Lecture 3 (Summer) - Room Acoustics Flashcards
What are Standing Waves?
A standing wave happens when a sound wave hits a highly reflective boundary, it reflects back on itself.
When 2 travelling waves (of same amplitude and frequency) add together, as they cycle through phase changes a wave is created that oscillates up and down as a function of time.
This standing wave between 2 parallel walls results in the reinforcement or resonance of sound energy at certain frequencies.
What is a fundamental frequency?
Standing waves between 2 parallel walls result in the reinforcement or resonance of sound energy at certain frequencies.
The lowest resonant frequency is called fundamental - f0
Other similar resonances occur at multiples of this fundamental - 2f0; 3f0
The room will have peaks and dips in frequency response at those frequencies.
What is the equation for the fundamental frequency?
To solve for f
f = c/λ
c - speed of sound
f - fundamental
λ - wavelength (double the distance betwen the two walls) can sometimes be written as 2L
What are the 3 modes of vibration in standing waves?
Axial modes - refers to only 2 parallel walls (has most affect)
Tangential modes - 2 sets of parallel walls (4 surfaces)
Oblique modes - all 6 surfaces in the room, 4 walls, ceiling and floor
What is the calculation of room modes?
f = (c/2)√(p/L)^2 + (q/B)^2 + (r/H)^2
L - Length, B- breadth, H - height
put 1’s into the p/q/r part of the equation depending on what you are calculating, use a chart for this.
What is sound absorption?
The removal of acoustic energy from a space as a consequence of the presence of different surface materials.
The amount of absorption by a material is indicated by the absorption coefficient, which may vary at different frequency bands.
What are absorbers and diffusers?
Can be used to solve problems caused by standing waves by acoustically treating the room.
Absorbers - take energy out of the room
Diffusers - spread the sound equally
What is an anechoic chamber?
A space where every sound will be absorbed, nothing is reflected back. Can sound very unnatural.
Why shouldn’t we mix in an anechoic chamber?
mixing space should have a little bit of acoustic energy knocking around the room. But we have to ensure that this energy does not come from any specific direction.
Audiences don’t watch in an anechoic chamber (pretty sure!)
We need sound to be diffuse (spread evenly in a space)
Absorbers and diffusers can be combined for general acoustic treatment.
How do we know that playback in the auditorium will be accurate?
Cinemas can be THX certified.
They consider;
Outside noise, inside noise, audio distortion, obstructed/uncomfortable viewing angles, reverb in the auditorium, unequalised or poor audio.
Noise rating must be below NR30.
RT60 is dependent on auditorium size.
No echoes!
A baffle wall behind the screen helps spread sound evenly.
