A pair of DPA mics designed for close‑miking inside a piano.
What is the approximate sound pressure level (SPL) of a concert grand piano, about 12 inches above the hammers during a triple forte passage — or, in other words, as loud as it could get? I’d like to make sure that if I’m getting any distortion it’s not at the microphone (I have the maximum SPL ratings of all of my mics). I’ve been getting decent results with mics about 18 inches above the hammers, but I’d like to bring them even closer in. I can experiment with this, but is there a general Goldilocks zone, for a worst‑case scenario fortississimo passage?
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: It’s difficult to be definitive, as every piano and pianist will be different, but there’s some published research into this question so ball‑park figures are available. Microphone company DPA state that “the sound pressure level inside a concert grand piano can exceed 130dB SPL peak at less than eight inches (20cm) over the strings...” and I’ve read other papers that claim peak levels of 136dB SPL immediately above the hammers. The initial impact of hammer on string is effectively a point source, so the level of that first transient will reduce by around 6dB for each doubling of distance: if it was 136dB SPL at eight inches, it would be close to 142dB at four inches, and 148dB at two inches — in theory! In practice, I doubt many pianos, let alone most pianists, are capable of generating that kind of SPL, and I suspect there would be tuning and other mechanical issues to resolve.
The vast majority of mics will cope perfectly well, even with a gorilla at the keyboard!
I’ve used Sennheiser MKH20s close to the hammers in a grand piano on several occasions and not had distortion problems. Without the pad, these are rated for a maximum SPL of 134dB. I’ve also used AKG C414B‑ULS mics (rated at 140dB SPL) without the pad. To be confident, I would suggest using a mic rated for something in excess of 130dB SPL where possible. However, in practice there are probably very few occasions that get anywhere near that level. Also, remember that the maximum SPL rating of a mic is not its clipping level, but rather the level where the output generates 0.5% Total Harmonic Distortion (some manufacturers use the 1% THD figure because it will be higher!). On transient peaks it’s pretty hard to hear even 3% distortion (which defined the standard maximum operating level for open‑reel tape machines), so I suggest you don’t get too bogged down with the numbers. The vast majority of mics will cope perfectly well, even with a gorilla at the keyboard!