topic. I like to think about ways we can improve sound, and those codecs at maximum don’t make it worse. My colleague @Kuan, Wee Hong can maybe speak to it.
Jermo Köhnke
Product Manager Audiophile
He did, here is what he had to say about that:
(...) when it comes to the driver of Sennheiser True Wireless products, they are actually using the same transducer as the wired earphones, which is our True Response 7mm transducer.
In the case of the Momentum Sport, it is using the 10mm transducer.
The 7mm transducer are used across the range of TWS and also product such as IE 200/600/900 and the difference in acoustic is achieved through the overall improvements in the acoustic system.
Such as through acoustic back volume, refined dampening, adjust the voice coil winding and through usage of different resonator chamber in the front volume (TWs have shorter nozel than IEM).
On the IE 600 and 900, the transducer are channel matched for perfect stereo imaging and any units with even slightest distortion artifacts are taken out.
About wireless audio, you are absolutely right, than when in a mobile and busy environment, the differences are likely negligible, as you may be distracted by environment noise and it is also nearly impossible to zoom into the music without compromising on safety.
Of course, these are subjective based on individual. But priority in an outdoor environment will be to prevent drop-off, and to achieve it, the scalable Bluetooth codec will usually reduce to low bit-rate for a more stable connection.
With regards to the sound quality of apt-x lossless, it is quite unfortunate that I have not tested it, as I mentioned, due to limited source. But I am trying to borrow a Sony phone to be able to test out further.
Prior to the launch of Momentum True Wireless 4 with its apt-X Lossless, I was able to test apt-X, apt-X HD, LDAC and AAC vs wired (USB and Aux) in a semi-controlled condition, where everything is the same, other than the connection.
The equipment I used is an Astell & Kern SP 2000T, ifI DSD Gryphon and IE 600, and playing Nat King Cole’s L-O-V-E and ACDC Shoot to Thrill.
With my colleague, the biggest impression formed on us is not just the increase in detail retrieval, but an obvious increase in soundstage. I can write a lot about it, but here is the brief (not-so) recap:
In the case of SBC and apt-X, the sound is akin to a HD 600 space where it is intimate and goes from ear-to-ear (in-head). The separation between instruments are neglible and it can be perceived to be murky.
And as we switch to the Hi-Res codec, tonality of the music change as the bass gives a speedier perception and the soundstage expanded outwards into the space around the head. With the expansion of the soundstage, the perception of layering (or depth) becomes noticable.
What is interesting is that the AAC codec perform quite closely with apt-X HD, even though AAC is not a high-resolution codec
But the best performance goes to LDAC, where the piano tap reverbate very crystal clearly and I can hear clearly the air reverb from the trumpet in L-O-V-E. But I was able to achieve that as I was able to toggle the highest 990kbps while apt-X HD capped at 570kbps
I personally will love to also experience apt-X Lossless and the latest LC3+, and I will share with you once I have tested.
Wee Hong, Kuan
Senior Product Manager, Global Audiophile Segment
JK: We constantly explore the cutting edge of technology of all transducer types. For in-ears, the answer is quite straight-forward: We believe that our dynamic driver technology in the True Response driver is on the technical level superior to all other implementations. During the start of the development of that driver in the late 2000s, the R&D team carefully benchmarked all available technologies and we arrived at the 7mm transducer we have today, which combines extremely low distortion across the entire frequency band, consistent frequency response between 20 Hz and 20kHz, small size, headroom up to 126 dB and is at the edge of what is consistently manufacturable. All of these factors have trade-offs with each other, and we believe this implementation is the best solution without handing over any compromises to the customer.
What was initially found over 15 years ago is still true today. Physics haven’t changed dramatically, and all other transducer types have significant draw-backs. Even the most recent innovations like MEMS speakers are severely limited by their small size and stiffness, which limits the output they can generate with low distortion. For instance, we tested some MEMS transducers which already started distorting at 2-3kHz due to the limited excursion. We do not believe that this technology is useful for either audiophile products or even noise cancelling devices, which require even more bass performance.
Diving into the question whether we plan to build hybrid earphones: There is simply no reason for us to do it. Our 7mm transducer does everything we believe is conducive to good sound, and there is nothing we would add. We understand the novelty and exciting story behind it, but we try to stay true to our commitment of best sound, and not flashy stories to sell product.
For large headphones, the answer is a bit more nuanced. There are some obvious advantages to large diaphragms, and we have extensively researched and pre-developed all commonly known technologies. The problem lies in the manufacturing maturity and required investment for other transducer types.
For the time being, we only see potential in those technologies for very high price points. Sennheiser is many times larger than almost all competitors in the audiophile space, and we require a certain pay-off for investments we do. At the same time, our scale also demands that we do not launch “prototype” products which are just the first thing you can build. There is always the expectation of serial production with high yield, excellent reliability and designed to sound the best we can achieve. This makes entering a new technology a lot harder. That being said, you won’t find bigger supporters for a new large-diaphragm headphone from Sennheiser than in this company. We’ll do our best to make it happen.
Second question: I love the sound of the HD-600 and prefer it to the 650. However I have heard and read the claim many times, that they are basically withing margin of error in their sound signature. Is that true in your opinion? They did not sound that much alike to me when I compared them.
If they were within margin of error, we would be happy with a single product. The original product was the HD 580, with a relatively open baffle and a different grille on the outside, which was a lot more closed. This headphone represented a diffuse field loudness equalized response, i.e. as scientifically neutral as you can get. With the HD 600, the very closed outside grille was exchanged with the metal grille we know today, which was previously reserved for our electrostatic products. As a result, the headphone was very open, with excellent detail retrieval, but slightly brighter than the initial HD 580.
With the HD 650, we readjusted the openness of the headphone by increasing the acoustic impedance of the baffle and in the transducer damping to get closer to the HD 580 again and smoothen the response.
Just looking at the tuning, the HD 650 is closer to the ideal. However, I personally prefer the increased openness and detail retrieval the HD 600 offers. Even within Sennheiser, you won’t find a consensus on the topic HD 600 vs HD 650
Also: The sub-bass roll-off, that the 600 and 650 possess, is that a technical limitation or is it part of the sound signature? Would raising the sub-bass change the overall sound too much and if so, is there a way to keep the experienced sound signature the same while raising the sub-bass?
I am asking that, because the only "real flaw" the HD-600 has for me is the sudden roll-off in the bass. Have you guys experimented with that?
In theory, it’s a technical limitation of the design. For headphones with an acoustically open front volume, the behavior is analogue to a speaker in a room: the amplitude will decrease below the resonance frequency. This behavior can be manipulated with 3 general approaches:
- Lower the resonance frequency
- Decrease openness
- Miscellaneous, i.e. bass tubes or similar
All approaches have trade-offs which will compromise the current performance of the headphones in some ways. For instance, one could lower the resonance frequency by increasing coil weight, but this would also reduce the brilliance of the treble. One could increase the compliance of the surround (this was implemented in the HD 660S2), but this may increase tumbling/distortion, and create more partial vibrations in the high frequencies.
You could also decrease the openness of the front volume, this is something you can do experiments on as a user: you could put tape on the stainless steel mesh on the baffle or install earpads with isolating fabric or dense foam. However, the increased occlusion has significant psychoacoustic drawbacks on the perception of sound and measurably increases the load of the transducer by increasing the weight/resistance of the air in the front volume, effectively increasing the weight of the transducer, so you’re losing treble again.
Bass tubes reduce the acoustic short circuit between front and back volume and can help polish the frequency response and reduce THD (like in the HD 560S), but the bass loses perceived speed and impact compared to the HD 660S2 without bass tube.
The HD 600 has been carefully developed to balance all these parameters with the primary goal of practically zero distortion and a smooth frequency response without audible resonances. All band aids for the bass response would take away from the these qualities, and I much rather have a draw-back that can easily be compensated by EQ than a weakness which you can not compensate, like increased THD.
How did you guys came to the conclusion of what sound fits into what price category? I am aware of all the other factors that increase or decrease manufacturing cost, but there had to be some conscious effort to create different sound signatures to sort them into the different price tiers of the products, right?
For us, there is no clear correlation between sound signature and price. We can create warm, analytical and neutral sound signatures at any price point. We demand a higher price for those products which incur a higher cost for us, primarily through the increased transducer cost. Our entry offerings have a copper coil, while the audiophile headphones exclusively use aluminium coils, which take much longer to bond and take double the machine time. Some transducers like the HD 800 S and of the HD 600 are hand-made, which is obviously more expensive, and they use higher quality materials. For instance, the HD 800 S uses a stainless steel mesh for higher consistency and longevity compared to paper on the 500 series. The HD 600 and HD 650 use silk as transducer damping, which is the finest mesh you can use and reduces distortion by air turbulence to a minimum. These are some examples of how we go the extra mile in the product to produce superior sound, but it has nothing to do with the tuning by itself.
One thing we’ve seen is that customers in the highest price segments prefer a slightly bright presentation, so that would be a general customer demand we usually follow.
How has your stance or philosophy on "good audio quality" in general and developing signatures and designs for new products changed over the years, working in the industry? Or is it still the same?
I started out with HD 600 before joining Sennheiser, and my highest priority used to be simply a balanced, natural sound signature. I then bought an HD 800, and while the extra space was nice, but what I love most about it that it had a tendency to “pull me into the music”. So instead of just being in kitchen radio mode, I would stop what I’m doing and start just intently listening to the music. When I joined the product management team and got insights into what makes headphones sound good, I started spending more attention to different attributes that help with creating that experience. So I know spend a lot of attention on the quality of openness (low occlusion), angling of transducers, “speed” of the transducer (combined low weight and low damping) and excuse slight gimmicks in the tonality. The tonality you can always change with EQ, the detail retrieval or bass impact is much harder to adjust.
Some other personal questions: What is your favorite headphone/IEM, what gear are you listening to in your own time and what kind of music?
It’s between the old Orpheus, the HD 600, the HD 560S and a forthcoming headphone to be announced in May. I think the old Orpheus is flat out the best sounding headphone, with a natural timbre and extraordinary brilliance, but obviously impractical. I still love the HD 600 because it’s basically a baby Orpheus: natural timbre and excellent detail retrieval. The HD 560S holds a special place in my heart because it was the first audiophile headphone I helped developing and I did the acoustics myself. I’m also a big fan of cost-conscious design, and the value for money is extraordinary. For the last one you’ll have to be a bit patient.
What are you most proud of during your time working with Sennheiser?
Visiting the manufacturing lines Ireland makes you aware that the decisions you take during development will have to be carried out by dozens of people for years or even decades to come. Beyond that, the commercial success of your decisions has a direct impact on how many people are employed, so you have a very direct responsibility for other humans and their families.
As you may know, there has been some back and forth for our Ireland factory, where headphone production was outsourced for a while. I’m probably the proudest of the fact that over my tenure, Ireland has been steadily rebuilt into the headphone and transducer manufacturing temple that it should be, and I’m thankful for the trust the organization placed on me.
What do you think of wireless audio and aptx lossless? Have you tried this new codec, is there even a noticable difference?
I’m personally not too involved in that topic. I like to think about ways we can improve sound, and those codecs at maximum don’t make it worse. My colleague @Kuan, Wee Hong can maybe speak to it.
Jermo Köhnke
Product Manager Audiophile
He did, here is what he had to say about that:
(...) when it comes to the driver of Sennheiser True Wireless products, they are actually using the same transducer as the wired earphones, which is our True Response 7mm transducer.
In the case of the Momentum Sport, it is using the 10mm transducer.
The 7mm transducer are used across the range of TWS and also product such as IE 200/600/900 and the difference in acoustic is achieved through the overall improvements in the acoustic system.
Such as through acoustic back volume, refined dampening, adjust the voice coil winding and through usage of different resonator chamber in the front volume (TWs have shorter nozel than IEM).
On the IE 600 and 900, the transducer are channel matched for perfect stereo imaging and any units with even slightest distortion artifacts are taken out.
About wireless audio, you are absolutely right, than when in a mobile and busy environment, the differences are likely negligible, as you may be distracted by environment noise and it is also nearly impossible to zoom into the music without compromising on safety.
Of course, these are subjective based on individual. But priority in an outdoor environment will be to prevent drop-off, and to achieve it, the scalable Bluetooth codec will usually reduce to low bit-rate for a more stable connection.
With regards to the sound quality of apt-x lossless, it is quite unfortunate that I have not tested it, as I mentioned, due to limited source. But I am trying to borrow a Sony phone to be able to test out further.
Prior to the launch of Momentum True Wireless 4 with its apt-X Lossless, I was able to test apt-X, apt-X HD, LDAC and AAC vs wired (USB and Aux) in a semi-controlled condition, where everything is the same, other than the connection.
The equipment I used is an Astell & Kern SP 2000T, ifI DSD Gryphon and IE 600, and playing Nat King Cole’s L-O-V-E and ACDC Shoot to Thrill.
With my colleague, the biggest impression formed on us is not just the increase in detail retrieval, but an obvious increase in soundstage. I can write a lot about it, but here is the brief (not-so) recap:
In the case of SBC and apt-X, the sound is akin to a HD 600 space where it is intimate and goes from ear-to-ear (in-head). The separation between instruments are neglible and it can be perceived to be murky.
And as we switch to the Hi-Res codec, tonality of the music change as the bass gives a speedier perception and the soundstage expanded outwards into the space around the head. With the expansion of the soundstage, the perception of layering (or depth) becomes noticable.
What is interesting is that the AAC codec perform quite closely with apt-X HD, even though AAC is not a high-resolution codec
But the best performance goes to LDAC, where the piano tap reverbate very crystal clearly and I can hear clearly the air reverb from the trumpet in L-O-V-E. But I was able to achieve that as I was able to toggle the highest 990kbps while apt-X HD capped at 570kbps
I personally will love to also experience apt-X Lossless and the latest LC3+, and I will share with you once I have tested.
Wee Hong, Kuan
Senior Product Manager, Global Audiophile Segment
