An overview of Room Modes - Bass Frequency concerns in small rooms

So far in this blog we've discussed horn vs. cone speakers, box vs. architectural speakers, and an overview of speakers in general.

Today I'd like to provide a little background on the physics of sound, which will lead to a future entry on subwoofers/satellites vs. full-range speakers.

In the days when I first started playing with and learning about audio, the only way to get full-range sound (both good low frequencies and good high-frequencies) was to use a good full-range speaker. At its core, a full-range speaker contains one or more cones and/or horns in a single box, and the aggregate device reproduces all sounds from the lowest to the highest.

Cheap full-range speakers had only a single cone (or horn) that had to deliver all sound frequencies at the same time. Better speakers had two cones, a driver or woofer for low frequencies and a tweeter for high-frequencies. There was a crossover circuit that fed low-frequency sound to the driver, and high-frequency sound to the tweeter. The best full-range speakers added a mid-range cone and more crossover circuitry. Some more exotic speakers had multiple tweeters, mid-ranges, and even woofers to deliver more power or punch in those frequency ranges.

Note that all of these sounds are delivered from a single box. This means that all of the sounds come from essentially the same place in the room.

However, research by Floyd Toole and others over the last 40 years or so has essentially demonstrated that, depending on the frequency, sound can have very different characteristics. High-frequency sounds are very directional, while low-frequency sounds are omni-directional - they radiate in all directions.

Low-frequency sounds, also known as bass, have the further characteristic of long wavelengths. The wavelength of a bass frequency ranges from about 56.5 feet (at 20 Hz) to 11.3 feet (at 100 Hz). Relative to a typical listening room in a home (which we consider to be a "small" room when compared to concert halls), these are very long waves, and have very different characteristics than higher frequency sounds that have wavelengths measured in inches or fractions of inches.

When a high-frequency sound is produced in a small room, the directional sound travels in one direction to a boundary (wall, floor, ceiling), and bounces to the next boundary, etc. until its energy is dissipated. This means when the speaker is pointed at the listener, the sound travels directly to the listener's ears, and also bounces off the walls. These secondary "bounces" hit the listener's ears a few milliseconds later than the original direct sound. Third- and fourth-order reflections hit the ear even later. At these frequencies, the human ear is very good at combining and differentiating those reflections. In fact, the earlier reflections actually reinforce the original sounds, making the listener's perception clearer and stronger.

Later reflections, on the other hand, are perceived as echoes. They are so far out in time from the original sound that the ear cannot recombine them into the original sound, but rather perceives them as a separate sound.

Low-frequency sounds act very different. The wavelengths are long relative to the room, and they emanate from the speaker in all directions. Every room will have one specific frequency for which the room length exactly matches the length of the sound wave. It will also have a frequency for which the room width exactly matches the sound wave length, and a third for which the room height matches the wavelength.

Let's consider just one of these for the moment - the frequency at which the wavelength exactly matches the length of the room.

When the sound emanates from the speaker at one end of the room, it travels the length of the room to the opposite wall, and bounces back and forth between the front and back wall. These reflections will reinforce themselves - the peak of the wave will be higher, and the valley of the wave will be lower. This is called a standing wave.

You can hear this effect very easily by putting a subwoofer in a room and sending a pure test tone through it. By varying the tone, it's easy to find a frequency that is too loud in one spot, but too soft in another. Quite literally, you can find a "hotspot" for that frequency, where it's too loud, and then take a step or two in any direction, only to have that sound completely disappear! This frequency is called a "room mode", and it also happens for wavelengths that are 1/2, 1/3, etc. of the room length.

Of course, this is all basic physics, so with a little math we can predict which frequencies will cause a room mode. Since every room has dimensions, it's impossible to completely eliminate room modes. However, by understanding what causes a room mode, we can compensate for them by proper subwoofer placement, multiple subwoofers, parametric equalizers, or other mechanisms of good audio design.

If you want the best possible sound out of your audio equipment, whether it's your home theater, stereo system, or even a mixing room, classroom, or auditorium, call your local experts! Inspired Electronics, Inc. is one of just a handful of companies in Illinois certified for both audio and video calibration, and we can make your room sound better. Call us today at 847-471-4420 to set up an initial consultation.

Distributed Audio options

Rob Schultz, President of Inspired Electronics, Inc., was recently published in Electronic House magazine's Ask a Pro section. Read the original article here: http://www.electronichouse.com/article/q_how_much_power_do_i_need_for_multiroom_audio or continue reading.

Q. I just moved into a house that is pre-wired for speakers & volume controls in 3 rooms, 2 outdoor porches, a media room and a family room. I will be buying a new receiver and amplifier but don’t know how much power I need or what Ohm speakers/controls I should get. Any budget conscious recommendations or guidance for a 1st time do-it-yourselfer? - John, Dallas TX

Rob Schultz, of Inspired Electronics, Inc. offers up this solution:

The most basic approach is to have a speaker selector and volume control unit at the head end, and split the output from the receiver. This is simple to install, and pretty inexpensive if the house is already wired. The speaker selector must be impedence matching, which means there’s a switch that lets you tell it how many speaker pairs there are. When you select the right number of speakers, the unit adjusts for the impedence of the speakers so you don’t blow the amplifier. The main drawback of this sytem is that you can only control what’s playing and the volume in the main room. So when your favorite song comes on, you have to run to the main room to turn it up!

You can also use a speaker selector in the main room with individual volume controls in each room. Hopefully the pre-wire has service loops to the right volume control locations, and you know where those are! Use impedence matching volume controls in this scenario. Now you can adjust volume in each room, but you still have to go back to the main room to change what’s playing.

For either option, we plan a minimum of 20 watts per zone, and 40 watts for outdoor zones or large rooms, and we spec 8 Ohm speakers. For more zones or when more volume is desired, we use either a high-power stereo receiver with a speaker selector, or a low-power receiver with a distribution amplifier, such as the Niles 1230 (12 x 30 watts) or Niles 1260 (12 x 60 watts).
By splitting your audio signal, you decrease power and volume by as many ways as you have zones. So if you have a 50 watt amplifier split 5 ways, you get only 10 watts per zone - not even enough for background music.

More advanced options allow you to control the equipment from each room, provide metadata feedback on the keypads, and play different sources in different rooms.

If you're interested in having audio throughout your home or business, visit our web site at http://www.inspired-electronics.com, or call Keith at (847) 471-4420.

Architectural speakers

In my last post, I talked about box speakers. These have a well-earned reputation for having the best sound at any given price point.

But architectural speakers (those that are built into the structure, such as in-wall and in-ceiling speakers) have made great strides over the last few years, so that the best architectural speakers are pretty close to box speakers at a similar price point.

The main advantage architectural speaker have over box speakers is that they can blend in with the room, and they don't take up any space. After all, if you bring in an interior designer, or just spend a lot of your own time and energy putting together the perfect room, do you really want to muck it up with speakers? Sure, those boxes may be great in the theater or game room, but what about the living room or dining room, where there aren't a bunch of other electronics in view?

Many people are even willing to give up some of the audio quality benefits of box speakers in order to "get the electronics out of the room." After all, you're not watching movies or concert videos in those rooms, you're really just listening to background music.

Of course, when your favorite song comes on, what do you do? If you're like me (or my wife!), you want to turn it up. And this is where poor speakers really start to show their deficiencies, regardless of whether they're boxes or in-walls.

There are a couple of factors that give box speakers an advantage over architectural speakers. Those factors can be mitigated or even overcome by good speaker design and proper installation.

First, the box is a resonance chamber. The chamber is designed during the speaker design process, and can be changed during early prototype testing to give the speaker the proper tone, timbre, and resonance. With a built-in speaker, the wall becomes the resonance chamber. You may have a big open box, there may be firebreaks in the wall (making a smaller resonance chamber) or there may be insulation in the wall (absorbing some of the sound). For in-ceiling speakers you may even have a huge void (the attic) behind the speaker.

Good installation can help here. First, insist that the installer use a back box that's designed for the speaker. Since the back box is designed before production, you will get many of the same characteristics as a box speaker, and you will avoid most of the problems of architectural speakers. Generally, back boxes are available for in-ceiling speakers and higher-end in-wall speakers. However, the back boxes may not fit well in a standard 2x4 wall, or may be too high for the cavity above the ceiling. In addition, some back boxes are designed as pre-construction boxes, which mean they must be installed before the drywall is put up.

The second advantage box speakers have over architectural speakers is their inherent rigidity. The box is joined on all edges, and is usually made of MDF (Medium Density Fiberboard). This construction is highly resistant to the types of forces that speakers generate when asked to deliver high sound output. Architectural speakers, on the other hand, usually have an ABS plastic or aluminum "spider" - a sort of cage that houses the driver and magnets. Since there are narrow legs separated by relatively wide gaps, the speaker is much more susceptible to those forces, which can flex the entire speaker out of shape, forcing the cone to be out of shape. Of course, this can impair the sound quality at those volume levels.

Again, proper installation is a good first line of defense. These speakers need to be firmly mounted to a well-constructed wall. In doing so, the speakers gain back some of their stiffness. In particular, if you can attach the speakers to 3/4 or 1" drywall which is glued and tightly screwed to good wooden studs, the speaker will have gained considerable stiffness. And a good back-box will also help to stiffen up the speaker - indeed, most better architectural speakers are designed to work best with a back-box. Beyond that, some speakers have specific design characteristics which help with this problem. James Loudspeakers, for example, have an integrated back-box made out of welded aircraft aluminum. These are essentially box speakers that are designed to fit inside the wall. And other speakers have aluminum or more exotic spiders, which further reduce flexing.

For a professional opinion on what speakers will work best in your listening environment, call Inspired Electronics, Inc. at 847.471.4420 for a free in-home consultation, or visit our web site at http://www.inspired-electronics.com/.

Free-standing or Box speakers

What do you envision when you hear the word "speaker"? I know some people think of a person on a podium addressing an audience, but for those of us in the audio and video realm, we think of something that generates sound - the final link in the process that starts with a microphone and enables sound to be captured, transmitted, and replayed.


And for most of us, that thing looks like a box.

We'll talk about other options (in-wall, in-ceiling, etc.) in another article. But right now, I'd like to talk about this most common type of audio speaker.

In previous articles, I talked about the different driver types (cones, domes, horns, ribbons, and others). The box is really just the enclosure that houses the driver and associated electronics.

However, that box is also an important element in the acoustical properties of the speaker. If the box is the wrong size, shape, or made of the wrong materials, it will affect the efficiency and the sound quality of the speaker.

For example, many speaker enclosures are made of MDF (medium-density fiberboard) covered with a lacquer or wood laminate. The lacquer or laminate is to help the box look good in your living room (MDF is not very pretty!). But the MDF is what really provides the acoustical properties.

The box itself serves as a resonance chamber and contains the sound emanating from the back of the speaker. This enclosure needs to be made of an acoustically inert substance to reduce "sympathetic vibrations" which can lead to the speaker "walking" across the floor, or even shaking itself apart! Lead or concrete would be excellent, except they're too heavy. MDF, on the other hand, provides a reasonable balance between weight and acoustical properties. Some manufacturers make their enclosures out of other materials as well. For example, high-end speaker manufacturer James Loudspeakers makes the enclosures out of aircraft aluminum, which has excellent audio qualities. But they still have to make a nice-looking outer shell.

The construction of the box is important, too. It needs to be completely sealed, or some of the sound will leak out of the enclosure instead of being sent in the direction of the listener. This keeps the sound from the back of the speaker from mingling with the sound from the front of the speaker. This is important because the speaker actually creates sound both when it moves forward and when it moves backward. The sound created by the backward motion is 180 degrees out of phase with the forward sound, which means if the two mingle, they will cancel each other out!

If you'd like to learn more about which speakers are right for your home theater or audio setup, please visit us online, or call us at 847.471.4420.

An overview of Horn Speakers

Horn Speakers are often considered to be of excellent quality, but the reality is that quality varies greatly depending on price and intended application. There are a couple of high-end speaker manufacturers such as Klipsch and Fostex that have well-designed horns, but remember that horns are also used in bullhorns and low-quality outdoor loudspeakers, which have very limited ranges.

Horn speakers use the interaction between the horn and the driver to increase the efficiency of the speaker. This allows a speaker to generate a higher volume for the same power input and distortion level. However, the same properties that increase the voume limit the frequency range. So a bullhorn can generate a high volume of sound using small batteries, but only in the frequency range of the human voice.

In order to compensate for this, some horn speakers use multiple drivers (with appropriately tuned horns) to cover the entire frequency range. Others use horns primarily for one frequency range (e.g. for the tweeters), and use standard cone drivers for the rest of the frequency range. Most Klipsch speakers fall into this category. Of course, there are always some manufacturers that save money by not adequately covering the entire range of hearing.

If you're looking for horn speakers or any other type of speakers for your home theater or home audio system, visit Inspired Electronics, Inc. on the web!

A little about cone speakers

The most common type of speaker is a cone speaker. This speaker has a diaphragm made out of a cone of a fairly stiff material, such as paper, polypropylene, carbon fiber, or aluminum, although other more exotic materials have been used. The diaphagm moves back and forth, pushing air out into the room, generating the sounds you hear.

The best material for a speaker cone is something that has low mass, good stiffness, and good damping properties. The low mass improves the speakers efficiency, meaning less power is needed to drive the speaker. If the material is not stiff enough, it won't generate clean sounds, especially at higher frequencies and higher volumes. And if there's not enough damping, the speaker will have a characteristic "ringing" sound.

Interestingly, paper is a good speaker material, because it has all of the properties needed. However, paper has the disadvantage of not handling moisture well (including changes in humidity). Polypropylene is used often in in outdoor or high moisture applications. Aluminum and other metal cones are light and stiff, but often don't have sufficient damping properties, and can sound too "bright". Fiberglass, Kevlar and Carbon Fiber have good properties, but can increase the cost of the speakers. So if you're interested in good speakers, and willing to pay a little more, we often recommend these materials.

The mechanism that moves the cone back and forth is a large magnet and an electromagnet. These two work together to vibrate the cone, compressing and decompressing the air, and generating sound waves. Note that the speaker generates sound in both directions. Since the reverse direction is exactly opposite the forward direction (180 degrees out of phase), care must be taken to ensure the reverse sound doesn't cancel out the forward sound!

In order to provide a stiff frame for the magnets to drive against, the speaker has a "spider", which is a stiff cage. Many spiders are made from a stiff plastic, while others are made of aluminum or another metal. At one time, spiders were made of Bakelite, but plastic and aluminum are much more durable. Plastic is light and inexpensive, but can distort under high load (high volume).

Speakers also have a flexible surround which binds the cone to the frame. This surround also keeps the sound generated by the reverse direction of the cone from coming around to the front and canceling the sound generated by the forward direction. In many older speakers, the surrounds have degraded to the point where this happens, and the speakers need to be repaired or replaced.

I'll talk more about speaker types and speaker design in future Blog postings, so stay tuned!

In the meantime, visit us on the web to learn more about how we can install a home theater or distributed audio system for you!

The importance of speakers

If you're listening to music, watching TV, or even talking on the phone, you're using speakers. Speakers, after all, are what translates recorded or broadcast material into sound waves that you can actually hear.

And the quality and type of speakers has a tremendous impact on how good that material sounds.

For example, have you ever sat on hold listening to your favorite song? I'll bet you found it at least somewhat annoying, because you couldn't hear all of the nuances of the music that you're used to.

That's because telephone speakers (in the earpiece, or even on a speakerphone), are not what we call full-range speakers. They're only designed to reproduce the human speaking tones, which range from about 110Hz for a low male voice, to about 1000 Hz for a higher female voice, up to about 3000 Hz for a child's scream.

On the other hand, the full range of human hearing is from about 20 Hz to about 20,000 Hz (20 KHz). Good full-range speakers reproduce all of that sound, and the entire range is used in music and movies. If you're not hearing the upper and lower ranges, you're missing a lot of important information in the sound track - and it just doesn't sound good!

Now, consider the speakers you're using for your current TV and movie-watching. If you're like many people, you listen through the speakers that came with the TV. Fortunately, TV manufacturers tend to be really good at video - unfortunately, audio is not really their specialty.

Many people consider these speakers to be "courtesy speakers" - that is, speakers that are included to allow you to set up and watch the TV without having to do a lot of work. But that doesn't make them good.

Lots of people have tried hooking their TV into their stereo, which usually results in far better sound than with just the TV speakers. We often hook up "surround sound" receivers and speakers - and the results are tremendous! Click here for more information on our home theater installations.

There are, of course, wide variances in the sound range of different types of speakers, so be sure the speakers you select will cover the entire range.

In the next few blogs, I'll talk about different aspects of speakers.

In the meantime, please visit us on the web at http://www.inspired-electronics.com/, or call us at 847.471.4420. We can answer your audio and video related questions, and help you get set up for the best home theater!