Resonant Frequencies

Wikipedia defines resonance as ” the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system’s resonant frequencies or resonance frequencies. At these frequencies, even small, periodic driving forces can produce large amplitude oscillation, because the system stores vibrational energy.” It goes further when it talks about resonance in specific. It states,” Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes (such as kinetic energy and potential energy in the case of a pendulum). However, there are some losses from cycle to cycle, called damping. When damping is small, the resonant frequency is approximately equal to the natural frequency of the system, which is a frequency of unforced vibrations. Some systems have multiple, distinct resonant systems.”

Our listening, home theater, and mixing room are all systems that have a resonant frequency. Our room due mainly to its dimensions and parallel surfaces is definitely a system that has resonances. Can our room store sound energy and easily transfer that energy between certain storage modes. A room can “load” with excess low frequency energy and then store for brief time intervals and then transfer that energy into the next room passing through the existing room physical structures. There is some type of an energy transfer cycle that the room exhibits and the frequency of resonance is room dimension dependent. Our room can also have multiple resonance systems within itself.

How do we damp our system or room to minimize these resonances. We want to cause energy losses within our room’s energy transfer cycle. We must use the physical structure of the room. Can we make it larger , so we can spread out the multiple resonance systems within our room and provided more time and distance between the peaks and valleys in our room’s energy transfer cycle. This is usually not an option. We now must go into the room and work within the physical shell of the room. We must use damping from the inside out.

Our damping methodology must focus on the energy created from within the room. Can we reduce sound pressure at major room boundary reflections by “venting” or providing pressure release valves such as physical holes at surface pressure points? If we do that, we must prevent that energy from causing other acoustical issues in adjoining rooms and then we must also keep unwanted external energy from getting through our “vents” Can we absorb excess sound pressure energy close to the vibrational producing source such as our speakers to reduce the bandwidth that our system or room operates at? Can we reduce the mechanical resonant frequencies within the room’s wall structure? I think the answer is yes to all of these if we view the room as a pressurized box with a resonant system.

Response of Our Ears To Sound

We have all heard really loud sounds. A jet taking off, a dragster or funny car blasting away from a dead stop, or an explosion all generate large amounts of sound energy. The perceived loudness of this energy by our ears depends on the particular frequency we are addressing and the intensity at which we perceive or hear that frequency. To interpret this frequency/intensity ratio and put it in a form that more closely resembles actual human hearing, we go to what is termed the Fletcher-Munson loudness curves.

The Fletcher-Munson curves take frequency and intensity and apply this data to a predetermined domain of measurement. The Fletcher-Munson frequency start point is 1,000 cycles. The F/M loudness scale determines that the ear is the most reactive in the frequency range that starts with 3,000 cycles and goes through 4,000 cycles. This loudness scale also shows that the threshold of hearing is more reactive at lower frequencies. For example, the threshold of hearing at 60 cycles is 48 db higher than at 1,000 cycles.

Loudness of any sound is a ratio of the perceived magnitude of that sound energy by the live organism it encounters. The units or intervals the loudness scale uses must reflect real human reaction points. The units on the scale must match common human hearing experience and also match the sensation magnitude. The scale also must be constructed so that when the units on the scale increase by a certain factor the sensation magnitude of human hearing increases proportionately. If the units are quadrupled then the corresponding human hearing sensation must also quadruple.

Pitch is defined as frequency that reacts with the medium in which it is transmitted in. When sound travels through the air in a room, we have the frequency produced by the sound source and we also have that frequency reacting with the air and producing another sound. It is source sound and air sound combined. Pitch is not an objective quality but rather a subjective one. Pitch is the subjective quality humans assign to a sound in order to place it in its appropriate position on the music scale. There is a measurable difference between frequency and pitch.

Orchestra and Stage “Shells”

Harry Olson wrote a book entitled “Acoustical Engineering”. It was published in 1991 but with only a new introduction from a 1957 original publication. The book “Acoustical Engineering” was based on an earlier work entitled, “Elements of Acoustical Engineering” copyrighted 1940 and 1947. I like to look at older acoustic books and read what the current thinking was at the time. It is amazing how some things have really changed and some things have not at all. I like one section entitled, “Orchestra and Stage Shell”. It talks about acoustic treatment in outdoor theaters.

The article focus states, “When orchestra and stage productions are conducted in outdoor theaters it is desirable to provide a shell to augment and direct the sound to the audience, to surround the orchestra with reflecting surfaces and to protect the performers and instruments against wind, dew, and other undesirable atmospherics” I like the use of, “undesirable atmospherics”. It is a nice way to say bad.

In this necessary “shell”, we must focus on the acoustical treatment that will line our shell with, in order to maximize the sound quality for all parties concerned. It appears from this section, that most of the shells in those days were concave in shape and design. This concave shape produced what the book calls, “intense and sharp concentrations of reflected sound in both the shell and audience area”. It also goes on to say that “these acoustic effects are particularly undesirable when the sound is picked up by microphones on the stage for sound reinforcement and broadcasting”. This reflected energy produced by this concave shell can not achieve a balance sound for the conductor’s position. Therefore, without the orchestra leader hearing what the audience hears, we have a definite acoustical issue when it comes to the treatment used inside our shell.

Poly-cylindrical shell or a concave shell lined with poly-cylindrical structures will produce the best sound for all parties concerned concludes this section of thought. It will be good at the microphone positions, the orchestra leader, and finally the audience. A poly-cylindrical structure is shaped by an 180 degree arch and then a flat surface for mounting. The arch is not a diffusor. It is a sound re-director. It uses the angle of incident equals angel of refraction physical law. Numerous poly-cylindrical devices installed in a wall, would redirect the energy that strikes them in different directions opposite to their original striking direction, thus creating a sound redirected sound field.

I have never heard a shell lined with poly-cylindrical devices, but I wish I could get that chance. Most concert shells I have heard are lined with quadratic diffusors which are usually positioned in both vertical and horizontal planes thus, providing two dimensions to our sound field which would be directed at the conductor and audience. It would be interesting to compare quadratic diffusion sound with poly-cylindrical sound just to hear the difference. It could be different in many ways.

With two dimensions of sound created by quadratic diffusors positioned both “vertically” and “horizontally” I would know that sound because I have created it on numerous occasions. It is characterized by a smooth and equal frequency spread across the room plane it is focused on. The diffused sound field would be equal parts “air” and equal parts sound. If it were a solid, it would look like a very loosely woven tapestry spread across a room boundary surface. I hope sound redirection through a poly-cylindrical lined shell retains this feature but adds something of its own. I do not know what that would be, but I would sure like to hear it.

Acoustical Treatment in Our Studios

In a professional recording studio, we have the control or monitoring room. We also have the “live” room. A live room is a room about which we seek to acoustically create a larger sound through higher than normal reverberation times. In a live room, it is necessary to acoustically treat some reflections from room boundary surfaces and let the other reflections run free. We can accomplish this variable type of acoustic presentation with acoustical treatments that allow the engineer to vary the surface of the treatment from absorption to diffusion and even sound redirection. Our acoustic goal for the room and for our microphones is to compliment the individual vocals and instruments. A live room needs to provide at the microphone, an additional amount of analog energy through room sound into our electronic, digital signal path.

In our “live room”, we must have a complete understanding of the size and locations of any room modes. Room modes occur between the parallel surfaces of our room. Because sound waves have certain lengths and travel at a given speed, they become influenced by the room dimensions. Certain parallel room dimensions are more problematic than others, but they exist in almost every room. A room mode will cause an energy field drop out or an energy field exaggeration at certain frequencies or certain frequency groupings. A microphone placed within a room mode that is a null will not let the microphone “hear” all the frequencies produced by the vocals or instruments. If the microphone is placed in a room mode that exaggerates certain frequencies, then one will have distortion at the microphone surface.

Reverberation is another acoustic variable that we need to address. Reverberation in our control room is almost non existent. We don’t want a lot of reverb in the in our control room because we don’t want it to smother or destroy any of the ambiance in our recordings created by our “live room”. In a “live room”, we want to have reverberation, not in our control room. To achieve the proper amount of reverberation is both an art and a science. All one has to do to verify this statement is tour different facilities and listen to their rooms. One will have a big sound that one could record drums in and add a large amount of ambiance to the recording. Another could be good for piano because of its acoustic surface treatment and rate and level of absorption material used.

Surround Sound Monitoring Room Acoustics

Tomlinson Holman, in his book entitled “5.1 Surround Sound Up and Running”, discusses the room acoustic requirements for monitoring a 5.1 mix. He states,

” Multichannel affects the desired room acoustics of control rooms only in some areas. In particular, control over first reflections from each of the channels means that half-live, half-dead room acoustics are not useful. Acoustical designers concentrate on balancing diffusion and absorption in the various planes to produce a good result.” If live end/dead end is not useful as it is in two channel monitoring, what are our options with multiple channel monitoring in terms of acoustical treatment?

It appears that primary reflections off of our multichannel monitors are as much of a concern as they are in two channel. Primary reflections distort the more pure (without the room), direct sound from the monitors. The engineer wants to hear all the vocals and instruments from the speaker without the room reflections. Room reflections add the sound of the room into the mix and that is not acceptable. Near field monitoring is a process to minimize room sound by sitting closer to the speakers to hear the wanted direct sound. It is important in two channel audio and it is even more important with multiple channels that are all producing energy at the same time. No need to add room sound to 2 channels, let alone 5.

He states that acoustical designers, “concentrate on balancing diffusion and absorption in the various planes to produce a good result”. If we look at each plane or room boundary surface for two channel sound, we can get an idea on how to balance absorption and diffusion in a multiple channel presentation. The ceiling must be a balance of diffusion and absorption in two channel sound as well as multiple channel monitoring and playback. We definitely do not want ceiling reflections in our mix, just as we don’t want them in our two channel presentations.

The same will apply to all the room surfaces. A balance of diffusion and absorption will work for all the room boundary surfaces. Each surface should have the same amount and type of diffusion and absorption to provide the engineer with a uniform and consistent room sound. In two channel sound, we tend to absorb the primary reflections and use diffusion in other areas. With multiple channel monitoring consisting of multiple, mono sources, we must have a balanced acoustical field for all channels.

Low frequency issues must also be addressed, so that the low frequency effects channel can be monitored correctly and produces the necessary energy level to fill a room with explosions and other special effects. Muddy and bloated bass energy will only confuse the listener and smother the middle and high frequencies. Vocals will be lost in a low frequency muck that may prevent localization of the vocals with the image on the screen. Video movements on the screen must have a corresponding audio movement from channel to channel. Low frequency absorbers whether freestanding or built in are a must.

I hope this explanation helped. Please leave any comments below so I can get back to you. Don’t be afraid to hit those Facebook like, Google+ and Twitter buttons on the left hand side so other people can see this post. And if you want to learn more about this subject please sign up for our free room acoustic treatment videos and ebook which provide step by step instructions. Get instant access by signing up now.


How To Choose The Correct Speaker

February 22, 2012 No Comments

Working out how to choose the correct speaker for the room you are going to be using it in involves many factors. All of these factors must be considered when selecting a loudspeaker for your listening room. We are just referring to two channel sound sound with a left and right channel speaker. We need […]

What is Good Sound?

What is Good Sound?

I walk through trade show rooms and listen to the exhibitors tell me what they consider good sound. They tell me their room is kind of their idea of good sound. Some tell me that their room sounds pretty good but does not fall into “good sound”. A hotel room is a compromise and one must work around many variables to achieve a sound that demonstrates what their product could sound like in a good room some say.

Some rooms have too much bass energy. Some rooms are bright and have so much specular reflections that it is difficult to hear all the vocals in a three part harmony or hear two bass instruments each producing their own sound.These rooms are characterized by low definition and a small image. Some rooms have the listening chair up against the back wall. Some say,”Our room will have the best sound of show”. Really?

I guess everyone has a different idea of what “good sound” is. Is good sound the type of sound where their is an emotional attachment immediately to the music? Is good sound the type of sonic presentation where one can hear every instrument and vocal in a balanced presentation? Is good sound the type of sound where the speakers and amplifiers disappear and one can only hear and “see” only the music? Is good sound a combination of some of these variables and not others?

For us, it is removing the room from the sound and having the ability to hear all the instruments and vocals in a balanced presentation.To achieve this objective, all low frequencies are heard without any bass bloat. There are layers to the bass and the bass attack and decay is as tight and clean as the attack and decay of middle and high frequencies. Middle and high frequencies are layered like our bass presentation and their is a distinct separation between the instruments and vocals.Comb filtering of middle and high frequencies is under control. Their is air present and instruments and vocals float in the room all across our sound stage. No speakers are seen or heard.

How does one achieve good sound? I am sure their are many approaches as there are opinions on what constitutes “good sound”. We choose to reduce low frequency pressure in the room from all low frequency producing devices. One must first deal with low frequency pressure in order for the middle and high frequencies to come through without being smothered by excessive low frequency energy. Excessive low frequency energy can be controlled at the source or at room boundary surfaces. Middle and high frequency reflections off of room walls can be controlled through the use and application of absorption or diffusion technologies.All of this control must be applied in a way that produces a balanced sound stage with a height, width, and depth.

Headphone Sound or Room Sound

Head phone sound is different than sound in our rooms. Both have their good and bad points.

Head phones seal our ears with a small oval shaped room. Inside that “room” is a small speaker that produces sound energy and fires it directly into our our outside ear. From there, the sound travels into our ear canal and then finally into our inner ear. We hear bass energy, middle, and high frequency energy all evenly portrayed in the two channels that are wrapped around our head.

Were our ears designed to have sound that close to it and at those pressure levels produced by headphones. If they were wouldn’t they be smaller. No need for three or four inch ears if the sound we hear is generated from such a short distance away as with headphones. No, our ears were designed to hear sounds from all directions and at some distances. Men have larger ears than women. This is probably due to the way men used to hunt for food. Hearing everything from all directions when hunting for food, enabled man to bring home the dinner rather than be the dinner himself. A larger sound receiving instrument was needed to allow for selective hearing and localization of predators.

Do headphones produce a sound stage in front of our head. I have never experienced that with headphones. I have experienced both left and right channel separation, but no sound stage. Headphones do not have a sound stage and therefore do not have any height, width, or depth to their sonic presentation.

They do have definition. Headphones portray every sound in detail. One can hear all the instruments and vocals with distinct separation between them. If we can get our rooms to have that instrument and vocal separation coupled with headphone like clarity, we will have achieved our room acoustic objectives.

A room has space and the sound source is at a much farther distance from our ears than headphones allow for. We can have a small as is the case with near field listening or monitoring. We can also have a large sound stage depending on room dimensions and the correct balance in the room of acoustical treatments. A room and the sound produced in it is easier for the design of our ears to interpret. The music or sound reaches our ears in a way and manner that allows for localization and spaciousness to occur. With this spaciousness, we now can have information going into our brains that has much more data in it than just headphone sound. We have room sound and room sound is closer to free space sound which is probably how our ears evolved into the size and shape they have today.

Db Meter

A popular search term in audio is “db meter”. Sometimes people search for terms that they think go together or they have seen or heard it used somewhere. Sometimes there is a combination of words into a phrase that tries to illustrate an audio point or concept. Lets examine each word within the search term “db meter” and see if it the right word or group of words to use in this situation.

Db stands for decibels and is a unit of measure that those in the audio world use to describe intervals or amounts of energy within in a given environment. It is only that: a unit of measure. It does not have any value of its own other than to say it is a unit of measure that is calculated and formed to correspond to the human ear hearing range. A large db number can cause inner ear damage. A smaller db number may be too low to hear the difference in gain jumps. A db is part of a scientifically calculated scale or ratio for human hearing comparisons and even regulations.

What does the db unit mean when it is attached to a number? A db meter measures sound pressure levels.The sound pressure level can be assigned many different units of measurement. Therefore, using a ratio is better for human sense of hearing comparisons. A db unit is a ratio of acoustic power levels expressed in decibels that “comply” within our human hearing range. These are decibels that express a power ratio made for human hearing measurements. For example, a Saturn rocket has a sound pressure (Pa) of around 100,000 Its sound pressure level measured in db is 194. Normal conversational speech has a sound pressure of .02 while the sound level is 60.

Searching for a db meter may be confusing for the clerk at the store, sine we are really measuring sound pressure levels expressed in decibels in our room. Although, if you go to Radio Shack and ask for a sound pressure meter, they will search their product data base and will find no entries in it for “sound pressure meter” If you change your request for a db meter, they will have one for you quickly. It is funny how things work.

Building a Sound Room with a Living Roof

We have a client in Arizona who wanted us to build him a sound room with a living roof. A sound room you are all familiar with. A living roof may be an other issue. It was also a pleasant surprise for us.

A living roof is a roof designed to support 18″ of top soil and the watering and drainage system necessary to maintain this miniature ecosystem. Supporting 18″ of earth and water is no easy task, especially when the roof size is 25′ x 50′. That is 1875 cubic feet of earth at approximately 20 lbs. / cu. ft. is 37,500 pounds of earth, not to mention the piping for water and drainage. The roof must support 16 tons of earth and pipes.

Earth is an excellent barrier to external noise. Go into your basement and sit quietly. There you are surrounded on 4 sides by earth and concrete. In this project, the roof and 6′ up the 12′ side walls will also be covered with earth. So, in this project we have 1 1/2′ earth on the roof and six more feet of earth on each wall side. Now, we need concrete walls at the correct thickness to match the acoustical properties of 1 1/2′ of earth on the roof.

We determined that an 8″ poured concrete wall all around will meet all our structural issues for ceiling support and acoustical issues for sound transmission class ratings and all external noise measured calculations. The 8″ concrete shell will build a room that is 25’wide and 50′ long. The ceiling height is 12′. One could not ask for a better room size when it comes to acoustical issues that must be dealt with.

At 50′ in the length dimension, even a 20 Hz.wave, which is the lowest wave we usually work with in rooms has some room to run. No low frequency issues or any others for that matter when it comes to the 50″ length dimension.The 25′ width is also good for low frequency, but will give us a few issues. Those issues will be resolved through the use of our activated carbon technology which will be added to the inside walls. A ceiling height of 12′ only increases our room volume and is welcome for all forms of sound playback and recording.