by Neil Bauman, Ph.D.
© March, 2018
A man wrote,
I am a member of the technical committee at our church and we are investigating installing a hearing loop system. When an addition was built for our new sanctuary, we ran the cable for a loop system. What we are planning on doing is running a line from one of the auxiliary output jacks from our sound/mixer board. I assume that we would hook into an amplifier of some type.
At present we are not sure if there is a specific type of amplifier module which will act as a loop transmitter. We have a collection of small public address amplifiers and I am wondering if we can utilize one of those. This means that we would go from the mixer/board to the transmitter, to the amplifier and connect to the loop wire. The room is approximately 50′ x 75′. I do not know what the total length of the loop wire is.
Am I on the right track? Any advice/recommendations would be greatly appreciated.
Looping your church is a wonderful idea and hard of hearing people will love you for it. I love listening via properly-installed hearing loop systems.
However, this is not a do-it-yourself project if you want to produce beautiful, clear sound via a hearing loop. In order for your loop system to work well, you need more knowledge than you currently have. You’d be wise to enlist the help of a professional hearing loop installer before you proceed further. Let me explain why.
The goal of a hearing loop system is to let anyone sitting anywhere inside the looped area clearly hear speech via the t-coils in their hearing aids.
In order to achieve this, your new hearing loop system needs to meet certain technical standards. These standards are laid out by the International Electrotechnical Commission (IEC). The appropriate IEC standard for hearing loop systems is number 60118-4.
The reason to follow this standard is not to comply with more “red tape”, but to ensure that your new hearing loop system will perform properly so your people will hear beautiful, clear sound.
This standard lays out the basic requirements of a hearing loop system. There are four technical points that deal with how well a hearing loop system will perform. They are:
1. The looped area will be free from annoying magnetic interference. Therefore, the first thing you need to do before you install a loop system is to test the area of your church you are planning to loop to determine whether there is any significant magnetic interference present. (Note: you cannot hear magnetic interference with you bare ears. You need to listen for it via a hearing aid with a t-coil on an equivalent device, or with a special field strength meter (FSM) with an audio jack such as professional loop installers use. With the FSM, you can visually see how strong the interference is, and at the same time hear what it sounds like.) Magnetic interference typically is a loud humming or buzzing sound. This is very annoying to people wearing hearing aids while in t-coil mode.
The IEC standard wants any magnetic interference to be less than -47 dB, although they allow interference levels up to -32 dB. (Larger numbers are better in this case.) Professional loop installers try to keep any interference below -40 dB for best results as interference at -32 dB will still bother some people.
If the interference level is too high, you’ll need to track down and eliminate the cause of the interference. Two major sources of magnetic interference are a) magnetic interference caused by faulty grounds anywhere in the electrical system in your church, and b) faulty wiring such as running the hot and neutral wires in separate conduits thus creating a ground loop. (Both of these problems often occur as a result of changes/upgrades to the church wiring over the years.) Other sources of magnetic interference may be faulty ballasts in certain fluorescent light fixtures, dimmer switches or “noisy” motors. You’ll need an electrician to fix any wiring problems to eliminate the magnetic interference before you are ready to install a hearing loop system.
2. The peak volume of the signal on the loop needs to be set at 0 dB at the center of the loop. Thus, anyone who has a hearing aid with a properly-adjusted t-coil will be able to hear the sound at a comfortable volume without having to adjust the volume of their hearing aids every time they enter a looped venue. This ensures that some people don’t complain that the loop is too loud and others complain it is too soft.
3. The volume of the sound signal must not vary more than ± 3 dB anywhere inside the looped area. This means you can sit anywhere inside the looped area you want—front, back, middle—and hear just as well wherever you sit without having to adjust the volume of your hearing aids. Nor will there be a “doughnut hole” in the center of the loop where the signal is too soft to hear well.
4. The frequency response of the loop will be flat (± 3 dB) between 100 Hz and 5,000 Hz. This part of the standard ensures that you will hear clear, intelligible sound. If the frequency response drops off in the higher frequencies—a common problem with incorrectly designed loops—you will still hear speech, but it will sound “muddy” and be largely unintelligible.
How you meet these latter three requirements at the same time is all in the details.
A simple perimeter loop, such as you have installed works wonderfully well in smaller rooms such as home loop systems, but fails as the room size increases. This is just the laws of physics in action.
Let me explain. Alternating current flowing though the loop wire sets up a varying magnetic field that extends out from the loop wire. The t-coils in your hearing aids “pick up” this magnetic field. The farther you are from the loop wire, the weaker the magnetic field (signal). Thus, in a large hearing loop, a person sitting close to the wire would hear a much louder signal than a person sitting in the center of the loop—the point farthest away from the loop wire. This is the “doughnut hole” effect, mentioned above.
Therefore, the maximum width of a loop is where the signal at the center of the loop is greater than 3 dB as compared to the signal at the loop wire.
This distance is also greatly influenced by any nearby metal. This means that the more metal there is in a floor (think rebar or iron mesh in a concrete floor) the smaller the width of the loop needs to be in order to meet IEC standards 2 and 3 above. (Conceptually think of magnetic metals such as iron and steel as “sucking” the signal out of the loop. The result is you need more power going into the loop and narrower loops to offset the presence of such metals.)
Another problem that crops up as the hearing loop size increases is that the high-frequency response noticeably drops off. This is a bit technical, but I’ll make it simple. As the signal frequency increases, the resistance to alternating current flow in a wire increases. This is called impedance.
Ohm’s law tells us that for a given voltage, as the impedance increases, the current flow reduces. Since impedance increases with frequency, this means that the current flow is less in the higher frequencies. In turn, this means the magnetic field is weaker in the higher frequencies too.
Since the higher-frequencies give speech much of its intelligibility, if you can’t hear them well, you don’t understand much of what you hear.
Yes, you hear a person speaking (because you can hear the louder low-frequency sounds which have less impedance) but you cannot understand speech well (because you don’t hear the softer high-frequency sounds which have more impedance).
There are two parts to the solution to this problem. First, you do not use typical audio amplifiers such as the public address (PA) amplifiers you already have. This is because they are voltage amplifiers. One result is that the current drops significantly in the higher frequencies due to increasing impedance. This reduces the strength of the magnetic field in these frequencies, which in turn, makes speech sound muddy and less intelligible as we just saw.
Instead, you want to use special loop amplifiers. (technically called loop drivers) which are current amplifiers. In contrast to voltage amplifiers, current amplifiers keep the higher-frequency signals more or less comparable to the lower-frequency signals by keeping the current reasonably constant across all frequencies—thus giving you clear speech signals.
Therefore, if you want beautiful, clear speech signals, getting a proper loop driver is necessary.
The second part of the solution is to make smaller loops so you don’t need as much current to give a strong signal (0 dB) in the center of the loop.
Taken together, these two things solve the problem of No. 4 (above).
As you can appreciate, a perimeter loop works well for small rooms. However, as the size of the room increases (and if there is metal present in the floor), you need to make the size of the loop smaller, which seems like an impossible situation when you are trying to loop an entire church sanctuary.
In your case, the sanctuary at 50 x 75 feet is basically too large for a single perimeter loop. With a room that size, the strength of the magnetic field in the center of the room would be greater than 3 dB less than the signal closer to the loop wire. Thus you’d have a “doughnut hole” in the middle that would not meet IEC standards. The result would be that people sitting near the wire will complain the signal is too loud, and those in the center of the room will complain that the signal is too soft.
So you’re stuck, right?
But wait. There is a solution. Instead of one large loop, you can make a series of smaller loops. Rather than making each smaller loop completely separate, professional loop installers use one long piece of wire, but arrange it such that there are a series of smaller loops covering the whole area to be looped.
For example, you could take a perimeter loop and fold it into a “square” figure-of-eight shape. Now you have essentially made two smaller loops. The result is that the distance from the loop wire to the center of each of the resulting loops is half of what it was before. Thus, you cut down on the variability of the signal strength between the signal near the loop wire and the signal at the center of the loop. When this variability is down to only ± 3 dB, you have met one part of the standard.
However, if that isn’t enough, you can take each half of the figure of eight and fold it again. This results in essentially four loops. Now the distance from the loop wire to the center of the loop is only one-quarter of what it was as a perimeter loop. This may be adequate for your size of loop. If not, you could fold each “loop” in half again and get eight smaller loops.
Professional loop installers make an educated guess as to the likely loop width, then lay out a test loop. They measure the output and variability of the loop and keep adjusting the loop width until the results meet the IEC standard. (They can also use a special computer program that takes the data from the test loop and tells them the desired loop configuration, wire size, driver size, etc.)
Without knowing the particulars of how much metal is in the floor/walls near the loop wire, I can’t recommend a specific loop configuration, but if I were installing it, I’d probably try a quadruple figure-eight configuration. In other words, you’d essentially have four smaller loops all constructed from one continuous piece of wire.
This is just the simplest kind of multiple loop system that you can use. Professional loop installers have a number of more complex loop configurations they use depending on the characteristics of the building they are looping, but the concept is the same.
Furthermore, professional loop installers often use not just one loop system (a single array), but two separate loops overlaying each other, but off by half a loop width. These are known as phased arrays. They are much more complicated as they require two loop drivers and a phase shifter to feed the two loops 90° out of phase with each other.
However, the even better results of phased arrays are worth the additional initial cost. Phased arrays give much “smoother” signal coverage as they eliminate “nulls” directly over the loop wires crisscrossing the floor. In addition, they are not susceptible to orientation problems in the forward direction.
With loop systems, orientation problems can occur because loops transfer the signal to the t-coils in your hearing aids best when both are oriented in the same plane (vertically). One problem single loops have is that, for example, when you bow your head in prayer the loop signal can fade out as your t-coils are no longer oriented vertically. Phased-array loop systems get rid of this problem.
As you can now appreciate, looping larger venues is not a do-it-yourself project unless you are skilled in the technical aspects of hearing loop systems. However, if cost is a major factor, you can cut the cost by working with a professional loop installer and doing much of the installation work yourself. However, you need to leave the design and calibration of the loop to a professional. (Incidentally, to calibrate and test loops, you need test equipment worth many hundreds of dollars.)
There is another factor you apparently overlooked. You didn’t say what size of wire you used for the loop you installed. You don’t just use any size of wire you feel will do the job. The size of the wire you need varies depending on its total length. You want to keep the total impedance of the loop to around 3 ohms.
(We briefly mentioned impedance before. Actually, there are two components to impedance—the impedance to alternating current flow is called inductance reactance. The other component of impedance is the resistance of the wire itself—simply called resistance. Taken together, they give us the total impedance to current flow.)
We want to keep the resistance of the loop wire itself to around 0.7 ohms. Thus, if you have a longer loop, you need a larger size wire since resistance increases as the wire size gets smaller. And for shorter loops you want a smaller wire to increase the resistance.
If you use too large a wire, the resistance is too low and some of the power remains in the driver and heats it up, rather than putting the power out into the loop. If you use too small a wire, there is too much resistance in the wire and you don’t get the full power output you’re your loop driver resulting in inadequate signal strength. So again, you should work under the guidance of a professional loop installer.
These are just some of the factors you need to consider when designing and installing a hearing loop system. There are a number of other considerations I have omitted in order to keep this simple, but you get the idea.
As you can appreciate, installing a hearing loop in a church is much more complex than looping the den in your home for watching TV. I suggest at the very least you get a professional hearing loop installer on board to design and calibrate your hearing loop system. In order to keep the cost down, some loop installers will let willing members of your congregation do much of the actual work of installing the loop under the loop installer’s general direction.
Having said that, I have worked with smaller churches and have designed the hearing loop, then remotely guided them in installing and calibrating simple hearing loops with them using my test equipment. So if you are a small church and want to loop your sanctuary, there are ways to do most of the work yourself under the direction of a professional loop installer—especially if cost is a serious problem.
You want all the hard of hearing people in your congregation to be able to clearly hear via your new hearing loop system—and that will only happen when you install the loop to meet the IEC hearing loop standards.
If you want a highly-qualified professional loop installer to work on your hearing loop project—whether it is a small church or a large stadium or anything in between—let me know. I am a member of a network of the most highly-trained loop installers in the world. We guarantee our results will meet or exceed the IEC 60118-4 standards so you know you will hear beautiful, clear sound. Contact me and I’ll put you in touch with one of them if you do not live in my area.