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Characteristics of a subwoofer

For some it is the most important piece of a home theater system , for others a huge and unnecessary box that gives them headaches in domestic installations. For the majority, one more element of our home cinema that allows us to improve the overall quality of sound at low frequencies.

The subwoofer or speaker for low frequencies has two basic missions: to reproduce without distortion frequencies to which the rest of the loudspeakers are not able to reach and reproduce the sound stored in the tracks of audio .1 of the multichannel formats.

This .1 is also known as low frequency effects channel or LFE and in it we find sub-bass sounds (below about 40 Hz) whose objective is to make the viewer vibrate in effects like explosions and shots, but also recreate a realistic environment reproducing the very low frequency sounds of motors, closed rooms, outdoor scenarios, etc.

The first audio tracks that made up this LFE channel used it sporadically and only to highlight certain key moments of the action. However, most movies now use it almost continuously to not only reinforce some sounds, but rather to recreate a realistic sound scene .

This has made them have become an almost essential element of any good home theater system worth its salt. And we say almost because it can be replaced in some cases by the front speakers, if these are able to reproduce low frequencies without problems and the A / V receiver supports the option of passing the LFE channel to the front.

Types of subwoofers

We can distinguish fundamentally two types of subwoofers (without entering forms, colors, power, formats or type of boxes, etc.), liabilities and assets .

The former, as their name suggests, do not have active amplification elements that give them enough power to produce sound. They are, so to speak, a transducer placed in a box , something similar to the rest of the front, central and effects cabinets that we have commented on throughout the special, but focused on the exclusive reproduction of low frequencies.

 

The passive, therefore, will need an external amplifier that provides the electrical power needed to move its membrane and produce sound. They do not have volume or phase or frequency controls , something that we will have to manage from the A / V receiver or from the amplifier that we use to power them. A clear example of how to build a passive subwoofer step by step can be seen in our special Sub-Delta project .

The active subwoofers, on the other hand, have an amplification stage integrated generally in the acoustic box itself. They are usually the most frequent to find in the market, since they incorporate everything necessary to put them to work from the first minute, just connect them to the electrical outlet and the receiver.

In addition, they usually incorporate extra features such as volume control, phase control, cutting frequency management , enhancement of certain low frequencies, even automatic on and off systems, remote control or wireless communication with the receiver.

Characteristics of a good subwoofer

A good subwoofer should be able to ideally reproduce all frequencies up to almost zero Hz without distortion and with sufficient acoustic pressure. As you can imagine, this usually does not happen, since reproducing frequencies below 10 or even 20 Hz without distortion (or with a non-perceptible distortion) is quite difficult.

So much that the majority of receivers of the market filters the signals of low frequency with which they work (many put the real limit in about 5 or 10 Hz) and a great part of the active subwoofers that we can find in the stores have subsonic filters that work with signals of up to 20 or 25 Hz to protect the loudspeaker from the lowest frequencies.

And it is that working at lower frequencies means that, in general, the speakers have to work hard to give an undistorted sound and that it is relatively simple that they leave the linear operating zone , depending on their characteristics and how the box is constructed. , favoring the possible breakage of the speaker (or reducing its useful life) if we put it at high volumes.

 

This does not mean that a subwoofer filtered at 20 or 25 Hz is bad, moreover, in most domestic installations the result may be more satisfactory than a full-range one, since the latter will tend to produce unwanted vibrations in objects and decorative elements of the room (furniture, shelves, lamps, paintings, and even floor and walls) that although striking in a first listen, eventually we will get tired.

If you ask me my opinion, a good home theater subwoofer should be able to go down passively, without the acoustics of the room and without distortion problems up to frequencies of about 30-35 Hz with falls of about 3-6 dB and reach about 20 Hz with maximum drops of about 9 dB (of course then all this is equalizable).

To achieve these requirements we will have to have a driver of at least 8 inches, one being recommended (it would be optimal to have 2 or 4 subwoofers, although this is beyond the possibilities of most non-dedicated domestic rooms) of 10, 12 and even 15 inches if we can afford to install it.

As for the shape of the box, when reproducing very low frequencies, the wavelengths will be higher than the size of the box, so they serve perfectly cubic and rectangular shapes.

With respect to the design format, the usual thing in the market is to find subwoofers of bass-reflex type (especially in those with less inches) or with passive elements that allow to lower more in frequency, but have the counterpart of introducing a group delay in the final sound that can sometimes be annoying giving the sensation of a softer sound, with less punch, or even hinder the location of the box in the room.

 

In my opinion, if you can choose, better a boxed subwoofer with a driver as big as your budget and room size allow. With it we will generally get a sound with more impact, with more punch, although we probably lose some extension at low frequencies (something that is otherwise solvable with equalization and that is generally not necessary if, as we said before we got it right up to 30- 35 Hz).

With regard to high frequency cutting, it is normal to work up to frequencies between 100 and 200 Hz , at which point the LFE signal of the audio tracks ceases to have content in most cases (although in recent years I have seen editions of tracks whose LFE channel reaches well above 500-600 Hz, something unusual and which does not make much sense the truth)

What power do I need?

And the power? How much will I need? Well, it depends on the final result we want to achieve. Especially because the audio tracks in the cinema have a rather high dynamic range at the level of the LFE channel, which in the ideal case requires a lot, a lot of power to be able to offer the acoustic pressure demanded by the soundtrack.

However, as in most cases we do not have a dedicated room or a pump-proof ear (if you search the Internet you can see cases of people who configure their home equipment to have sound pressures of more than 115 or even 120 dB, something exaggerated that can be harmful to our ears), the normal thing is to lower the expectations of sound pressure of the subwoofer to levels that we will feel comfortable in the nearly two hours that usually last movies.

In general, for a room of average size of about 20-25 square meters, with about 150-250 watts RMS should be enough for our subwoofer, although of course this will depend on multiple internal and external factors. Among the first we will logically have the sensitivity of the speaker, something that we can not vary and depending on which we will obtain for a same power a higher or lower sound pressure (the higher the sensitivity, the higher the sound pressure for the same excitation power).

 

And among the external factors we will have to consider how the room is acoustically isolated, the position of the subwoofer in it and above all how are the rest of the speakers in our home cinema . If you have followed the special you will have noticed that it made me very heavy with that of all the speakers reaching at least up to 40 Hz (and that the frontals if possible were full band).

One of the reasons for this personal craze is that if the rest of the speakers are able to lower a lot in frequency, the range of these that we will have to send to the subwoofer will be smaller, minimizing the power needed in the subwoofer to the Once we facilitate their installation in the room.

If instead our main speakers are small and unable to reproduce frequencies below 80 or even 100 or 150 Hz (as typical surround speakers 3 – inch sell us with kits 5.1), all these frequencies are the have to send the subwoofer , needing it to support more power and getting a less localized sound.

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Active or passive subwoofer Which is better?

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