FAQ
Frequently Asked Questions (& Answers) About Sound Reinforcement
| Impedance |
| Power Handling |
| Amplification |
| Loudspeaker Acoustics and Vibration |
| General Acoustics |
| Audio Electronics |
| DAS Product |
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 How do I calculate the impedance that results from connecting several loudspeakers? |
 The total impedance for a number of speakers in parallel is equal to the impedance of one divided by the number of speakers. For instance, two 8-ohm speakers in parallel total 4 ohms (8 ohms divided by 2 speakers equal 4).If the speakers are in series, the total impedance is the sum of the individual impedances of each speaker. For instance, two 8-ohm speakers in series total 16 ohms (8 ohms plus 8 ohms equal 16). In pro audio, series connections are rarely used outside low power applications such as background music, paging, or surround sound. |
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| Why is the impedance that I measure with a multimeter different from the system's nominal impedance? |
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The multimeter looks at resistance, not impedance. Impedance is the opposition to the passage of alternating current and varies with frequency. Resistance is the opposition to the passage of direct current. Resistance is normally lower than nominal impedance. For instance, an 8-ohm nominal impedance cone speaker would typically have a (DC) resistance around 6 ohm. See any DAS spec sheet for an impedance curve. Dedicated impedance meters for audio use exist; they typically use a sinewave generator to measure impedance at 1 kHz, so you need to look at the speaker's impedance chart to see what value you should expect at that frequency.
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| What is RMS power? |
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"RMS power" is a misnomer derived from the usual method of calculating speaker power, which is to divide the RMS voltage squared by the impedance (V2/Z). RMS is the acronym for root-mean-square, which relates to the mathematical equation that obtains the average of the modulus of an alternating signal (that is, with positive and negative cycles). The correct expression for "RMS power" is average power. The ratio between RMS and peak values is different for each signal.
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| What is average power? |
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It is the result of deriving power from an RMS value of voltage or current. It is the correct way of writing "RMS power".
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| What is programme power? |
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Programme power is an archaic term dating back to when power tests used sine waves. Many manufacturers, including D.A.S. Audio, apply a convention whereby programme power is twice the average ("RMS") power, although other manufacturers may use different power ratios, which may even vary from product to product. Nowadays the term "programme power" has no specific meaning, although it might be used as a power amplifier recommendation for controlled applications.
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| What is peak power? |
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Peak power is the power that corresponds to the signal peaks that the amplifier feeds the speaker during a power test. Typical test signals have peaks that carry four times the average signal power, which is why peak power is normally four times the average ("RMS") power.
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| What is AES power? |
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The AES (Audio Engineering Society) publishes a standard for the measurement of loudspeaker component parameters. Generally speaking, this standard calls for a 2-hour test using pink noise with specified dynamics and with a frequency content that matches the component's frequency range. Although it is a standard for components, it is often extended to the different ways on an active system. The rating always derives from RMS voltage or current measurements, therefore the rating corresponds to average ("RMS") power.
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| What is IEC power? |
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The IEC (International Electro technical Commission) publishes standards for loudspeaker measurement. We can speak of IEC power by following those standards. Specifically, IEC 268-1 defines a signal with specified dynamics and a frequency content that tries to resemble that of real music. The rating always derives from RMS voltage or current measurements, therefore the rating corresponds to average ("RMS") power. D.A.S. Audio utilises IEC shaped noise for enclosure power testing.
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| Can I run the amplifier with 2 ohm loads? |
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Virtually all professional amplifiers will accept loads down to four ohms safely in stereo mode. Many are rated for 2 ohm loads but often will run into overheating protection when used with 2 ohm loads, particularly in high ambient temperature and high output power applications. Check the user's manual of you amplifier to find out the lower impedance it will take. Never use a total impedance load that is lower that the lowest impedance that the amplifier will take (for instance, hooking up too many enclosures in parallel).
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| Should I enable the amplifier’s high-pass (sub-sonic) filter? |
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It is good practice to use a high-pass filter for the lowest frequency component in your system (i.e. a full-range enclosure; or a subwoofer if using subwoofer reinforcement). If your electronic crossover has a high-pass filter, use it, and disable the amplifier’s. Use the amplifier’s high-pass filter only if there is no other filter of this type anywhere else in your system chain. Beyond their operating ranges, speakers produce little sound. Instead, they radiate large amounts of heat. In addition, frequencies below a speaker’s operating band will generate a lot of cone movement. By using a high-pass filter, we can put the low frequency energy that would otherwise be wasted into usable amplifier power for the speaker’s operating band. We also prevent mechanical damage from excessive cone movement. Thus, high-pass filtering maximises acoustic power and makes systems more reliable. We recommend enabling a high-pass filter that is equal or higher than the system’s lower usable frequency. For instance, for an R-215 used as a full-range box, set the high-pass frequency no lower than 40 Hz. No lower than 80 Hz for a DAS Factor-5. DAS signal processors provide system specific high-pass filtering. DAS Energy series amplifiers also provide sub-sonic filtering with two useful cut-off frequencies. |
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| What amplifier should I use with transformer loudspeakers? |
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There are specific amplifiers for 70/100V lines, sometimes referred to as constant voltage. For applications where little power is required, these amplifiers are appropriate. As the power requirements for the system increase, conventional amplifiers are often used. For instance, a conventional amplifier delivering 1200W per channel at 4 ohm can be considered a stereo 70V amplifier. Likewise, an amplifier delivering 600W per channel at 4 ohm can be considered a monophonic 100V amplifier in bridge mode. Similarly, an amplifier delivering 300W per channel at 4 ohm can be considered a monophonic 70V amplifier in bridge mode. To calculate the required amplifier power for a 70 or 100V line, simply sum the power for the amplifiers connected to an amplifier channel. Example: if we want a 100V line with 50 DAS Factor-5T enclosures (with built-in multi-tap transformer to select different power levels for 50, 70 and 100 volts) such that 25 of them use the 30W tap and 25 of them use the 15W tap, we would need an amplifier delivering a minimum of 25x30+25x15=1125W. In this case, assuming all speakers share the same signal (zone), it may be worthwhile, and probably more economic, to use a conventional 600W output per channel at 4 ohm amplifier in bridge mode, such as a DAS Energy Series E-12. |
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| What is the damping factor? |
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Damping factor is related to amplifier output impedance, and can be explained as the ability of the amplifier to control the movement of a loudspeakers' voice coil. For a given speaker load, the damping factor is, in practice, given mostly by the cable impedance, so that one should not place too much importance on this amplifier parameter. It is also important to note that damping is directly related to the speaker load, so that using an 8 ohm load will yield twice the damping factor compared to a 4 ohm load. To maintain a high damping factor, always use cable of appropriate thickness (gauge).
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| What cable should I use? |
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To ensure quality sound (high damping factor) and minimum power loss, the speaker cable needs to have a large enough cross section depending on the cable length, number of speakers per channel and speaker impedance. The longer the cable and the more speakers, the thicker the cable has to be. The lower the impedance of each speaker, the thicker the cable has to be. The most important feature for a cable is its wire area; you can discard cables with magical properties and astronomical price. Amplifier user's manuals often have tables to aid the selection of wire gauge for different cables lengths and speaker impedances. The DAS Energy Series amplifiers' user's manual provides tables of this type. |
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| What is efficiency? |
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Efficiency is the ratio between the acoustic power output and the electrical power input to a loudspeaker. Power not converted into sound ends up as heat. Loudspeakers are very inefficient devices that put out more heat than sound.
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| What is sensitivity? |
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Sensitivity is the sound pressure level (SPL) which is measured at a distance of 1 metre in front (on-axis) of a loudspeaker which is fed 1 watt of electrical power. If the measurement uses a different distance or power, the result is scaled so that it refers to the standard 1W/1m.
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| What is this excursion thing then? |
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(You can put the sun lotion and Swiss army knife down for now). On a cone or diaphragm, excursion is the movement outward and back or from the voice coil rest position. It is inversely proportional to frequency, so that low frequencies generate more movement than high frequencies. Over-excursion takes place when excessive movement causes scratching or deforming of the voice coil, or even fracture when the coil support "bottoms-out" and hits the speaker's bottom plate. To prevent damage, avoid equaliser gain on frequency ranges below a speaker's operating range (such as 20 Hz on a SUB218, or 40 Hz on a Factor -5), use a high-pass filter and be careful when utilizing large power amplifiers. |
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| What is the directivity index? |
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The directivity index (Di) is an expression in decibels of the directionality of a sound source. Narrow coverage systems have a high directivity index, whereas wide coverage systems have a low one. Di is the decibel form of directivity Q factor, so both are related as Di = 10*log(Q).
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| What do we mean by long / short throw? |
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A long throw system is one that has high sensitivity and narrow directivity (high Di), whereas a short throw system is one with lower sensitivity and a wider coverage (low Di). These are informal terms without precise definitions.
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| What is the difference between direct radiation and horn loading? |
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With direct radiation, a vibratory element (such as a cone loudspeaker) radiates directly into the air. Horn loading takes place when the loudspeaker couples with air through a horn. In doing that greater efficiency is achieved, that is, more sound and less heat. We also get more directionality. The theoretical size for a horn is inversely proportional to frequency. That is why, for bass bins, the horn of which would be impracticably large, the horn has to be "folded" inside the box, thus referred to as a "folded horn" box. |
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| What is horn EQ? |
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A compression driver and horn combination exhibits inherent high frequency roll-off. This is compensated at the passive filter network in passive systems, while active ones (with no passive network elements) take care of the correction at the electronic crossover.
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| What is a third octave? |
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A third octave is more like a "third of an octave", i.e. an octave divided by three. An octave is a frequency band that spans within two frequencies with a ratio of 2:1. For instance, the 1000 Hz octave band spans from 707 to 1414 Hz. Adjacent octave bands are also spaced by a ratio of 2:1, such as the 500 and 1000 octave bands. The octave band centre frequencies have been standardised by ISO (International Organization for Standardization) to rounded figures as follows: 63:125:250:500:1k:2k::4k:8k:16k Hz A third of an octave is a frequency band that is three times smaller than an octave band, so that an octave band logically comprises three third octave bands. Thus, the 1000 Hz third octave has a bandwidth of 891-1122 Hz. And the 1000 Hz octave band encompasses the 800, 1000 and 1250 Hz third octave bands. Third octave bands are often used in audio because they relate to the way human hearing (and mammals' in general) works. The most used form of pro-audio equaliser is the 1/3rd octave band "graphic" equaliser, the term "graphic" deriving from the fact that glancing at the fader positions we can (arguably) get an idea of the resulting EQ. Analysers that measure frequency content in 1/3rd octave bands are often referred to as RTAs. The 31 third octave audio band centre frequencies have been standarised by ISO to rounded figures as follows: 20:25:31.5:40:50:63:80:100:125:160:200:250:315:400:500:630:800:1k:1k25:1k6:2k:2k5:3k15:4k:5k:6k:8k:10:12k5:16k:20k Hz For the musically inclined, a third octave is equivalent to four semitones. |
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| What is a balanced line? |
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There are two basic ways to transport an audio signal with microphone or line level: Un-balanced line: Utilizing a two-conductor cable, it transports the signal as the voltage between them. Electro-magnetic interference can sum to the signal as undesired noise. Connectors that carry un-balanced signals have two pins, such as RCA (Phono) and 1/4 in. (6.35 mm, often referred to as jack) mono. 3-pin connector such as XLR (Cannon) may also carry un-balanced signals if one of the pins is unused. Balanced line: Utilizing a three-conductor cable, one of them acts as a shield against electro-magnetic noise and is the ground conductor. The other two have the same voltage with respect to the ground conductor but with opposite signs. The noise that cannot be rejected by the shield affects both signal conductors in the same way. At the device’s input the two signals get summed with opposite sign, so that noise is cancelled out while the programme signal doubles in level. Most professional audio devices use balanced inputs and outputs. Connectors that can carry balanced signal have three pins, such as XLR (Cannon) and 1/4 in. (6.35 mm) stereo. |
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| I need a replacement diaphragm for a 1 in. compression driver with 16 ohm impedance, but the current spare is 8 ohm. |
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DAS only manufactures high frequency compression drivers in a single impedance version, the impedance of which lies somewhere between the 16 and 8 ohms "standard" nominal impedances. Historically, the same driver may have been labelled as either 8 or 16 ohm depending on circumstances, but the actual impedance is and has always been the same, and therefore the replacement diaphragm is the same. |
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