One day in the acoustics room with Frank Kuhl

That is how the MONACOR expert optimises frequency responses of speakers


Frank Kuhl's business card says 'Development Engineer Acoustics'. He is the kind of legendary tinkerer you rarely find these days, an autodidact with several decades of experience. As a trained fitter he wrenched on motor bikes and stages before turning his true passion into his profession. Since he was 12 years old he has been working with loudspeakers. You can tell when watching him move through his apparently cluttered workshop like a fish in the sea. There, he runs acoustic tests with new speaker series. One of his main tasks is to optimise the frequency responses of new speakers for various applications.

What is the job of a Development Engineer Acoustics?

Customers frequently ask for speakers with new requirements for specific ranges of application. For instance, a speaker is required to provide background music for a store and at the same time high speech intelligibility. Frank Kuhl knows what is required of the speaker and passes the information on to his distributor. Then he orders a sample and can test - and in most cases tinker as well. His main task is to develop the optimum sound experience.

Using software, he analyses how the speaker sounds in the acoustics room. During these tests there must not be any sound reflections or background noise from outside.

No shunt yard for PA technology

“Some PA dealers are called 'shunt yards' for speakers,” Frank Kuhl says. “They buy a white-label product from some international distributor, stick their label on and sell it directly. That's different at MONACOR.” Frank Kuhl estimates most dealers make little to changes to speakers from distributors. In his opinion a shame, for distributors hardly ever develop their product for various applications and the specifications sometimes are rather guide values.

“If specifications are incorrect, work is unnecessarily hard for installers and planners.”

— Frank Kuhl

The incredible silence of an anechoic room

Entering Frank Kuhl?s acoustics room feels strange. The steel door is 50 cm thick and is oppressively reminiscent of a bunker - yet mass stops sound. Inside the room you almost feel as if you were under water. It is so silent; the room is so anechoic, you feel pressure on your ears. “I hardly notice that anymore”, says Frank Kuhl.

“We used to call it 'dead room' but there is always a little sound. That's why we call it more accurately 'anechoic room' now. It is perfect for measurements.”

— Frank Kuhl

In this room, Frank Kuhl places a microphone at an exact distance to the speaker, which is to be tested. Then he lets the speaker reproduce its full spectrum from the lowest to the highest tone. The recorded frequency response is the base of his work.

Improve PA technology in 4 steps

Basically, every speaker has to go through the same 4 steps with Frank Kuhl to make it into our product range.

1. Testing and reproducing the frequency response with a sine-sweep measurement

One of the main questions the Development Engineer wants to answer regarding each new speaker is, how linear is the frequency response and is it worth touching it up? Ideally, the frequency response is consistent. However, no speaker meets this condition.

A consistent frequency response means that all frequencies are reproduced at the same volume. Frank Kuhl wants to know where there are significant fluctuations and outliers in the frequency response. A good speaker should reproduce pleasant sound and should not overload, even at high frequencies. On the other hand, low tones should be audible at all.

Drawing: frequency response of the speaker ESP-232/WS

A consistent frequency response means that all frequencies are reproduced at the same volume. Frank Kuhl wants to know where there are significant fluctuations and outliers in the frequency response. A good speaker should reproduce pleasant sound and should not overload, even at high frequencies. On the other hand, low tones should be audible at all.

Drawing: frequency response of the speaker ESP-232/WS

“Many speakers have shortcomings regarding the lower tones. However, if the speaker is mounted in a corner anyway, that fills the bass range.”

— Frank Kuhl

2. Testing the impedance

Aside from the frequency response, Frank Kuhl also tests the impedance, i.e. the resistance. All speakers must comply with the impedance stated in the specifications, for otherwise the amplifier/its power amplifier is in danger.

3. Testing THD

THD stands for Total Harmonic Distortion. When testing the THD, Frank Kuhl measures the sounds which are not part of the audio signal proper, i.e. sounds generated and interspersed by the speaker. Imagine it like that: Without the individual sound of instruments, the standard pitch A would always sound the same. Since instruments resonate differently, we can distinguish e.g. violin and guitar. It is the same with speakers. Main influences are the cone, the magnetic field in the cabinet and the voice coil. That is how overtones are created.

4. Optimising the speaker

Frank Kuhl moves components, such as resistors and crossover networks, or replaces them with his own. Then he tests again, until the speaker is perfect for the intended application.

“Ears are the most sensitive for frequencies between 2,000 and 4,000 Hz. When a frequency response spikes in this range, you know the speaker is screaming in your ears. I straighten that out.”

— Frank Kuhl

What happens after anechoic room and workshop?

Frank Kuhl takes apart every new speaker series. In some cases, however, the test result is so far from what is expected that even Frank Kuhl is not able to do up the speaker or that the price-performance ratio is simply not satisfactory. He then orders a new sample or sometimes even designs a completely new product himself.

“I optimise the components of a speaker until it sounds good for the application. It does not matter if I have to send back 3 samples or to develop a completely new design.”

— Frank Kuhl

With his new speaker concept or the modified sample, Frank Kuhl asks the supplier if they could put that into series production. If the answer is yes and the price is right, a new speaker series is born - thanks to Frank Kuhl and his passion for good speakers, always tailored to the respective application and the price range.

MONACOR INTERNATIONAL offers a high quality and perfectly adapts products to their target groups. Look on our respective brand website to find out what our product brands got in store for you.
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