IT'S easy to get so immersed in the electronic end of things that we forget momentarily the simple truth that all hi-fi systems, in reality, are acoustic devices. In the fullest sense of the term, acoustics is the study of sound in all its manifestations and in relation to all types of materials--gases, liquids and solids.
In a high-fidelity sound reproduction system, electrical energy is first converted by the speaker and its associated enclosure into sound waves in the air of the listening area. It is here at the transducing element, where the conversion is made, that our concern with acoustics begins. Except in occasional outdoor installations, this sound is then propagated in the more or less enclosed space of a room. The room thus is the source of our second group of acoustic considerations. Getting the program to sound right in the area where it is wanted, and keeping it confined to that area only, are the two major acoustic problems that concern the installer.
It should hardly be necessary to add that the user is just as much concerned with this end result as the installer, or rather more so, as he is going to live with the system for a reasonable time. This section discusses the basic acoustic factors that affect the installation, while the following section explains how to deal with some of the more common difficulties you may encounter in practical work.
Fig. 801. Cross-section of a typical cone type speaker.
Speakers
At the present state of the art, a wide variety of speakers and enclosures is available; so great a variety that several volumes would be required to cover the subject adequately.
Since, for our purpose, acoustics begins with the speaker, let's make it clear right now that you cannot get good sound from a poor speaker or from any multiplication of poor speakers. We tried it years ago. The idea was that since cheap speakers are so non-uniformly relative to each other, at a frequency where one was bad some others would be good. This worked out a little like making an omelet with only a few bad eggs in the lot, although the analogy is not perfect. Suffice to say, it didn't work out that way, although it is true that with a great deal of doctoring and cut-and-try experimenting, it is possible to get such an arrangement to sound passable.
You may well ask, then, what constitutes a good speaker? The answer requires that we classify speakers according to the frequency range and power they are expected to handle. A tweeter or speaker that is to be good for high frequencies has very different requirements from a woofer or low-frequency speaker.
At present, most speakers are of the permanent-magnet type, although electrostatic, piezoelectric and possibly corona-discharge (ionization) types, bid fair to become of increasing importance in the future.
Most woofers are cone-type direct-radiator speakers basically similar in cross-section to Fig. 801. Since they handle the greater part of the audio load in terms of power, they should be ruggedly constructed. A powerful magnet of generous size is required to provide the magnetic flux necessary to give good control of the voice-coil motion. The cone must be light, but sufficiently stiff to move as a unit over a wide range of frequencies. And the suspension must be flexible enough to permit free movement of the cone, but yet not introduce spurious effects of its own.
Fig. 802. Cross section of a horn tweeter.
All in all, making a good speaker is a reasonably complicated job, which is why they are not cheap.
A good tweeter need handle only a small fraction of the power required of a woofer but, it must be even more precise. Think, for example, of the diaphragm of the horn tweeter shown in Fig. 802 generating a complex waveform at a frequency of 15,000 cycles. A rather staggering order of engineering and production skill is required to make this possible. For this reason, a good tweeter isn't cheap either, even though it may physically be comparatively small.
Speaker enclosures
A speaker enclosure is an acoustical device whose basic purpose is to improve the low-frequency response of the speaker.
Even the best speakers will give poor performance at low frequencies if they are not enclosed or coupled to the air in some way. The reason is that as soon as the frequency is reduced to a point where half of the wavelength of the sound in air is about equal to the sound path from the back to the front of the speaker, the back wave, being 180° out of phase with the front wave, starts to cancel it. The speaker is still generating sound waves, but the back wave and front wave reach the listener at the same time and cancel. So they don't do you any good. Therefore, you've got to help the speaker along by eliminating or controlling that back wave, if you're to get respectable reproduction at the low frequency end of the sound spectrum.
Infinite baffle
The simplest way to get rid of trouble with the back wave is to eliminate it entirely. This is the thinking behind the infinite baffle.
A truly infinite baffle is something that exists only in theory, but there are two ways of approximating its effect in practical installation work. One is the well-known trick of mounting a speaker in a hole in a wall between two rooms (Fig. 803). By isolating the back wave from the front wave, the wall in which the speaker is mounted becomes effectively an infinite baffle. Cancellation of the front wave by the 180° out-of-phase back wave at low frequencies is eliminated. The speaker is now free to respond smoothly down to its resonant frequency. At this point, it peaks and then drops off quite rapidly. Very satisfactory low-frequency response can be obtained in this way, provided that the speaker has a reasonably low resonant frequency to start with. Of course, response can be carried well below resonance also, but some tricks are required.
Efficiency
There is some small objection to this method because it is inefficient, since the entire back wave is thrown away unless the room behind the speaker is also a listening room. Considering the excess power available in contemporary amplifiers, however, we don't consider this objection as very serious, but there's another objection that is serious: the majority of people do not have a room available for the back side of the speaker.
Fortunately, there is another way of approximating an infinite baffle--the use of a large, fully-enclosed cabinet with a hole only for the speaker. By large, we mean a cabinet of the order of 5 1/2 cubic feet for an 8-inch speaker, 8 cubic feet for a 12-inch speaker, or 10-cubic feet for a 15-inch speaker. Such cabinets must be solidly constructed with plenty of internal battens to control panel resonances, and plenty of padding to control standing waves and absorb the back-wave energy.
With this arrangement, the inefficiency objection remains. The cabinet must be so large as to be unwieldy for many residential applications but, if you reduce its size, you'll raise the resonant frequency of the entire speaker system and thus lose part of the bass end. However, it's an excellent enclosure method if you have the room space for it.
Reflex baffle
The phase inverter or reflex baffle provides a way of reducing the overall size of the speaker enclosure while at the same time increasing efficiency and boosting the bass response below the resonant frequency of the speaker. This is accomplished by means of a tuned port which enables you to utilize a portion of the back wave at low frequencies. Fig. 804 is a chart which, with the accompanying text, will assist you in determining port sizes for reflex enclosures. As an installer, you are not expected to be an ...
Fig. 803. Approximation
of an infinite baffle by mounting in the wall between two rooms.
... acoustical engineer, so don't hang yourself by your thumbs trying to monkey with cabinets and ports already specified and engineered by a manufacturer for his speakers. We imagine his engineers know what they are about. Specify only ports when you have to.
Even if enclosure size is no object, you might want to use a reflex, rather than an infinite baffle, in the event you or your client has chosen a speaker that has excellent mid- and high-range response but is a bit weak at the low end. The reflex may allow you to boost the low end enough to bring the overall response into a more pleasing balance.
Horns
According to Paul Klipsch, one of the foremost horn engineers and designers in the field, a horn should not properly be called an enclosure. It is a housing that is at the same time an integral part, albeit an extension, of the speaker itself. To the installer, this means you do not under any circumstances fiddle with the insides of a horn. If you like a particular speaker in a particular horn, use it "as is." If you don't like certain exterior aspects such as grille cloth, legs, molding or color, you can change these things. But do nothing that will alter the interior arrangements in any way. The chances are 110 to 0 that any such alterations will be detrimental to the performance.
Other speaker housings
The acoustic facts of life underlying the three types of speaker housings mentioned have given rise to a number of patented speaker enclosures and complete speaker systems. Many of these enjoy considerable popularity by virtue of providing respectable sound in a compact package. Such enclosures and systems, like horns, should not be tampered with internally by the installer, if best results are to be obtained.
Room acoustics
Assuming that, by whatever means, you have a speaker and housing operating satisfactorily together, the sound must still move through the room to the listener's ears. The room itself can acoustically favor, hinder or distort the propagation of the desired sounds. By the room, we mean the room and all the en closed furnishings. This means upholstery (chairs and sofas), drapes, floor coverings, cabinets and tables, as well as areas of smooth plaster, glass, wood or tile in floor, walls and ceiling. For, as we shall see, changing some of these can substantially alter the room acoustics for better or for worse.
Speaker placement
To give the room a fighting chance to do well for you, begin by making a determined effort to locate the speaker system or systems in acoustically favorable positions. Such positions are diagrammed in Section 11. In that section we will remind you to remember acoustics when dealing with esthetic problems, and here we must remind you of the reverse. Remember all the way through your consideration of acoustic matters that an installation that performs beautifully, but has to look badly to do it, will still be a monumental failure with just about all of your clients.
At times, you may have to compromise on speaker location for esthetic reasons, but merely because a speaker is not in the ideal spot for a given room doesn't necessarily mean that you have an acoustic calamity on your hands.
A speaker that is not located in the optimum place in a room will probably give poor sound distribution to parts of it. But this does not mean that it must provide bad sound in terms of causing echoes, standing waves or any type of acoustic distortion. These problems involve other factors in addition to speaker placement.
Reverberation time When a speaker starts to produce sound in a room, it does not instantly reach full intensity, due to the fact that sound travels through air at a finite velocity (approximately 1,100 feet per second) and therefore takes a finite time to reach the room limits.
As the waves of sound radiating from the speaker spread out into the room, they are partially absorbed and partially reflected each time they strike a wall, floor, ceiling or some object in the room.
Each wave is reflected many times before it is ultimately dissipated. Full intensity is reached when the amount of sound energy being absorbed equals the amount being produced by the speaker.
Fig. 804. Graph shows the relationships between the resonance of reflex
en closures, port area, and cabinet volume. The reflex cabinet at the
right is built in the traditional shape.
Fig. 805. Path of a pencil of sound in a room from source through
various reflections to ultimate dissipation.
Fig. 805 shows in very abbreviated form, and in two dimensions only, the path of a given point on a sound wavefront as the wave goes bouncing about in a room. The thinning of the line with each reflection indicates the partial absorption of the sound each time it strikes something. In reality, a sound would bounce many more times in a room than can be shown conveniently in the drawing, before being absorbed entirely.
When the speaker stops producing sound, the energy already bouncing about in the room will require a finite time before it is absorbed and dissipated, just as a finite time was required to build it up. This time, specifically the period required for the sound to decay to one-millionth of its original intensity, is termed the reverberation time of the room.
Absorption
The reverberation times of rooms will vary somewhat with differences in size (cubic volume) and with the absorptiveness of the interior surface and contents. For example, an acoustic tile ceiling or a carpeted floor will absorb more of the sound hitting them and reflect less than a smooth plaster ceiling or a bare hard wood floor. As surface absorption is increased, the number of reflections necessary to absorb a given amount of sound energy fully is decreased, thus shortening the reverberation time.
Fig. 806. When two rooms are identical except for size, the larger
will have a longer reverberation time because of the larger air paths
the sound must travel.
Let's take another example. Room A and room B have interior surfaces of the same materials and in the same proportions, but room B is half again as large as A (Fig. 806). The same number of reflections will be required to absorb fully a given sound in both rooms. But in B, the distances and therefore, the time be tween reflections is longer, increasing the reverberation time.
If the reverberation time in a listening room is too short be cause there is too much highly absorptive material in the room, reverberation dies out very rapidly, and a very dull, flat and un realistic acoustic effect results. On the other hand, when a room has too long a reverberation time, unpleasant hangover effects will be noted due to a long-period echoes and standing waves.
Thus, it is entirely possible to have an excellent hi-fi system correctly installed technically and in perfect working order that can produce only a very unsatisfactory sound in a particular room, purely because of room acoustics.
Absorbency vs frequency
Thus far, in discussing reverberation time, we have considered sound without regard to differences in frequency. This could be misleading, because any given material will absorb differing amounts of sound energy at different frequencies. A typical carpet, for instance, will absorb about three times as much at 2,048 cycles as it will at 128. And at 4,096, it has better than four times the absorption at 128 cycles.
Generally, absorption will increase in any material as the frequency goes up. Not only does absorption increase with frequency but the rate of its increase also slopes up as frequency rises. The curves of absorption coefficients relative to frequency increase will vary for different materials, and they won't necessarily be smooth curves, either. Some may have flats or dips here and there, but the general direction of such curves can be depended upon to move up with rising frequency.
This partly explains the common problem of getting adequate dispersion of highs in a listening room. Not only are highs difficult to spread as they emanate from the speaker, but they are also the first things to be absorbed by the room and therefore, won't readily spread out by reflection either. If a room is live enough to disperse highs well by reflection, it will very likely be intolerably reverberant at mid- and low-frequencies. On the other hand, a very dead room with extremely low reverberation time will be one in which you won't hear any highs very well unless you keep your ear pretty close to the center axis of the speaker system.
Optimum reverberation time
As we have noted in the previous discussion, if the reverberation time in a given room is too short or too long, the reproduction of music in that room will be impaired by acoustics, regardless of the quality of the reproducing system. Hence, it might be helpful if we could compute or measure the actual reverberation time of a room and compare this with some sort of absolute optimum for its cubic volume.
Unfortunately, the available methods for either measuring or computing reverberation time accurately are sufficiently complex to suggest that the installer will do well to avoid them. These are jobs which should be done by an acoustical engineer if considered essential and, if you are doing a sound system in an auditorium, it may very well be essential. However, the average residential installation will not require elaborate analysis.
Sabine, one of the pioneers in architectural acoustics, used only his ear as a detector, and a stop watch for time measurements. For most of your purposes this method, plus an educated look at the room and furnishings, will be good enough.
Make it a standard procedure to look over any room in which you are planning an installation with a thought to acoustics.
Fig. 807. Reverberation time of a room versus room volume is a function
of frequency. (Modified from Harry Olson's Musical Engineering, McGraw-Hill
Book Co., Inc.)
Suppose you see a fairly large room, say 18 x 30 feet, with lots of glass and smooth plaster, a bare tile floor, very thin and few drapes, and a few light, modern upholstered pieces. You have good reason to suspect at once that the room may be too reverberant. Now, try a single sharp clap of your hands at various spots in the room and listen carefully to the rate of decay of the echo each time. If the clap rings on stubbornly, your suspicion is taking on substance.
Making reverberation measurements you want to make a rough reverberation measurement, hook up an audio oscillator ahead of an amplifier and speaker. Then, with ear and stopwatch, check the reverberation time at a few frequencies, perhaps 100, 500 and 1,000 cycles. If you come up with times around 2 seconds or greater, you have too long a reverberation time.
Reverberation vs room volume
The graph of Fig. 807 gives the optimum reverberation time for rooms as a function of room volume for a few selected frequency ranges. Note that the optimum time differs at different frequencies, but note also that the curves don't start really to fan out until the room volume gets well over 10,000 cubic feet. Unless you're doing a lot of theaters and auditoriums, you won't deal with many rooms over 10,000 cubic feet.
Live and dead rooms
A room that is too dead (not enough reverberation time) is a bit more difficult to spot in advance without instruments. Fortunately, its deleterious effects are also far less painful. What with echoes, standing waves and various spurious resonances winging about, a room that is too live produces some very positive types of unpleasantness, while a dead room produces effects primarily of a negative nature. Highs just disappear, the mid-range loses brilliance and clarity due to loss of the upper overtones, and bass tones tend to degenerate into a series of dull thuds.
Very often, visual inspection will tip you off to a room that later proves to be too dead. Thick carpeting, lots of heavy drapes, extensive well-stocked bookshelves and plenty of heavy, upholstered furniture should make you suspicious. The clapping trick doesn't help you much here, except to confirm your suspicions by giving you no ring.
Both excessive liveness and excessive deadness in a listening room are bad, but of the two, we tend to find excessive liveness the more objectionable, due to the positive nature of the aberrations it introduces. Suggestions regarding the alleviation of both of these conditions will appear in Section 9.
Miscellaneous object resonances
An ill-assorted polyglot group of miscellaneous grunts, groans, squeaks, rattles, buzzes, whistles and rumbles occasionally appears to plague owner and installer subsequent to an installation. They are usually intermittent, and often seem at first to appear and disappear at random. Then, you'll begin to notice that they are associated with specific frequencies as they recur in the program material coming through the system.
They are not in the speaker box, because they come from somewhere else in the room, and at times they can be quite troublesome to run down.
We don't know where you might find them, but perhaps we can help you by giving you a few examples of some we've run across.
Fig. 808. (a) Sound waves traveling through a room, and (b) reflected
from the end wall.
Often, a double-hung window will rattle in its frame at a particular frequency, while another window in the same room will rattle at a different frequency. Closet and cabinet doors, al though clear across the room, are common offenders in the same way. Piano strings will sometimes vibrate sympathetically at all sorts of odd frequencies. And, occasionally, a slight vibration set up in a shelf is enough to cause bric-a-brac to rattle.
We recall one case when at odd intervals a beautiful, clear bell like tone would ring out through the room. It was always the same tone, and it was usually frightfully out of key with the pro gram at the moment, which didn't make sense. After much head scratching, we finally ran down the cause. A moderately sharp transient at a particular frequency caused a shelf inside a cabinet to vibrate. This in `turn caused a piece of crystal glassware on the shelf to tap against a piece next to it, producing a lovely, clear ring. It was excellent quality crystal. Naturally, slightly separating the pieces on the shelf stopped the ringing.
This was a particularly troublesome one to track down because it appeared only on transients and not when the frequency was generated steadily. But most object noises will give themselves away if you plug an audio-frequency generator into the system and sweep through the whole band very slowly with the gain pretty well up. When you hear an extraneous noise, diddle the frequency up and down a bit until you've got it at the point that produces the worst noise. Then, leaving the generator and system running as is, poke about the room until you locate the cause of the noise.
Fig. 809. Diffraction of sound through a small opening.
It's not a bad idea to make a routine noise check with the generator at the completion of all installations, and point out to the client any noises you uncover. This way, you'll forestall any possibility of the system being blamed for nonexistent defects.
The question is now up to the client as to what he wants done to correct the fault, and what he's willing to live with.
Sound transmission
More than a few pleasant relationships between neighbors, or between landlords and tenants, have been strained if not shattered after hi-ft has entered the picture. Yet, usually, most of the strain can be avoided by a little understanding of sound transmission.
Since the sound you hear in your listening room is transmitted from the speaker through the air to your ear, it is not difficult to understand that it will continue to travel via air through any openings in the room such as windows, doors, etc. and thus be transmitted to adjacent areas. But sound transmitted through solid media--walls, floors and ceilings--is not as readily expected or understood, and it's a darn sight harder to control.
Sound transmissions in air
The same physics that gives us the sounds we want to propagate in air also goes on to produce a spate of effects we definitely don't want. Fortunately, a little understanding of what is happening will provide some clues to the control of the undesirable effects.
Fig. 808-a shows progressively the movement of a sound wave from a speaker out into a room. In. Fig. 808-b, we see it reflected...
Fig. 810. Transmission of sound through a large opening.
...from the end wall. As we noted in discussing reverberation, part of the sound striking the wall is lost in the wall and part reflected into the air, to travel until it strikes another obstacle where the process is repeated.
If, however, there is an opening in the end wall, some of the sound will be transmitted through the opening into the adjoining area. The amount and character of this transmission will vary, de pending upon the size of the opening relative to the wavelength of the sound.
Diffraction If the size of the opening is small relative to the wavelength, as in Fig. 809, the sound passing through will spread out in all directions. But since the amount of sound energy that can pass through the opening is small, the intensity of the sound trans mitted to the adjoining space will also be small.
When sound changes direction by passing an obstacle, the pro cess is defined as diffraction, which is exactly what is happening here. The sound struck the opening from a single direction but is proceeding from it in all directions.
When, as in Fig. 810, a sound encounters an opening that is large relative to the wavelength, the results are quite different.
It passes through with no loss in intensity, but note that this time it does not spread out in all directions but rather it bends only moderately around the edges of the opening. This is what happens when higher-frequency sounds pass through an open door or window.
If you ever heard of a case where someone across the street complained of noise stairs wasn't disturbed in the least, you now know why.
Reflection
In the same way that the size of an opening relative to wave length will vary the way sound is transmitted through the opening, the size of an object relative to wavelength will affect its reflection of sound and its shadowing effect on the area behind it.
When an object is small relative to wavelength, as in Fig. 811, the sound waves bend around it by diffraction, and meet again in a relatively short distance. The object, therefore, casts a very slight sound shadow and does not reflect such a sound appreciably.
Fig. 812 shows how conditions change when the object is large relative to wavelength. It now reflects a substantial portion of the sound striking it and casts a sizable sound shadow. This explains why lows will go around a chair placed in front of a speaker system while the highs won't.
If, instead of being perpendicular to the path of the sound striking it, a reflecting surface is angled (Fig. 813), the deflected sound will bounce off at an angle equal to that at which it struck.
Such a reflecting object will also cast just as dandy a sound shadow as one that is perpendicular to the sound source.
Echo
An echo is merely a sound that has traveled the scenic route via reflection, and arrives at the ear with sufficient delay to give the impression of being distinct from the sound wave that reached the ear directly from the source. For a well-developed echo, the...
Fig. 811. Sound will readily diffract around an obstacle that is small
relative to the wavelength.
...sound must bounce off a rather highly reflective surface or surfaces, so that the echo arrives at the listener's ear without too much loss in intensity relative to the direct sound.
A multiple echo is a series of individual echoes traveling along paths of different lengths and therefore reaches the listener with differing time delays.
Severe echo is seldom a problem in the average living room, but can become quite serious in auditoriums, meeting halls or dance halls, and various commercial and industrial areas.
Standing waves
Fig. 812. A large object reflects sound and casts a sound shadow.
Another type of undesirable acoustic effect produced by reflection is termed a stationary or standing wave. A normal sound wave is progressive in nature, moving continually outward from its source. A standing wave is merely the result of a reflected sound wave combining with the direct wave from the sound source.
Fig. 813. Effect of sound striking an angled reflecting surface.
For a very simple case to illustrate the principle of standing wave formation, consider that a steam whistle is suspended be fore a vertical cliff at a distance of 100 feet. When compressed air is fed to the whistle, it generates a continuing sound that has a wave length of, say, 10 feet. Along the path leading from the whistle perpendicularly to the cliff, sound will travel straight to the cliff and be reflected straight toward the whistle. Thus, as soon as the sound has had time to reach the cliff and return to the whistle, a steady condition will exist in which sound waves are moving in both directions over the same path. The wave length of the sound being 10 feet, it is easy to see that at a point on the sound path 5 feet from the cliff, the direct wave and reflected wave will be in phase and will reinforce each other, causing the sound pressure to be almost double its normal value.
(If no energy was lost at the reflecting surface, the pressure would be doubled.) This reinforcing effect will be found at all points in the sound field where the direct and reflected wave are exactly in phase. On the direct path between source and reflector in this case, the in-phase points would be at 5 feet from the cliff, 15 feet, 25 feet and so on. But at points 10 feet, 20 feet, 30 feet and so on, the direct and reflected waves will be exactly out of phase and would almost completely cancel each other. Since these maxi mum- and minimum-intensity points remain stationary, although the direct and reflected sound waves that combine to produce them continue to move back and forth as usual, they are called standing waves.
In large auditoriums and even in outdoor stadiums, standing-wave patterns frequently occur, particularly when the audience is sparse, permitting large areas of hard surface to reflect the sound waves without the absorbing effects of human bodies and clothing. This may result in "dead" spots where announcements or music are badly distorted or almost inaudible, if the sound system is not carefully installed to minimize such standing-wave effects.
However, in homes and small auditoriums, standing waves are seldom a problem and, in auditoriums built since the advent of amplified sound, the acoustics are usually taken into account when the building is first put on the drawing board, so that such effects are minimized.
Sound transmission in solids
Sound can be transmitted not only through air but also through solid media. This fact turns up to plague the installer in the form of sounds that are generated in the desired listening area, picked up in turn by the walls, floor or ceiling, only to emerge in areas where they are not wanted.
The intensity-of such transmission, and therefore the extent of the disturbance it causes in other people's lives, is governed by three factors. One is the power of the source; the second is the type of construction used in the building; and the third is the manner in which the sound is picked up by the structure.
Sound can be transmitted into solid media in two ways, first via air and second by direct conduction. As we noted when discussing reverberation and reflection, when a sound wave propagated in air strikes a wall, part of the energy is absorbed by the wall and part is reflected by it. Of the energy absorbed by the wall, some is dissipated in the wall but part is also transmitted through it to the air beyond (Fig. 814).
The amount of sound actually absorbed and dissipated in the wall is termed transmission loss, and is measured in decibels.
Although there are occasional exceptions, it will usually be true that the heavier the materials and the thicker the wall, the greater will be the transmission loss.
Direct conduction
A speaker mounted in a wall will conduct energy directly into the wall through its frame. A speaker enclosure standing directly on a bare floor can do the same thing; that is, conduct some sound vibration directly into the floor. Such direct conduction won't radiate very well from the wall or floor, but it can travel astonishing distances through a building. As we shall see in Section 9, except at very low frequencies and very high power, direct conduction for our purposes can be fairly well controlled.
Fig. 814. Effect of a partially reflecting, partially absorbing and
partially transmitting wall.
There is one other circumstance under which direct conduction becomes hard to control, and that is at frequencies that cause parts of the room, walls or floor to resonate.
Structural resonance
In the same way that a speaker cone, a speaker box or a piano string has a resonant frequency or frequencies, so will a wall or a floor. In fact, it may have several and, depending on the construction, it may resonate very freely. This is an aspect of construction that might never bother anyone until a hi-fi set is introduced. But then it can cause quite a disturbance in places adjacent to the listening area, as well as some pretty undesirable effects there, too.
While structural resonances won't often cause the most blatant of your acoustic problems, they'll be among the most difficult to correct if they're bad enough to require correction. However, the situation is never hopeless, and some means for dealing with this and other difficulties are discussed in the next section.