by Charles Miller
In my quest to understand headphone design, I have modified several models. I never changed or touched the transducers, but all the parts around the transducers were fair game. My favorite headphones of all are now my modified Sennheiser 500s. The Sennheiser transducers are simply amazing, with a smooth very wide range response and incredible linear dynamic range. It is the standard “baffles” and the standard “enclosures” around the Sennheiser 500 transducers that give them their light, bright, phased sound. But all of that is easy to fix with the Sennheiser 500 phones. There are two easy steps and one harder step:
- a) Take out the foam cushions and set them safely aside. Get a roll of masking tape and a pair of sharp scissors. Hold the headphones in front of a light source so you can see numerous small circular holes in the baffle around the transducer. NEVER touch the transducer! Use the masking tape, and cover up the holes above and on both sides of the transducer. Continue covering all of the holes, EXCEPT for the bottom two holes. Leave the bottom two holes beneath the transducer and the tiny slot at the bottom of the baffle open.
b) The transducer is protected by radial struts which attach to an annular hub at the center. The geometry resembles a spoke wheel with a hole in the center for an axle. Use a small piece of masking tape and seal off the center hole. NEVER touch the transducer. Complete steps a) and b) for the left and right units, and replace the foam cushions.
c) Next comes an important reminder. Steps a) and b) are reversible. Up to this step, the original Sennheiser sound and appearance can be restored to stock by just carefully peeling off a little masking tape. But this next step calls for irreversible structural changes to the Sennheiser housings. You may be satisfied with only steps a) and b). If you continue with this next step, you will improve the sound, but you will destroy the resale value of the headphones!
The housing behind the transducers creates a cavity resonance which colors the sound, particularly the middle of the frequency range, such as voices. The housing has numerous small openings which do reduce the “Q” or severity of the cavity resonances somewhat. But, they are still quite audible. Many very expensive headphone models have wire mesh behind the transducers. The mesh is more open and transparent to sound. As a result, the “Q” of the cavity resonances is lowered, and their audible effects become less audible.
So my third step is to cut away a substantial part of each housing, and replace it with a more open material such as wire mesh or perforated metal. I used light perforated sheet metal taken out of a cylindrical hair styling brush. These can be found in the cosmetics department of many stores. I used a Dremel tool with a rotary cutting blade to cut the openings in the back of the housings. Then I cleaned up the edges with a small knife. It was a tedious method, and one slip could have totally ruined the phones. I also considered just enlarging all the existing openings in the housing, but that would leave lots of plastic shavings and debris inside. While I am happy with my odd looking Sennheisers, having shiny perforated metal plates glued on to them, there is probably a much better way to do this. The perforated plates were quick and dirty.
There is no tape on the metal patch. In the picture of the completed modification (below), there is a piece of cotton INSIDE the baffle. The cotton is unimportant. I had tried putting sound damping material inside the baffle, but I could never detect any benefit from doing that. When I took the picture, a forgotten piece of cotton had fallen into view. Oooops.
I KNOW—-SO WHY? Well, place some of the best headphone transducers on the planet into rationally designed baffles, and you will have some of the best headphones on the planet. They do not make ordinary music sound bright, grainy, phased, or fantastic. But they have a very wide frequency range and tremendous dynamic range. A lot of equalization can be applied to the input signal without any worry about overloading these transducers. And the Sennheiser 500 transducers can be driven to fairly loud levels by the latest generation of CD players which operate from just two alkaline cells, unlike some high impedance phones. However, a decent headphone amplifier really improves the sound compared to the output of most portables.
No one needs these modified Sennheisers for listening to the vast majority of popular music. But the fact is, the modified Sennheisers sound just as wonderful with very complex and demanding orchestral music as the ordinary phones sound with pop music. For instance, I suggest listening to the Telarc recording of the Cincinnati Pops Orchestra performance of “Hymn to the Fallen” from the motion picture “Saving Private Ryan” ( Telarc CD-80495). When listening to this recording through the modified Sennheisers, I never get the feeling that the headphones are limiting the experience in any way.
With the Sennheiser 500s, placing tape on the baffles to cut out distortion is the same thing as enabling extra cost PC features by just changing a secret jumper. The modified Sennheisers have tremendous bass capacity. If you cover up all the holes in the baffle, you will easily get too much bass!
c. 2000, Charles Miller.