A Review By: Alphonso Soosay
Basic Intro: “Audio signal source”, let’s say acoustic musical instruments and vocals coupled with microphones for a recording session produce an electrical “Image of the Sound”. To be precise, it produces an electrical signal that has the same frequency and harmonic content, and a size that reflects the relative intensity of the sound as it changes. The job of the amplifier is to take that electrical image and make it larger, large enough in power to drive the coils of a loudspeaker. Having a “high fidelity” amplifier means that you make it larger without changing any of its properties. Any changes would be perceived as distortions of the sound since the human ear is amazingly sensitive to such changes. As soon as the amplifier has made the electrical image large enough, it applies it to the voice coils of the loudspeaker, making them vibrate with a pattern that follows the variations of the original signal. The voice coil is attached to and drives the cone of the loudspeaker, which in turn drives the air. This action on the air produces sound that more or less reproduces the sound pressure variations of the original signal.
Testing Speakers In Production:
Let’s start off with Testing Audio Speakers as it is very complex because there are a number of stages to testing the speaker components: Importantly is the “Crossovers”, the “Drive units”, and then the “Complete system”. It is not simply a test to see if the speaker makes any wide-ranging sound. In reality, very detailed measurements are taken along every step of the testing stages.
One of the most important components of a loudspeaker is something called the “Crossover” also known as a filter network or a dividing network. It’s basically a small circuit board that contains electronic components such as resistors, capacitors and inductors. It’s responsible for dividing the entire frequency band into the individual ‘Low, Mid and High frequencies’ which is also known as Woofer, Midrange and Tweeter components that the individual speaker drivers will deliver.
Let’s say, if you want to direct the low frequency sound to the woofers only in a loudspeaker. Consequently the filter on the crossover will actually prevent high frequency sound from getting to the woofers. Similarly, for the tweeter, manufacturers make sure that they do not allow low-frequency sounds getting to the tweeter, just high frequency sounds, so the filter will make sure only those high frequency sounds gets transferred to the tweeter. The crossover network design, along with the driver design, is also what allows them to manipulate the frequencies of curves which are the most important design element of a great-sounding pair of loudspeaker.
For that reason, a manufacturer will design pretty much all of their components in-house, including all of their crossover networks, they have the benefit of being able to keep very beneficial control on the quality of these electronic parts used. Since they are such a critical component of the speaker, every single component in the Crossover is tested and measured for accuracy.
Transducers: The majority of quality Speaker manufacturers manufacture our own “Transducers” (drive units or woofers, tweeters and midranges) , although it would be very easy to buy them off the shelf because there are lots of manufacturers making these types of speaker drivers. One of the ideas and really the most important reason to make them in-house, other than having complete design control over the part is that they then have full control over the quality of each part that goes into the loudspeaker that you might purchase.
There are a few stages along the way while manufacturers are building transducers where they actually perform precise measurements on them. As a concluding test, just like the crossover networks, and just like the entire system is tested before it gets boxed up and shipped to a customer, each individual drive unit is boxed up and tested.
Manufacturers usually think about investments in state-of-the-art measurement procedure. It allows them with a very quick signal sweep to perform an entire suite of measurements. In just a couple of seconds, they are able to measure not only the frequency response or the amplitude response of the drive unit, but at the same time they are also measuring for input impedance, buzzes and distortion. Also all those items are measured individually in their in-house anechoic chamber that has a microphone mounted in it. Each one of the speaker box will get a stamp of approval or a ‘Pass’ sticker if they are within precise specification.
Manufacturers will determine their own tolerances and they are normally very detailed. This is really one of the most important part of any audio speaker manufacturing along with detailed crossover network design that make up the speaker system. Solid Cabinets are very important, but it’s also those components that go into a loudspeaker that are so important, and that’s why manufacturers go to such great lengths to make sure they are tested and they meet their stringent quality requirements every step of the way before it goes to the superstore audio showroom.
Mini-Chamber Test: After manufacturers have been through and individually tested the crossover network and the entire speaker drives units individually, they then assemble them into their design speaker cabinet. Followed by testing the complete speaker as a system, to make sure that each pair is “Balanced” properly and everything is wired correctly, and that there are no issues with a loose connections anywhere in the system.
Quality manufacturers will at least normally have a small anechoic chamber on their production line. They position the speaker on belt and mount the speaker pointing inwards into the chamber, where there is a condenser microphone mounted. Manufacturers normally do a frequency response curve test using the same measurement and same system that they do in their large anechoic chamber. They also do a polarity test. The reason they do a polarity test is that there is a possibility (although it happens very infrequently) for the polarity of the input terminal to be reversed from what it should be. What would happen in that case is that you would get one speaker that was wired in-phase and one speaker that was wired out-of-phase, the sound would be all incorrect and you wouldn’t get any Bass. So it’s a very important detail to check.
Finally manufacturers will do a sweep test at a high level where the calibrated measurement microphone is actually the operator’s ears, because when in that position they are trying to detect internal issues that are very difficult to pick up with a microphone: things like wires buzzing inside the cabinet, even a loose screw or something like that. So all of those aspects are tested essentially by Ear, after all of the additional steps are done.
Ultimate Test in Anechoic Chamber: The measurements that take place on an L/C/R speaker are numerous. Manufacturers measure the front centre and rear independently, and they also measure them collectively. Manufacturers perform a full frequency of curves to measure the audio output power. The reason they do that on every model is because there are different frequencies communications occurring, and those interactions between the front and back drivers can change those individual response curves, so they have to do a very detailed measurement here whereas the other models may just need a single on-axis frequency response.
Hopefully the above basic information has given enough insight of what manufacturers do for their production testing of their loudspeakers in production. I am sure that you can foresee that it’s very involved, and the luxury of manufacturing almost all of the components in-house is that manufacturers can do this sort of testing along the way and guarantee a high-quality speaker product that matches very closely to their endorsements.