This is the amount of signal amplitude range between the nominal signal level and the siganl level at which clipping will occur. Headroom is normally expressed in dB.

The headstock is situated at the top of the neck. Normally this has the tuning pegs (sometimes called machine heads), and often displays the guitar company logo.

Helmholtz Resonator
This is an acoustic room treatment that can trap a specified range of low frequency audio signals.

Hertz (Hz)
This is the unit of measurement for frequency.

1 Hz = 1 cycle/sec

More generally:

Frequency = Wavelength/time

Instrument used as part of a drum kit consisting of a stand, rod, pedal, and 2 cymbals, one on top of the other. As the pedal is raised or lowered it causes the rod running through the stand to raise the top cymbal. Creating different tones as it is struck.

High-Pass Filter
This is a type of filter that rejects low frequency audio signals.

Low frequency tone that is normally a multiple of 50Hz or 60Hz. This is because it is often related to the mains power frequency.

One can't deny what has happened to us in the past. The secret is to enjoy and be proud of the music we've created and the people with whom we have been linked. It's all a long chain of involvement in the world, and we are proud to be yet another link in this chain.”
Graham Nash