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Introduction to Room Acoustics

Room acoustics are often overlooked and should not be kicked to the bottom of your list. The impact on the sound quality can be immense and it’s not always clear how to handle the situation. Let’s get started.

When Sound Waves Unite

Acoustics aren’t just about treating your walls with an absorber or two, though it helps. Placement and location of the speakers have an effect as well. The most important thing to remember about room acoustics is that there are hot and cold spots of sound energy in your room. Controlling these hot and cold spots is critical to achieving quality sound.

Physics teaches us that when two waves of the same frequency combine, the result can cause reinforcement or cancellation. This is what we mean when sound waves unite. When sound exits a speaker, some energy will go directly to your ears and the rest will bounce off any object in your room then arrive at your ears delayed in time. All these reflecting surfaces bombard you with sound waves much like small speakers but delayed in time with poor frequency response. When sound bounces off a reflective surface, it's not perfectly reflective and has its own coloration on the sound. When the sound arrives at your ears the bass, midrange or treble could have too much or too little emphasis. If this wasn't bad enough, the distance traveled of these reflected waves will determine how loud they are and how much the phase difference is to the direct sound wave. Ultimately, the room can make things sound pretty messy.

 

Dividing the Problems

It is best to divide room acoustics into two categories, low frequencies and midrange/high frequencies. This is because sound waves that are about the same size of the room (low frequencies) behave differently than sound waves that are smaller (midrange and treble).

Low Frequencies

The best way to understand low frequency sound waves is to visualize that your speakers have sound waves emanating out of them like waves in a pond after you toss in a pebble. Just like in the diagram, low frequencies are non-directional and will go everywhere in your room.

The most difficult part of room acoustics is achieving good bass response. Proper placement of your speakers and subwoofers will determine bass quality. It is a lot like real estate – the speaker's locations and one's listening location significantly determine bass quality. Just walk around your room while the music is playing and try to notice the bass peaks and valleys from one end of the room to the other.

There are basically two low frequency issues you will need to deal with, Boundary Interference and Room Modes. Bass response is influenced by the distance you and your speakers are from all large Room Boundaries (walls, ceiling and floor). This is called Boundary Interference Response, which causes dips in bass response. The dimensions and shape of your room alter bass response as well. These are called Room Modes or Standing Waves. Again, proper placement of your speakers and subwoofers along with your listening position will determine bass quality. If you couldn’t tell, it's very important to balance these two issues.

Boundary Interference Response

Most people notice that moving speakers close to a wall seems to increase the bass. Two factors play into this - first, gain takes place because the distance from the wall is less than a ¼ wavelength and second, the boundary interference dips are pushed above bass frequencies. This happens because the sound wave measured from the woofer is less than a ¼ of a wavelength from any large room boundary. When the direct and reflected waves combined they are constructive so amplification takes place. When the ¼ wave distance from a boundary is reached the waves are destructive so a dip in frequency response occurs. This dip can be even worse if your speakers are the same distance from more than one boundary or coincide with how far your listening seat is from the back wall.

The distance of your speakers from the back and side walls should differ by at least 33%. The distance of your listening seat from the wall behind you will factor in to this minimum 33% ratio as well. For example, if you set up your front speakers at 3 ft from the back wall, 4 ft from the adjacent sidewall and 12 ft from the opposite side wall, then your listening position should be at least 5.32 ft and no more than 8 ft from the back wall. If you keep a good boundary ratio (no less than a 33% difference) there are many possibilities. Because of the large wavelengths of bass frequencies, the distance your listening seat is from other boundaries like the floor, side walls and ceiling, under most circumstances is irrelevant. For your speaker boundaries, the ceiling and floor boundaries make a difference, but the distances are fixed so there is not much you can do about them other than making sure the other boundary distances don't match.

Room Modes

What is a room mode? Next time you blow air across the opening of a bottle to make a sound, keep in mind you are finding the pitch of sound in which the wavelength is exactly a multiple of the distance between the open end of the bottle and the bottom.

When a speaker plays bass, some of these low frequencies are the same wavelength as your room dimensions. This creates a standing wave when the sound bounces back to where it came from. The second half of the full cycle of this wave is coming out of your speaker so when the first half of this wave is reflected to your listening position, it combines with the second half. For example, if your room is 21 ft long like the diagram, the first mode will be 42 ft long and 27 Hz. How does a 42ft wave will fit between 21ft walls? The first half of the 27 Hz wave projects out from your speaker then bounces off the opposite wall as the second half of the wave is leaving the speaker. So, unless you are in the center of the room (no pressure point), 27Hz will be amplified.

The first room mode is not the only problem, there are multiples of this frequency that fit nicely between your walls. Take the 27 Hz from above and multiply it by 2, 3, and 4 and we get 54 Hz, 81 Hz, and 108 Hz. This is just for the length of the room. The width and height have their own modes. You can see in the chart that some of the mode multiples match, which is not good. This can result in result in a big hump or dip at the mode frequencies. The good news is the high order modes are easier to absorb with treatments like corner bass traps.

The room modes we have described are known as axial modes. Axial modes have the most impact on bass because they are between two parallel surfaces. Tangential and oblique room modes reflect off room surfaces at angles and lose power quickly, so they aren’t as crucial to treat.

Bass Tips and Tricks

The balancing act of dealing with boundary interference and room mode problems is a little tricky. Here are some tips and guidelines:

  1. First find the distances that your speakers (at the woofer) are from the back and side walls. For example: from your front right speaker, measure the distance from the right and left walls and the back wall. If any of these distances are the same, you will need to make a change by at least a 33% ratio if you can. Also check your listening seat distance from the back wall, change the distance if it matches any of the wall boundaries around your speakers. Remember, the 33% rule applies here as well.
  1. Find room modes by using an online room mode calculator. If the any two or three modes are the same frequency you will have problems.
  1. After you find what seems to be the correct placement regarding boundary inference, room modes can mess things up. Using your ears, put on a recording with repetitive bass tracks. If the bass isn't just right, move your speakers or your chair a few inches, while keeping 33% boundary ratio. It's best to work out a general grid area that you can move your speakers or listening position about 4 inches at a time. This could mitigate room mode problems.

Acoustic Treatments for Low Frequencies

Most acoustic treatments for low frequencies are great for refining the quality of bass in your room. The long wavelengths of low frequencies will require special treatments. These special treatments are called bass traps and they come in all sorts of shapes and designs. Bass traps are designed to deal with reflected bass energy only (vs direct), so the name can be a little misleading. The most effective bass traps are resonant type absorbers like those described above. Because this style of trap is not exactly plug and play, we don't recommend them unless you know what you are doing.

One the easiest bass traps to use is the porous corner trap. Any porous material (open cell urethane foam, fiberglass, mineral wool, cotton jute) will work. The foam products are in a triangular shape that fits nice and snug in a corner. There are also acoustic panel treatments that can go across room corners to create an air gap. Even though these treatments are not ¼ wavelength in thickness they have enough maximum thickness or air gap (12 to 18inches) to offer absorption down to about 80Hz.

Midrange and High Frequencies

As we increase in frequency, sound waves become more directional and room acoustics becomes easier to deal with. The front channels are going to be the most important for acoustic treatment. The surround channels will need some treatment but it's not as critical.

Primary Reflections

Primary reflections are all the sound waves that radiate from your speakers and bounce off objects in your room only once before it hits your ears. Each primary reflection is different from another in the frequency range. These variables are based on angle of incidence from your speakers, the material and shape of the reflecting surface, the reflecting surfaces distance from your speakers and the angle at which the reflected sound hits your ears. The diagram on the right shows many primary reflections there from each speaker. It's difficult to show floor and ceiling reflections but they are just as important as the wall reflections.

Midrange and treble frequency reflections behave similarly to rays of light. This makes it easy to find where the reflections are. A good trick is to tape a small mirror or CD (on the reflective side) to a 4-ft long board. Next, line up the reflector to the same level of your speakers. Finally, set it up where you think the reflections are, then sit down in the sweet spot - keep moving it until you see the speaker reflection at your listening seat. The buddy test works as well – where you have somebody hold the mirror and move it until you see your speaker’s reflection. These reflection points are where you would place treatment.

Flutter Echoes

A room with many hard and flat reflective surfaces can be harsh or very live sounding. When you clap your hands, you hear a high-pitched sound immediately as well a hollow ringing sound (think of a bathroom). You will hear these echoes off the front and back walls as well as your right and left sidewalls. They make the center phantom image between your speakers blurry and unbalanced. How to treat flutter echoes? They need to be treated on the walls directly to your right and left as well as front to back. The diagram below illustrates where flutter echoes are located as well as where treatments should go.

Midrange/Treble Tips and Tricks

The further away from boundaries your right and left speakers are the better. You will find that angling them to point more directly at your ears can help imaging.

If you can't use commercial acoustic treatment there are a few options. Using anything with a large round or uneven surface will work to diffuse primary reflections or flutter echoes. If your room width is asymmetrical then the wall closest will need most of the treatment to balance out the reflection. The following is a list of treatment ideas:

Floor Reflections: Thick wool floor rugs, cotton jute and thick padding under carpet

Ceiling Reflections: Coffered ceilings with varied widths not to cause uniform resonance, wood beams, tapestries with thick foam behind them, and convex or angled ceilings.

Front Wall Reflections: Bookcases with different size books, tapestries with mineral wool or thick foam behind, trusses and pillars, large leaf plants.

Side wall Reflections: Bookcases with different size books, artwork with uneven surface, trusses and pillars, wainscot, and large leaf plants. Treatments should be symmetrical, so they are the same on the right and left sidewalls.

Back wall Reflections: Bookcases with different size books, trusses and pillars, artwork with uneven surface, large leaf plants, lamps with large round shades.

Windows: drapes, honeycomb shades or shudders.

Introduction to Acoustic treatments

There are two ways to deal with primary reflections and flutter echoes. Those are absorption and diffusion. The first assumption for acoustic treatment is to absorb as much as you can. However, absorption is only one tool in the tool box- with diffusion being the other. The value of diffusion is well known in opera houses and recording studio. As this is becoming more common knowledge, these acoustic treatment techniques are more popular for home theater and two channel audio setups. A successful use of acoustic treatment involves both absorption and diffusion.

How absorbers work

Absorbers are split into three groups, Porous absorbers, Resonant absorbers and Membrane absorbers.

Porous absorbers (fiberglass, mineral wool, open cell foam, cotton jute, etc.) work by causing friction or resistance to the air particles in a sound wave. This means that a typical foam or fiber like material must vibrate a little bit internally. This resistance works best when a sound wave is at its highest particle velocity point, about ¼ wavelength from a reflective boundary. The further out you are from a boundary the more absorption. This can be accomplished by a thicker material or by spacing the material away from a wall. Porous absorbers are more of a brute force method of sound absorption, but they will absorb a wider range of frequencies than other methods.

Resonant absorbers can use the vibration of enclosed air and suspension to create antiphase sound waves. These antiphase sound waves can decrease the dB level of sound dramatically. The only limitation is the narrow bandwidth and complexity. This style of absorber is almost exclusively used for trapping bass frequencies. These usually are perforated plasterboard or perforated metal corrugated sheets or boxes.

Membrane absorbers are basically a flat box mounted on the wall with a sheet of plywood or something similar on the front, with a light material like wool inside the box. These can be built as part of your wall if doing a new build-out or reconstruction on your room. These types of absorbers usually have a resonance frequency which the membrane absorber absorbs. They can also be part of doors, windows and floors.

How Diffusers Work

In a nutshell, they scatter sound. Diffusers spread out the sound energy and dB level in time, so when you hear diffused sound it might seem livelier or come from a larger space. There are number of mechanisms that diffuse sound. The first group is the geometric diffuser, which can be shaped like a half cylinder, full cylinder, a pyramid, or just a triangular shape. When sound hits these diffuser shapes, the sound is redirected based on where it impacts and at what angle this is. The second group is the number theory based Quadratic Residue Sequence and Primitive Root Sequence Diffusers. They work by using wells or cavities of different depths to create phase differences, resulting in the scattering of sound. The third group employs distributed absorption and reflection, which creates a difference in dB level at the surface but when summed together they scatter sound. Many of these hybrid designs work effectively to absorb midrange frequencies and then diffuse the high frequencies.

The Treatment Goes Where?

For treating primary reflections, the first thing you need to do is determine distances of all your primary reflections. First measure the distance from your speakers to where you will be sitting. Next, find all primary reflections in the room with the mirror trick. Then measure this distance to where you will be sitting. Then subtract the primary reflections distance from the direct sound from your speaker. The shorter the distance, the louder the reflection will be. For distances over 5ft you can employ more diffusion.

Arranging absorbers and diffusers in a room can be confusing. Use the mirror trick to locate them. The wall behind your front speakers will need treatment for two primary reflections, and if using a center channel there is three. The front wall reflections have a big impact on the mid bass, so a thick absorber will be required for this. At least two-inch thick fiberglass or mineral wool should be used. Next, treat the middle of that same wall with an absorber or diffuser to address front to back flutter echoes. Place the diffuser at or just above ear level. Next you will need to treat the floor with a large thick rug made of wool or cotton jute. The ceiling will also need treatment with thick acoustic absorber panels. On the sidewalls, you can treat the primary reflections with diffusers if the wall is a least 4-5 feet away. If the walls are closer look at an absorber or possibly a combination of absorber/diffuser. Diffusers will need to be placed on the sidewalls. This will mitigate the sidewall flutter echoes. Lastly the back wall will need a diffuser or two in the middle at about ear level.

The diagram shows a treatment arrangement that illustrates how many spots in a room can be treated. The red colored panels are wideband diffuser/absorbers, the blue panels are diffusers and the green panels in the corners are bass traps. It may seem like a lot of panels and many of these treatment spots should experimented with. This is especially true for treatments that deal with your surround speaker's primary reflections.



2 COMMENTS

James Shuster
November 16, 2018 at 13:31

Excellent article. Another trick I like to use with acoustic room design is to put any sound absorbing item where people would probably not wish to sit. If there are already enough seats in the room, then chances are, no one would prefer to sit next to a speaker, anyway. So putting a large plant, bookcase, any 3D type of sound absorbing base under a counter height table, or even a small bookcase with non-uniform book sizes. You don’t want to obstruct direct view of a speaker from any possible listening seat. There are spots in front and to the side, between a speaker and the wall, that are probably not really being used for anything. If it fits the room, you could even hang a pull back curtain over this corner and no one would miss the space.

It’s pretty easy to start looking cluttered with too much in the room, but I like to keep this in mind when designing a listening room or lower budget home theater.

Bart Andeer
November 16, 2018 at 13:33

Thank you for this article. It is very misunderstood, even by “experts”. I very successfully control low frequencies to 27 Hz, but it is expensive. Diffusion should always be broadband, it makes a huge difference. Use arrays front and /or rear. Generally in rooms less than 17’ use QRD horizontally. The 1st side wall reflection is not an enemy, it is where the breadth beyond the speakers comes from. Best generally tamed with mild absorption or diffusion. I have done “hear the difference” demonstrations with Resolution Acoustics with adjoining rooms, one treated, one not at AXPONA and Capital Audio Fest. If making a significant investment, and done right, it will be significant, you really should audition rooms done by who ever you wish to do your room, make sure you like what he/she does. The biggest mistake most DIY’ers make is killing the room with excessive absorption and believing forum know-it-all’s.
Good Luck,
Bart Andeer
Resolution Acoustics


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