Technical Discussions of Audio and Loudspeaker Issues
the Audio Engineering
Topic No. 9
Examination of Loudspeakers
Just as a physician can use his stethoscope to quickly detect a heart murmur, you can use a stethoscope to quickly reveal the worst flaws of a loudspeaker system. By providing a high degree of acoustic magnification a stethoscope allows the speaker designer to quickly locate troublesome panel resonances, air leaks, and a host of other ills that plague most loudspeaker systems. After locating the sources of various problems the designer can take action to reduce the spurious sounds that emanate from even our best speakers.
If you have ever battled the hisses, wheezes, buzzes, and leaks that spoil the audio entrusted to our care, then you will be pleased to know that help is at hand in the form of a simple listening tool that has been used for centuries: The stethoscope.
The only sound coming from a loudspeaker enclosure should be from the driver components themselves, not the surfaces or seams of the box. With most speakers, even very good ones, you will find it quite easy to hear unwanted sound being radiated by the enclosure panels. The designers goal is to reduce this junk sound to an absolute minimum. We especially want to minimize any highly colored resonances.
We normally hear in stereo as a result of our ears being placed at two separate points in space separated by several inches. When you use the stethoscope both of your ears hear from the same single point at the position of the listening piece. You are now listening not in stereo, but rather in mono mode. This allows us great acuity in examining a particular sound source . . . such as a beating heart, or a murmuring cabinet seam.
How to perform a stethoscopic examination of a loudspeaker enclosure:
Start by playing your choice of program material (usually either pink noise or music) through the one speaker you wish to examine. Keep the volume moderate since you will be using a powerful acoustic magnifier to listen. Have a small wood scrap handy, say a 3" by 3" piece of MDF. If you do not have a scrap of wood handy then just use a book.
Next place the earpieces of the stethoscope in your ears and place the listening piece on the wood block. The sound you hear now is your normal background sound that will serve as your reference. Proceed to compare the background sound you hear with the chest piece on the wood block to the sound you hear when the chest piece is placed on the enclosure. Carefully move the listening piece about the surface of the enclosure noting the sound level and occasionally referring back to the wood block. Ideally the enclosure should sound just like the neutral wood block. Normally you will want to use a very light touch, just enough pressure to seal the listening piece against the surface of the box. You can increase the pressure on the listening piece to hear the effect of added damping at the test point. As you increase the pressure you should notice a reduction in the level of the sound coming from the panel.
While listening to a panel tap the box adjacent the listening piece to hear the nature of the local panel resonances. Then listen for coloration in the panel sound that is similar to the coloration in the tap tone. Often the sound transmitted from the panel takes on the response characteristics of the panel resonances. A high Q panel resonance can translate into a sharp peak in the sound radiated from that panel.
Once the hot spots are located the effects of damping and bracing can be evaluated immediately without the need for complex test gear. Any cabinet buzzes and vibrations revealed by the examination can be treated directly at the source.
For the following tests you should remove the chest piece from the end of the stethoscope and listen via the tube only. This reduces the size of the "sampling area" and is easier on the ears as well.
By placing the listening tube several inches in front of each driver in a multi-way speaker system you can hear each individual driver to a great extent. You can pan between the drivers by moving the listening piece between the woofer and tweeter. This allows very sensitive panning between drivers to listen for phase effects. . . such as a notch in the frequency response at crossover.
Drive the speaker from a signal generator set in the low bass region, say 30 to 40 Hz, to listen for air leaks at the enclosure seams, driver mounting edges, and jack plates. Even the smallest leak will be heard clearly. Snooping around the surround of a woofer driven this way can reveal the presence of "edge tics".
Turn the volume down and insert the bare stethoscope tube through the enclosure vent to evaluate the internal enclosure resonances. Once you see how useful the stethoscope is you will find new tests of your own.
Caution! High SPL Zone!
As you handle the stethoscope you will quickly notice how loud the simplest noises can be. Handle the stethoscope carefully and place the ear pieces in your ears only at the last minute and remove them the instant you have made your evaluation. Guard your hearing!
Thank You Dr. Ashley
With such immediate usefulness it is surprising that more speaker builders do not employ stethoscopes in their routine speaker testing! I first learned to use a stethoscope for loudspeaker evaluation from Dr. J. R. Ashley when I took his class in Loudspeaker Design (EE 496) at the University of Colorado in 1976.
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