Therefore, some engineers and reviewers adjust the input voltage so that it equates to 1 watt at the impedance of the input frequency or band of frequencies of the speaker under test. Assuming the amp is muscular enough to drive low impedances, then if a speaker has a lower impedance, it will play louder for the same voltage input from the amp and this is information the consumer should have.
There is an international standard that defines sensitivity, and it specifies that sensitivity be referred to SPL 1m for 2. The actual measurement can be made at any distance and input level, but must be calculated back to 1m and 2. A 4-ohm speaker will under these conditions appear to have an advantage, and many manufacturers take unfair advantage of this. This is why it is also required to quote nominal impedance, so that one can see that if the nominal impedance is low then the speaker will in fact be drawing more power from the amplifier and hence the amplifier will have to be able to deliver this.
Not all manufacturers follow the industry standards in every instance of published specifications, so the consumer needs to pay close attention and read the fine print. Because a speaker is a voltage-driven device, we would be much better off to move away from amplifier power to amplifier voltage, specified in dB relative to 2. At the same time, the minimum impedance of the load that the amplifier can maintain at this maximum voltage should be quoted.
Using this, we could directly calculate how loud the speaker could go. An example will illustrate this: suppose we take a conventional W amp into an 8 ohm load. Note: Of course this doesn't factor in loudspeaker compression or distortion which will vary in degree depending on the quality of drivers and crossover components of each particular speaker. Now, what happens about impedance If we quote 6. If we quote 3. Just cutting a hole for the magnet leaves a mm step from the tweeter face plate to the baffle plane.
The result is seen below. Response of XT25TG from non-flush mounting. Quite a difference. Below the two readings on the same graph:. This is what it should look like. Rebate a little deeper than the height of faceplate, add some rubber strips and screw the tweeter in place until the faceplate is flush with the baffle. No more, no less. Not 0. If wider you can use tape to cover the ditch.
Even a narrow ditch will cause response irregularities. Use thin painter's tape like seen above. The moving system of the loudspeaker, including the cone, cone suspension, spider and the voice coil, has a certain mass and compliance.
This is most commonly modeled as a simple mass suspended by a spring that has a certain resonant frequency at which the system will vibrate most freely. At this frequency, since the voice coil is vibrating with the maximum peak-to-peak amplitude and velocity, the back-emf generated by coil motion in a magnetic field is also at its maximum. For frequencies just below resonance, the impedance rises rapidly as the frequency approaches F S and is inductive in nature.
At resonance, the impedance is purely resistive and beyond it, as the impedance drops, it looks capacitive. The impedance reaches a minimum value, Z MIN , at some frequency where the behavior is mostly but not perfectly resistive over some range of frequencies. Knowing the resonate frequency and the minimum and maximum impedances are important when designing cross over filter networks for multiple driver speakers and the physical enclosure the speakers are mounted in.
To help understand the measurements we are about to make, a simplified electrical model of a loudspeaker is shown in figure 1. The circuit in figure 1 has a dc resistance placed in series with a lossy parallel resonant circuit made up of L, R, and C, which models the dynamic impedance of the speaker over the frequency range of interest. The dc resistance measurement is usually less than the driver's nominal impedance Z NOM. Rdc is typically less than the specified loudspeaker impedance and the novice loudspeaker enthusiast may be fearful that the driver amplifier will be overloaded.
However, because the inductance L of a speaker increases with an increase in frequency, it is unlikely that the driver amplifier actually sees the dc resistance as its load. Typically, the industry standard is to measure the voice coil inductance at Hz.
As frequencies increase above 0 Hz, there is a rise in impedance above the Rdc value. This is because the voice coil acts as an inductor.
This means that you can check the bandwidth of the electrical pulse before you begin the acoustic measurement. It is a good idea to also check the output from the power amplifier to make sure that it is not overloaded by the pulse input. This sort of overload will be hard to detect from the microphone output so check that the output of the power amp is within its rating.
Note : If you want to make a set of measurements for comparison you must use identical settings for each. Otherwise the relative levels of the measurements will be different. For this note it is assumed that the output of the pulse gen is in channel A and the return signal from the microphone in channel B.
If the pulse is made longer, then the highest frequency of measurement drops. There is a tradeoff between: getting enough power into the pulse to overcome background noise reaching the mic, and bandwidth.
There is an upper limit on the pulse height set by the power amp and speaker. If you do not want to measure above 10 kHz increasing the pulse length will improve the noise performance. Notice that the Spectrum View window is rectangular. Do not confuse this with the measurement time window. To get good results for the loudspeaker measurements you can try changing the Spectrum View window. The Blackman window is probably best. Having set the pulse length you can now start looking at the acoustic output, you will need to adjust the repetition rate of the pulses.
Set the impulses going through the speaker and observe the results on PicoScope. The screen below shows the results. Notice that the red pulse from the microphone is delayed just over 2. The measurement distance in this case was 95 cm which gives us a measured speed of sound of m.
Figure 2. When setting the repetition rate the reverberation time of the room must be considered. As stated earlier the first reflection from the room determines the measurement window we can use. However, the energy from the room reflections continues to arrive at the microphone for some time after this.
In an average lab the sound from the pulse would probably take from 0. If your room is very reverberant large with little absorption the decay may take longer, up to 5 s or 10 s.
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