IDEAL LISTENING PARAMETERS AND CONDITIONS
It's a highly subjective question of what sounds good/bad to each person Sometimes you can find completely opposite opinions about the same device, sound, live concert and this uncertainty can be the driving force behind endless hopeless debates. But fortunately there are solid handholds that bring some objectivity to the subject.
What technical requirements does a playback system have to meet to be considered of high quality at all? In the world of HiFi and Home Theatre there are no standards that set technical requirements for sound quality and it is practically a free-for-all, using confusing and incomplete technical specifications as a weapon or even as a misleading tool.
Often referred to as the HiFi standard the much outdated German DIN 45500 was written in 1973. Even then it was not very strict, the requirements it sets are easily met by today's equipment and it is therefore not suitable for quality selection in itself. It was not really a technical document but rather an attempt to define High Fidelity as a new market category that was emerging at the time. Since then there has not been a single similar standard on the subject, but in the Pro Audio world a number of benchmarks have been recorded, if not as standards, more as recommendations. The European Broadcasting Union (EBU) is one of the main administrators and creators of these Their document Listeningconditionsfortheassessmentofsoundprogrammematerial dates from the 1990s, but is still relevant today. Although it deals primarily with listening conditions in sound studios, many of its guidelines obviously apply to HiFi playback systems, even if these requirements are not usually met in a home environment
Certain aspects of the aesthetic, artistic and technical quality of a listening experience can only be judged by the listener, and cannot (as yet) be measured by instruments and conventional measurement techniques But in order to make a confident, unbiased judgement about an audio experience, we need to ensure favourable conditions for both the listening room and the playback system, but the parameters discussed below also affect the subjective experience of listening to music or watching a film.
The quality of the listening conditions is characterised by the properties of the sound field created by the loudspeakers at the listening point at the height of the listener's ear (1.0 - 1.2 m above the floor). The main components are direct sound and early and late reflections, which are timing and frequency dependent. The direct sound is influenced only by the capabilities of the playback system and in particular the loudspeakers, and is not affected by the acoustic characteristics of the room, the reflections and echoes generated. By reflections we mean sound waves bouncing off the boundary surfaces of the room and objects placed in the room, which reach the listener with a time delay of varying degrees compared to the direct sound Early reflections occur within 15 ms after the first sound, followed by late reflections.
It is recommended that the total sound pressure level of the acoustic reflections should be at least 10 dB below the level of the direct sound in the frequency range 1 kHz to 8 kHz.
This parameter has a fundamental impact on the resolution, dynamics and detail achievable in interception The comb-filtering effect of early reflections from surfaces and equipment can be observed in the form of repeated highlights and cancellations in the response curve, which also make the frequency response ideally described by a straight line jagged. In recording studios, the surface of the mixing desk in front of the listener is a
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potential source of early reflections, and in living rooms, surfaces close to the speakers and the listener cause the same effect Reflections should be sufficiently large and evenly distributed over time and frequency. The Reverberation Time (RT60) is an important characteristic of the sound field; it defines the time interval during which the sound pressure level in the room decreases by 60 dB after the sound source has been silenced. It is usually measured during the initial period of decay and in the sound pressure ranges of 5 - 25 or 5 - 35 dB (T20, T30), from which the RT60 value is calculated, as the latter is usually difficult to measure on its own. The reflection time is measured with 1/3 or 1/1 octave filtering using the loudspeakers used for listening to music as the sound source. The reverberation time is frequency dependent and cannot be described by a single number, but has a nominal value (Tm), which is a mathematical average of the RT values measured in the 1/3 octave bands between 200 Hz and 4 kHz, which is particularly critical for music listening
The listening time in listening rooms in recording studios and in small listening rooms (less than 100 m3) should be between 0.2 and 0.4 seconds, or increase in line with the size of the room. RT values measured in the frequency range 63Hz to 8kHz in 1/3 octave bands shall fall within the tolerances shown in the figure below Abrupt variations of the afterglow time with frequency shall be avoided, the difference between RT values of adjacent octaves shall not be greater than 10 %. If this is not met, the timbre and intrinsic sounding proportions of the sounding material will be grossly altered.
FIG 1 - Tolerance of reverberation time
Another important parameter in the interaction between loudspeakers and the room and in the assessment of listening conditions is the frequency-dependent sound pressure level, or frequency response curve, produced at the listening point, the accepted tolerances of which are shown in the figure below. Lm is the mean value of the sound pressure levels of the 1/3 octave bands between 200 Hz and 4 kHz. The tolerances shall be observed for each channel separately. For stereo and multi-channel sound reproduction it is important that the individual response of each channel is closely matched
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FIG 2 - Tolerance in frequency response
This tolerance is sometimes very difficult to maintain at low frequencies especially in small rooms. Professional acoustical management and careful selection of loudspeakers and listening positions can usually achieve this, but sometimes electronic correction (equalisation) may be necessary if the response curve cannot be adequately linearised by other means. This correction can be achieved by adjusting the transfer parameters of the loudspeakers, amplifiers or other equipment (EQ, DSP). In order to avoid deterioration of the reproduction quality, the electronic correction should be carefully and moderately parameterised, and should be applied only in the low frequency range (below 300 Hz), with both channels being equally tuned in stereo.
The correct adjustment of the listening volume or the sensitivity of the playback system is closely related to the dynamic range of the system Adjustment should be made separately for each channel, with a signal level noise floor test signal of -9 dB (ITU-R recommendation) for analogue systems and -18 dBFS (full scale digital level) (EBU R68 technical recommendation) for digital systems. The gain of the channels shall be set so that the RMS sound pressure level (weighted A, averaged slowly) at the listening point is: LLISTref = 85 - 10 log(n) dB(A), where n = number of channels (speakers). This is 82 dB(A) SPL for a 2-channel system and 78 dB(A) SPL for a 5channel system. The difference between the individual sound pressure levels of the channels and their sensitivity should not exceed 1 dB, this is particularly important for stereo systems to ensure a good spatial response. Increasing the gain by 10 dB could in principle achieve sound pressure levels above 100 dB(A), but this is generally an unachievable requirement in practice due to the acoustic conditions in residential spaces and the limitations of Hi-Fi systems.
In listening rooms of sound studios, the background noise requirement is that the continuous acoustic background noise level (RMS, averaged slowly) from equipment operating inside the building or from external sources, measured at the listening point at 1,2 m from the floor, should preferably not exceed NR10 and should under no circumstances exceed the recommended NR15, especially for pulsating, cyclical noise. Unfortunately, these values are often exceeded in rooms in urban environments
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Fig 3 - Acceptable background noise limits (1/1 octave band sound pressure levels)
The positioning of sound sources and listening points is also critical. The height of the acoustic centre of the sound source should be 1.0 m and the distance from the surrounding surfaces should be at least 1.0 m. Their angles of inclination should be adjusted so that their acoustic radiation axis is coincident with the listener's ear (some special loudspeakers may be exceptions to this rule). No object should obstruct the sound propagation between the loudspeakers and the listening point, but this is more important above 150 Hz and less strict in the operating range of subwoofers
The delay time difference between channels of a stereo system shall not exceed 100 µs. The following figure illustrates the placement rules for stereo systems resulting from the above statement
Fig 4 - Recommended layout of the stereo audio system
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The loudspeakers must be positioned symmetrically in relation to the centre axis and at the same distance from the listener. The optimum angle of view of the loudspeakers from the reference listening position in the centre of the listening area is 60°, hence the listening distance h ≈ 0,9 b. The distance between the speakers should be between 2 and 4 m. The radius of the listening area (rL) is typically 0.6 - 0.8 m, which covers the range of sound technology work and physical activity during listening to music.
For a proper stereo field effect, the listening position should be placed on a central axis perpendicular to the straight line connecting the speakers. Subwoofers may also be used to reproduce the sound of the lowest part of the audible frequency range, the lower octaves. The main advantage of this is that the subwoofer placement does not have to comply with the strict rules for stereo speakers mentioned above, and the size of the cabinet and speakers of the front speakers can be reduced, and the speakers free of the lower octave radiation will also have lower distortion. The optimum crossover frequency between sub- and front-range speakers depends on a number of factors, including the placement of the speakers, the acoustics of the room and the characteristics of the expected frequency response. In order to prevent the source location of sub-bass sounds from being determined by hearing, a lower crossover frequency is recommended for subwoofers placed further away from the front speakers
The quality of the listening environment is determined by the properties of the sound field generated in the listening area at the height of the listener's ear. This is fundamentally influenced by the size of the room. The minimum floor area required for reference level conditions is 40 m2 , but adequate quality characteristics can be achieved in the 30-40 m2 range. In order to achieve an even distribution of low frequency problems (modes, standing waves), the room proportions should be kept within certain limits.
The following limits for length/height and width/height ratios should be respected:
1,1xW/H ≤ L/H ≤ 4,5xW/H - 4 L < 3xH W < 3xH where L = length, W = width, H = height
Room dimensions and their integer multiples should be within 5% of each other.
The room shall be acoustically symmetrical about the listening axis. This requirement applies not only to the geometry but also to the reflectivity of surfaces, in particular those near the listening point and the sound radiators. The location of sound absorbing and sound reflecting surfaces on walls, ceilings and floors shall be chosen to avoid rattling echoes and disturbing primary reflections Objects and structures in the room should not resonate longer than and louder than the room's own reverberation time. The above limits for background noise shall also be taken into account in the inhibition of noise, airborne sound and body sounds entering the room.
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Types of loudspeakers that meet the technical requirements for loudspeakers are not necessarily suitable for reference quality listening, and subjective listening is necessary to select and judge them Measurements on loudspeakers are typically carried out in anechoic, acoustically highly attenuated rooms where the environment does not have a significant effect on the measurement results. The measurement distance is determined by the size and physical design of the loudspeaker, but ideally should be close to the typical listening distance of the application. An exception to this is the measurement of the absolute frequency response, which is typically performed at a reference distance of 1 m from the loudspeaker For electrical parameters a measurement accuracy of 0,2 dB shall be achieved, for acoustic parameters the measurement error shall be less than 1 dB over the full range. To achieve this, it is advisable to use gated measurement procedures.
The frequency response curve of the loudspeaker shall be within a tolerance of 4 dB in 1/3 octave bands over the frequency range 40 Hz to 16 kHz. The frequency response measured at an angle of 10° and 30° horizontal to the axis of radiation shall not deviate from the frequency response measured on the axis by more than 4 dB. The radiation maximum shall be in the direction of the principal axis at all frequencies. The dispersion of the sound source, direction dependent radiation, shall be symmetrical with respect to the principal axis. In the case of asymmetrical loudspeakers, mirror symmetry shall be applied to the cabinet design and speaker placement for stereo pairs.
The harmonic distortion is measured using fixed voltage sinusoidal signals at 90 dB SPL average sound pressure level over the entire audible frequency range. None of the harmonic components shall exceed -30 dB (3%) between 40 and 250 Hz or -40 dB (1%) between 250 Hz and 16 kHz. The specifications for intermodulation distortion are not yet uniform. Decay time (ts) is the time of reduction of the sound pressure generated at the time of sounding to a specified fraction, measured by burst signals of variable frequency
The decay time of a loudspeaker, expressed in seconds, shall not exceed the following limit: ts ≤ 2,5/f where f is the frequency of the sound under test. For example, for 100 Hz the maximum permissible decay time is 25 msec The maximum sound pressure level (Leff-max), related to the dynamic range, that the loudspeaker can produce for at least 10 minutes without thermal or mechanical damage or activation of the overload protection. The minimum expected value, measured at a distance of 1 m with a RMS (slow mode) sound pressure meter, is 108 dB SPL.
The inherent noise level (Lnoise) of the playback system shall be measured on the axis of radiation of the sound source at a distance of 1 m from the sound source, A-weighted (RMS, slow), with a maximum permissible level of 10 dB(A) SPL at all frequencies.
If all these parameters and tolerances are met, there is a good chance of high quality interception, but these parameters are not guarantees of success, only conditions.
However you may need proper HiFi devices to exploit your excellent acoustic environment. May the Muzix be with you.
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Sources:
EBU Tech. 3276 – 2nd edition May 1998
[1] IEC Publication 225 (1966): Octave, half–octave and third–octave band filters intended for the analysis of sounds and vibrations
[2] ITU–R Recommendation BS.645: Test signals and metering to be used on international sound programme connections
[3] EBU Technical Recommendation R68: Alignment level in digital audio production equipment and in digital audio recorders
[4] EBU document Tech. 3282: Alignment signals for digital coding levels and listening conditions – Handbook for the EBU R–DAT alignment tape
[5] IEC Publication 651 (1979, 1993): Sound level meters
[6] IEC Publication 268: Sound system equipment. Part 1 (1985): General; Part 5 (1989): Loudspeakers.
[7] EBU Technical Recommendation R37: The relative timing of the sound and vision components of a television signal
[8] ITU–R Recommendation BS.708: Determination of the electro–acoustical properties of studio monitor headphones
[9] IEC Publication 581: High fidelity audio equipment and systems; minimum performance requirements. Part 10 (1986): Headphones
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