EVERYTHING YOU WANTED TO KNOW ABOUT PROFESSIONAL AUDIO BUT WERE AFRAID TO ASK
Professional Audio
Thinking of setting up a professional audio system, here some guidelines for selecting the right equipment that can play well together and make some awesome music in your home or commercial environment.
Understanding Crossover Frequency
The crossover directs frequencies to the driver best-suited for each particular band. Tweeters (HF drivers) produce high-frequencies but cannot reproduce bass frequencies. Conversely, woofers (LF drivers) do a horrible job of reproducing high frequencies.
In a three-way design, there are two crossover points, and is a two-way design, there is one crossover point.
For example, with a three-way design with crossovers points at 400 Hz and 2.9kHz, all frequencies below 400Hz (low-pass filter) are directed to the LF drivers. Frequencies between 400Hz and 2.9kHz are directed to the MF driver (band-pass filter) and frequencies above 2.9kHz are directed to the HF driver (high-pass filter).
In a two-way design, there is one crossover point: Frequencies above the crossover point are directed to the HF driver and frequencies below the crossover point are directed to the LF/MF driver.
A 2 ½-way design is a hybrid 3-way crossover. Typically there are two LF drivers with low bass frequencies going to one driver and higher bass frequencies going to the other.
The most important thing to understand about Impedance is that is varies with frequency and power – it is not a set value.
Nominal Impedance
Impedance (Z) is the calculated value of Inductance (L), Capacitance (C), and Resistance (R).
There are two specifications for Impedance: Nominal which is more or less an average value, and minimum, which is a critical value to know when matching loudspeakers to amplifiers. Typically, the lower the frequency the lower the applied impedance the loudspeaker presents to the amplifier. Under normal conditions, a speaker’s impedance may fluctuate from 3.2 Ohm Z to as high as 17- or 18-Ohm Z.
The most important thing to understand about Impedance is that is varies with frequency and power – it is not a set value.
Impedance (Z) is the calculated value of Inductance (L), Capacitance (C), and Resistance (R).
There are two specifications for Impedance: Nominal which is more or less an average value, and minimum, which is a critical value to know when matching loudspeakers to amplifiers. Typically, the lower the frequency the lower the applied impedance the loudspeaker presents to the amplifier. Under normal conditions, a speaker’s impedance may fluctuate from 3.2 Ohm Z to as high as 17- or 18-Ohm Z.
Rule of Thumb:
A 4Ω amplifier can drive an 8Ω loudspeaker but it will produce 50% less power. Conversely, an 8Ω amp driving a 4Ω loudspeaker produces 50% more power.
You CANNOT measure the Impedance of a loudspeaker with a DC Ohmmeter and get an accurate reading. All you can check for is an open or short circuit.
UTS – Maximum Output
Maximum Output is the highest SPL (Sound Pressure Level) a loudspeaker can produce over an extended period. Burst or transient volume is not measured as that number can be quite high but is misleading. Proper measurement of Maximum Output is on the form of an average.
Typically, measurements are taken in the dBA scale. dBA is the international standard for measuring environmental sound pressure level. A-weighting compensates for how the human ear responds to sound (i.e. it is less sensitive to lower frequencies).
Recommended Amplifier Power
The minimum amount of power needed to move the speaker diaphragms enough to make sound and the maximum safe power before over-excursion of the drivers occur.
Loudspeakers require sufficiently large input power to reproduce sound, but an under-powered loudspeaker may produce hard clipping (distortion) which ultimately causes the voice coil windings to overheat, resulting in a dead speaker. Too much power and the voice coil over-extends, resulting in a blown speaker. More speakers are damaged from under-powering than from over-powering simply because the damage is cumulative and not heard as distortion until it’s typically too late.
Rule of Thumb:
In the real world, it’s best to ignore the low-end spec. That’s shown to show the minimum amount of power needed to move the voice coils in any appreciable manner.
A good match is an amplifier capable of producing between 60% and 110% of the recommended maximum.
For example, the R5 shows a Recommended Amplifier Power of 15 to 200 WPC. A good match for this speaker would be an amplifier that produces between 120 Watts per Channel ((200x.6)*100) and 220 WPC ((200×1.1)*100). In terms of performance and damage potential, an amp rated lower than 60% of maximum poses far more danger to the loudspeaker than an amp rated above the maximum.
Harmonic Distortion
Harmonic Distortion is the presence of odd-order harmonics that interfere with the natural even-order harmonics of the fundamental frequency.
Sounds develop harmonics in multiples of the fundamental frequency. Even-order harmonics sound warm and pleasant while odd-order harmonics are generally unpleasant to listen to. Odd-order harmonics are produced by clipping (the top or bottom of a waveform is abruptly cut off) and interfere with the 3rd and 5th harmonic of the fundamental.
This measurement is taken at an SPL of 90dB with a microphone placed 1 meter from the speaker. The specification is presented as the ratio (%) of the sum of all the harmonic power present compared to the SPL of the fundamental frequency.
For example, the R5 produces <0.3% THD for the frequencies from 120Hz to 20kHz. A THD below 1% is considered acceptable.
Frequency Range and Frequency Response
Frequency Range is the actual span of frequencies a speaker can produce.
Even though a speaker may produce some sound above or below the listed number, meaningful Frequency Range is within the -6dB attenuation line.
Typical in-room bass response refers to how a loudspeaker operates in real-world environments. Rooms tend to boost lower frequencies because of a phenomenon known as signal loading. Typically, in a normal room under typical circumstances, a loudspeaker may go as far as 25% beyond the lowest specified frequency. This is also sometimes referred to as Low Frequency Extension.
Frequency Response calculates Frequency Range versus Amplitude (which is variable by frequency and applied power). A Frequency Response Curve (FRC) shows the flat response (within +/- 3dB) throughout a speaker’s frequency range.
Sensitivity
Sensitivity measures how much volume is produced from a given amount of power.
The higher the sensitivity rating the more volume you get when compared to a loudspeaker with a lower rating. Below 84dB is considered poor while 92dB and above is considered excellent.
Sensitivity is often confused with efficiency, but it is not exactly the same thing. Efficiency measures the ratio of power converted to sound energy compared to heat energy. Sensitivity measures the amount of SPL (Sound Pressure Level) a loudspeaker produces for a given amount of power. The specification is typically derived in an anechoic chamber with 1 Watt of power at 1kHz with a microphone placed 1 meter away).
Rule of Thumb
With each 3db gain in volume, output power is doubled. If your speaker/100-Watt amp combo plays at 87dB and you want to play at 90dB you will need 200 Watts of power, and so on.
Distortion
Every component adds some level of distortion along the audio chain, changing the original signal along the way. Generally, mass-market audio devices will tend to add a little coloration to compensate for things they don’t do well. Once you get into the realm of higher-end audio components you will begin to hear real differences and a closer reproduction of the original sound.
Distortion: refers to any signal that appears in a reproduced sound that was not in the original recording or program source.
Audio reproduction is affected by distortion, and every component in the chain adds some level of distortion. The higher the quality of the device, the less distortion or deviation from the original. An old adage among audio enthusiasts is that a system is only as good as its weakest link, so if you spend tons of money on your amplifiers and speakers but you are streaming a lo-res file for example, the sound you get out of your system will only be as good as your streamed lo-res files.
Since the level of distortion determines how good our listening experience can be, let’s look at the three types of distortion that affect audio.
Harmonic Distortion
A harmonic is a signal that is created from a fundamental signal. Amplifier circuits tend to be the biggest culprit in the creation of harmonics in your audio chain. An amplifier creates even order harmonics of a fundamental signal. For example, at 100 Hz an amplifier circuit creates a harmonic 200, 400, 800, 1600, 3200, 6400, 12800 Hz. Each harmonic gets lower in amplitude, so the harmonic at 200Hz will cause you far more grief than the harmonic at 800Hz, and by 12.8kHz the harmonic energy is so low you won’t hear it. Each instrument, especially acoustic instruments, also create their own harmonics, so oftentimes the artificial harmonic from the amplifier circuit is not noticeable.
Harmonic distortion can also be filtered out, as in the case of a Class D amplifier that is being used to drive a subwoofer or other LF frequency driver. In that case, since the designer only cares about fundamental frequencies below 200Hz (for example), everything above 200Hz is filtered out (lo-pass filter) so we never hear the distortion.
Clipping can also produce a tremendous amount of harmonic distortion. In the extreme, distortion caused by clipping can completely destroy the listening experience and even damage equipment (especially speakers). Clipping is caused by overloaded components or weak components that are being asked to do more than they were designed to do. A low power amplifier driving a high-quality pair of speakers will often clip. The danger here is the energy not being used by the speaker is transferred into heat energy. Build up enough heat energy in a speaker coil and the windings will open and you’ve got yourself a dead speaker.
Harmonic distortion (listed as Total Harmonic Distortion (THD)) in component specifications is expressed as a percentage of the original signal and is typically produced in electronic components, but can also be produced by loudspeakers.
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