The Schroeder Frequency

In a standard 3 way system the 80-250 Hz audio band is emitted by a single driver.

Free-standing designs come in a closely-set classic vertical Tweeter/ Midrange/Woofer array, quite high above the floor, even though lowfrequency electrical power is split among two or more woofers so as to increase dynamic headroom.

The most important aspect is the physical distance between the subwoofer and the woofer, both designated to emit acoustic power below the Schroeder Frequency, which is about 250-300 Hz for a standard living room (80m3). This frequency limits a listening environment to no more than three resonance modes for the first octaves (32-250 Hz), triggering a hearing phenomenon called <bass booming>, activated by an irregular distribution of energy accumulating around a few spectral components.Apart from the wellknown timbre alteration, another very detrimental effect exists in stereokinetic perspective reconstruction. It is commonly described as a mono effect at low frequencies or “collapsed volume”. Lack of spatiality, meaning decrease of air volume around the sources, is due mainly to the similarity of bounced energy to the direct energy coming along the line-of-sight from the speakers. In fact, when the minimum size of a closed space is greater than the maximum wavelength being radiated inside it, the schroeder frequency assumes a very low value and no stationary field is established. This means that in stereo the perception of air between and around the speakers is completely described by the particle vector velocity, which is crucial since the reverberated field is really stochastic, that is to say chaotic. In small, closed spaces, like those for home listening, the stationary field overlaps and from the psychoacoustic standpoint tends to erase low-frequency ambience information contained in the original recording. The shelf-mounted systems are fitted with a single radiator that emits the Schroeder Frequency, so the distance between a woofer and subwoofer will be d=0 m (Acoustic Doublet, Page 102). The first free-standing system is a standard three-way with two parallel woofers, for which about d=0.2 m distance can be assumed in most cases. The second free-standing system includes three woofers, which can have split frequency, so a d=0.2 m distance can be assumed. In all practical cases there is no significant gap between two adjacent frequency components within the critical band. It is easy to understand that for all possible frequency combinations there is only one sound ray reaching the listener’s ears, since all the components in frequency share the same origin in space. This is not a specific feature in itself: it is a condition if a coherent wave-front is required to carry correct information to the listener by a direct path.

Correlation and Coherence

To associate doublet-type emission to a significant measurement, the correlation concept should be brought into play for indicating the degree of similarity between the two audio

In this case only pressure values measurable near the listener’s head in a home setting are of interest. However, the two signals are sinusoidal and of the same frequency so

the correlation can lead to the occasional contradiction, since it is defined as the phase difference cosine. Each time the two components reach the receiver at 90°, the correlation coefficient will be null, in open contradiction with reality. It then becomes necessary to add more, unequivocal information, moving on to the coherence function. This implies that for the scope of home audio, crossovers with infinite slope do not allow for an efficient use of the overlapping area. The relative acoustic phase between the two speakers for low frequencies was decided on the basis of a minimum 2.5 m listening distance and a minimum 1.3 m wavelength for stabilizing the stereo-kinetic front and reducing (compatibly with overall quality) the direct field/bounced field correlation.

Let’s start getting the point.