Applying CLIO for Hard Hearing People
By Rob C.J. Kersten
Introduction
Without doubt, the CLIO is primarily applied in the High Fidelity world, more in particular in load-speaker development / improvement projects with lengthy debates about three dB’s or less.
Often less understood are the acoustical limitations for the average presentation- meeting- and living-room. Somehow there seems to be an erroneous consensus by many people that acoustical problems can be attacked with powerful amplifiers and loudspeakers. This acoustical problem is even worse for about 20 percent of the listeners because they face impaired hearing (hearing loss of 20 dB or more often varying across the audible frequency band). Listening to music may be difficult for these persons - understanding what is said may be next to impossible.
A non-profit organization faced tremendous acoustical problems at one of there locations. Listening to speeches, lectures et cetera was very difficult and tiring in the suspect meeting room. The author was consulted for defining, investigating, and correcting the reported acoustical issues at a near zero budget. Consequently the field proven adagio: "Keep things as simple as possible" was followed.
The first attack was based upon the most valuable instrument available – the human ear. Listening to both male and female speakers – with and without amplification – hinted to PA installation distortion and reverberation problems. Complaints about the induction loop for hearing aids have also been mentioned. Knowing this, a plan was made to apply the CLIO to verify the following areas:
Obviously the CLIO system did not provide all required capabilities but the needed adapters and not to complicated sensor have been developed, verified and calibrated efficiently using the CLIO. Critical fundamental issues have been double-checked by independent instruments to prevent erroneous conclusions.
The PA system
The principle diagram for this installation looked rather Spartan – a good microphone – a low noise 60 dB microphone amplifier with symmetrical line output – some patching – a PA amplifier – and a combination of wall- and ceiling-loudspeakers.
Two problems had to be solved to interface the CLIO system with the PA installation namely:
Figure 1 – Microphone test adapter
First of all the microphone amplifier was verified. It demonstrated to be optimized for speech applications with a low roll off characteristic (-3 dB point at 150 Hertz). See figure 2 below for the Frequency response. There's apparently nothing wrong here.
Figure 2 – Frequency response Microphone Amplifier
Next the whole PA system was verified from microphone input up to the loudspeaker terminals. The result is shown in figure 3 below.
Figure 3 – Frequency response PA installation
Needless to say that the author was shocked by the difference. No low – no high - but instead a lot of distortion. The problem was not caused by the main amplifier itself but somewhere hidden in the cabling / patching. To keep the story short: The installer had implemented but not documented a microphone impedance matcher with amplitude control between the microphone amplifier (line level!) and the main amplifier. Removing this overdriven / non-linear device solved all electrical issues.
Verifying room acoustics
Reverberation requirements are very different for music and speech applications. A bathroom may sound like a cathedral (even to poor singers) but the same reverberation makes speech intelligibility impossible. In other words, you may hear the speech but can’t understand what’s being said. A tiring experience to say the least. Speech applications require a small reverberation time (RT60) of 0.6 seconds for rooms with ≤ 300 persons seating capacity.
RT60 measurements are normally excluding the impact of microphone feedback. This will unveil the characteristic of the room itself. Purposely the same measurement was made with feedback present. Microphone sensitivity was set to this room’s standard. The RT60 was verified with the CLIO at several spots inside the room. Figure 4 below shows a typical measurement including the feedback effect.
Figure 4 – Room Reverberation Times (RT60)
Some conclusions have been drawn at this point namely:
It is obvious that the measured values exceeded the requirements in particular at low frequencies. This was partly solved by the installation of soundproofing tiles to the ceiling and also by attacking the feedback itself as mentioned in the following chapter.
Feedback
The feedback can be made visible with CLIO operation in FFT mode (see figure 5 on next page).
Figure 5 – FFT analyzes of Feedback signal
A sheet of acoustical isolation material was moved around the microphone to detect the feedback path. It was noted that one loudspeaker nearest the microphone was the prime contributor. Reducing the speakers output level solved this issue.
A direct wave from the loudspeaker to the ear is the main contributor for sound eligibility as is an acceptable sound to noise ratio (≥ 25 dB). Vague understanding possibilities have been noted in the center of the room. The CLIO impulse function was used to monitor the direct wave sound pressure level at various locations. This made clear that the loudspeakers inadequately covered the center region. Two additional ceiling loudspeakers solved this issue. A repeat of the initial listening test was appreciated as very acceptable.
The induction loop sensor
A simple measurement instrument could be borrowed but it was calibrated for 1 kHz only. The measurement outcome was highly questioned, an accurate sensor expensive and hard to obtain leading to the decision to build and calibrate a sensor. Figure 6 shows the principle diagram for the sensor.
Figure 6 – Principle Diagram 1 V/A/m Sensor
The CLIO Generator signal was connected to a single turn magnetic loop of 1-meter diameter. The CLIO output resistance is 100 Ohm. The impedance of the loop is much lower and can be neglected so 1-Volt output converts to 10 mA current converting in turn to a field-strength of 10 mA/m in the center of the magnetic loop (law of Biot and Savart). The sensor was calibrated after some experimenting to 1 V/mA/m ± 1 dB between 50 Hz to 10 kHz with a maximum output voltage of 500 mV (field-strength 500 mA/m).
The induction loop system
The requirements for an induction loop are an average field-strength of 100 mA/m ± 3 dB at ear-level between 100 and 5000 Hz. The required current can be obtained from a special induction loop amplifier (current signal) or from an impedance matching transformer connected to an ordinary amplifier. The expected field-strength variations and required amplifier rating can be calculated with the before mentioned law of Biot and Savart. Picture 7 shows the results for a room of 10 x 6 meter at ear-level (1.20 m while seated).
Figure 7 – Expected Magnetic field-strength
The actual field-strength was measured at several locations in the room. Figure 8 shows the obtained result at four spots that should have identical amplitude.
Figure 8 – Measured Magnetic field-strength
The problem for the induction loop was caused by insufficient drive current and by magnetic attenuation at some spots due to metal construction material. Increasing the amplifier output solved the first. The latter can not be solved so the advice was given that people with hearing aids should not be seated in this impacted area.
Conclusion
The CLIO is designed with very specific applications in mind. The author used the instrument creatively in related but less obvious areas. The project mentioned above was rapidly executed with hardly using any other instrument. The end result was very satisfactory at minimal cost (free labor).
The author can be contacted by e-mail at: