[boggy]

Pozdrav svima,

Prilazem zanimljiv odlomak iz Multimedia Manufacturer, u kome je komentar na rad Floyd Toole-a objavljen na 119 AES konvenciji, mart 2006. godine, a koji se tice akustike MALIH soba.

MALA soba je u praksi najcesci slucaj audio rezije... samo da podsetim

Akustika prostorija, kakva se uci u skolama, se generalno bavi VELIKIM prostorima...

Nevolja je sto se te zakonitosti vrlo cesto primenjuju NEKRITICKI na male sobe... i to skoro uvek daje razocaravajuce i neprihvatljive rezultate... ovaj komentar je neki pokusaj da se skrene paznja na dosadasnja proucavanja ovog konkretog problema... koji se sastoji u tome da su MALE sobe, pogotovo audio rezije, veoma specifican slucaj u akustici prostorija...

Eto.... koga ova tematika zanima... i koga ne mrzi... moze procitati, pa ako jos ima neki smisleni komentar... super!

Napomena:
Razmisljanja i zakljucci Floyd Toole-a, su nam, solidnim delom, bili ideja vodilja u dizajnu akustike u Hammer Production u Novom Sadu.

For example, Floyd Toole gave this much more technical and extended version of his CEDIA talk. From the convention program tutorial synopsis:

“The physical measures by which acousticians evaluate the performance of rooms have evolved in performance spaces. They rely on a set of assumptions that become progressively less valid as spaces get smaller and more acoustically absorptive. In live performances, sound sources radiate in all directions, and the room is a part of the performance. In sound reproductions, loudspeakers tend to exhibit significant directivity, and what we hear should ideally be independent of the listening room.

What, then, should we measure in small rooms?

To what extent can we eliminate the room?

Or do we need to?

Is there a point beyond which the human hearing system is able to adapt to the listening space—hearing through the room and around the reflections to accurately perceive the source?

A certain amount of the right kind of reflected sound appears to enhance the music listening experience and, interestingly enough, to improve speech intelligibility.”

Toole has concluded that normal reflections in a typical small living room seem not to interfere with perception of the recorded space. He has also determined that early lateral reflections (<50ms) have a beneficial effect on intelligibility similar to raising the dialog level, and that the reflection pattern is more important than reverberation.

This has led to Toole’s recommendation that too many or too few reflections can be a problem. In particular, acoustic absorption, diffusion, and reflection must be broadband, ideally starting below 200Hz. He pointed out that the typical 1" or 2" sound panel most often affixed to walls works only at relatively high frequencies, and acts to effectively turn down the tweeter with no effect on the midrange or upper bass, thus unbalancing the sound.

Toole reminded us that many of the practices to deaden a room’s acoustics came from standards for broadcast and recording control rooms, where sound details must be heard more clearly, not for general listening rooms.

Harman testing has shown that sonic anomalies which are audible while the listener is moving around become much less obvious once the listening position is essentially fixed (the listener sat down!). Toole spoke of comb filtering, and has deduced that it is an artifact of measurement, not an audible problem in normal listening. We are less sensitive to room resonances in music than in broadband noise—our hearing is sensitive to the frequency response effects, not the time domain ringing, particularly in a reflective space, due to our “multiple looks” at the signal (except in the bass range, where both are factors). Audibility thresholds decrease with repetition and duration in reflective rooms.

Humans seem to be able to adapt to a room’s acoustics, up to a point. Thus, we should be able to predict real-room performance from a proper set of anechoic measurements, except for bass issues below about 300Hz. In Harman’s listening tests, low-frequency performance accounts for about 30% of a speaker’s rating.

Harman testing has shown that speakers with a fairly flat on-axis response and with a smooth and gentle off-axis level fade and rolloff are preferred by expert and amateur listeners. A speaker with an off-axis response that differs substantially from its on-axis response cannot be corrected with an equalizer. In-room measurements lower the accuracy of the data since they include a mix of direct and reflected sound. Since with most speakers the reflected sound has a different frequency response than the direct sound, an equalizer will cause more problems than it will cure.

He also noted that spatial-averaging of speaker measurements is critical-single-point measurements are erroneous and meaningless. Also, many types of measurements are required to characterize the sound, and that 20th-octave data is necessary to unmask narrow-bandwidth frequency response problems that would be averaged out and masked in third-octave measurements.

Regarding bass, the Schroeder frequency is that threshold above which room resonances are considered so close together that they can be ignored with respect to frequency response anomalies.

Toole has found that this works for large rooms, but the formula for its calculation yields too low a frequency for small rooms. In fact, Toole believes that calculation of room resonances is irrelevant in real home listening rooms, because not all modes have an equal effect or are heard equally, since there are multiple sources (speakers), and because all the formulas are based on a source in a corner with the measurement taken in the opposite corner (where there is never a listener). This has led to Harman’s recommendation to use four subwoofers in the center of each wall to provide the most uniform bass over the largest listening area in the widest range of small-room sizes; it reduces, and sometimes eliminates, the need for bass EQ.

An informal discussion with Fred Ampel revealed his finding that for a fused sound field and precise localization, matching the sound arrival time at the listening position from multiple speakers to within a few microseconds is critical, in spite of common wisdom. He also believes, from his experience and tests, that especially at low frequencies, the acoustic distance from a speaker to a listener can be up to 20% different than the measured distance, particularly in a small room. Naturally this reduces serious listening to a single location.

pozdrav

bogi