Influence Of Room Acoustics Research Articles

The "Missing 6 dB" Revisited: Influence of Room Acoustics and Binaural Parameters on the Loudness Mismatch Between Headphones and Loudspeakers.

Generations of researchers observed a mismatch between headphone and loudspeaker presentation: the sound pressure level at the eardrum generated by a headphone has to be about 6 dB higher compared to the level created by a loudspeaker that elicits the same loudness. While it has been shown that this effect vanishes if the same waveforms are generated at the eardrum in a blind comparison, the origin of the mismatch is still unclear. We present new data on the issue that systematically characterize this mismatch under variation of the stimulus frequency, presentation room, and binaural parameters of the headphone presentation. Subjects adjusted the playback level of a headphone presentation to equal loudness as loudspeaker presentation, and the levels at the eardrum were determined through appropriate transfer function measurements. Identical experiments were conducted at Oldenburg and Aachen with 40 normal-hearing subjects including 14 that passed through both sites. Our data verify a mismatch between loudspeaker and binaural headphone presentation, especially at low frequencies. This mismatch depends on the room acoustics, and on the interaural coherence in both presentation modes. It vanishes for high frequencies and broadband signals if individual differences in the sound transfer to the eardrums are accounted for. Moreover, small acoustic and non-acoustic differences in an anechoic reference environment (Oldenburg vs. Aachen) exert a large effect on the recorded loudness mismatch, whereas not such a large effect of the respective room is observed across moderately reverberant rooms at both sites. Hence, the non-conclusive findings from the literature appear to be related to the experienced disparity between headphone and loudspeaker presentation, where even small differences in (anechoic) room acoustics significantly change the response behavior of the subjects. Moreover, individual factors like loudness summation appear to be only loosely connected to the observed mismatch, i.e., no direct prediction is possible from individual binaural loudness summation to the observed mismatch. These findings – even though not completely explainable by the yet limited amount of parameter variations performed in this study – have consequences for the comparability of experiments using loudspeakers with conditions employing headphones or other ear-level hearing devices.

Acoustics and orchestral sound.

The tone quality of an orchestra rests on the cooperation of a multitude of differing instruments which are used partly in solo fashion and partly gathered in groups; it is furthermore shaped by the spatial extension of the ensemble and by the influence of room acoustics. Acoustic measurement procedures permit an analysis and description of tonal characteristics of individual instruments in regards to sound power output, spectral composition, time-dependent fine structure, and orientation dependence of sound radiation. Appreciation of the full orchestral sound, on the other hand, requires consideration of sound transit times within the orchestra with the associated problem of playing together, as well as variations in wall reflection times for different instrument positions. The effect of dynamics and masking, the chorus effect and the seating arrangement of the strings , and spectral and time-dependent tone construction will be demonstrated with the assistance of the Weber State University Symphony Orchestra, prepared by its regular conductor, Professor Michael Palumbo. Sound examples are taken largely from the overture to the opera ‘‘Der Freischūtz’’ by Carl Maria von Weber. In conclusion, the orchestra will perform the entire overture directed by Professor Meyer.

Intonation precision of choir singers

Results from two experimental investigations on the acoustics of choirs are reported. The first concerns sound-pressure levels (SPL) inside choirs, and the balance of loudness between a singer’s own voice and the ‘‘reference’’ sound from the rest of the choir. Long-term SPL averages during rehearsals typically gave 80–100 dB, with little variation between choirs or with position in the choir. The fundamental frequency precision of choir singers was found to deteriorate abruptly when the reference was too loud. The influence of room acoustics and of the spacing between singers is discussed. The second investigation concerns the effect of spectral variations in the reference sound. Experiments were made primarily with individual singers under laboratory conditions. Changes in vowel quality, and absence/presence of certain partials and of vibrato, were all found to affect somewhat the degree of fundamental frequency agreement between singers. Standard deviation in fundamental frequency between section colleagues in live rehearsal was found to be between 0.10 and 0.15 semitones, or 0.6% and 0.9%. Reference vowels with common partials reinforced by formants were found to increase fundamental frequency precision.

Possibilities and limits of sound power measurements with a real-time intensity analyzer

The sound power of a number of test objects was determined from spatially averaged intensity measurements. The results show that the influence of room acoustics is insignificant even for rooms of widely different room constants, if the measuring surfaces are exactly defined and if a good space-averaging technique is used. The intensity integrated over a closed surface defining a source-free space compared to the sound pressure integrated over the same surface gives a measure of the capability of a specific intensity measuring system to suppress external noise. For the test arrangements measured with broad band noise, this suppression was found to be 14–18 dB(A). A similar value of 15 dB was found from sound power measurements on a source with high external sound and an analysis of the results in one-third octave bands. From these measurements an analytical function was derived which describes the average error of the spatially averaged intensity as a function of the difference between the external sound level and the source sound level. For practical measurement situations a further analytical function was derived which gives this intensity error as a function of the difference between the measured (spatially averaged) pressure and intensity levels. Thus it is possible to estimate the error of intensity measurements directly from measured intensity and pressure data.

Flutter in Sound Records

Frequency modulation of a sound signal produces some very interesting and sometimes not readily identifiable auditory sensations. Distortion of this type is caused in sound records by the nonuniform speed of the record during the recording or reproducing process. While the physical nature of frequency modulation and some of the physiological effects produced by it are discussed, the paper consists largely of demonstrations including a demonstration of the side-band nature of flutter, the influence of room acoustics on the perception of flutter, the auditory effects of flutter in single frequency tones, and a demonstration of the auditory effects of frequency modulation in speech and music.