Hi Michael,
Flutter echo will depend on the finish of the walls, floor, ceiling. Even in a room of +100 m² you can find flutter echo. Flutter echo is effectively mostly audible in higher frequencies.
Flutter echoes are generated by impulses of very short duration. A periodic sequence of single echoes is generated by reflection of the impulse at opposite parallel walls. These echoes return to the observer after a time which depends on the distance of both the source and the observer to the walls. The observer perceives a series of sound pulses having frequency and duration of the original impulse. The time between these pulses, or pulse frequency f, depends on distance between source and observer, and is determined by
f = c/l
with
l = distance between walls
c = speed of sound
The smaller that distance the higher the pitch or frequency of the resulting sound: for 3.4 m that would be 100 Hz. For very small distances the flutter echo has a metallic sound, above 2 m a rattling sound, above 16 m the single echoes are audible as such. There is hence no such thing as “flutter echo in the higher frequencies” since the pitch of the echo is determined exclusively by the room dimensions.
Maa, “The flutter echoes”, J. of Acoustical Society of America 1941, vol. 13, p.170
I have doubts that normal music program material contains impulses of durations short enough to trigger flutter echoes in domestic rooms.
The science of psycho-acoustics has determined that 0.4 seconds in a normal room for listening to music is experienced as most neutral. Personal tastes may and will vary.
There is not much research on reverb time, here is I what I could find, if you know of other sources, please let me know.
The BBC made some experiments in the late 70ies to see how reverb times in their control rooms should be. These tests were done with speech and it was found that reverb time should be not greater than 0.4 s.
Gilford, “The acoustic design of talk studios and listening rooms”, J. of the Audio Engineering Society 1979, p.17
Dutton has made experiments to see whether or not the reverb time has an effect on phantom source localization of speech recordings. Tests were done in 8 different rooms with reverb times between 0.1 and 1.4 s. The effect was judged to be insignificant.
Localization in the anechoic chamber was no better than in a normal room with reverb time of 0.3 (10 kHz) and 0.7 (100 Hz). The room with 1.4 was judged to be very disturbing.
Dutton, “The assessment of two-channel stereophonic reproduction performance in studio monitor rooms, living rooms and small theatres”, J. of Audio Engineering Society 1962, p.98
Leonard made experiments to see if the reverb time of a studio control room had an effect on the level of reverberation that was to be added to the mix. Reverb time of the room was increased from 0.2 to 0.4 s by adding reflective panels. For the condition of higher reverb time less reverberation was added to the mix.
Leonard et al., “The effect of acoustic environment on reverberation level preference”, Audio Engineering Society preprint 8742 (2012)
Weisser made experiments in 8 different rooms with reverb times between 0.24 and 1.6 s, using speech and music. With speech the lower reverb times around 0.3 s were preferred, with music the range 0.6 s – 1s.
Weisser et al., “Evaluation of sound quality, boominess and boxiness in small rooms”, J. of the Audio Engineering Society 2006, p.495
I had another customer who had a medium-sized room with huge windows (single layer glazing). The reverb time in the lowest bass was 9 seconds!! (not a typo) and I can't recall the rest of the FR but it was not 9 seconds (obviously). So yes, FR is relevant in reverb measurement. (even in a 0.4 sec room the bass is always going to be going till double or triple that value)
Higher reverb times in the bass are no problem, even the recommendations such as EBU 3276 allow values up to 0.7 s at 63 Hz. 9 seconds measured in the lowest bass more likely is mode decay time rather than true reverb time. I don’t know of any research re: FR of reverb times in the range of 0.3 – 1 s, so any statement in this regard is pure speculation.
I don't think I wrote that an absorber absorbs only reinforced frequencies. I did write that it lowers the energy level of the reflected wave (I recall having spoken about the fact you cannot achieve 100% on this). At all frequencies is only correct insofar that an absorber (depending on placement, material, thickness etc.) will have maximum effect to a certain frequency only and a rapidly decreasing effect on the surrounding frequency and no effect on the rest of the spectrum.
Maybe it’s me but the way you write "... at all frequencies" let me understand that an absorber will always have effect on all frequencies and I am very much disagreeing on that.
Porous absorbers have maximum effect when the velocity of the air molecules is greatest, which is the case when pressure is zero, hence at a null of the standing wave, which is at a distance of a quarter wavelength from the wall. All frequencies for which the quarter wavelength of their respective standing wave is within the thickness of the absorber are subjected to maximum possible absorption. All other frequencies are absorbed to a smaller degree, but they are still absorbed. Porous absorbers hence are not frequency specific as you seem to imply. For perpendicular incident an absorber of 1 m thickness will have maximum effect on 85 Hz and all higher frequencies, lesser effect on frequencies below 85 Hz, an absorber of 2.85 m will have maximum effect on all frequencies above 30 Hz.
A corner trap will have an effect on standing waves, but no effect on direct sound and first reflections.
As you write yourself, absorbers do lower the amplitude, so you loose sound pressure, so you will try to recover that.
This might be true but where’s the problem, you just crank up the volume a bit. However, this is not what you initially said. What you said is
You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy. By putting the volume harder, you will be faced with a load of other problems in the mids and highs which, to solve them as well, you will be adding in even more absorbers.
This seems to imply than only bass is attenuated and mids/highs remain untouched, so that turning the volume knob higher leads to problems with mids/highs.
Porous absorbers are not absorbing selectively, so they won’t attenuate bass alone. If anything, they will attenuate bass and everything above, and the latter even stronger than the bass frequencies, so what you would get get is over-attenuation of mids and highs, because absorption coefficients are much higher for mids and highs than for the lows.
With concepts like dead end, live end etc. which were developed when psycho-acoustics was not yet a science.
Psychoacoustics took off when Charles Lindbergh was welcomed back in New York after his famous flight across the Atlantic. By the time that the Davis brothers published their paper about LEDE, which was in 1980, psychoacoustics was a well established scientific discipline with thousands of publications: in 1950 scientists from the Bell labs designed the ABX test for psychoacoustic experiments, the paper describing the Haas-effect was published in 1972, perception thresholds of reflections were known for both speech and music, German physicists had made numerous experiments in synthetic sound fields.
Klaus
