However, the published chart detailing the way it changes the frequency response is often lost and was hard to find on the Internet — and very difficult to read even when found — so I've carefully re-drawn it here for the benefit of future enquirers.
The ring has an outer diameter of 35mm, an inner diameter of 25mm, and is 6mm thick. I've not found a generic supply, but I'd suggest searching for neoprene washer M25 x 35.00 x 6mm.
The pressure ring must be fitted such that it is flush with the top of the microphone housing.
The information Sennheiser provides with the ring states "This pressure build-up ring increases sensitivity of the MKH20 within the frequency range 4 to 10kHz by approximately 2dB. In conjunction with the diffuse field equalisation the ring enables an easy matching of the frequency response to a given recording situation."
The microphone's polar response is naturally more directional at high frequencies, with the front/back ratio falling above 1kHz to roughly 5dB at 6kHz, and reaching 15dB at 16kHz. Fitting the ring is likely to increase this ratio further above 4kHz but, sadly, Sennheiser has not (AFAIK) published polar-pattern plots detailing the resulting changes.
