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Professional Approach to Room Acoustics

Balanced Room Acoustics

A Review for Hi-Fi & Home Theatre Enthusiasts

Introduction

Acoustic performance characteristics are essential in delivering a complete Hi-Fi and Home-Theatre environment. The last thing any Hi-Fi or Home- Theatre owner wants is having a high-end, full-scale 5.1 or 7.1 audio system compromised by its environment which has not been optimised for wonderful music listening. Acoustics in rooms have a vast influence on the reproduction of CD/DVD music and DVD movie sound tracks. A good room must transfer its music to the listener, preserving the original musical qualities of intimacy, definition, timbre, balance and dynamic-range; it must contribute fullness of tone, loudness and a wide range of crescendo and decrescendo. It all means that, a good acoustically balanced room enhances the music.

Professional consideration to audio acoustic involvedness has traditionally been to construct special flex walls, hanging fibreglass panels from ceiling and construct special Bass traps. Recording professionals discovered that “Sonex” acoustic materials were massacring noise & reverberation in heavy industrial plants inMinneapolisUSA. It was only then that the audio recording professionals started conducting tests with these 3 different thicknesses of unique high performance “Sonex” acoustic materials to control sound in Audio Recording Studios and TV Studios all over the world many years ago. It was a great success for acoustic control in the audio recording studio industry.

With an acoustically balanced room, anyone can achieve the state-of-the-art sound reproduction in their music room. How? Using proper acoustic control materials in your home music room can improve the acoustics, creating a truly realistic soundstage effectively. Before you spend any more money on upgrading your audio equipment, upgrade that one “objective” first (acoustic control) which will give you back your greatest rewards. “Balanced Room Acoustics”. Or “BRA” as it is called.

Fortunately, everyone is born with a simple set of tools for testing of room acoustics. Stand in the centre of the room and just clap your hands once. If you hear a dry thud, then you are in a highly damped environment. In a typical living room, clapping your hands will generate a good deal of reverberant. It does not matter if the sound comes from your hands, Pink noise or your Hi-Fi / Home Theatre -speakers, the outcome is the same.

Sound in a lively room is composed of two parts, the direct sound and the reflected / reverberant sound. The true tone of vocals and acoustic musical instruments carried in that part, travels directly to the listener’s ears is the direct sound. A fraction of a second later, we hear the first reflection from the walls and ceiling. And then, as these waves travel back and forth in the room, they produce many normal modes of vibration, each of which decays at its own speed producing the reverberant part of the sound. Reverberation sustains the tones of sound that are produced in a room. However, as the intensity of the room swells, the participation of the hall becomes more and more apparent. Reverberation intensifies the growth of the sound.

Described below are four specific problems that can be controlled are:

1) Reduction of Reverberation Time:

Reverberation time is abbreviated as RT60, and is defined as the time it takes in seconds for a sound to decay 60 decibels (dB) in level once the sound source has been abruptly shut off. In most cases, the 60dB reduction is equivalent to the time it takes foe one to no longer hear the sound. To experience the Reverb time of a room, just clap you hands in an empty room or your music room and hear how quickly the sound decays. Then go into a larger room with hard materials and do the same thing. You will hear the reverb difference. It is important to maintain a relatively low reverberation time in a Hi-Fi and Home Theatre room since this has direct relation to the clarity and intelligibility of sound. It also allows any type of artificial environmental sound that is recorded onto the DVD movie to be translated to the home viewers, and not masked by a longer decay in the listening room. The amount and location of sound absorbing materials should be very carefully selected to arrive at a balance between reducing early sound reflections and achieving a target reverberation time goal of around 0.30 to 0.35 seconds al all frequencies between 250 Hz, and 0.40 to 0.50 seconds at frequencies between 60 Hz and 250 Hz.

2) Noise Reduction:

Using premium acoustic materials at the precise locations allows anyone listening to music to focus on what they are listening to. It has excellent absorbing capabilities for reducing ambient noise levels, helping to minimise obtrusive and unwanted extraneous noise. It increases the sense of dynamic range, because low levels are easier to hear.

3) Reduction of Slap Echo:

Slap echo causes a smearing of high frequency information due to reflection problems in overly live rooms. Slap echo tends to make a live room sound louder, edgy and irritating. If acoustic materials are used accurately it can be effective in eliminating slap echo problems in any room.

4) Reduction of Standing Waves:

This is a common problem in any enclosed space; standing waves reinforce certain frequencies and cancel others, often leading to artificial frequency control using equalisers. Use of premium acoustic materials provides a natural solution by eliminating standing waves and smoothing overall frequency response. It is achieved through the placement of Acoustic materials at key locations within the room.

Notes to remember is that having too much sound absorption in a room can create a difficult listening experience. Yet, having exposed flat and hard surfaces like dry walls, glass windows and wood flooring can result in several acoustic problems. Also long delayed echoes off the rear wall that can actually shift localisation of an image from in front of a listener to behind a listener.

How to improve on this acoustic complicatedness? On relevant areas of the walls, incorporate quality acoustic materials that reflect the sound in a diffused manner. The sound must be scattered in a variety of directions. This eliminates any perceived reverb/echoes as well as enhances the surround sound quality by widening the apparent width of the Home Theatre.

Basic definitions of sound:

Sound is geared up of one or additional pressure waves that cycle between high and low amplitude regions. The time period of each cycle is known as its frequency, expressed in terms of Hz (Hertz). The physical length of each cycle is known as its wavelength. Lower frequencies have longer wavelengths. For instance, a sound with a frequency of 1 kHz (1000 Hz) has a wavelength of one foot, while a 60 Hz sound has a wavelength of in the region of 18 feet. The effect of the longer wavelengths in a smaller home theatre room is a potential variation of loudness level dependant upon listener position in the room.

Sound levels are calculated in Decibels (db) in the strength of our mind; our ears detect differences in volume level in a non-linear fashion. Note: Our ears become less sensitive to sound as it increases. Decibels are a logarithmic scale of relative loudness. A difference of approximate. 0.5 and 1 db is the minium perceptible change in volume, 3 db is a reasonable change in volume, and about 10 db is an approximate doubling the volume of the source.

Dreadful Acoustics

The most common dreadful acoustical environment is a bright reflective room. Polished wooden-floor, tiled-floor, naked concrete room, these are exactly the kind of rooms which are unpleasant acoustical spaces. Sound bounces back and forth, mercilessly ricocheting off the walls, glass windows, floors and ceilings, dying out about three seconds after the sound is issued. Acoustic materials are specifically designed to trap, absorb and control the echoes, reverberations and distortions which smear the soundstage, in which your good quality speakers are trying to recreate. By reducing reflected sounds, a quieter and more revealing listening environment will be created. Your music can be played louder without distress, and it helps keep original quality sound in your room where it belongs and not bouncing all over the whole room.

The most common materials used for sound absorption are open-cell foam such as that used in Recording and TV studios like “Sonex”. “Fibreglass” insulation can also be used. These days, you can get decorative acoustical materials form “Echo Busters”. I find that they work very well in homes with Hi-Fi and Home Theatre use. All these available acoustic materials are effective sound absorbers at low to high frequencies.

Many rooms exhibit very long reverberation times at frequencies below 250 Hz, resulting in bass boom and other undesirable effects. Resonating absorbers are used when absorption at low frequencies is required. There are many types of resonating absorbers. Some simply consist of a diaphragm which vibrates in response to an impinging sound wave, converting a portion of the wave’s energy to heat by frictional losses in the vibration diaphragm. Others are based on the Helmholtz resonator which uses a resonant cavity, containing one or more openings to absorb sound. Such a cavity has a certain resonant frequency at which the air in the cavity will begin to vibrate in performance with a sound wave entering it. The Helmholtz resonator absorbs the energy of a sound wave through frictional losses between the sound wave and the air contained in the cavity. One interesting property of Helmholtz resonators is that sound which is not absorbed is re-radiated in a hemispherical fashion, resulting in diffusion of the remaining sound energy.

Over the years I have advised many friends and friends of friends on what type of amplifiers and speakers to buy, but I can’t remember a single friend asking me what kind of acoustic treatment should they be using, and getting the best natural sound.

Absorption

Some materials absorb sound rather than reflect it. The efficiency of absorption is dependent on the wavelength.  Thin absorbers like carpet and acoustic ceiling tiles can affect high frequencies only, while thick absorbers such as drapes, padded furniture and specially designed “Bass Traps” are required to attenuate low frequencies. Reverberation in a room can be controlled by adding absorption: the more absorption the less reverberation. Clothed human absorb mid and high frequencies well, so the presence or absence of an audience has a significant effect on the sound in an otherwise reverberant room.

Diffusion

Diffusion of a sound wave occurs when the wave strikes a room boundary and is reflected back into the room such that its energy is evenly distributed in all directions. Diffusion is desirable because it inhibits the creation of standing waves which might otherwise result in frequency response problems. A standing wave pattern is an interference phenomenon. It is formed as the result of the perfectly timed interference of two identical frequency waves passing through the same medium. A standing wave pattern is not actually a wave; rather it is the pattern resulting from the presence of two waves of the same frequency with different directions of travel with thew same medium. All standing wave patterns consist of nodes and anti-nodes. The nodes are point of no displacement caused by the destructive interference of the two waves. The anti-nodes result from the constructive interference of the two waves and thus undergo maximum displacement from the rest position.

It is very common for a listening room to provide inadequate absorption and diffusion across the frequency band, resulting in blurring, ringing, Bass-boom and other problems. It’s psychosis listening to music in a room like this (distortion).

Reflection

A sound wave can be reflected by a surface or other object if the object is physically as large or larger that the wavelength of the sound. Because low frequency sounds have long wavelengths they can only be reflected by large objects. Higher frequencies can be reflected by smaller objects and surfaces as well as large. The reflected sound will have different frequency characteristics than the direct sound if all frequencies are not reflected equally. Reflection is also the source of echo, reverb, and standing waves. Echo occurs when a reflected sound is delayed long enough to be heard by the listener as a distinct repetition of the direct sound. Reverberation consists of many reflections of a sound, maintaining the sound in a reflective space for a time even after the direct sound has stopped. Standing waves in a room occurs from certain frequencies related to the distance between parallel walls. The original sound and the reflected sound will begin to reinforce each other when the distance between two opposite walls is equal to a multiple of half the wavelength of the sound. This happens primarily at low frequencies due to their longer wavelengths and relatively high energy.

Early Reflections

In any characteristic room, sound arrives at the listener’s ears via two key directions:

1) Direct, in a straight line between the source and the listener.

2) Reflected, off a large amount of surfaces in the room.

Because, any reflected sound wave arrives at the listener’s ear slightly delayed in time from the direct sound, there is another experience that occurs known as “phase” cancellation. This “phase” cancellation occurs when two signals of the same frequency are out of phase with each other resulting in either boost or cut in the overall level of the combination signals.

The Solution

The objective is to use a line of acoustical room treatment products which are effective, lightweight, affordable and visually attractive. An effective line of room treatment is one which provides adequate amounts of absorption and diffusion across the frequency band.  The ideal placement for acoustic panels which yield the best results are treating the front wall behind the front speakers, the side walls at speaker heights, corner of ceiling & walls, and low energy Bass traps. This helps to solidify the centre imaging and to capture the first reflections. However, experimentation with placement is likely to be required to get the panels positioned just spot on in your listening room. Acoustic panels are available in widths ranging from 2’ to 4’ and heights ranging from 4’ to 6’ and are intended to be wall-hung with supplied hangers, or are available with optional bases. Note: Any room size can be made to sound better by using acoustic materials to trap the unwanted sounds.

For Hi-Fi & Home Theatre use, I would recommend the “Echo Busters”, as they are designed for home acoustics and are very easy to install.

General Wall Traps

Acoustic traps & panels work by converting sound energy that strikes the panel into mechanical energy and what you hear is less confused audio experience. It means much less deflections and more direct sound.

Corner Traps

Corner trap panels are placed in the ceiling corners. This eliminates the megaphone effect the corners are famous for. Corners are also known for having all frequencies that eventually find their way there. By neatly capping this corner juncture with corner traps, you will be creating a surface that absorbs almost all the frequencies that hit upon it and prevent them from being ricocheted back into your listening room.

Bass Traps

This improves Bass reproduction in your Hi-Fi or Home Theatre environment by removing unwanted low frequencies. Standing waves are formed by sound bouncing off walls, ceilings, floors and crashing into each other at certain points in the room. These unwanted low frequencies need to be removed to allow you to hear that full Bass extension your speakers are capable of reproducing. Bass traps increase the dimensionality of your soundstage imaging by reflecting unabsorbed sound in a hemispherical pattern back into the room. Depending on the room size, you could use 4’ up-to 6’ heights.

Bass traps available are designed to absorb sound in the critical 40Hz to 250Hz frequency range. They are designed to sit in the corners of your room, and it allows Bass to pass through, and diffuses high frequencies. When heavy Bass music is played, the Bass traps resonant frequency gets excited and they resonate, or sometime vibrate considerately and wick off some of the unwanted Bass energy from the room. This allows kick Bass drums to sound tighter and Bass guitar to sound deeper and better defined.

The Results

With any listening room that is well balanced or well treated acoustically, the treated room’s ambient noise level seemed to be reduced significantly, and reproduced music appeared to emerge from an astonishingly black background. A silence between musical notes becomes more silent than any untreated listening room.

After installing a small amount of acoustic materials in my listening room, I immediately noticed the difference in acoustic control. Hand-clap test revealed much less echo and high-frequency ringing at my sweet spot. With music listening sessions, musical lines were more articulate as the decay of one note didn’t interfere with the attack of the next to the same degree as in the untreated room. The high level of expression allowed complex classical musical passages to be unravelled by the ear / brain much more readily, resulting in far less listener fatigue during extended listening sessions. The control over the listening room acoustics offered me to direct more of my attention to what was happening in the music mix and less to filtering out the rooms “noise” superimposed over it.

The difference in an untreated or poorly treated listening room is that, the sound one hears is the sum of the retrieved recorded acoustic and the rooms own acoustic.

I have noticed that music played back in my treated room had subjectively more visceral impact that before the room was given acoustic treatment. I suppose its possible that I was hearing a note’s attack more completely, without the smearing or blurring associated with early reflections from the side and rear walls. The effects were real and heightened the illusion of reality. Also music played back in my treated room also took a new found ease and smoothness, giving me the freedom to lean more heavily on my volume control in an effort to recreate realistic sound levels. It’s just like being in the recording studio’s mixdown control room.

After hearing the major sonic improvements using acoustic materials, it was hard to go back to listening to music in any untreated rooms.

Music played back in my treated room emerged from a silent background, and exhibited more detail, a better sense of the recorded acoustics, more dynamic impact, and tighter deeper Bass.

Treating the music room is a must for anyone trying to get the best result out of their HI-Fi or Home Theatre Audio Systems.

Why is Acoustic Control compulsory in Hi-Fi and Home Theatre’s?

The essential goal in designing a room for Hi-Fi music and watching DVD movies is to achieve the sound at the listener’s ears that the music producers originally intended. Assuming a flat response from the speakers, this assumes that the room itself does not distort or otherwise alter the frequency response, the relative amplitude or the time arrival of any sound events. The standing waves (comb filtering effect), if left untreated, would in fact cause a significant distortion of sound. The distortion would be different as well at every seat due to varying reflection patterns and path length differences. Another important reason to eliminate the early arriving reflections is that a strong reflection can also cause a shifting of an image in the frontal sound stage. For example, a strong reflection of a vocal or speaking voice playing through the centre channel speaker off the either side wall can make the voice seem like it is coming more from the side of the screen rather than the centre. This would be an obvious concern if the movie director’s intent was to have the voice located in line with the actor’s position on the screen.

In order to avoid image shift and distortion problems, establish which surfaces could reflect sound off and reach the listeners ears within a very short time after the direct sound. Now, treat these surfaces with sound absorbing materials that are highly efficient at all frequencies above 125Hz. These materials serve to significantly reduce the energy level of the reflected sound so that it is well below the level of the direct sound. Sometimes minor image shifting might occur, but the impact will be so little that no distortion would be heard.

While evaluating surfaces on which to apply sound absorbing materials, we also evaluated the reverberation time or the decay time of the room. Often mistakes are made of applying sound absorbing materials to every possible surface in the room. Not only does this create an unnatural listening environment, but it also eliminates non-absorptive surfaces that can be beneficial for surround sound speakers to interact with. So, having too `many sound-absorbing materials in a music/ home theatre room can create a problematical listening experience.

The amount and location of sound absorbing materials should be very carefully selected at arrive at a balance between reducing early sound reflections and achieving a target reverberation time goal of around 0.30 to 0.35 seconds at all frequencies between 250 Hz and 4 kHz, and 0.40 to 0.50 seconds at frequencies between 60 Hz and 250 Hz.

Understanding Ambient noise criteria and Measurement

Having a noise free atmosphere is extremely important to being able to appreciate a wide dynamic range of sound. There are many instances where an action scene in a Home Theatre movie brings together a climax of visual and sound level and then suddenly stops with dead silence in the movie. To have this instantaneous contrast in sound level appreciated to its fullest, your Home Theatre needs to have a reasonably low background noise level.

Noise in your Hi-Fi and Home Theatre room is measured with a special sound level meter that records the average decibel level over a short period of time throughout the seating area. The level is measured in ranges of frequencies called octave bands to allow for a more detailed analysis of the quality of noise in the theatre. The measured octave bands are then compared to a set of plots that rate decibel level versus frequency. These are known as the Noise Criteria.

The technique of designing loudspeakers has not changed much in the last hundreds of years. A loudspeaker builder listens to the tone, and then repeats the entire process with a slight change in its design and construction. This is a tedious process and one often thinks that if there could be an easier and if there was a way to ‘see” the sound. Spectrum analysis is a tool that gives us the ability to see the timbre. However, today these expensive spectrum analysers are now within the reach of average Hi-Fi Home Theatre users. The output of any good spectrum analyser is of major importance. This is what is going to be interpreted by the Home Theatre user. An unintelligible output renders the whole system worthless.

Musical sounds are usually visualised as “waves” of air that vibrate with a particular frequency. This frequency is expressed in cycles per seconds; however, instead of saying “cycles per seconds” we now call it “Hertz”. The range of human hearing is said to extend from 20-Hertz to 20-kilo Hertz (20 cycles to 20,000 cycles per second). This range is referred to as the “Audio Spectrum”. On the other hand, ambient sounds, vocals and musical instruments sound consist of a mixture of different frequencies. It is the nature of this mix which helps to determine timbre. Therefore by looking closely at these component frequencies we get insight into the timbre of any sound. This is spectrum analysis.

Advice

With setting up using acoustic materials for Hi-Fi and Home Theatre systems, precise speaker’s placement leads to smoother and more balanced frequency response across the hearing bandwidth leading to accurate sound quality. Also, using quality speakers with finer clarity, the easier it is for the brain to recreate an accurate representation of the live music experience from the recorded information.

It is all too easy to abuse room acoustics products and over-treating a listening room. Such over-treatment often results in a dull, lifeless listening environment not conducive to musical enjoyment.

I recommend installing acoustic material in a staggered manner, which creates an alternating array of absorption and diffusion for sound. I have noticed that most people tend to buy “Sonex” materials in 4ft by 8ft sheets.  For maximum efficiency, it’s better to use 8 ft long by 8-inch wide strips with space between them. It is more acoustically efficient.

Most acoustic foam treatment materials are made from polymer-based products. Flammability is always an issue. Melamine was developed as a less flammable alternative to polyurethane. It is also more temperature and chemical resistant and significantly more flame retardant than polyurethane and with a typical flame spread index as low as 2.5 compared to 95 for a similar polyurethane-based panel. Melamine will not flame but it will turn into ashes when exposed to flame, because of its limited combustibility, it does not require fire protection such as sprinklers.  It’s practical for home use where there are no sprinklers.

If you are uncertain how to do it, then you should get your room acoustically analysed by a professional and then find out how it improves the acoustics to suit your Hi-Fi and Home Theatre room.

Acoustics of any room can affect the sound of your Hi-Fi & Home Theatre almost as much as the quality of your speakers does, from my experience, room acoustics have a far, far bigger effect on the sound than the quality of your amplifier, speakers and cables.

It will be far easier to use a computer to analyse your room. The available room and speaker positioning program available are the “Rivers”, “ODEON”, “CATT”, Bruel & Kjaer” or “CARA”. A simple but more basic program is the “Visual Ears”. Its only limitation is that it is not designed to work with L-shaped rooms. It cost about A$90/- and it is available online from www.kbacoustics.com/visualears. If you rather not use a computer, then some Hi-Fi and Home Theatre consultants will be available to do the programming and calculation of your room that is if you provide the room dimensions. Expect to pay a minium fee of A$200/- or more for this service.

I hope that after reading this article, you will be inspired to start treating your room right if you are not happy with the sounds you are hearing.

Alphonso Soosay.

Audio Recording Engineer.

Perth. WA.

Comments

  1. alphonso says:

    Positve Feedback for Home Theatre Enthusiasts

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