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LIVE ROOM - DEAD ROOM - DRY ROOM
Which is the best room to listen to music?
First
Part

Jorge Knirsch

Introduction 

The concept holds, now deeply rooted and consolidated in the market of audio and video and also among us, lovers of music and image, and among acoustic engineers too, that "a good room for reproducing music and shows has to be necessarily live, or if it’s not for this purpose, at least it must be a semi-live room". There are controversies about the issue of how live a room must be, but all agree that it must be live or semi-live. And putting such premise to work, we observe an interaction between the specific room under consideration and that which is being reproduced. In other words, by setting up our audio system in a live room, we create a new audible and particular solution, which shows itself to be superior to the sound results coming from one system alone.
          If you pay close attention nowadays, the great majority of environments tend to be live, and public places mainly are like this, such as airports, bus stations and many others, in spite of very few exceptions, such as some movie and play theaters. And we’re not aware there is a large influence from such environments in our well-being, a much neglected aspect among us.
          In this article we intend first of all to study sound waves in a broad way, presenting some physical concepts which normally aren’t mentioned but are very important to acoustics. Then, we’re going to study and define one of the most important parameters in acoustics, Reverberation Time (RT), showing its influence on listening, in order to suit it to the several existing rooms. We’ll define the various room types in this occasion, the live ones, the dead ones (also called deaf rooms by recording staffs), and dry rooms, so as to decide then which of them is the best option to us. We’ll also examine what comes to be listening comfort, how to attain it, and what its consequences are for our emotional.

© 2004-2008 Jorge Bruno Fritz Knirsch
Todos os direitos reservados
http://www.byknirsch.com.br

Sound Waves

  From the start, I did not think I would be writing a whole article on sound waves, but there wasn’t time enough. Happenings have stepped over actions, in a way that now I intend to be quite brief regarding this subject, in spite of the fact that the subject is of vital concern, and enter reverb time analysis right away. I do this so that you can follow me through regarding the presentation of physical phenomena and of the technical aspects we are going to deal with next.
          There are various types of waves in physics. Mechanical waves among them are produced by disturbances, that is, particles vibrating in a come-and-go, in a given medium. Through such vibrations mechanical waves are transmitted. Mechanical waves can be felt and seen. For example, we see them in springs, in the strings of an acoustic guitar, in water. They do not exist in a vacuum, for there are no particles there. They are progressive, that is, they move away from the source (point of their emission), carrying energy without transferring matter. They have several characteristics, such as frequency, wavelength, speed, amplitude and phase. We’re going to see some of these characteristics ahead, in more detail. They may also undergo some important physical processes such as reflection, refraction, diffraction, interference or superimposition, and polarization. Some of these physical processes we’re going to study now.
          There are two kinds of mechanical waves: transversal and longitudinal waves. In transversal mechanical waves, the disturbances, that is, come-and-go vibrations, take place orthogonally with respect to energy transfer. In other words, vibrations are orthogonal to their transmission or propagation movements. For example, we see this in waves that propagate in strings tied by one end. The waves we see in the water surface are another example.
          In longitudinal mechanic waves, the disturbances, that is, particles’ come-and-go vibrations, take place in the direction of movement. One example of such wave type is found in springs, where waves propagate longitudinally, in the direction of energy transmission. Sound waves are also longitudinal mechanic waves. They can be generated in a loudspeaker or in any musical instrument for example.
          A sound wave represents a mechanical wave front, with air pressure
variation, though it propagates in all directions after its emission. Contrary to what I saw written in many Websites, sound waves are longitudinal, but their propagation is spherical. This is the first very important physical concept: sound waves are longitudinal mechanical waves with spherical propagation, diffusing themselves in all directions after their point of emission. This aspect, not very much known and also little mentioned, is represented in the sideway figure, in just one plane, but try to visualize it now in spherical form - imagine a loudspeaker placed in space, surrounded by air all round it, and having no reflective surface near it. Imagine also a listener being positioned some feet in front of such speaker, and another person some feet behind it, both people on an imaginary line drawn orthogonal to the loudspeaker. Let us now suppose that the speaker starts to emit a sound wave of, say, 20Hz, which is spherical, at a volume suitable for our ears. Which of the two persons do you think is going to listen to it? Just the one in front, or both? Think a little, the question is really interesting, for it makes us become conscious of something which we intuitively know already! Of course nobody will doubt the person in front of the loudspeaker will listen to the 20Hz sound, but most important to us is that the person behind it will listen to it the same sound wave, at less intensity, it’s true, but he or she will listen to it nonetheless! Such physical process is called diffraction.
          Another physical concept which is very important, and, for sure, something new for many of us is: A sound wave of low frequency (bass) has the physical capability to rebuild itself, in other words, it diffracts, and the wave is rebuilt behind the objects and surfaces that cross its way. But, as the sonorous frequency increase, this phenomenon decreases. As stated, this second concept is specially true for low frequency waves and it becomes a great challenge in the treatment of acoustic rooms and studios. Therefore, it is not any surface put in the acoustic room that will solve the problem of the bass.
          The magnitude of these two concepts is huge and of great relevance in the treatment of acoustic rooms. In order to explain this a little bit better, lets analyze the basic formula of a sound wave:

 

In order to explain it, sound speed is constant in media, it’s the wavelength multiplied by frequency. Sound speed varies with room temperature. Let us adopt 344m/s for temperature values around 20 degrees centigrade. Regardless of frequency, sound propagates in the air at the same speed, changing wavelength according to frequency.
          In order to explain these terms more effectively, let us see the figure below. Every sound wave can be represented by a sinusoidal wave, being of course emitted in all directions from the point of emission, something that is not shown here. Sinusoidal waves have both an upper and a lower part. We can consider the upper part as being the area of high air pressure in the room, and the lower part as being the low pressure one. On the upper wave crest we have maximum pressure, and on the lower wave crest we have air rarefaction (low pressure). In the horizontal axis we have the room’s pressure under standby conditions, which stays around the pressure of one atmosphere. Let us comment a little on the physical aspects of these statements, in a practical way. Let us take our 20Hz wave for example, which generally is the lowest frequency we can listen to and feel, and let us study it more closely.

 


Fig. – Upper-wave wavelengths are three times greater than those of lower waves, but their frequency is just one third of lower waves’ frequencies. Both have the same amplitude.

The wavelength of a 20Hz wave is 17.2m. Pay attention, I’m going to repeat it – the wavelength of a 20Hz wave is something around seventeen meters. It’s something quite large! I wish to comment that the distance between one high-pressure crest and the next is 17m. It’s an enormous distance! And the distance between a high-pressure and a low-pressure crest is half of this, that is, 8.5m. Many people ask me if they’re going to hear a 20Hz wave in a room whose dimensions (height, width and length) are less than such a wavelength. The answer is undoubtedly yes. We’re going to listen to 20Hz in a small room. The loudspeaker, when it produces such wave, does not know about room size, it just emits it. Now, when this wave meets the walls, it keeps on forming itself, going back from the wall toward the direction of the source, coming and going and reflecting itself in all directions, since the room is smaller than this wavelength. And the meeting between the waves that go and come back also starts to happen, creating sums and subtractions through the generation of interferences; the meeting of two crests creates doubled pressure, the meeting between a crest and a dip creates a region of normal pressure, and the meeting between two dips creates a region of twofold-lowered pressure, and all the others exist between these three gradations. As you’re now beginning to realize, bass notes are a really serious problem in small rooms, let’s say below 100 cubic meters! The subject becomes more complex still if this wave is part of the room’s resonance, but this is a problem we’re going to study in the chapter dealing with standing waves, when we get there.
          Now let’s imagine a 200Hz wave. Such a wave has a wavelength of 1.72m, that is, it has a wavelength ten times smaller than the 20Hz wave. You’ll agree with me here that the difference is very large. If we now take a 2000Hz sound wave its wavelength will be 17.2cm only. The reduction, with respect to the 20Hz sound wave's wavelength of 17.2, is one hundredfold. It’s an enormous reduction! There’s no doubt the way to carry out acoustic treatments for the several wavelengths and magnitudes so different must be quite differentiated.
          As we had said, sound waves are spherical in low frequencies, but gain directivity as frequency rises. This is the third, very important physical concept – the 360-degree spherical angle tends to diminish to narrower and narrower conical angles as frequency rises, going from obtuse to more acute angles, and becoming a directional angle in high frequencies above the human listening range. Then, at 20,000Hz, our hearing limit, waves begin to be directional, but contrary to what people say, they still propagate in acute-angle sound cone form. This is very important, for above medium frequencies and well above, in the highs, we can very well deal with sound waves according to the laws of optics, regarding reflection, diffusion, refraction and diffraction, what makes the approach simpler. This is not totally correct, but allows us a good approximation to the real physical phenomenon.
          Just to recall what bass, mids and highs are, we show the graph below:

Bass, mids and highs in the perception of music.

Bass sounds start in 20Hz and go up to about 160Hz, where mids start, going up to about 1300Hz, and there start the highs. Such values are flexible. Many adopt the limit frequency between bass and mid sounds as being 200Hz, and between mids and highs as being 2000Hz.

 Summary

In this second article on acoustics, we assess some little known aspects of sound waves, but of great relevance however for the technical solution of acoustical treatments, mainly in small rooms (smaller than 100 cu. meters) destined to either normal or critical auditioning. As a complement to what several websites and educational books say, sound waves are longitudinal, but their diffusion (propagation) is spherical, with steady increase in diameter and reduction of intensity (by the distance squared) after emission. We repeat below the three basic concepts of sound waves, so that these are not forgotten, for we’re going to use them countless times later on.

-Sound waves are longitudinal mechanical waves, which propagate spherically, diffusing themselves in all directions from their point of emission.

-Low frequency sound waves have the physical ability to reshape themselves, that is, to diffract, getting reconstituted after objects or barriers. And as frequency rises, this phenomenon gets lessened.

-As sound frequencies increase, the spherical angle of its propagation in 360 degrees diminishes to an ever increasing conical angle, going from an obtuse angle to a more acute angle, becoming a directional wave in high frequencies, well above human hearing.

These three basic concepts about sound waves are fundamental to explain later on why some techniques work and some others don’t. Let’s put down some myths which exist regarding acoustic treatments, and show a safer and more correct way to do it, bringing more information to our field.  

I wish that properly balanced room to all, so that you can listen what was recorded really!!! Excellent auditioning and warm regards!!!!!

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