General Speaker Frequently Asked Questions by Andrew Jones

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    Chris Walker
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    Q1. What is the advantage of different cone materials for speakers?

    A1. Loudspeaker drivers come in many sizes, varieties, intended applications, cost, and performance requirements. There is no such thing as one size fits all. As a result, the best cone material is not necessarily the newest flavor of the month. The best cone material is the one that best fits the purpose for which the driver is intended to be used. Take a subwoofer driver for instance. It is generally large in order to move enough air, but the cone can be heavy. In addition, it needs to work down to low frequencies but not up to high ones. In fact, the range is so restricted that often the cone resonances are way above the required operating range for even traditional cones materials such as paper. In his instance, the cone is required to resist the high back pressure exerted on it from the cabinet so must be stiff when acted on by that pressure.
    Another example would be a driver intended for very high efficiency. In this case the cone must be very light. If we also need it to act as a midrange, it must have an extended high end response. So the cone resonances must be correspondingly high. The factor that sets this parameter is the stiffness to weight ratio. Normally, we only care about getting the ratio as high as possible. This can be achieved by high mass and very high stiffness, or low mass and not so high stiffness. Clearly the optimum, in this example, would be the low mass version.
    At ELAC we are very careful to design our drivers specifically for the system in which it will be used, rather than develop a generic driver that is sold for use in as many systems as possible. In his way the driver is optimized to meet the many variables that exist and must be optimized in any particular system design.

    Q2. What is the advantage of a 3-way speaker compared to a 2-way?

    A2. Let’s consider the perfect speaker. It would contain just one full range driver. This driver would work down to the lowest frequency (20Hz) at full output level, and respond up to the highest frequencies (20kHz or beyond). It would have a constant directivity pattern and high efficiency to minimize amplifier requirements.
    Not only does this driver not exist, it is not even possible given the laws of physics! To reproduce low frequencies we must move a large volume of air, albeit relatively slowly. This demands a large diameter driver which by necessity is quite heavy.
    For high frequencies we only need to move a small volume of air, but we must do so very quickly.
    These two requirements are mutually opposed. In addition we are not going to e=achieve a constant directivity response.
    For these reasons we are forced to consider a two way system. One driver dedicated to high frequencies and one to low, with a xover network to direct the appropriate frequencies to the appropriate driver. This works well enough fora lot of applications, and is certainly a cost effective solution for the most part. However, it does have it’s limitations.
    Firstly, the woofer must ideally have a good clean response up to a couple of octaves above the xover frequency. Likewise the tweeter must have clean response to a couple of octaves below the xover frequency. These requirements are not easy to achieve.
    If we now consider a three way system, we have increased complexity, but with attendant benefits. Each drive, (bass, midrange, tweeter) now is only required to operate of a narrow frequency range. Their design can be better optimized for his range. The overall directivity response can be better engineered.
    The cost in this approach is the need for a complex three way xover network. Often these are poorly designed, leading to less than perfect blending together of the drivers, and an audible discontinuity in the response. ELAC engineers however have decades of experience in computer aided xover network design, and contributed to some of the earliest research in this field. As a result we are able to engineer three way system with extraordinary driver integration.

    Q3. Which type of speaker is better for music? A bookshelf or floor standing speaker?

    A3. There is no simple answer because it depends on your associated equipment and the listening environment. First let me say that the answer is applicable to both movies and home theater. Typically, bookshelf speakers have limited bass output, both in terms of frequency extension and bass dynamic capability.
    Clearly, the small size of a bookshelf limits the size of bass driver that can be accommodated and so limits how much air the driver can move. To reproduce low frequencies, we need to move a lot of air.
    Secondly there is a relationship between the cabinet size, the low frequency extension and the efficiency that is attainable, governed by the pesky laws of physics. One cannot have bass extension, high efficiency and a small box. If we therefore limit the size of box, as in a bookshelf speaker, we must give up efficiency or bass response. Obviously for a floor stander we have greater cabinet volume along with the capability of accommodating either larger drivers or multiple smaller drivers, so the limitations of the bookshelf are largely overcome. We can have somewhat higher efficiency, or better bass response and more bass output.
    So, do we need all of the gains that the floorstander can give us? This will depend upon musical tastes, how loud you like to listen, how big is your listening space, how far away from the speaker you listen, and how powerful your amplifier is. What we do attempt in our designs is to give the bookshelf speakers almost the same bass extension as the floorstanders, so that you are still able to hear most of your music, just at a lower maximum volume level.

    Q4. When should I use a subwoofer?

    A4. You may well get away without using a subwoofer at all, even with our bookshelf speakers. If your listening habits are predominantly with music that doesn’t have super low bass recorded, or not at high levels, then you can enjoy the simplicity of just the main pair of speakers. However, if you have a large listening space, like to listen to music very loud or to music that contains lots of low bass or want to use the speakers as part of a home theater setup then a subwoofer would be advantageous.

    Q5. What is the advantage of speaker bracing?

    A5. Speaker bracing is a technique used to minimize speaker cabinet vibration. Why would we want to do this? Well, for most accurate reproduction of recorded music, we require that only the woofer or tweeter radiate sound as governed by the input signal. However in practice life is not so simple. Along with the drivers moving, we get unwanted vibration of the speaker cabinet. The stiffer and stronger we make the cabinet, the less unwanted vibration we get. We can get this in many ways. We can use thicker panels for the cabinet, we can use superior material that are inherently stiffer and stronger, or more massive. We can also add bracing inside the cabinet to tie opposite panels together so that they cannot move so readily.
    All of this costs money, of course. The skill in designing to a price point in a budget speaker is to decide where to spend the money in the overall design of the speaker. Sometimes, bracing does not significantly reduce the vibrations but just moves them to a higher frequency where they might in fact become more audible! So careful balancing of choices, less cost in the cabinet but more cost in the drivers for example, can result in a higher overall performance.

    Q6. What is stereo imaging?

    A6. Stereo imaging is the illusion created of a spread of sound between the two speakers that mimics the sound heard at the original performance, or that is created in the studio. It typically has parameters that include perceived width, height, depth, specificity and instrumental separation. Some systems and technologies can give a better image than others (our concentric drivers for instance), and some listeners judge this parameter less important than the musical performance itself, such as perceived rhythm and timing, musical communication etc.

    Q7. Why are speaker impedances different?

    A7. Speaker impedance is an indication of how much power will be drawn from the amplifier. For a given voltage from the amplifier, a lower impedance will result in a higher current draw and more power being delivered form the amplifier to the speaker. All other things being equal, higher impedance is a good thing. Of course, all things are not equal!
    The speaker with the lower impedance will always sound louder when directly compared to one with higher impedance, so this is often a way that manufacturers can gain some advantage in such comparisons. However, as stated earlier, there is no free lunch. It’s louder because it is taking more power from the amplifier.
    Unfortunately, there is not a good, easy way to characterize the speakers’ impedance or its demands placed upon the amplifier with music signals.
    The most universally accepted definition simply states that the minimum impedance should be no lower than 80% of the rated impedance, For an 8 ohm speaker, this means 6.4 ohms minimum and for a 4 ohm speaker 3.2 ohms minimum. However, not only do many manufacturers ignore this basic specification, it doesn’t state over how much of the frequency range the impedance stays close to the minimum. One that only occasionally dips low will place very different demands upon the amplifier than one that is always close to the minimum.
    Therefore it is not easy to state unequivocally whether a particular speaker will work on a particular amplifier. However, we do stay within the accepted limits: the minimum impedance does not drop below 80% of the rated impedance, and we try and minimize how much of the frequency range that it is at that minimum value.

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