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.

The thing that will never go away is that connection you make with a band or a song where you're moved by the fact that it's real people making music. You make that human connection with a song like 'Let It Be' or 'Long and Winding Road' or a song like 'Bohemian Rhapsody' or 'Roxanne,' any of those songs. They sound like people making music.”
  Dave Grohl