The TM arrangement proposes to use 4 subwoofers to achieve greater summation, and proposes to keep the acoustic centers nearby to maintain an omnidirectional coverage pattern.
Michael "Mija" Krieg Schreiber
Making subwoofer system designs is one of the most interesting and fun stages in the design of a system, sub arrangements allow you to change and control the directivity of low-frequency energy. One of the needs in low frequency arrangements is to achieve uniform coverage throughout an enclosure and avoid contamination within the stage, for this TM arrangements were born.
To achieve the above, we must understand the behavior of a single subwoofer at different frequencies, for this we can rely on acoustic prediction tools, such as Nexo's NS-1 software.
NS-1 allows you to create an acoustic space, place sound sources and finally predict their behavior. For this exercise we are going to use a square space only in order to study the behavior of the S118 subwoofer of the STM family.
Let's look at the coverage of this subwoofer at different frequencies.
We can see how a single subwoofer behaves as an omnidirectional element, although it seems to be more omnidirectional in the low frequency than in the high frequency. This change in directivity is related to the wavelength, remember that the wavelength of 31.5 Hz is equivalent to 10.8 meters while the 120 Hz is equivalent to only 2.8 meters, so for 125 Hz the same speaker cabinet can represent an obstacle. This idea is a fundamental part of TM arrangements, the question we can now ask is, how can we make coverage omnidirectional for all frequencies?
If we add a second S118, back to back to the previous sub, we will have 2 sound sources working simultaneously and therefore we would expect to obtain greater sound pressure. But what about the coverage pattern?
The coverage looks split in the form of a cross. This is because, for that frequency, the separation between the acoustic centers of the speakers is equivalent to a full wavelength. So if we wanted to avoid these cancellation aisles the separation between the acoustic centers should be minimized, preferably it should be less than 1/4 of the wavelength.
If we now perform the same exercise, but turn the speakers so that they are face to face, we can appreciate how the destructive interaction is no longer present, even this coverage pattern seems to be more omnidirectional than the pattern of a single S118 at 125 Hz. By turning the subwoofers we have achieved greater closeness between the acoustic centers of the speakers, this closeness results in a very consistent and uniform sum throughout the coverage of this arrangement.
We can also think that the acoustic center of this S118 subwoofer is located near the front of the speaker. This coverage pattern is much more uniform than the pattern of a single speaker, but it is not yet entirely omnidirectional, will it be possible to achieve an even more uniform pattern?
One of the challenges that Thomas Mundorf faced when developing this arrangement was to achieve a high SPL (sound pressure level) with uniform coverage, thinking about the high SPL we know that the greater the number of speakers we will achieve greater summation. With this in mind the TM arrangement proposes to use 4 subwoofers to achieve greater summation, and proposes to keep the acoustic centers nearby to maintain an omnidirectional coverage pattern.
If we calculate the SPL gain that we achieve in this ratio of 4 subwoofers we could say that:
SPL sum=20log((Final sub num)/(Original sub num))
So:
Sum of SPL=20log(4/1)
So this configuration achieves 12dB SPL of sum and an omnidirectional coverage pattern. It is because of this behavior that Thomas Mundorf says that this arrangement functions as a point source in horizontal coverage, but Thomas also speaks of this arrangement as an inline arrangement in vertical coverage. LineArray arrangements have very particular vertical coverage characteristics and the best way to study this behavior is by means of subwoofer lines.
Vertical management control
If we now model the vertical coverage of the array we have created and add more subwoofers to each block or cluster, we can see that the vertical coverage narrows as we add elements.
This is actually the principle of operation of online arrangements, the general idea of this phenomenon is, that having a line of omnidirectional sources the coverage pattern will be related to the length of the array and the frequency that is reproduced. Once again we can associate this phenomenon with wavelength and phase.
If we imagine the sub line and place ourselves below it, it will be evident that all the elements will reach our listening position at different times, this causes that in that position the signal of the different subwoofers adds up with phase differences so there will be considerable attenuations. On the other hand, if we position ourselves at the front of the arrangement, the differences in time and phase of each element will be minimal and therefore we will have good summation in the front of the arrangement.
The coverage of a sub line is subject to frequency. When the sub line is the same size as the wavelength of a frequency, for that frequency the array will have approximately 72° coverage. When the sub line is twice the length of a frequency, for that frequency the array will have 36° coverage. So the coverage of a line is proportional to the frequency.
I mean. If we have a line of subwoofers that measures 3.4 meters, which is the wavelength of 100 Hz, we will have 72° of coverage for 100 Hz. If we analyze the coverage of that same line for 200 Hz, which has a wavelength of 1.7 meters, we will see that the coverage is 36°. On the other hand, if we analyze the coverage of 50 Hz, which has a wavelength of 6.8 meters, we will see that the coverage is 144°.
If we now analyze the coverage of our TM array with 7 S118 subwoofers we can confirm that:
Underneath the arrangement there is considerable attenuation and that we have achieved managerial control.
For more recorded frequencies the managerial control is lower
For higher frequencies the managerial control is greater
These arrangements have great advantages when performing 360° assemblies, since they allow to achieve a uniform coverage throughout large enclosures and at the same time they manage to eliminate low-frequency contamination on the stage, so that achieving a good mixture will be easier.
Here is an example of a TM arrangement for a 360° assembly at the Arena Ciudad de México.
*Michael "Mija" Krieg Schreiber, has worked in various companies, productions, nightclubs and facilities as a designer, technician and operator of sound systems gathering to date 8 years of experience. Among others are Audio Performances, Meyer Sound Mexico, Hi tech Audio, the papal Mass in San Cristóbal de las Casas 2016, Corona Capital 2014, National Auditorium, among others. Nowadays he concentrates his career in educational activities offering different presentations and professional audio courses. Among the schools and organizations with which he has collaborated are: AES Mexico, IPN Instituto Politécnico Nacional, Instituto Tecnológico y de Estudios Superiores de Monterrey, UNITEC Universidad Tecnológica de México, SAE INSTITUTE México, EMEH Escuela de la Música del Estado de Hidalgo, G Martell, Pro Audio Puebla, among others.
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