"The European Broadcast Union recommends in technical paper 3276 that the level of early reflections should be at least 10dB below the level of the direct sound for all frequencies in the range of 1kHz to 8kHz, within a time window of 15ms after the arrival of the direct sound.
In a typical setup, the path traveled by the reflection may be about 2-3 times longer than the path of the direct sound. Distance alone would hence remove 3-6dB from the level of the reflection. The remaining part has to be removed by either absorptive materials or by controlling speaker directivity. A speaker producing an off-axis response which at 90 degrees and starting at about 500Hz is 6-9 dB below the on-axis level should represent an absorber-free solution, should it not ?
The question is, are the EBU figures correct in view of existing psycho-acoustical knowledge and experience and if yes, would a tight-directivity speaker indeed be an absorber-free solution as far as the early reflections are concerned?"
Klaus Rampelmann, December 18, 2003
"This is a trick question - well, not really. Rather, it is one that has some rather complex and interesting angles (excuse the pun) to it.
First, there are a few assumptions that have to be made -- or at least agreed upon -- in order to discuss this. What exactly is meant by the term tight directivity? By definition, do we imply that the speaker should objectively have so tight a mid-to-high dispersion as to totally preclude any and all reflections from the side-walls of the listening environment? Even if this were so, how would one account for sound reflected from the rear walls - behind the listener or from the ceiling? The (rear) reflections, for instance, would have an even longer travel path than the arrival times of any side wall reflections and might even, depending on the room dimensions, arrive outside of the human ear's natural ability to successfully integrate them with, and as part of, the original (direct) signal. This is referenced to the Haas Effect.
Second, the phrase "In a typical setup, the path traveled by the reflection may be about 2-3 times longer than the path of the direct sound" implies that there is one single and simple reflection of the source sound. In fact, there hardly ever is just a single reflection to any sound in an average room. The walls, ceiling, uncarpeted floor and other hard surfaces such as doors, windows and the glass in picture frames etc are all involved to some degree or other, and can all add to the acoustic dispersion of sound.
The real question posed here has more to do with our perception of sound -- relative to the environment that it is being listened to in -- rather than the actual dispersion qualities of a loudspeaker or horn. Our second assumption therefore has to be in connection with the statement that "distance alone would hence remove 3-6 dB from the level of the reflection". This is not always true. As a simple example, how many times have you stood in an open hallway, hall or empty room and experienced your voice -- in that hallway, hall or empty room -- coming back to you louder than it was coming out of your mouth, even at the volume of normal speech?
Sound travels differently in free field than it does in an enclosed area. This relates to the Inverse Square Law that states that the intensity of sound pressure in free field is inversely proportional to the square of the distance of that sound from its source. This means, in common terms, that there will be a 6dB loss per doubling of distance of a sound from its source. Sound in an enclosed environment conforms to a whole different set of acoustical rules and laws.
Perhaps one of the biggest misunderstandings in the audio/audiophile world is the relationship between the speakers and the room they are used in. Both the speakers and the room are, in actuality, a tuned circuit. They act with and upon each other. This being so, buying the most elaborate and expensive audiophile equipment and speakers by no means guarantees that you will have the best sound. Loud music in a small room can very easily overcome the natural acoustics of the room and produce horrendous results. Conversely, even soft music in a large, open room can sometimes be fraught with intelligibility problems due time variant reflections that can overlap and overcome the initial signal.
So, we come to the question of controlling sound in a room. How much absorption is right versus how much reflection. How much reflection is required to maintain the kinetic energy in the room. To me, a listening room should first and foremost sound natural and musical. That is, it should be easy to listen to music in and it should not be fatiguing to spend extended amounts of time listening to music. To understand this more easily, it might be constructive to substitute the words dispersion or diffraction for reflection. This is because, in reality, we want to excite the room with sound, not deaden it so that it becomes hard, brittle and unnatural . Acoustic dispersion means that there will be many reflections. The difference is that diffracted/dispersed sound will not all necessarily converge directly on the same focal point in the room. Rather, it will be caused to move in and around the room so as to excite the air without undue buildup or coloration. This is what's meant when I refer to kinetic energy. Also, it must be understood that the way the low frequencies react in an enclosed space (room) is also an integral and important part of this phenomena.
By way of explanation then:
Low frequencies move around a room in search of their boundaries. The physical boundaries of the room are the walls, ceiling and floor. The low frequency sounds are seeking their potential as natural wavelengths (for instance, 100Hz is approximately 11'4" in length) and when and if they find them, that particular frequency is reinforced and liable to become audibly louder. When the sound pressure of these lower frequencies reach the physical barriers represented by the mass of the walls, floor etc, they are turned back into the room and the air itself is moved and modulated. Apart from producing complex wave patterns referred to as Room Modes, this air movement also modulates the high frequency information traveling through the air in the room. In a well-designed room (or if you were just plain lucky) this kinetic energy can be perceived as musical excitement, adding apparent dimension, depth and energy to the music being listened to.
So, to answer the question "are the EBU figures correct in view of existing psycho acoustical knowledge and experience and if yes, would a high directivity speaker indeed be an absorber-free solution, as far as the early reflections are concerned" is a qualified yes and no. A high-directivity speaker might, indeed, preclude the need for side-wall absorption as far as early reflections are concerned. As to whether that would improve the listening sound field, that is another story and I, for one, doubt it. I see the use of the term psycho-acoustical knowledge and experience as a disclaimer. Experience tells me that a room, any room, by virtue of the nature of Physics, is involved in the reproduction of the sound, whether we like it or not. The exercise is to get the speakers and the room to work harmoniously together to this musical end. Experience has also shown me that with a little thought and work, in most rooms this can be accomplished."
Chris Huston, Senior Acoustical Engineer, Rives Audio.
|To have your question answered, click here to e-mail it to 6moons - we will forward it to Rives Audio on your behalf. Please remember to ask questions of a general nature as the professionals at Rives get paid to provide customized assistance. If the auto-launch doesn't properly open for you, e-mail to email@example.com, subject header RAM:EF. We'll publish the answers as soon as we receive them so check back often to learn more about this complex but vital subject. For our interview with Rives' president Richard Bird Rives which became the impetus for launching this feature, click here.