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The noise floor defines the amplitude of the sum of all noise sources and other unwanted signals within a system. This could be the ambient acoustic noise in a room, or the electrical noise in a recording system or other audio equipment. In  a high quality audio system the noise floor is typically around -100dBu or better. The total dynamic range capability of a system is normally defined as the difference in amplitude between the noise floor and the highest signal that can be passed without distortion.

A 'Dummy Head' is a binaural recording array which intentionally resembles (to a greater or lesser degree) the size and shape (and sometimes also the density) of the average human head, with two microphones placed on opposite sides in the positions of the ear canals. The idea is to capture passing sound waves in the sa,e way as a normal listener would perceive them, with the correct inter-aural time delays and head shadowing effects which are critical to assessing directional cues.

See Interference Tube Microphone

A microphone — typically with a hypercardioid capsule — which has a perforated tube fitted in front of the diaphragm to try and create a substantially narrowed polar pattern.

Sound waves directly on-axis pass straight down the tube to reach the diaphragm without being affected in any significant way. However, this tube also has a number of slots or holes along its length which allow off-axis sound waves to enter at different points. The resulting different path lengths to the diaphragm create phase shifts resulting in partial cancellation and thus reduced sensitivity to sounds from the side of the tube. Hence the design's name of an 'interference tube microphone'. Alternative names include 'rifle' or 'shotgun' mic, principally because in the film and TV industries the mic is often fitted to a handle, resembling a firearm.

The cancellation of off-axis sounds is rarely perfect or consistent, resulting in varying amounts of attenuation for sound sources of different frequencies and at different angles of incidence. This means that off-axis sounds, while genreally attenuated, are also substantialluy coloured , and if the angle between mic and sound source is varied the cancellation dips move up and down the spectrum to give a phaser-like colouration or whoosing effect.

A method of controlling dynamic range in which the source signal is split, with one path being heavily compressed before being recombined with the original signal. The result is a form of 'upwards compression' in which low level signals are raised in level while high level elements are left unchanged. This technique preserves transients and is therefore particularlty popular for processing drum tracks. (Conventional downwards compression inherently modifies transients.)

Often known as New York Compression, and as London Compression! 

There is a full explanation of the technique here: https://www.soundonsound.com/techniques/parallel-compression

Jitter in a digital system is a random or deterministic timing deviation from the required periodicity of a reference sample clock.

If affecting the clocking of an A-D converter it causes the analogue signal to be sampled at incorrect moments and thus generates amplitude errors in the digital data which cannot (easily) be removed. If the jitter is random this typically results in a slight increase in high-frequency noise and distortion.

If jitter affects the D-A converter it reproduces the accurate digital samples at slightly incorrect times, resulting in the reconstructed analogue waveform having amplitude errors. Again, if the jitter is random this typically results in a slight increase in HF noise.

More problematic is when the jitter has a deterministic character, often having fixed relationship to the system clocks or interfacing arrangements. This form of jitter creates unwanted spectral or tonal elements, and their typically anharmonic nature often makes them audible even when at extremely low amplitudes. 

There are a variety of technical means of reducing and eradicating Jitter and, ideally, in a high quality digital audio system jitter should be in the low picoseconds range. Some digital clocking systems manage to get jitter down inro the femtosecond range!

Many digital interfaces, including S/PDIF and AES3, introduce some jitter (called interface jitter) as an inherent artefact of waveshape distortion caused by cable capacitance or fibre optic dispersion. 

The acoustic envelope describes the three phases of any natural acoustic sound: the initial transientor attack portion, the sustain period of near-constant sound, and the closing decay as the sound creation is ceased. Of these three stages, the initial transient part is arguably the most important in both recognising the type and nature of the instrument and determining its location in the environment around the listener.