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Audio Acoustics in Small Rooms. By Earl R. Geddes GedLee LLC www.gedlee.com. My Background. My PhD. thesis was on the modal response of small non-rectangular rooms Conclusions: The first mode was independent on room shape – depended only on volume
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Audio Acoustics in Small Rooms By Earl R. GeddesGedLee LLC www.gedlee.com
My Background • My PhD. thesis was on the modal response of small non-rectangular rooms • Conclusions: • The first mode was independent on room shape – depended only on volume • Concluded that room shape had little effect, except in cases of extreme symmetry • Distribution of absorption was far more important in the more symmetrical shapes – shape did help to distribute the damping evenly among the modes. • Only damping can help the smoothness of the LF response
Conclusions Con’t • With damping comes loss of LF energy • Above Fs, damping has no modal effect • These conclusions have significant implications to LF in small rooms • substantial LF damping is required for good LF response smoothness • the damping must be well distributed • There will be substantial energy loss to make up for
The small room problem • Small rooms have two/three regions of importance that need to be attended to. • The LF modal region • Modes are discrete • Free waves are not permitted • Above Fs (Schroeder frequency) • Modes are irrelevant • Waves propagate freely – geometrically • Transition region is possible
The small room problem • There is no reason to believe that these three regions will have the same characteristics, problems or solutions • What works in one region may be completely wrong in another • Hence, these regions must be approached independently, finding those solutions that work for that region and then seeing if the two sets of solutions can be somehow melded together
The small room problem • The goal is to produce at the listeners position a perceived playback of a sound source that is optimized for imaging, timbre (coloration) and spaciousness • Basically the same things that a large room is designed for • Since “perception” is involved some psychoacoustics is relevant
Psychoacoustic Fundamentals • The perception of image location is dominated by frequencies in the range of 1kHz – 8 kHz in the Auditory System (AS), with a greater emphasis on the middle of this range (See Blauert) • If good imaging is desired then this range must be relatively free from frequency response aberrations, diffraction and wall reflections < about 10ms.
Psychoacoustic Fundamentals • Recent work (to be published) has shown that diffraction and very early reflections (< 1 ms.) are far more perceptively important than a spectral analysis of their effects would indicate • This is hypothesized to be due to the masking of the ear being far poorer in the time domain than the frequency domain
Psychoacoustic Fundamentals • If this hypothesis is true (the data indicates that it is) then it is also suspected that these diffraction effects would be level dependent • Hence, while the diffraction is linear in a mathematical sense, its perception may be highly nonlinear • To a listener, this diffraction might appear to be nonlinear distortion since it becomes more audible at higher levels than lower levels – yet it has a physically linear cause
Psychoacoustic Fundamentals • This hypothesis is completely consistent with another study (AES) that showed that perceptually nonlinear distortion in compression drivers is virtually nonexistent • Yet it seems to be common knowledge than horns sound worse at higher levels than lower levels • Horns add only very low orders of nonlinearity, but virtually all horns have diffraction effects in them
Implications • It then appears that it is not only critical that the room not have early reflections, but it is just as important that the sources not have any near field diffraction from the cabinets, any waveguide devices, or nearby structures • Source diffraction of any sort must be held as just as undesirable as early reflections
More Psychoacoustics • As stated before, at low frequencies, we need not be too concerned about reflections, and probably diffraction and we certainly need not be concerned about these problems down into the modal region - this region is dominated by the room and has hardly anything to do with the source
Why Not? • A simple solution would seem to be to just put sound absorbing material everywhere, or better yet just move outside! • A non-reflection room is usually not found to be perceptually adequate – that’s because it lacks a very important acoustic property known as spaciousness. • Spaciousness occupies a large part of the study and design of spaces for the performing arts – it is nearly as important in small room acoustics.
Spaciousness • To fully understand spaciousness we need to understand the concepts of direct and reverberant fields. • The direct field (not to be confused with the near field) is where the sound from the source dominates over the reverberant sound. • There is a 6 dB/octave falloff with distance • The reverberant field is when the reverb dominates • There is no level dependence on location
Into the Recording • The importance of spaciousness can be easily demonstrated • tonight if time permits • By moving closer and closer to speakers that are canted inward, the sound field becomes more and more dominated by the direct field – the direct to reverb ratio goes up. • Moving back beyond a certain point has no effect.
Into the Recording con’t • Moving forward creates a subjective effect that I call “in the recording” • Backward - “in the room” • The former gives the subjective impression of “being there” – you are moved into the recorded space • The later gives the impression that the musicians have been transported into the room with you • Some like the “in the recording” effect, but I find it unnatural - precise imaging beyond reality, no spaciousness, a kind of headphone effect
Spaciousness • Clearly spaciousness does not just happen, to have it or not have it requires some design considerations • Not paying attention to it will likely leave an audio reproduction with poor imaging and or a colored sound character along with a lack of spaciousness
Sources and Spaciousness • Clearly the first arrival sound should be nearly flat (a subtle HF roll-off is usually preferred) but definitely smooth • What is not commonly attended to is that for spaciousness to occur there must be a substantial reverberant field component at the seating location • The reverberant field response in a reverberant room is dependent on the sources power response – not its anechoic response
Sources and Spaciousness • Very few loudspeakers have both a flat anechoic response and a flat power response. • That’s because it cannot be achieved with piston sources - a piston source does not have a flat power response when it has a flat anechoic response – it beams at HF • Pistons can be both, but only below ka=1, and then only as omni-directional sources
Return to room acoustics • Now lets consider the source placement in the room along with the sources directivity • All sources have negligible directivity at LF and most have a directivity that varies with frequency throughout its operating range • This means that the anechoic response and the power response cannot match • Consider a omni-directional speaker and one with 90° coverage
Sources in rooms • The omni source will have a multitude of early reflections while the directive source, if properly aimed, will have only a single reflection (horizontal plane), which arrives at the ear opposite to the direct sound from this source. • The opposite ear effect is notable because it is far less objectionable than a reflection to the same ear.
Specification of Source • The sources should have the following characteristics: • They should be directive at < 90° • They need to have off-axis responses that are flat as well as on-axis • They need to have a flat power response for low coloration in a lively room (required for good spaciousness – to be discussed) • This is because of a time-intensity tradeoff in the AS – longer signal times yield louder perception
Specification of Source • These criteria need not be carried to the very lowest frequencies – i.e. below about 500 Hz – since imaging is not influenced and coloration effects are low • Some increase in the LF power response would help to offset the well damped LF sound field. • The loudspeaker therefore can, and should, widen in directivity below about 500 Hz with no problems – this is a “no brainer” • 1000 Hz. is a more workable starting point for this transition and will probably not affect the imaging if the widening is down slowly. • Above 1 kHz, especially 2-6 kHz, high directivity is crucial, but it must be smooth and nearly flat at all points within its coverage field
The Summa Loudspeaker • The GedleeSumma was specifically designed to meet these small room requirements • It uses a waveguide for narrow directivity with constant coverage, but also contains an internal foam plug (patent pending) to help to control internal reflections and diffraction and higher order modes
Summa Con’t • The cabinet edges and the waveguide termination are all substantially rounded for an absolute minimum of cabinet and waveguide diffraction • The freq. resp. is equalized (near) flat at an off-axial location of 22.5° and is uniform and smooth at virtually all points contained within its coverage.
Summa Con’t • The polar response transition to a piston source is done precisely where the piston and waveguide match polar patterns. The waveguide is axi-symmetric to match the woofer. Polar response through crossover is flawless • The cabinet is molded composite with very high internal damping and is internally braced to be very rigid • It is extremely efficient: 97 dB/watt @ 1 m
Room Acoustics at LF • The room dominates the LF situation where the source has little effect • At LF there are two things that will improve the expected frequency and spatial response. • The first is to dampen the room as much as possible • And the second is to place multiple sources at various locations around the room.
LF Damping • Providing LF damping is a daunting task, because we know that we want as little HF damping as we can get to lower the direct/reverb ratio for better spaciousness. • Virtually all commercial sound absorbing materials do exactly the wrong thing – lots of HF absorption, negligible LF absorption • The only effective solution is that the absorption must be built into the structure
Construction • The construction techniques are not difficult and details can be found in my book “Premium Home Theater”.
LF smoothness • Once the maximum amount of LF absorption has been utilized (and hopefully the room is still live!) The only other thing that can be done is to use multiple subs. • Since the Summa’s use 15” Pro loudspeakers each one can handle lots of LF energy – that’s three • I use two more – one at the front of the room and one in a back corner.
LF smoothness • Studies have shown that the use of multiple subs can substantially smooth out the LF sound field both in frequency and in space. The improvement goes as about 1/N, where N is the number of independent sources. • Others studies claim that particular locations work best – I claim that random locations work just as well (if not better!).
Final analysis • Finally, the following slides are in-room measurements of frequency responses • Remember that these are in-situ measurements and not gated (except for a tapered 10 ms window)
