Unit 1 - Room Acoustics Flashcards
primary objective in Building Acoustics
A primary objective in Building Acoustics is providing an environment that enhances the
the listening experience of any person in the room.
- In a concert hall the listener needs to hear the performance with sufficient volume and quality without external distraction.
- a bedroom needs to be quiet, free from distracting sounds from within the remainder of the building and from outside.
The Requirements of the listener/user dictate the important factors in our acoustic designs
two groups of sound types
- Steady sounds, these can be periodic (test tone), quasi-periodic (a musical chord), or
random (white noise), and
- Impulsive sounds, these can be single events such as gunshot or the transition that occurs when a steady source is turned on or off
The three main approaches to room acoustics
- Geometrical; the distribution of sound is modelled as rays issuing from the source and
being reflected specularly from each surface in the room (angle of incidence equals
the angle of reflection).
- works best at the higher audible frequencies
2. Wave-based; the sound is assumed to be distributed in the form of three-dimensional
standing waves (room modes) within the room boundaries. This approach is important at low frequencies when the room modes are clearly separated.
- Statistical; the sound field is modelled as the superposition of many randomly distributed travelling waves
Listening in the open airListening in the open air
Imagine a listener in an open space with very little ambient noise.
- When a sound is made nearby, the listener receives the sound first in one ear and then the other.
- These sound signals are processed by the brain so that the listener can quickly identify the direction and the nature of the source
once the sound waves pass the listener and carry on and the sound pressure level ever-decreasing at 6 dB per distance doubling.
-With no reflections, the travelling sound waves pass the listener in an instant and are quickly lost.
point source in the room makes a single brief sound
If a point source in the room makes a single brief sound the listener first receives the direct sound travelling straight from source to listener.
-The strength of the direct sound will be independent of the room environment.
The time lag between the direct sound arriving first at one ear and then the next locates the source within the room
Precedence Effect
Within any room the first sound that reaches the ear establishes the location of the source. This is a psycho acoustical phenomenon known as the Precedence Effect.
- helps us to determine the direction of a sound source even in the presence of sounds reflected by walls.
- The listening experience changes, as the ears receive reflections from the many reflecting surfaces within the room
Reflections of the original sound arriving between 2 ms and 80 ms
Reflections of the original sound arriving between 2 ms and 80 ms are localised solely in the direction of the first sound arriving at our ears.
- For speech the precedence effect is effective for delays between 2 ms to 50 ms after the first sound,
- for music the effective time period increases up to 80 ms.
- Reflections arriving after the precedence effect window may be heard as distinct echoes if they are strong enough
- for music the effective time period increases up to 80 ms.
Reflections of the original sound arriving before 2 ms
For time delays shorter than about 2 ms then summation of the first and second signals occurs and both sound signals contribute to the perceived direction.
This summation effect (summing localisation) is commonly found in sound reproduction when loudspeakers emit the same
what do multiple reflections as time goes represent
As time goes on, the number of reflections increases as the result of multiple reflections within the room.
-These multiple reflections constitute the reverberant sound and the distribution of the
reflections make up the reverberant field
reverberation time is a function of frequency
The reverberation time is a function of the frequency of the excitation sound as the result of sound absorption varying with frequency.
-Cathedrals have very large air volumes and hard stone surfaces which can result in long reverberation times (>30 s) at low frequencies
Noise Intrusion
Noise can emanate from outside the hall
Internal noise can result from air conditioning systems, stage machinery and lighting system.
Noise Criterion Contours (or one of their equivalents) are used to define the
acceptable octave band levels for noise within the unoccupied hall.
- For the highest specification performance venues, NC 10 is required with NC 15 being the limit of acceptability
- For cinemas NC 25 would be a suitable criterion.
Loudness
This can be quantified as the Strength Factor, G.
The resulting SPL at the listener’s location from an omnidirectional source on the stage, is compared with the free field SPL of the same source operating at the same power, but at a distance of 10 m.
Uniformity
An ideal auditorium would deliver the same listening experience at all locations. This is a
worthy but unattainable aim but careful design can ensure good sound distribution and avoid
significant variation throughout the hall.
uniformity
Shadowing of seating
Shadowing of seating areas by balconies is problem that requires attention.
- if the area beneath the balcony is too deep then seats at the back will receive poor sound.
- depth of the overhang should be no more than the height of the balcony above the seats
Uniformity
Smooth concave curved surfaces
Smooth concave curved surfaces can lead to focussing of sound at ‘hot spots’.
- Treating the surface with absorbent material would reduce the reflections
- surface could be covered with an acoustical rough layer to create diffuse reflections
Uniformity
Reflections shows a progressive decrease in their intensity with arrival time
Reflections shows a progressive decrease in their intensity with arrival time.
- If the intensity of a reflection is greater than that of those immediately surrounding it can be heard as an echo and detract from the listener’s enjoyment
- Once an echo is pointed out it may be difficult to ignore
- More Lilley in large spaces with focusing domes such as royal albert hall
duration of the reverberant sound alters perceived sound
The duration of the reverberant sound is important in controlling the quality of the perceived sound.
- If the duration is too short then very little is added to the volume and quality of the sound.
- If too long it becomes difficult to discern the individual sounds in speech or musical notes as they will tend to merge together.
- For clarity in speech, individual speech sounds need to be heard without too much reverberation
For music, the amount of reverberation will alter the quality of the listening experience;
- too little reverberance and the music would be described as ‘cold’ or ‘thin’ while
- too much reverberance would make it difficult to hear individual notes resulting in a ‘muddy’ or ‘muddled’ sound
Clarity
Good definition of speech or music requires a relatively strong direct sound. The less the
sound has to compete with continuing reverberance the easier it is understood.
-achieved by having the listener close to the source and having a direct path between source and listener
objective measure of clarity is the ratio of early to late sound strength. This compares the
energy from an impulsive source arriving before a set time interval (commonly 80ms) with the sound arriving after that time.
Clarity Index C80
Envelopment
a feeling of surround sound is enhanced if the early sound includes strong lateral reflections.
Traditionally narrow rectangular concert halls produced strong diffuse side reflections through the hall.
Once the importance of the lateral reflections was appreciated, then concert hall design incorporated means of enhancing lateral reflections.
modern designs for envelopment
modern designs have used overhead reflectors suspended over the audience to provide the lateral reflections.
- Quadratic Residue Sequence (QRS) reflectors are a means of providing diffuse lateral reflections
- Can achieve uniform scattering of large range
Apparent Source Width’ (ASW)
The subjective parameter widely used to describe the contribution of the lateral sound energy is called the ‘Apparent Source Width’ (ASW)
A measure of the lateral sound energy arriving at the listener’s position in the first 50 to 80ms after the arrival of the direct sound.
- measured using a dummy head This coefficient
- has been shown to be a more accurate assessment of envelopment than the Early Lateral Energy Fraction, LE80
Spaciousness
Spaciousness refers to the effect where the sound appears to the listener to emanate from a source wider than the width of the actual source.
-This is also measured as ASW.-
Spaciousness is also assisted by sufficient strength of arriving energy at low frequencies; quantified by the low-frequency Strength Factor, Glow.
flexibility of Multipurpose Halls
modern approach is to incorporate flexibility into the design that allows the hall operators to vary the room acoustics.
- Birmingham Symphony Hall,
uses a reverberation chamber around the rear of the hall and a series of pneumatically-opened doors above and around the Hall, to control the acoustic access to this space.
- This reverberant chamber also serves as an acoustic shield from external noise and as a site for dramatic off-stage lighting and sound effects
